Fiber-Optic Technology

Definition and Overview

Definition

Fiber-optic communications is based on the principle that light in a glass medium can carry more information over longer distances than electrical signals can carry in a copper or coaxial medium. The purity of today's glass fiber, combined with improved system electronics, enables fiber to transmit digitized light signals well beyond 100 km (60 miles) without amplification. With few transmission losses, low interference, and high bandwidth potential, optical fiber is an almost ideal transmission medium.

Overview

The advantages provided by optical fiber systems are the result of a continuous stream of product innovations and process improvements. As the requirements and emerging opportunities of optical fiber systems are better understood, fiber is improved to address them. This tutorial provides an extensive overview of the history, construction, operation, and benefits of optical fiber, with particular emphasis on outside vapor deposition (OVD) process.

1. From Theory to Practical Application: A Quick History

An important principle in physics became the theoretical foundation for optical fiber communications: light in a glass medium can carry more information over longer distances than electrical or radio frequency (RF) signals can carry in a copper, coaxial or wireless medium.

The first challenge undertaken by scientists was to develop a glass so pure that one percent of the light would be retained at the end of one kilometer (km), the existing unrepeatered transmission distance for copper-based telephone systems. In terms of attenuation, this one-percent of light retention translated to 20 decibels per kilometer (dB/km) of glass material.

Glass researchers all over the world worked on the challenge in the 1960s, but the breakthrough came in 1970, when Corning Incorporated scientists Drs. Robert Maurer, Donald Keck, and Peter Schultz created a fiber with a measured attenuation of less than 20 dB per km. It was the purest glass ever made.

The three scientists’ work is recognized as the discovery that led the way to the commercialization of optical fiber technology. Since then, the technology has advanced tremendously in terms of performance, quality, consistency, and applications.

Working closely with customers has made it possible for scientists to understand what modifications are required, to improve the product accordingly through design and manufacturing, and to develop industry-wide standards for fiber.

The commitment to optical fiber technology has spanned more than 30 years and continues today with the endeavor to determine how fiber is currently used and how it can meet the challenges of future applications. As a result of research and development efforts to improve fiber, a high level of glass purity has been achieved. Today, fiber’s optical performance is approaching the theoretical limits of silica-based glass materials. This purity, combined with improved system electronics, enables fiber to transmit digitized light signals hundreds of kilometers without amplification. When compared with early attenuation levels of 20 dB per km, today’s achievable levels of less than 0.35 dB per km at 1310 nanometers (nm) and 0.25 dB per km at 1550 nm, testify to the incredible drive for improvement.

2. How Fiber Works

The operation of an optical fiber is based on the principle of total internal reflection. Light reflects (bounces back) or refracts (alters its direction while penetrating a different medium), depending on the angle at which it strikes a surface.

One way of thinking about this concept is to envision a person looking at a lake. By looking down at a steep angle, the person will see fish, rocks, vegetation, or whatever is below the surface of the water (in a somewhat distorted location due to refraction), assuming that the water is relatively clear and calm. However, by casting a glance farther out, thus making the angle of sight less steep, the individual is likely to see a reflection of trees or other objects on an opposite shore. Because air and water have different indices of refraction, the angle at which a person looks into or across the water influences the image seen.

This principle is at the heart of how optical fiber works. Controlling the angle at which the light waves are transmitted makes it possible to control how efficiently they reach their destination. Lightwaves are guided through the core of the optical fiber in much the same way that radio frequency (RF) signals are guided through coaxial cable. The lightwaves are guided to the other end of the fiber by being reflected within the core.

The composition of the cladding glass relative to the core glass determines the fiber’s ability to reflect light. That reflection is usually caused by creating a higher refractive index in the core of the glass than in the surrounding cladding glass, creating a “waveguide.” The refractive index of the core is increased by slightly modifying the composition of the core glass, generally by adding small amounts of a dopant. Alternatively, the waveguide can be created by reducing the refractive index of the cladding using different dopants.

The Design of Fiber

Core, Cladding, and Coating

An optical fiber consists of two different types of highly pure, solid glass, composed to form the core and cladding. A protective acrylate coating (see Figure 1) then surrounds the cladding. In most cases, the protective coating is a dual layer composition.

Figure 1. Core, Cladding, and Coating

A protective coating is applied to the glass fiber as the final step in the manufacturing process. This coating protects the glass from dust and scratches that can affect fiber strength. This protective coating can be comprised of two layers: a soft inner layer that cushions the fiber and allows the coating to be stripped from the glass mechanically and a harder outer layer that protects the fiber during handling, particularly the cabling, installation, and termination processes.

Single-Mode and Multimode Fibers

There are two general categories of optical fiber: single-mode and multimode (see Figure 2).

Figure 2. Single-Mode and Multimode Fibers

Multimode fiber was the first type of fiber to be commercialized. It has a much larger core than single-mode fiber, allowing hundreds of modes of light to propagate through the fiber simultaneously. Additionally, the larger core diameter of multimode fiber facilitates the use of lower-cost optical transmitters (such as light emitting diodes [LEDs] or vertical cavity surface emitting lasers [VCSELs]) and connectors.

Single-mode fiber, on the other hand, has a much smaller core that allows only one mode of light at a time to propagate through the core. While it might appear that multimode fibers have higher capacity, in fact the opposite is true. Singlemode fibers are designed to maintain spatial and spectral integrity of each optical signal over longer distances, allowing more information to be transmitted.

Its tremendous information-carrying capacity and low intrinsic loss have made single-mode fiber the ideal transmission medium for a multitude of applications. Single-mode fiber is typically used for longer-distance and higher-bandwidth applications (see Figure 3). Multimode fiber is used primarily in systems with short transmission distances (under 2 km), such as premises communications, private data networks, and parallel optic applications.

Optical Fiber Sizes

The international standard for outer cladding diameter of most single-mode optical fibers is 125 microns (µm) for the glass and 245 µm for the coating. This standard is important because it ensures compatibility among connectors, splices, and tools used throughout the industry.

Standard single-mode fibers are manufactured with a small core size, approximately 8 to 10 µm in diameter. Multimode fibers have core sizes of 50 to 62.5 µm in diameter.

Figure 3. Optical Fiber Sizes

3. Outside Vapor Deposition (OVD) Process

Basic OVD optical fiber manufacturing consists of three steps: laydown, consolidation, and draw.

Laydown

In the laydown step, a soot preform is made from ultrapure vapors as they travel through a traversing burner and react in the flame to form fine soot particles of silica and germania (see Figure 4).

Figure 4. OVD Laydown Process

The OVD process is distinguished by the method of depositing the soot. These particles are deposited on the surface of a rotating target rod. The core material is deposited first, followed by the pure silica cladding. As both core and cladding raw materials are vapor-deposited, the entire preform becomes totally synthetic and extremely pure.

Consolidation

When deposition is complete, the bait rod is removed from the center of the porous preform, and the preform is placed into a consolidation furnace. During the consolidation process, the water vapor is removed from the preform. This high-temperature consolidation step sinters the preform into a solid, dense, and transparent glass.

The Draw

The finished glass preform is then placed on a draw tower and drawn into one continuous strand of glass fiber (see Figure 5).

Figure 5. Optical Fiber Drawing Process

First, the glass blank is lowered into the top of the draw furnace. The tip of the blank is heated until a piece of molten glass, called a gob, begins to fall from the blank—much like hot taffy. As the glob falls it pulls behind it a thin strand of glass, the beginning of an optical fiber.

The gob is cut off, and the fine fiber strand is threaded into a computer-controlled tractor assembly and drawn. Then, as the diameter is monitored, the assembly speeds up or slows down to precisely control the size of the fiber’s diameter.

The fiber progresses through a diameter sensor that measures the diameter hundreds of times per second to ensure specified outside diameter. Next, the inner and outer primary coatings are applied and cured, using ultraviolet lamps. At the bottom of the draw, the fiber is wound on spools for further processing.

Fiber from these spools is proof-tested and then measured for performance of relevant optical and geometrical parameters. Each fiber has a unique identification number that can be traced to all relevant manufacturing data (including raw materials and manufacturing equipment). Each fiber reel is then placed into protective shipping containers and prepared for shipment to customers worldwide.

4. OVD Benefits

Fiber produced using the OVD process is purely synthetic, exhibits enhanced reliability, and allows for precise geometrical and optical consistency. The OVD process produces a very consistent “matched-clad” fiber.

OVD fibers are made of a core and cladding glass, each with slightly different compositions. The manufacturing process provides the relationship between these two glasses. A matched-clad, single-mode fiber design allows for a consistent fiber (see Figure 6).

Figure 6. Index Profile of a Matched-Clad Fiber Design

The OVD process produces well-controlled fiber profiles and geometry, both of which lead to a more consistent fiber. Fiber-to-fiber consistency is especially important when fibers from different manufacturing periods are joined, through splicing and connectorization, to form an optical system.

Depressed-Clad Fiber Profile

The modified chemical vapor deposition (MCVD) process produces what is called depressed-clad fiber because of the shape of its refractive index profile.

Figure 7. Index Profile of a Depressed-Clad Fiber Design

Depressed-clad fibers are made with two different cladding glasses that form an inner and an outer cladding region. The outer cladding consists of a glass from a substrate tube that is generally purchased from an outside supplier, as opposed to the OVD method, where all of the glass is made synthetically within the fiber manufacturer’s control. The inner cladding region adjacent to the fiber core has an index of refraction that is lower than that of pure silica, while the outer cladding has an index equal to that of pure silica. Hence, the index of the glass adjacent to the core is depressed.

Questions of Strength

One common misconception about optical fiber is that it must be fragile because it is made of glass. In fact, research, theoretical analysis, and practical experience prove that the opposite is true. While traditional bulk glass is brittle, the ultrapure glass of optical fibers exhibits both high tensile strength and extreme durability.

How strong is fiber? Figures like 600 or 800 thousand pounds per square inch are often cited, far more than copper’s capability of 100 pounds per square inch. That figure refers to the ultimate tensile strength of fiber produced today. Fiber’s real, rather than theoretical, strength is 2 million pounds per square inch.

ABCs of Fiber Strength

The depth of inherent microscopic flaws on its surface determines the actual strength of optical fiber. These microscopic flaws exist in any fiber. As in a length of chain, the weakest link (or, in fiber’s case, the deepest flaw) determines the ultimate strength of the entire length of the chain. The flaws are distributed along the fiber length – the larger the flaw, the more distance between them along the fiber.

Many fiber manufacturers tensile-load, or proof-test, fibers after production. This process eliminates proof-test size flaws and larger, thereby ensuring that the flaws of most concern are removed and creating a minimum design strength for the fiber.

Life Expectancy

Fiber is designed and manufactured to provide a lifetime of service, provided it is cabled and installed according to recommended procedures. Life expectancy can be extrapolated from many tests. These test results, along with theoretical analysis, support the prediction of long service life. Environmental issues are also important to consider when evaluating a fiber’s mechanical and reliability performance.

Bending Parameters

Optical fiber and cable are easy to install because it is lightweight, small in size, and flexible. Nevertheless, precautions are needed to avoid tight bends, which may cause loss of light or premature fiber failure.

Experience and testing show that bare fiber can be safely looped with bend diameters as small as two to three inches, depending on allowable optical loss. Splice trays and other fiber-handling equipment, such as racks, are designed to prevent fiber-installation errors such as this.

5. Fiber Geometry: A Key Factor in Splicing and System Performance

As greater volumes of fiber in higher fiber-count cables are installed, system engineers are becoming increasingly conscious of the impact of splicing on their systems. Splice yields and system losses have a profound impact on the quality of system performance and the cost of installation.

Glass geometry, the physical dimensions of an optical fiber, has been shown to be a primary contributor to splice loss and splice yield in the field. Early on, one company recognized the benefit provided by tightly controlled fiber geometry and has steadily invested in continuous improvement in this area. The manufacturing process helps engineers reduce systems costs and support the industry’s low maximum splice-loss requirement, typically at around 0.1 dB.

Fiber that exhibits tightly controlled geometry tolerances will not only be easier and faster to splice but will also reduce the need for testing by ensuring predictable, high-quality splice performance. This is particularly true when fibers are spliced by passive, mechanical, or fusion techniques for both single fibers and fiber ribbons. In addition, tight geometry tolerances lead to the additional benefit of flexibility in equipment choice.

The benefits of tighter geometry tolerances can be significant. In today’s fiber-intensive architectures, it is estimated that splicing and testing can account for more than 30 percent of the total labor costs of system installation.

Fiber Geometry Parameters

The three fiber geometry parameters that have the greatest impact on splicing performance include the following:

• cladding diameter—the outside diameter of the cladding glass region.
• core/clad concentricity (or core-to-cladding offset)—how well the core is centered in the cladding glass region
• fiber curl—the amount of curvature over a fixed length of fiber

These parameters are determined and controlled during the fiber-manufacturing process. As fiber is cut and spliced according to system needs, it is important to be able to count on consistent geometry along the entire length of the fiber and between fibers and not to rely solely on measurements made.

Cladding Diameter

The cladding diameter tolerance controls the outer diameter of the fiber, with tighter tolerances ensuring that fibers are almost exactly the same size. During splicing, inconsistent cladding diameters can cause cores to misalign where the fibers join, leading to higher splice losses. The drawing process controls cladding diameter tolerance, and depending on the manufacturer’s skill level, can be very tightly controlled.

Core/Clad Concentricity

Tighter core/clad concentricity tolerances help ensure that the fiber core is centered in relation to the cladding. This reduces the chance of ending up with cores that do not match up precisely when two fibers are spliced together. A core that is precisely centered in the fiber yields lower-loss splices more often.

Core/clad concentricity is determined during the first stages of the manufacturing process, when the fiber design and resulting characteristics are created. During these laydown and consolidation processes, the dopant chemicals that make up the fiber must be deposited with precise control and symmetry to maintain consistent core/clad concentricity performance throughout the entire length of fiber.

Fiber Curl

Fiber curl is the inherent curvature along a specific length of optical fiber that is exhibited to some degree by all fibers. It is a result of thermal stresses that occur during the manufacturing process. Therefore, these factors must be rigorously monitored and controlled during fiber manufacture. Tighter fiber-curl tolerances reduce the possibility that fiber cores will be misaligned during splicing, thereby impacting splice loss.

Some mass fusion splicers use fixed v-grooves for fiber alignment, where the effect of fiber curl is most noticeable.

Figure 8. Cladding Diameter, Core/Clad Concentricity, and Fiber Curl

6. How to Choose Optical Fiber

Single-Mode Fiber Performance Characteristics

The key optical performance parameters for single-mode fibers are attenuation, dispersion, and mode-field diameter.

Optical fiber performance parameters can vary significantly among fibers from different manufacturers in ways that can affect your system's performance. It is important to understand how to specify the fiber that best meets system requirements.

Attenuation

Attenuation is the reduction of signal strength or light power over the length of the light-carrying medium. Fiber attenuation is measured in decibels per kilometer (dB/km).

Optical fiber offers superior performance over other transmission media because it combines high bandwidth with low attenuation. This allows signals to be transmitted over longer distances while using fewer regenerators or amplifiers, thus reducing cost and improving signal reliability.

Attenuation of an optical signal varies as a function of wavelength (see Figure 9). Attenuation is very low, as compared to other transmission media (i.e., copper, coaxial cable, etc.), with a typical value of 0.35 dB/km at 1300 nm for standard single-mode fiber. Attenuation at 1550 nm is even lower, with a typical value of 0.25 dB/km. This gives an optical signal, transmitted through fiber, the ability to travel more than 100 km without regeneration or amplification.

Attenuation is caused by several different factors, but primarily scattering and absorption. The scattering of light from molecular level irregularities in the glass structure leads to the general shape of the attenuation curve (see Figure 9). Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. It is these water ions that cause the “water peak” region on the attenuation curve, typically around 1383 nm. The removal of water ions is of particular interest to fiber manufacturers as this “water peak” region has a broadening effect and contributes to attenuation loss for nearby wavelengths. Some manufacturers now offer low water peak single-mode fibers, which offer additional bandwidth and flexibility compared with standard single-mode fibers. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation.

Figure 9. Typical Attenuation vs. Wavelength

Dispersion

Dispersion is the time distortion of an optical signal that results from the time of flight differences of different components of that signal, typically resulting in pulse broadening (see Figure 10). In digital transmission, dispersion limits the maximum data rate, the maximum distance, or the information-carrying capacity of a single-mode fiber link. In analog transmission, dispersion can cause a waveform to become significantly distorted and can result in unacceptable levels of composite second-order distortion (CSO).

Figure 10. Impact of Dispersion

Dispersion vs. Wavelength

Single-mode fiber dispersion varies with wavelength and is controlled by fiber design (see Figure 11). The wavelength at which dispersion equals zero is called the zero-dispersion wavelength (λ _º ). This is the wavelength at which fiber has its maximum information-carrying capacity. For standard single-mode fibers, this is in the region of 1310 nm. The units for dispersion are also shown in Figure 11.

Figure 11. Typical Dispersion vs. Wavelength Curve

Chromatic dispersion consists of two kinds of dispersion. Material dispersion refers to the pulse spreading caused by the specific composition of the glass. Waveguide dispersion results from the light traveling in both the core and the inner cladding glasses at the same time but at slightly different speeds. The two types can be balanced to produce a wavelength of zero dispersion anywhere within the 1310 nm to 1650 nm operating window.

Transmission in the 1550 nm Window

Optical fibers also can be manufactured to have low dispersion wavelength in the 1550-nm region, which is also the point where silica-based fibers have inherently minimal attenuation. These fibers are referred to as dispersion-shifted fibers and are used in long-distance applications with high bit rates. For applications utilizing multiple wavelengths, it is undesirable to have the zero dispersion point within the operating wavelength range and fibers known as non-zero dispersion-shifted fiber (NZ-DSF) are most applicable. NZ-DSF fibers with large effective areas are used to obtain greate capacity transmission over longer distance than would be possible with standard single-mode fibers. These fibers are able to take advantage of the optical amplifier technology available in the 1530 to 1600+ nm operating window while mitigating nonlinear effect, as that can be troublesome at higher power levels.

For applications such as the interconnectino of headends, delivery of programming to remote node sites, high-speed communication networks, and regional and metropolitan rings (used primarily for competitive access applications), NZDSF fiber can improve system reliability, increase capacity, and lower system costs.

Mode-Field Diameter

Mode-field diameter (MFD) describes the size of the light-carrying portion of the fiber. For single-mode fibers, this region includes the fiber core as well as a small portion of the surrounding cladding glass. MFD is an important parameter for determining a fiber’s resistance to bend-induced loss and can affect splice loss as well. MFD, rather than core diameter, is the functional parameter that determines optical performance when a fiber is coupled to a light source, connectorized, spliced, or bent. It is a function of wavelength, core diameter, and the refractive-index difference between the core and the cladding. These last two are fiber design and manufacturing parameters.

Cutoff Wavelength

Cutoff wavelength is the wavelength above which a single-mode fiber supports and propagates only one mode of light. An optical fiber that is single-moded at a particular wavelength may have two or more modes at wavelengths lower than the cutoff wavelength.

The effective cutoff wavelength of a fiber is dependent on the length of fiber and its deployment and the longer the fiber, the lower the effective cutoff wavelength. Or the smaller the bend radius of a loop of the fiber is, the lower the effective cutoff wavelength will be. If a fiber is bent in a loop, the cutoff is lowered. The cutoff wavelength of a fiber is reduced when it is cabled. The reduction is predictable, enough so that fiber manufacturers can specify a maximum “cable cutoff wavelength” for the fiber.

Environmental Performance

While cable design and construction play a key role in environmental performance, optimum system performance requires the user to specify fiber that will operate without undue loss from microbending.

Microbends are small-scale perturbations along the fiber axis, the amplitude of which are on the order of microns. These distortions can cause light to leak out of a fiber. Microbending may be induced at very cold temperatures because the glass has a different coefficient of thermal expansion from the coating and cabling materials. At low temperatures, the coating and cable become more rigid on the glass to cause microbends. Coating and cabling materials are selected by manufacturers to minimize loss due to microbending.

Specification Examples of Uncabled Fiber

To ensure that a cabled fiber provides the best performance for a specific application, it is important to work with an optical fiber cable supplier to specify the fiber parameters just reviewed as well as the geometric characteristics that provide the consistency necessary for acceptable splicing and connectorizing.

Splicers and Connectors

As optical fiber moves closer to the customer, where cable lengths are shorter and cables have higher fiber counts, the need for joining fibers becomes greater. Splicing and connectorizing play a critical role both in the cost of installation and in system performance.

The object of splicing and connectorizing is to precisely match the core of one optical fiber with that of another in order to produce a smooth junction through which light signals can continue without alteration or interruption. There are two ways that fibers are joined:

• splices, which form permanent connections between fibers in the system
• connectors, which provide remateable connections, typically at termination points

Fusion Splicing

Fusion splicing provides a fast, reliable, low-loss, fiber-to-fiber connection by creating a homogenous joint between the two fiber ends. The fibers are melted or fused together by heating the fiber ends, typically using an electric arc. Fusion splices provide a high-quality joint with the lowest loss (in the range of 0.01 dB to 0.10 dB for single-mode fibers) and are practically nonreflective.

Mechanical Splicing

Mechanical splicing is an alternative method of making a permanent connection between fibers. In the past, the disadvantages of mechanical splicing have been slightly higher losses, less-reliable performance, and a cost associated with each splice. However, advances in the technology have significantly improved performance. System operators typically use mechanical splicing for emergency restoration because it is fast, inexpensive, and easy. (Mechanical splice losses typically range from 0.05.0.2 dB for single-mode fiber.)

Connectors

Connectors are used in applications where flexibility is required in routing an optical signal from lasers to receivers, wherever reconfiguration is necessary, and in terminating cables. These remateable connections simplify system reconfigurations to meet changing customer requirements.

Acronyms Guide

CNR - carrier-to-noise ratio

CSO - composite second-order distortion

IVD - inside vapor deposition

MCVD - modified chemical vapor deposition

MFD - mode-field diameter

NZDSF - nonzero dispersion-shifted fiber

Billing in a 3G Environment

Definition and Overview

Definition
Billing in a 3G environment refers to the capacity of a wireless communications service provider to capture, rate, and bill next-generation mobile communications events. These events include voice, data, and electronic content such as mobile Web browsing and e-mail, mobile commerce activities, and streaming video.

Overview
The migration from second generation (2G) to third generation (3G) wireless communications systems represents a landmark shift in the development of the mobile communications industry—and the communications and information industries in general.

This tutorial explores the evolution of the wireless market from 2G through interim 2.5G to advanced 3G networks. Its central focus is an examination of the challenges that service providers face in deploying billing systems that are capable of meeting existing requirements and flexible enough to support the gradual adoption of 2.5G and 3G next-generation services by consumers.

The tutorial also reviews the enabling technologies and standards that define each generation of communications services. To satisfy the unique billing requirements of carriers that deploy advanced network services, billing systems must support the standards described in the following sections.

1. 2G Billing Challenges

The introduction of 2G cellular radio in the 1990s led to a genuine and significant change in human behavior. Technically, it provided the basis for the transition of voice technology from an analog, wired environment to a digital, wireless environment. Psychologically and socially, the advent of 2G transformed telecommunications from a communications tool to an agent of social change that improved people's professional and personal lives by enabling unprecedented communications flexibility.

Deploying a billing system for wireless services has never been simple. However, in the early days of 2G, billing was based on voice minutes. As a result, many wireless carriers, familiar with traditional voice telephony, implemented wireless billing systems using previous models that billed for voice minutes using call detail records (CDR).

2. Transitioning from 2G to 2.5G Network Services

By the late 1990s, wireless subscribers' voracious appetites for value-added services such as two-way messaging, unified communications, electronic voice-mail and e-mail, and personal number services drove an evolution toward more sophisticated 2.5G network services.

The rate of acceptance of 2.5G services varied around the world. The growth of value-added services in the United States was outpaced by widespread acceptance in Europe and Asia. One major reason for this was a lack of integration among various network technologies such as code division multiple access (CDMA), time division multiple access (TDMA), and the Global System for Mobile Communications (GSM). Billing systems could not support the disparate standards required for different network types.

In Europe and Asia, however, the adoption of a single standard, GSM, facilitated rapid acceptance and implementation of value-added 2.5G services.

As a result of the lack of uniformity in standards, wireless carriers focused on two leading criteria in the selection of wireless billing systems:

• Speed to market
• The ability to interface with other systems

These market conditions also gave rise to a new market driver that would confer an important strategic edge—convergence—the ability to offer and bill for multiple services, such as long distance, Web browsing, and voice on a single bill. Convergence prompted wireless carriers to plan the rollout of enhanced services and products that would extend wireless capabilities well beyond voice.

By 1998, wireless messaging had gained a foothold in Asia. Mixed mobile and fixed services, as well as value-added services, enjoyed growing popularity in Latin America. Meanwhile, in more mature markets such as the United Kingdom, the United States, and parts of Europe, increasing numbers of carriers began upgrading their billing systems to support the growing market for wireless data offerings.

The rollout of these services presented a critical challenge. To offer wireless data services, carriers needed a billing system that could accommodate the new services—particularly if charges were to be calculated based on the quantity of data transferred rather than the duration of time on-line.

As a result, convergent services strained legacy billing systems that were designed to measure and rate usage-sensitive wireless voice. With the emergence of 2.5G, existing billing systems simply were not equipped to rate wireless data, which typically was charged based on a flat rate.

3. The Emergence of 2.5G Services and Resulting Billing Challenges

Mobile data technologies such as short message service (SMS), wireless application protocol (WAP), and general packet-switched radio service (GPRS) have facilitated the move into the 2.5G world of content, with applications such as mobile e-mail and access to other Web-based services via mobile handsets.

SMS originated as a platform for e-mail and value-added services such as news, weather, and stock reports. WAP, which allows Internet content to be retrieved via mobile phones or other wireless devices, became available to the mass market in 2000.

Concurrent with the introduction of WAP, GPRS facilitates GSM–based wireless broadband access to the Internet via a personal computer (PC). GPRS also enhances WAP service levels. GPRS enables mobile service providers to offer complex services that are transferred as packet-switched, non-voice, value-added services. This is in contrast to traditional circuit-switched services that are available across mobile networks.

GPRS adds value because it enables instant wireless connections ("always on" service), which in turn allow information to be sent or received immediately as the need arises, subject to radio coverage. No dial-up modem connection is necessary.

Despite the accelerated pace of development in wireless technology and the digital data world, business drivers have focused on acquiring customers and increasing revenue to the exclusion of developing and implementing adequate billing and business solutions. Consequently, many entrants into the 2.5G marketplace had to rely on less-than-adequate billing solutions, using shortcuts to accommodate the drivers. Two of the most common shortcuts include the following:

• Bundling data charges into access charges
• Adding "all-you-can-eat" service for a flat rate

In assessing the complexity of billing for wireless data, the first challenge has been event collection and mediation. Carriers needed to deploy Internet protocol (IP) billing models capable of capturing information from multiple servers, routers, gateways, and content providers.

Suddenly, as service providers watched their margins disappear, flat-rate billing began to lose its appeal. The new catch phrase used to describe more recent usage-sensitive billing is billing for content. This new billing model is currently being addressed by billing standards organizations such as the Global Billing Association (GBA) to represent industry interests.

The objective of billing for content is two-fold—to help all service providers, and mobile service providers in particular, to determine the following:

• The type of data being transmitted over their networks
• How to capture revenue from the data being transmitted

As the rollout of more sophisticated and complex 3G services becomes a reality, these questions present an even greater challenge.

4. Next-Generation 3G Services: Bringing Challenge and Opportunity

In the future, as 3G services are adopted, GPRS will provide a massive boost to mobile data usage and usefulness. The promise of next-generation technology is likely to be realized because of its flexible feature set, and inherent latency, efficiency, and speed.

In Europe, next-generation or 3G cellular mobile radio is known as Universal Mobile Telecommunications System (UMTS). UMTS is expected to offer broadband multimedia services in addition to basic services such as voice.

Both GPRS and UMTS will support many new types of services. These include the following:

• Streaming video services
• Videoconferencing
• Interactive on-line shopping
• Location-sensitive directories
• On-line banking, stock trading, and sports reporting

It is unlikely that these services initially will justify the significant capital investment required to fund the network development that enables them. Instead, industrial and commercial applications will most likely lead the way as major consumers of pure bandwidth.

The types of commercial applications being proposed include expanded versions of existing sales and service applications—extending on-line computer facilities to staff in the field and using mobile security applications to monitor buildings and moving vehicles.

If the mobile Internet is to succeed and enjoy widespread acceptance, service providers must offer a variety of services. However, service providers alone cannot supply all of the services that consumers will require. In fact, growth would be severely hampered if service offerings were limited to only those that service providers can develop and offer. As a result, in addition to their own, service providers will have to offer services supplied by outside sources.

Service-Provider Challenges

While the possibilities that result from the launch of next-generation services and content-based services are exciting, they present service providers with numerous challenges, such as the following:

• Where to assess the value of the content moving across networks
• How to deliver content developed and provided by third parties
• How to capture revenue generated by content provided by outside sources

Service-Provider Advantages

As service providers scramble to establish effective models for billing for these new services, several factors work in their favor:

• Service providers already interface with subscribers on a monthly basis in the form of invoices
• Service providers can bill content providers, uniquely positioning them as aggregators of content

Redefining Billing Requirements to Keep Pace With Change

As described previously, GPRS and UMTS are packet-switched networks that will change the elements of billing for a number of reasons:

• Users will be able to access content via a visual subscriber interface—as opposed to voice, mobile subscribers will be able to send and receive text, pictures, and video.

• New users will always be on-line. The concept of "making a call" will disappear.

• Networks will be able to locate users within a few miles or meters. This capability yields new forms of advertising and sponsored services, which means that third parties may be prepared to pay operators for access to their subscribers.

• To support GPRS and UMTS services, a new generation of mobile "phone" is being developed. The lines between traditional phones and laptops will blur as technologies converge. Different types of consumers will use different types of devices, depending on whether they want games, music, video, or voice.

• New partnership opportunities will abound as communications service providers partner with outside sources to produce the content that they cannot produce in-house. As a result, the volume of settlement activities required to manage the exchange of content between networks is expected to grow. Depending on the length of the value-chain, the speed with which the settlements are made will become critical.

• New, next-generation networks will generate different forms of data using different types of records in larger quantities. Forecasts range from twice as much data to 50 or even 100 times as much. Scalability in a billing system will be imperative.

Diagram of 3G Billing System

5. New Business Models for Billing 3G Services

In view of the new services and resulting billing challenges, the overriding question is how should these new services be billed? Data and content-based services enabled by high-bandwidth packet networks will require new business models. Consequently, service providers will have to modify or replace existing, voice-centric billing infrastructures with new systems.

Devising a Framework for Next-Generation Billing Systems

To be viable, next-generation billing systems must be capable of pricing data and content events in addition to voice calls. They will have to be highly flexible, event-based, and truly convergent. In developing a billing solution equal to the task, a number of new parameters for calculating charges can be used:

• Number of packets
• Uploading or downloading of data
• Quality of service (QoS)
• Location
• Content

Communication service providers will certainly seek to bill subscribers directly for some services. However, they may also consider any of the following scenarios:

• Billing third parties for access to subscribers: Banks, travel agents, stock-brokers and similar entities could be billed for secure access to mobile subscribers.

• Billing subscribers directly for everything, including content: This scenario would generate a convergent bill that resembles a credit card statement, giving the service provider complete control of the relationship and maximizing the value of the customer relationship. This model would require a complex system to track the delivery of goods, ensure the QoS across the various forms of content that they deliver, and settle payments with suppliers.

• Billing content providers for access: Subscribers might pay content providers directly and service providers might receive a commission. This type of approach would simplify logistics for the operator, but also would burden the subscriber with multiple bills.

Keys Issues in Selecting a 3G Billing System

When it comes to selecting a billing system, a number of key issues must be considered. The right billing system must do the following:

• Be real-time. To perform balance management and authorization for 3G services, the billing system must return a price for an ordered good or service in a sub-second time frame.

• Be based on open industry standards to allow for interoperability with other OSS solutions

• Be modular to minimize total cost of ownership (TCO) for the communications service providers

• Accommodate all current and future types of services (including voice, data, and content)

• Support bundling of these services into cross-product packages to meet the needs of individual market segments

• Provide a "customer-centric view" of the account versus a "service-centric view." This means that the customer-service representative (CSR) has a 360-degree view of the customer with all his or her services in order to give targeted and relevant service.

• Minimize time to market for new products and services. State-of-the-art billing systems are not an obstacle anymore when it comes to launching new products—they are essential tools.

• Enable the customer to perform his or her own customer care via the Web or any other device (WAP phone, handheld, etc.)

6. Conclusion

Criteria like these are important factors to consider when evaluating and making decisions about potential billing solutions in the 3G environment. In the best of all worlds, an ideal solution will address all of these requirements. However, as the nature of services is redefined, certain compromises will be necessary. For example, the sheer complexity of service offerings will make fully itemized bills impractical. At the same time, self-care will enable users to examine their bills in the detail of their choice. In all cases, the bill must be clear and easy to decipher.

In weighing the pros and cons of the respective billing approaches, one fact is clear. The common denominator is the need for a technically practical way to bill for services—one that makes sense to the subscriber.

Service providers must approach GPRS and UMTS services in a consistent and straightforward manner and should bill subscribers in the same ways that they would be billed for the traditional version of the service.

As the GPRS and UMTS communications "revolution" unfolds, promising extraordinary changes in the ways in which we communicate, exchange information, and make purchases, no one knows exactly what the future will bring.

What we do know is that the future will not be exactly what we expect it to be. And we know that it is reasonable to expect that the business models that will succeed are those that can evolve over time.

In conclusion, billing systems must be as flexible as possible. And because service providers cannot be tied down to particular ways of doing business, they will have to forge alliances with business partners that are just as flexible and adaptable.

Compared to the considerable capital investment that service providers are making in GPRS and UMTS services, the customer care and billing investment is a relatively small one. It is, however, a central and vital concern in a marketplace where differentiation is the key to success.

A leading determinant in market differentiation will be the customer-care and billing system—and in some cases, it may be the pivotal investment that will mean the difference between success and failure.

Glossary

Bill Cycle
The period for which a consumer receives an invoice; can also denote when a cycle begins or ends

Call Detail Record (CDR)
A billing-system feature that tracks details about calls, such as type, time, duration, originator, and destination. CDRs can be used for network monitoring, accounting, and billing purposes.

Carrier
(1) A telecommunication company that offers its services to the public; typically, a carrier files tariffs that are equally applied to all consumers; (2) a continuously varying electromagnetic signal that carries analog signals such as frequency modulation (FM), amplitude modulation (AM), or digital signals. (AKA: service provider, operator)

Cellular Telecommunications Industry Association (CTIA)
A trade organization that represents the cellular/PCS wireless industry and is involved with regulatory and public affairs issues in the mobile wireless phone industry

Code Division Multiple Access (CDMA)
CDMA is a generic term that describes a wireless air interface based on code division multiple access technology.

cdmaOne™ is a brand name, trademarked and reserved for the exclusive use of CDG member companies, that describes a complete wireless system that incorporates the interim standard (IS)–95 CDMA air interface, the American National Standards Institute (ANSI)–41 network standard for switch interconnection, and many other standards that make up a complete wireless system. CDMA2000 is a name identifying the 3G technology that is an evolutionary outgrowth of cdmaOne offering operators that have deployed a 2G cdmaOne system—a seamless migration path that economically supports an upgrade to 3G features and services within existing spectrum allocations for both cellular and personal communications system (PCS) operators. CDMA2000 supports the 2G network aspect of all existing operators regardless of technology (cdmaOne, IS–136 TDMA, or GSM). This standard is also known by its International Telecommunication Union (ITU) name International Mobile Telecommunications (IMT)–CDMA Multi-Carrier (1X/3X).

Convergence
The ability to offer and bill for multiple services

Customer-Service Representative (CSR)
A carrier representative who deals with the consumer for ordering services and handling troubles or discrepancies in billing records

Event Processing
The process of gathering events in a network for the purposes of billing and/or network monitoring. Most often associated with capturing the details of 2.5 and 3G services such as short message service, mobile Web browsing, mobile e-mail, and multimedia.

General Packet Radio Service (GPRS)
A GSM data transmission technique that does not set up a continuous channel from a portable terminal for the transmission and reception of data, but transmits and receives data in packets. It makes very efficient use of available radio spectrum, and users pay only for the volume of data sent and received.

Global Positioning System (GPS)
A series of 24 geosynchronous satellites that continuously transmit their position. Used in personal tracking, navigation, and automatic vehicle-location technologies.

Global System for Mobile Communications (GSM)
A digital cellular or PCS network used throughout the world

Internet Protocol Detail Record (IPDR)
IPDR.org is an open consortium of leading companies working together to bring this vision to reality. Collaborating service providers, equipment vendors, system integrators, and billing and mediation vendors facilitate the exchange of usage and control data between network and hosting elements and operations and business support systems by the deployment of IPDR standards.

Mediation Device
A device that can interface with complex multivendor switches and billing systems to gather the required information for provisioning; also can refer to the software used by carriers to interconnect operations support systems (OSS).

Personal Communications Service (PCS)
A two-way, 1900 MHz digital voice, messaging, and data service designed as the second generation of cellular.

Personal Digital Assistant (PDA)
A portable computing device capable of transmitting data. These devices make possible services such as paging, data messaging, electronic mail, computing, facsimile, date books, and other information-handling capabilities.

Provisioning
The process by which a requested service is designed, implemented, and tracked for a particular customer.

Quality of Service (QoS)
A measure of a carrier's service to a consumer

Rate Plan
The plan to which a consumer agrees upon requesting service

Time Division Multiple Access (TDMA)
A method of digital wireless communications transmission allowing a large number of users to access (in sequence) a single radio frequency channel without interference by allocating unique time slots to each user within each channel

Universal Mobile Telecommunications System (UMTS)
Europe's approach to standardization for 3G cellular systems

Web Hosting

Definition and Overview

Definition
The World Wide Web (WWW), a web of worldwide servers connected to the Internet, provides an easily used and understood method of accessing electronic content. Accessing information requires data communication between a Web-browser client and a Web-server application. Web hosting, then, is a means of hosting the Web-server application on a computer system through which electronic content on the Internet is readily available to any Web-browser client.

Overview
This tutorial will provide a basic overview of the main components that enable the Web, present two basic methods of Web hosting known as dedicated and shared, and discuss the challenges of resource management.

1. Overview of the Web

In late 1990 while working at CERN, the European Laboratory for Particle Physics Research in Geneva, Switzerland, Tim Berners-Lee invented the Web, including the definitions of universal resource locator (URL), hypertext transfer protocol (HTTP), and hypertext markup language (HTML). The Web provides a method for easily linking content contained on computer systems distributed throughout the world and connected to the Internet. Utilizing the Web, content on servers from many locations can be seamlessly linked and presented as a comprehensive resource collection. The Web further strengthens the power of the Internet's foundation of distributed computing.

The Web and the Internet remained the world's best-hidden resource until 1993 when Marc Andreessen, an undergraduate at the University of Illinois in Champaign, and a team at the National Center for Supercomputing Applications (NCSA) created the NCSA Mosaic browser. The NCSA Mosaic browser was the first Web-browser client that provided a friendly, point-and-click method for navigating the Internet using the Web.

The invention of the NCSA Mosaic browser was the start of the unprecedented growth of Internet users, Internet service providers (ISPs), and Internet business opportunities. By means of a user-friendly approach to searching and viewing the vast amount of information on the Internet, the Web-browser client enabled nontechnical individuals to benefit from the power and resources of the Internet.

Accessing content through the Web consists of communication between a Web-browser client and a Web server utilizing HTTP (see Figure 1).

Figure 1. Web Overview

The following is a step-by-step description of the communication path, as shown in Figure 1. It assumes that the Web server, the primary domain naming system (DNS) server, and the client computer are connected to the Internet and that all communication is conducted through the Internet.

• steps 1 and 2—The end-user types a URL into the Web browser. The client computer finds the Internet protocol (IP) number associated with the domain name in the URL from the primary DNS server.
• steps 3 and 4—The client computer uses the IP number obtained from the primary DNS server to request, through HTTP, the default HTML file from the Web server associated with the URL. The Web server sends the default HTML file to the client computer. The default HTML file provides information to the client computer for requesting all associated files—such as graphics—for the Web site's complete home page.

When the client computer and Web browser request and receive files from the same URL, the client computer is not required to perform a DNS lookup as described in steps 1 and 2. When the client computer attempts to retrieve a Web site from a different URL, the client computer must then perform steps 1 and 2 again.

2. Overview of Web Hosting

The complex web of servers consists of computer systems installed with Web-server software and connected to the Internet. These servers can be found in any facility with Internet connectivity. The process of maintaining and operating one of these servers is called Web hosting. Web hosting can be conducted in-house by the owner of the Web site, or it can be outsourced to a Web presence provider (WPP).

WPPs are typically companies with one or more data-center facilities that are connected to the Internet. Web hosting provided by WPPs can vary widely with respect to service quality and cost. Some providers consist simply of a room in the basement of a house and a tier-1 (T1) line connected to the local ISP. Others, however, are corporations with state-of-the-art hosting centers consisting of redundant fiber paths for high-speed Internet connections, redundant electrical power sources, a dry pipe–fire suppression system, and an experienced operations group, available 24 hours a day, seven days a week.

Web hosting can be provided on a shared computer environment or on a dedicated computer system. When a Web site consists only of standard HTML code and receives a small number of visitors, shared hosting service is the best solution. When a Web site consists of complex common gateway interface (CGI) scripts and proprietary programs and receives a large number of visitors, dedicated hosting service is the best solution.

3. Web-Hosting Implementation on a Dedicated Platform

The basic concept of Web hosting on a dedicated computer system consists of hosting one Web site on one computer system. The dedicated environment offers complete flexibility and security to both the WPP and the customer.

Web hosting on a dedicated computer system is the simplest and most straightforward method of operating a Web site. Because the computer system contains only one Web site, the configuration of software is standardized, as outlined in the software-installation documentation. Furthermore, system resources are dedicated to only one Web site and, therefore, are not constrained by any other process not associated with the operations of that site.

The essential components of Web hosting on a dedicated computer system are as follows (see Figure 2):

• computer system hardware
• operating system (including transfer control protocol [TCP]/Internet protocol [IP] stack)
• Internet connection (IP number and domain name)
• Web server software (HTTP)

Figure 2. Dedicated Hosting Basic Elements

Additional software applications can be added to the computer system to enhance the Web site and to simplify the process of uploading content. One of these applications is a file transfer protocol (FTP) server for remote access to the computer system for transferring HTML content files.

4. Web-Hosting Implementation on a Shared Platform

The basic concept of Web hosting on a shared computer environment consists of hosting many different Web sites on one computer system. The shared environment offers economic benefits to both the WPP and the customer. Because the Web-hosting environment is the same for all customers, the provider gains economic benefits from allocating portions of the total cost of the hardware, software, maintenance and operation, and customer support amongst all customers. Therefore, the total fixed cost is less on a per-customer basis than with dedicated hosting. The customer gains economic benefit by the reduced price of the Web-hosting service.

The essential components of Web hosting on a shared computer environment are the same as with dedicated hosting, except for the configuration of the software and the management of system resources. There are two basic ways to configure Web-server software for multiple Web sites. The first method is to configure the Web server with each Web site's specific configuration information. The second method is to operate multiple Web-server software on a single computer environment. The first method—a single configuration file with all of the Web site's information—has greater scalability but does not provide a means of limiting the resources consumed by each Web site. Therefore, a combination of both methods is ideal for creating a scalable shared-hosting service. A combination is achieved by using the single configuration file method for Web sites requiring small amounts of resources and using the multiple Web-server method to limit the resources consumed by Web sites that demand large amounts of resources.

When a Web site demands large amounts of system resources, the logical next step is to move the Web site to a dedicated computer system (i.e., dedicated hosting).

5. Web Hosting–Resource Management Challenges

Managing computer-system resources in the shared platform and the dedicated platform is challenging. As a Web site becomes more popular and is sought after by millions of Internet users, the Web site demands more and more system resources. Being able to measure, monitor, and manage the amount of system resources is essential for Web-site availability and server performance.

Critical system resources to manage include the following:

• central processing unit (CPU) utilization
• memory utilization
• disk-swap space
• disk space
• disk input and output
• network input and output
• Internet bandwidth (not a computer-system resource but still requires monitoring and managing)

These critical system resources have a direct relationship with the performance of a specific Web site. A Web site can be created or modified to minimize the demand on these system resources. Some Web sites are developed without the consideration of system-resource utilization. When a Web site contains and executes a common gateway interface CGI script, CPU resources are demanded. If the Web site contains a large number of CGI scripts and requires these scripts to be executed by every Web-site visitor, then CPU resources become a major bottleneck and cause the Web site to appear slow. It is important for the Web-site designer and developer to balance system-resource demands with Web-site functionality and creativity.

To measure, monitor, and manage the computer-system resources, additional software must be installed on the computer system. Each type of computer system hardware requires specific software for resource management. The computer-system manufacturer and operating system–software developer should be able to identify the necessary software applications for measuring, monitoring, and managing the system resources for their specific computer systems.

6. Advanced Web-Hosting Methods

During the last several years, Web hosting has evolved from simple one-computer system architectures to redundant, load-balanced server farms. A server farm is a network of computer systems. As a Web site demands more and more system resources, the traditional hosting environment is constrained by the limited amount of available resources. There are two basic means of providing more resources: a larger computer system or a distributed computer environment. To provide redundancy and scalability, the distributed computer environment is the preferred method of expanding system resources.

The simplest distributed computer environment consists of two identical Web servers on the same local-area network (LAN) with a load-balancing device (see Figure 3). The load-balancing device is the gateway for all traffic entering and leaving the Web servers. The load balancer directs the incoming traffic to the best performing Web server, to alleviate all resource bottlenecks. With the load balancer as the gateway, the two Web servers appear as one large computing environment to all end-users on the Internet. This simple distributed computer environment can be expanded to accommodate more Web servers, providing greater scalability and consistently high performance levels.

Figure 3. Load Balancing Two Web Servers

The simple distributed-computer environment provides a method for increasing the available computer-system resources, but it will not prevent performance problems associated with specific network issues within the LAN or with the Internet connection at that specific location. To overcome local network problems, Web hosting has continued to evolve into a geographically distributed computing–environment architecture.

By distributing the traffic of a Web site across multiple servers located in dispersed geographic locations, system resources can be added without interruptions in the Web-hosting service, and the Web site can always be available despite LAN or Internet-connection problems. Moreover, with intelligent wide-area network (WAN) load balancing, Web-site performance will increase for all visitors, regardless of their geographic location.

Figure 4 illustrates Web hosting in a geographically distributed computing environment.

Figure 4. Two Site Architectures

Glossary

CGI
common gateway interface

DNS
domain naming system

FTP
file transfer protocol

HTML
hypertext markup language

HTTP
hypertext transfer protocol

SSL
secure socket layer

URL
uniform resource locator

Wireless Short Message Service (SMS)

Definition and Overview

Definition
Short message service (SMS) is a globally accepted wireless service that enables the transmission of alphanumeric messages between mobile subscribers and external systems such as electronic mail, paging, and voice-mail systems.

Overview
This tutorial provides an introduction to basic SMS concepts, specifications, networks, and services.

1. Introduction

SMS appeared on the wireless scene in 1991 in Europe. The European standard for digital wireless, now known as the Global System for Mobile Communications (GSM), included short messaging services from the outset.

In North America, SMS was made available initially on digital wireless networks built by early pioneers such as BellSouth Mobility, PrimeCo, and Nextel, among others. These digital wireless networks are based on GSM, code division multiple access (CDMA), and time division multiple access (TDMA) standards.

Network consolidation from mergers and acquisitions has resulted in large wireless networks having nationwide or international coverage and sometimes supporting more than one wireless technology. This new class of service providers demands network-grade products that can easily provide a uniform solution, enable ease of operation and administration, and accommodate existing subscriber capacity, message throughput, future growth, and services reliably. Short messaging service center (SMSC) solutions based on an intelligent network (IN) approach are well suited to satisfy these requirements, while adding all the benefits of IN implementations.

Figure 1 represents the basic network architecture for an IS–41 SMSC deployment handling multiple input sources, including a voice-mail system (VMS), Web-based messaging, e-mail integration, and other external short message entities (ESMEs). Communication with the wireless network elements such as the home location register (HLR) and mobile switching center (MSC) is achieved through the signal transfer point (STP).

Figure 1. Basic Network Architecture for an SMS Deployment (IS–41)

SMS provides a mechanism for transmitting short messages to and from wireless devices. The service makes use of an SMSC, which acts as a store-and-forward system for short messages. The wireless network provides the mechanisms required to find the destination station(s) and transports short messages between the SMSCs and wireless stations. In contrast to other existing text-message transmission services such as alphanumeric paging, the service elements are designed to provide guaranteed delivery of text messages to the destination. Additionally, SMS supports several input mechanisms that allow interconnection with different message sources and destinations.

A distinguishing characteristic of the service is that an active mobile handset is able to receive or submit a short message at any time, independent of whether a voice or data call is in progress (in some implementations, this may depend on the MSC or SMSC capabilities). SMS also guarantees delivery of the short message by the network. Temporary failures due to unavailable receiving stations are identified, and the short message is stored in the SMSC until the destination device becomes available.

SMS is characterized by out-of-band packet delivery and low-bandwidth message transfer, which results in a highly efficient means for transmitting short bursts of data. Initial applications of SMS focused on eliminating alphanumeric pagers by permitting two-way general-purpose messaging and notification services, primarily for voice mail. As technology and networks evolved, a variety of services have been introduced, including e-mail, fax, and paging integration, interactive banking, information services such as stock quotes, and integration with Internet-based applications. Wireless data applications include downloading of subscriber identity module (SIM) cards for activation, debit, profile-editing purposes, wireless points of sale (POSs), and other field-service applications such as automatic meter reading, remote sensing, and location-based services. Additionally, integration with the Internet spurred the development of Web-based messaging and other interactive applications such as instant messaging, gaming, and chatting.

2. Benefits of SMS

In today's competitive world, differentiation is a significant factor in the success of the service provider. Once the basic services, such as voice telephony, are deployed, SMS provides a powerful vehicle for service differentiation. If the market allows for it, SMS can also represent an additional source of revenue for the service provider.

The benefits of SMS to subscribers center around convenience, flexibility, and seamless integration of messaging services and data access. From this perspective, the primary benefit is the ability to use the handset as an extension of the computer. SMS also eliminates the need for separate devices for messaging because services can be integrated into a single wireless device—the mobile terminal. These benefits normally depend on the applications that the service provider offers. At a minimum, SMS benefits include the following:

• Delivery of notifications and alerts
• Guaranteed message delivery
• Reliable, low-cost communication mechanism for concise information
• Ability to screen messages and return calls in a selective way
• Increased subscriber productivity

More sophisticated functionality provides the following enhanced subscriber benefits:

• Delivery of messages to multiple subscribers at a time
• Ability to receive diverse information
• E-mail generation
• Creation of user groups
• Integration with other data and Internet-based applications

The benefits of SMS to the service provider are as follows:

• Ability to increment average revenue per user (due to increased number of calls on wireless and wireline networks by leveraging the notification capabilities of SMS)
• An alternative to alphanumeric paging services, which may replace or complement an existing paging offer
• Ability to enable wireless data access for corporate users
• New revenue streams resulting from addition of value-added services such as e-mail, voice mail, fax, and Web-based application integration, reminder service, stock and currency quotes, and airline schedules
• Provision of key administrative services such as advice of charge, over-the-air downloading, and over-the-air service provisioning
• Protection of important network resources (such as voice channels), due to SMS’ sparing use of the control and traffic channels
• Notification mechanisms for newer services such as those utilizing wireless application protocol (WAP)

All of these benefits are attainable quickly, with modest incremental cost and short payback periods, which make SMS an attractive investment for service providers.

3. Network Elements and Architecture

The basic network structure of the SMS in an IS–41 network is depicted in Figure 1.

External Short Messaging Entities

An ESME is a device that may receive or send short messages. The short message entity (SME) may be located in the fixed network, a mobile device, or another service center.

• VMS—The VMS is responsible for receiving, storing, and playing voice messages intended for a subscriber that was busy or not available to take a voice call. It is also responsible for sending voice-mail notifications for those subscribers to the SMSC.
• Web—The growth of the Internet has also affected the world of SMS. Therefore, it is almost mandatory to support interconnections to the World Wide Web for the submission of messages and notifications. The increasing number of Internet users has a positive impact on the SMS traffic increment experienced in the last few years.
• E-Mail—Probably the most demanded application of SMS is the ability to deliver e-mail notifications and to support two-way e-mail, using an SMS–compliant terminal. The SMSC must support interconnection to e-mail servers acting as message input/output mechanisms.
• Others—There are several other mechanisms to submit short messages to the SMSC that include, but are not limited to, paging networks, specialized software for PC–based messaging and operator bureaus.

SMSC
SMSC is a combination of hardware and software responsible for the relaying and storing and forwarding of a short message between an SME and mobile device.

The SMSC must have high reliability, subscriber capacity, and message throughput. In addition, the system should be easily scalable to accommodate growing demand for SMS in the network.

Normally, an IN–based solution will allow for a lower entry cost compared to point solutions because it can support other applications on a single hardware platform and share resources, thereby spreading the deployment cost over several services and applications.

Another factor to be considered is the ease of operation and maintenance of the application, as well as the flexibility to activate new services and upgrade to new software releases.

Signal Transfer Point
The STP is a network element normally available on IN deployments that allows IS–41 interconnections over signaling system 7 (SS7) links with multiple network elements.

HLR
The HLR is a database used for permanent storage and management of subscriptions and service profiles. Upon interrogation by the SMSC, the HLR provides the routing information for the indicated subscriber. Also, if the destination station was not available when the message delivery was attempted, the HLR informs the SMSC that the station is now recognized by the mobile network to be accessible, and thus the message can be delivered.

Visitor Location Register (VLR)
The visitor location register is a database that contains temporary information about subscribers homed in one HLR who are roaming into another HLR. This information is needed by the MSC to service visiting subscribers.

MSC
The MSC performs the switching functions of the system and controls calls to and from other telephone and data systems. The MSC will deliver the short message to the specific mobile subscriber through the proper base station.

Air Interface
The air interface is defined in each one of the different wireless technologies (GSM, TDMA, and CDMA). These standards specify how the voice or data signals are transferred from the MSC to the handset and back, as well as the utilization of transmission frequencies, considering the available bandwidth and the system’s capacity constraints.

The Base Station System
All functions related to the transmission of electromagnetic radio signals between the MSC and the mobile devices are performed in the base station (BS). The BS consists of base station controllers (BSCs) and the base transceiver stations (BTSs), also known as cell sites or simply “cells.” The BSC may control one or more BTSs and is in charge of the proper resource assignment when a subscriber moves from one sector of one BTS to another, regardless of whether the next sector lies within the same BTS or in a different one.

The Mobile Device
The mobile device is the wireless terminal capable of receiving and originating short messages. Commonly, these devices have been digital cellular phones, but more recently the application of SMS has been extended to other terminals such as POS, handheld computers, and personal digital assistants (PDAs). The wireless network signaling infrastructure is based on SS7. SMS makes use of the mobile application part (MAP), which defines the methods and mechanisms of communication in wireless networks and employs the services of the SS7 transactional capabilities application part (TCAP). An SMS service layer makes use of the MAP signaling capabilities and enables the transfer of short messages between the peer entities.

The capabilities of the terminal vary depending on the wireless technology supported by the terminal. Some functionality, although defined in the SMS specification for a given wireless technology, may not be fully supported in the terminal, which may represent a limitation in the services that the carrier can provide. This trend, however, is disappearing as service providers’ merger and acquisition activity demands uniform functionality across all the constituents of the parent companies. Also, some manufacturers may include additional functionality, not considered in the specification, attempting to offer a more attractive product for service providers as well as end users. This will be the case more often as service provider continue to incorporate SMS into their revenue-generating and customer-loyalty strategies.

4. Signaling Elements

The MAP layer defines the operations necessary to support SMS. Both American and international standards bodies have defined a MAP layer using the services of the SS7 TCAP. The American standard is published by Telecommunication Industry Association and is referred to as IS–41. The international standard is defined by the European Telecommunications Standards Institute (ETSI) and is referred to as GSM MAP.

The following basic MAP operations are necessary to provide the end-to-end SMS:

• Routing Information Request—Before attempting delivery of a short message, the SMSC must receive routing information to determine the serving MSC for the mobile device at the time of the delivery attempt. This is accomplished by way of an interrogation of the destination handset’s HLR, which is accomplished via the use of the SMSrequest and SendRoutingInfoForShortMsg mechanisms in IS–41 and GSM, respectively.
• Point-to-Point Short Message Delivery—The mechanism provides a means for the SMSC to transfer a short message to the MSC that is serving the addressed mobile device. After the address of said MSC has been obtained from the station’s HLR, the short message delivery operation provides a confirmed delivery service. The operation works in conjunction with the base station subsystem while the message is being forwarded from the MSC to the MS. Therefore, the outcome of the operation comprises either success (such as delivery to the mobile) or failure caused by one of several possible reasons. The point-to-point short message delivery is accomplished via the use of the short message delivery–point-to-point (SMD–PP) and forwardShortMessage mechanisms in IS–41 and GSM, respectively.
• Short Message Waiting Indication—he operation is activated when a short message delivery attempt by the SMSC fails due to a temporary failure, such as the station being unregistered, and provides a means for the SMSC to request the HLR to notify the SMSC when the indicated mobile device becomes available. This short message waiting indication is realized via the use of the SMS_notification indicator and set_message_waiting_data mechanisms in IS–41 and GSM, respectively.
• Service Center Alert—The operation provides a means for the HLR to inform the SMSC, which has requested a notification that a specific mobile device is now recognized by the mobile network to be available. This service center alert is accomplished via the use of the SMS_notification and alert_service_center mechanisms in IS–41 and GSM, respectively.

Service Elements
SMS is comprised of several service elements relevant to the reception and submission of short messages:

• Message Expiration—The SMSC will store and reattempt delivery of messages for unavailable recipients until either the delivery is successful or the expiration time—set on a per-message basis or on a platform-wide basis—arrives.
• Priority—This is the information element provided by an SME to indicate the urgent messages and differentiate them from the normal priority messages. Urgent messages usually take priority over normal messages, regardless of the time of arrival to the SMSC platform.
• Message Escalation—The SMSC stores the message for a period no longer than the expiration time (it is assumed that the escalation time is smaller than the expiration time associated with the message), and after said escalation time expires, the message will be sent to an alternate message system (such as a paging network or an e-mail server) for delivery to the user.

In addition, SMS provides a time stamp reporting the time of submission of the message to the SMSC and an indication to the handset of whether or not there are more messages to send (GSM) or the number of additional messages to send (IS–41).

Subscriber Services
SMS comprises two basic point-to-point services:

• Mobile-originated short message (MO–SM)
• Mobile-terminated short message (MT–SM)

Mobile-originated (MO) short messages are transported from the MO–capable handset to the SMSC and can be destined to other mobile subscribers or for subscribers on fixed networks such as paging networks or Internet protocol (IP) networks (including the Internet and private e-mail networks). Mobile-terminated (MT) short messages are transported from the SMSC to the handset and can be submitted to the SMSC by other mobile subscribers via MO–SM or by other sources such as voice-mail systems, paging networks, or operators.

For MT–SM, a report is always returned to the SMSC either confirming the short message delivery to the handset or informing the SMSC of the short message delivery failure and identifying the reason for failure (cause code). Similarly, for MO–SM, a report is always returned to the handset either confirming the short message delivery to the SMSC or informing of delivery failure and identifying the reason.

Depending on the access method and the encoding of the bearer data, the point-to-point short messaging service conveys up to 190 characters to an SME in GSM networks and from 120 to 205 in IS–41 networks.

In GSM networks, the type of messaging service is identified by the protocol identifier information element, which identifies the higher-level protocol or interworking being used. Examples are telex, group 3 telefax, X.400 messaging, European Radio Messaging System (ERMES), and voice telephone.

In IS–41 networks, the service type is distinguished by use of the teleservice identifier. Basic teleservices include the following:

• Cellular messaging teleservice (CMT)
• Cellular paging teleservice (CPT)
• Voice-mail notification teleservice (VMN)

CMT differs from the CPT due to the inclusion of a reply mechanism that enables a user or network acknowledgment to be selected on a per-message basis. The user acknowledgment includes a response code that paves the way for powerful interactive services between SMSCs.

Many service applications can be implemented by combining these service elements. Aside from the obvious notification services, SMS can be used in one-way or interactive services providing wireless access to any type of information anywhere. By leveraging new emerging technologies that combine browsers, servers, and new markup languages designed for mobile terminals, SMS can enable wireless devices to securely access and send information from the Internet or intranets quickly and cost-efficiently. One of these technologies where SMS can provide a cooperative, rather than a competitive, approach is the WAP, which allows transport of data for mobile wireless users.

A generic network infrastructure for realizing the innovative SMS services is depicted in Figure 2.

Figure 2. Network Infrastructure

Some of the potential applications of SMS technology, utilizing both MT–SM and MO–SM where appropriate, include the following:

• Notification Services—Notification services are currently the most widely deployed SMS services. Examples of notification services using SMS include the following:
  • Voice/fax message notification, which indicates that voice or fax mail messages are present in a voice mailbox
  • E-mail notification, which indicates that e-mail messages are present in an e-mail mailbox Reminder/calendar services, which enable reminders for meetings and scheduled appointments.
• E-mail Interworking—Existing e-mail services can be easily integrated with SMS to provide e-mail to short messaging and mobile e-mail and message escalation.
• Paging Interworking—Paging services integrated with SMS allow digital wireless subscribers to be accessible via existing paging interfaces, as well as escalation of messages.
• Information Services—A wide variety of information services can be provided by the SMS, including weather reports, traffic information, entertainment information (e.g., cinema, theater, concerts), financial information (e.g., stock quotes, exchange rates, banking, brokerage services), and directory assistance. SMS can support both push (MT) and pull (MO) approaches to allow not only delivery under specific conditions but also delivery on demand, as a response to a request.
• WAP Integration—SMS can deliver notifications for new WAP messages to wireless subscribers but can also be used as the transport mechanism for WAP messages. These messages can contain diverse information from sources that include databases, the World Wide Web, e-mail servers, etc.

Mobile Data Services
The SMSC can also be used to provide short wireless data. The wireless data may be in interactive services where voice calls are involved.

Some examples of this type of service include fleet dispatch, inventory management, itinerary confirmation, sales order processing, asset tracking, automatic vehicle location, and customer contact management. Other examples may be interactive gaming, instant messaging, mobile chat, query services, mobile banking, etc.

Customer Care and Management
The SMSC can also be used to transfer binary data that can be interpreted by the mobile device without presentation to the customer. This capability allows the operators to administer their customers by providing a mechanism for programming the mobile device. Examples of such services include mobile device programming, which allows customer profiles and subscription characteristics to be downloaded to the mobile device (customers can be activated/deactivated based on the data downloaded) and advice of charge, which enables the SMS to be used to report charges incurred for the phone call (e.g., calls made when roaming).

One interesting method to provide customer support is to offer a list of answers to frequently asked questions via short message. SMS also can be used to distribute general information about other products and services being offered by the service provider, thus guaranteeing maximum penetration of the advertising over the existing customer base. In a different scenario, a service provider may want to deliver short messages to subscribers to remind them of, for example, past-due payments, instead of reminding them over traditional mail or courier delivery, therefore reducing cost and ensuring that the message is delivered to its destination in a timely manner.

5. Mobile-Terminated Short Message Example

Figure 3 depicts the successful MT—SM scenario for GSM.

Figure 3. MT—SM Scenario (GSM)

1. The short message is submitted from the ESME to the SMSC.
2. After completing its internal processing, the SMSC interrogates the HLR and receives the routing information for the mobile subscriber.
3. The SMSC sends the short message to the MSC using the forward short message operation.
4. The MSC retrieves the subscriber information from the VLR. This operation may include an authentication procedure.
5. The MSC transfers the short message to the MS.
6. The MSC returns to the SMSC the outcome of the forwardShortMessage operation.
7. If requested by the ESME, the SMSC returns a status report indicating delivery of the short message.

Figure 4. MT Short Message Scenario (IS—41)

1. The short message is submitted from the ESME to the SMSC.
2. The SMSC sends an acknowledgement to the ESME, indicating reception of the short message.
3. After completing its internal processing, the SMSC interrogates the HLR.
4. The HLR sends the routing information for the mobile subscriber to the SMSC.
5. The SMSC sends the short message to the MSC using the SMSDPP Invoke operation.
6. The MSC transfers the short message to the MS.
7. The MS returns an acknowledgement to the MSC.
8. The MSC returns to the SMSC the outcome of the SMSDPP operation.
9. If requested by the ESME, the SMSC returns a delivery receipt indicating successful delivery of the short message.

6. Mobile-Originated Short Message Example

Figure 5 depicts the successful MO–SM scenario, utilizing the GSM method. The IS–41 method for the MO-SM scenario is depicted in Figure 6.

Figure 5. MO—SM Scenario (GSM)

1. The MS is powered on and registered with the network.
2. The MS transfers the SM to the MSC.
3. The MSC interrogates the VLR to verify that the message transfer does not violate the supplementary services invoked or the restrictions imposed.
4. The MSC sends the short message to the SMSC using the forwardShortMessage operation.
5. The SMSC delivers the short message to the SME (and optionally receives acknowledgment).
6. The SMSC acknowledges to the MSC the successful outcome of the forwardShortMessage operation.
7. The MSC returns to the MS the outcome of the MO-SM operation.

Figure 6. MO—SM Scenario (IS—41)

1. The MS transfers the SM to the MSC.
2. The MSC interrogates the home SMSC to verify that the message transfer does not violate the supplementary services invoked or the restrictions imposed. The MSC sends the short message to the home SMSC using the SMSPP Invoke operation.
3. The SMSC delivers an acknowledgment to the MSC.
4. The MSC returns order release to the MS.
5. The SMSC queries the HLR for the location of the destination MS.
6. The HLR returns the destination (MSC) serving the destination MS.
7. The SMSC delivers SM to the MSC serving the destination MS.
8. The SMSC delivers the short message to the MS.
9. The MS acknowledges to the MSC the successful outcome of the SMSDPP operation.
10. The MSC returns to the SMSC the outcome of the MO–SM operation (delivery successful).

7. SMS Applications

SMS was initially designed to support limited-size messages, mostly notifications and numeric or alphanumeric pages. While these applications are and will continue to be widely used, there are more recent niches that SMS still can exploit.

Short bursts of data are at the heart of many applications that were restricted to the world of data networks with fixed terminals attached to a local-area network (LAN) or wide-area network (WAN). However, many of these applications are better served if the data communication capabilities could be added to the mobility of the station. Thus, a waiter who can charge a customer's credit card right at the table, at any time, instead of going to a fixed POS terminal located by the register will be able to help customers in a faster, more convenient way.

Also, the ability to track the location of a moving asset such as a truck or its load is very valuable for both providers and clients. This application, again, just needs to interchange small amounts of information, such as the longitude and latitude at a current time of the day, and perhaps other parameters like temperature or humidity.

This application does not necessarily require the monitored entity to be in movement. The requirements are basically short, bursty data and a location that has digital network coverage. For example, in a neighborhood, it would be faster, easier, and cheaper to drive a truck from the local power company, which interrogates intelligent meters to obtain their current readings and then forwards them via short message to a central data processing center to generate the billing. Similarly, delivery trucks could be alerted of the inventory of a customer running low, when the truck is close to the customer’s facilities. The truck driver could place a quick phone call to the customer to offer a short-time replenishment at a low cost for the distributor.

Another family of applications that can use SMS as a data transport mechanism is banking. It is no secret that automated teller machine (ATM) and Internet transactions are less costly than transactions completed at a branch. Internet transactions are even cheaper than ATM transactions. Therefore, enabling wireless subscribers to check their balances, transfer funds between accounts, pay their bills and credit cards is valuable, not only for the subscriber but also for financial institutions.

Entertainment applications are also good drivers of SMS usage. Examples of these are simple short message exchanges between two parties (“texting”) or between multiple participants (“chat”). Also, delivery of information that the subscriber can tailor to his or her lifestyle represents an attractive proposition for wireless users.

Wireless Web browsing allows the users to search for information without the physical restrictions of a PC. College students certainly appreciate not having to go to the computer lab or their dorm to check e-mail or find out what the required book is for the semester that is about to start.

E-mail continues to be by far the most used wireless data application. However, handsets are evolving quickly and are including more and more functionality that supports newer applications at the same time that user friendliness increases. Probably the next big success beyond wireless Web will be Internet shopping and other e-commerce applications such as electronic coupons, advertising, etc.

The potential for applications is enormous, and new needs appear to arise constantly, demanding a solution that may travel over SMS.

Glossary

ATM
asynchronous transfer mode

BS
base station

BSC
base station controller

BTS
base transceiver station

CDMA
code division multiple access

CMT
cellular messaging teleservice

CPT
cellular paging teleservice

ERMES
European Radio Messaging System

ESME
external short message entities

ETSI
European Telecommunications Standards Institute

GSM
Global System for Mobile Communications

HLR
home location register

IN
intelligent network

IP
Internet protocol

LAN
local-area network

MAP
mobile application part

MO
mobile originated

MO–SM
mobile-originated short message

MSC
mobile switching center

MT
mobile terminated

MT–SM
mobile-terminated short message

PDA
personal digital assistant

POS
point of sale

PP
point to point

SIM
subscriber identity module

SM
short message

SMD
short message delivery

SMD–PP
short message delivery–point to point

SME
short messaging entity

SMS
short message service

SMSC
short message service center

SS7
signaling system 7

STP
signal transfer point

TCAP
transactional capabilities application part

TDMA
time division multiple access

VLR
visitor location register

VMN
voice-mail notification

VMS
voice-mail system

WAN
wide-area network

WAP
wireless application protocol

Dense Wavelength Division Multiplexing (DWDM)

Definition and Overview

Definition
Dense wavelength division multiplexing (DWDM) is a fiber-optic transmission technique that employs light wavelengths to transmit data parallel-by-bit or serial-by-character.

Overview
This tutorial addresses the importance of scalable DWDM systems in enabling service providers to accommodate consumer demand for ever-increasing amounts of bandwidth. DWDM is discussed as a crucial component of optical networks that allows the transmission of e-mail, video, multimedia, data, and voice—carried in Internet protocol (IP), asynchronous transfer mode (ATM), and synchronous optical network/synchronous digital hierarchy (SONET/SDH), respectively, over the optical layer.

1. The Challenges of Today's Telecommunications Network

To understand the importance of DWDM and optical networking, these capabilities must be discussed in the context of the challenges faced by the telecommunications industry, and, in particular, service providers. Most U.S. networks were built using estimates that calculated bandwidth use by employing concentration ratios derived from classical engineering formulas such as Poisson and Reeling. Consequently, forecasts of the amount of bandwidth capacity needed for networks were calculated on the presumption that a given individual would only use network bandwidth six minutes of each hour. These formulas did not factor in the amount of traffic generated by Internet access (300 percent growth per year), faxes, multiple phone lines, modems, teleconferencing, and data and video transmission. Had these factors been included, a far different estimate would have emerged. In fact, today many people use the bandwidth equivalent of 180 minutes or more each hour.

Therefore, an enormous amount of bandwidth capacity is required to provide the services demanded by consumers. For perspective, in 1997, a long-distance carrier made major strides when it increased its bandwidth capacity to 1.2 Gbps (billions of bits per second) over one fiber pair. At the transmission speed of one Gbps, one thousand books can be transmitted per second. However today, if one million families decide they want to see video on Web sites and sample the new emerging video applications, then network transmission rates of terabits (trillions of bits per second [Tbps]) are required. With a transmission rate of one Tbps, it is possible to transmit 20 million simultaneous 2-way phone calls or transmit the text from 300 years–worth of daily newspapers per second.

No one could have predicted the network growth necessary to meet the demand. For example, one study estimated that from 1994 to 1998 the demand on the U.S. interexchange carriers'(IXCs) network would increase sevenfold, and for the U.S. local exchange carriers' (LECs) network, the demand would increase fourfold. In actuality, one company indicated that its network growth was 32 times that of the previous year, while another company's rate of growth in 1997 alone was the same size as its entire network in 1991. Yet another has said that the size of its network doubled every six months in that four-year period.

In addition to this explosion in consumer demand for bandwidth, many service providers are coping with fiber exhaust in their networks. An industry survey indicated that in 1995, the amount of embedded fiber already in use in the average network was between 70 percent and 80 percent. Today, many carriers are nearing one hundred–percent capacity utilization across significant portions of their networks. Another problem for carriers is the challenge of deploying and integrating diverse technologies in one physical infrastructure. Customer demands and competitive pressures mandate that carriers offer diverse services economically and deploy them over the embedded network. DWDM provides service providers an answer to that demand (see Figure 1).

Figure 1. Optical Transport to Optical Networking: Evolution of the Phototonics Layer

Use of DWDM allows providers to offer services such as e-mail, video, and multimedia carried as Internet protocol (IP) data over asynchronous transfer mode (ATM) and voice carried over SONET/SDH. Despite the fact that these formats—IP, ATM, and SONET/SDH—provide unique bandwidth management capabilities, all three can be transported over the optical layer using DWDM. This unifying capability allows the service provider the flexibility to respond to customer demands over one network.

A platform that is able to unify and interface with these technologies and position the carrier with the ability to integrate current and next-generation technologies is critical for a carrier's success.

2. Resolving the Capacity Crisis

Faced with the multifaceted challenges of increased service needs, fiber exhaust, and layered bandwidth management, service providers need options to provide an economical solution. One way to alleviate fiber exhaust is to lay more fiber, and, for those networks where the cost of laying new fiber is minimal, this will prove the most economical solution. However, laying new fiber will not necessarily enable the service provider to provide new services or utilize the bandwidth management capability of a unifying optical layer.

A second choice is to increase the bit rate using time division multiplexing (TDM), where TDM increases the capacity of a fiber by slicing time into smaller intervals so that more bits (data) can be transmitted per second (see Figure 2). Traditionally, this has been the industry method of choice (DS–1, DS–2, DS–3, etc.). However, when service providers use this approach exclusively, they must make the leap to the higher bit rate in one jump, having purchased more capacity than they initially need. Based on the SONET hierarchy, the next incremental step from 10 Gbps TDM is 40 Gbps—a quantum leap that many believe will not be possible for TDM technology in the near future. This method has also been used with transport networks that are based on either the synchronous optical network (SONET) standard for North America or the synchronous digital network (SDH) standard for international networks.

Figure 2. Increased Network Capacity—TDM

The telecommunications industry adopted the SONET or SDH standard to provide a standard synchronous optical hierarchy with sufficient flexibility to accommodate current and future digital signals. SONET or SDH accomplishes this by defining standard rates and formats and optical interfaces. For example, multiple electrical and optical signals are brought into a SONET terminal where they are terminated and multiplexed electrically before becoming part of the payload of an STS–1, the building block frame structure of the SONET hierarchy. The STS–1 payloads are then multiplexed to be sent out on the single fiber at a single rate: OC–3 to OC–12 to OC–48 and eventually to OC–192. SDH has a similar structure with STM–n building block resulting in signal rates of STS–1 through STM–64.

SONET and SDH, two closely related standards, provided the foundation to transform the transport networks as we know them today. They govern interface parameters; rates, formats, and multiplexing methods; and operations, administration, maintenance, and provisioning (OAM&P) for high-speed transmission of bits of information in flashing laser-light streams. A synchronous mode of transmission means that the laser signals flowing through a fiber-optic system have been synchronized to an external clock. The resulting benefit is that data streams transmitting voice, data, and images through the fiber system flow in a steady, regulated manner so that each stream of light can readily be identified and easily extracted for delivery or routing.

3. Capacity Expansion and Flexibility: DWDM

The third choice for service providers is dense wavelength division multiplexing (DWDM), which increases the capacity of embedded fiber by first assigning incoming optical signals to specific frequencies (wavelength, lambda) within a designated frequency band and then multiplexing the resulting signals out onto one fiber. Because incoming signals are never terminated in the optical layer, the interface can be bit-rate and format independent, allowing the service provider to integrate DWDM technology easily with existing equipment in the network while gaining access to the untapped capacity in the embedded fiber.

DWDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber to increase capacity (see Figure 3). Each signal carried can be at a different rate (OC–3/12/24, etc.) and in a different format (SONET, ATM, data, etc.) For example, a DWDM network with a mix of SONET signals operating at OC–48 (2.5 Gbps) and OC–192 (10 Gbps) over a DWDM infrastructure can achieve capacities of over 40 Gbps. A system with DWDM can achieve all this gracefully while maintaining the same degree of system performance, reliability, and robustness as current transport systems—or even surpassing it. Future DWDM terminals will carry up to 80 wavelengths of OC–48, a total of 200 Gbps, or up to 40 wavelengths of OC–192, a total of 400 Gbps—which is enough capacity to transmit 90,000 volumes of an encyclopedia in one second.

Figure 3. Increased Network Capacity—WDM

The technology that allows this high-speed, high-volume transmission is in the optical amplifier. Optical amplifiers operate in a specific band of the frequency spectrum and are optimized for operation with existing fiber, making it possible to boost lightwave signals and thereby extend their reach without converting them back to electrical form. Demonstrations have been made of ultrawideband optical-fiber amplifiers that can boost lightwave signals carrying over 100 channels (or wavelengths) of light. A network using such an amplifier could easily handle a terabit of information. At that rate, it would be possible to transmit all the world's TV channels at once or about half a million movies at the same time.

Consider a highway analogy where one fiber can be thought of as a multilane highway. Traditional TDM systems use a single lane of this highway and increase capacity by moving faster on this single lane. In optical networking, utilizing DWDM is analogous to accessing the unused lanes on the highway (increasing the number of wavelengths on the embedded fiber base) to gain access to an incredible amount of untapped capacity in the fiber. An additional benefit of optical networking is that the highway is blind to the type of traffic that travels on it. Consequently, the vehicles on the highway can carry ATM packets, SONET, and IP.

4. Capacity Expansion Potential

By beginning with DWDM, service providers can establish a grow-as-you-go infrastructure, which allows them to add current and next-generation TDM systems for virtually endless capacity expansion (see Figure 4). DWDM also gives service providers the flexibility to expand capacity in any portion of their networks—an advantage no other technology can offer. Carriers can address specific problem areas that are congested because of high capacity demands. This is especially helpful where multiple rings intersect between two nodes, resulting in fiber exhaust.

Figure 4. Capacity Expansion Evolution: A Strategy for the Long Term

Service providers searching for new and creative ways to generate revenue while fully meeting the varying needs of their customers can benefit from a DWDM infrastructure as well. By partitioning and maintaining different dedicated wavelengths for different customers, for example, service providers can lease individual wavelengths—as opposed to an entire fiber—to their high-use business customers.

Compared with repeater-based applications, a DWDM infrastructure also increases the distances between network elements—a huge benefit for long-distance service providers looking to reduce their initial network investments significantly. The fiber-optic amplifier component of the DWDM system enables a service provider to save costs by taking in and amplifying optical signals without converting them to electrical signals. Furthermore, DWDM allows service providers to do it on a broad range of wavelengths in the 1.55µm region. For example, with a DWDM system multiplexing up to 16 wavelengths on a single fiber, carriers can decrease the number of amplifiers by a factor of 16 at each regenerator site. Using fewer regenerators in long-distance networks results in fewer interruptions and improved efficiency.

5. DWDM Incremental Growth

A DWDM infrastructure is designed to provide a graceful network evolution for service providers who seek to address their customers' ever-increasing capacity demands. Because a DWDM infrastructure can deliver the necessary capacity expansion, laying a foundation based on this technology is viewed as the best place to start. By taking incremental growth steps with DWDM, it is possible for service providers to reduce their initial costs significantly while deploying the network infrastructure that will serve them in the long run.

Some industry analysts have hailed DWDM as a perfect fit for networks that are trying to meet demands for more bandwidth. However, these experts have noted the conditions for this fit: a DWDM system simply must be scalable. Despite the fact that a system of OC–48 interfacing with 8 or 16 channels per fiber might seem like overkill now, such measures are necessary for the system to be efficient even two years from now.

Because OC–48 terminal technology and the related operations support systems (OSSs) match up with DWDM systems today, it is possible for service providers to begin evolving the capacity of the TDM systems already connected to their network. Mature OC–192 systems can be added later to the established DWDM infrastructure to expand capacity to 40 Gbps and beyond.

6. The Optical Layer as the Unifying Layer

Aside from the enormous capacity gained through optical networking, the optical layer provides the only means for carriers to integrate the diverse technologies of their existing networks into one physical infrastructure. DWDM systems are bit-rate and format independent and can accept any combination of interface rates (e.g., synchronous, asynchronous, OC–3, –12, –48, or –192) on the same fiber at the same time. If a carrier operates both ATM and SONET networks, the ATM signal does not have to be multiplexed up to the SONET rate to be carried on the DWDM network. Because the optical layer carries signals without any additional multiplexing, carriers can quickly introduce ATM or IP without deploying an overlay network. An important benefit of optical networking is that it enables any type of cargo to be carried on the highway.

But DWDM is just the first step on the road to full optical networking and the realization of the optical layer. The concept of an all-optical network implies that the service provider will have optical access to traffic at various nodes in the network, much like the SONET layer for SONET traffic. Optical wavelength add/drop (OWAD) offers that capability, where wavelengths are added or dropped to or from a fiber, without requiring a SONET terminal. But ultimate bandwidth management flexibility will come with a cross-connect capability on the optical layer. Combined with OWAD and DWDM, the optical cross-connect (OXC) will offer service providers the ability to create a flexible, high-capacity, efficient optical network with full optical bandwidth management. These technologies are today's reality: DWDM has been utilized in the long-distance network since 1995, OWAD will be available in products in 1998, and the first OXC was showcased at industry conventions in 1997.

7. Key DWDM System Characteristics

There are certain key characteristics of acceptable and optimal DWDM systems. These characteristics should be in place for any DWDM system in order for carriers to realize the full potential of this technology. The following questions help determine whether a given DWDM system is satisfactory.

Does the system reuse embedded equipment and fiber plant?

DWDM systems at 2.5 Gbps should use the full capability of the embedded equipment and fiber base.

Is the system robust and reliable?

Well-engineered DWDM systems offer component reliability, system availability, and system margin. Although filters were often susceptible to humidity, this is no longer the case.

Do the pump lasers have connectors, or are they spliced in the optical amplifier?

An optical amplifier has two key elements: the optical fiber that is doped with the element erbium and the amplifier. When a pump laser is used to energize the erbium with light at a specific wavelength, the erbium acts as a gain medium that amplifies the incoming optical signal. If a connector is used rather than a splice, slight amounts of dirt on the surface may cause the connector to become damaged.

Is manual intervention required when adding or removing channels?

Automatic adjustment of the optical amplifiers when channels are added or removed achieves optimal system performance. This is important because if there is just one channel on the system with high power, degradation in performance through self-phase modulation can occur. On the other hand, too little power results in not enough gain from the amplifier.

Does the system use fluoride- or silica-based fiber amplifiers?

In the 1530- to 1565-nm range, silica-based optical amplifiers with filters and fluoride-based optical amplifiers perform equally well. However, fluoride-based optical amplifiers are intrinsically more costly to implement. The long-term reliability of fluoride-based fibers has not yet been verified.

Can the system's number of wavelengths and bit rate be upgraded?

While the answer is yes for all DWDM systems, planning for this is critical. If service providers put together their networks in a specific way and then want to upgrade, one of two things must happen: They need either more power or additional signal-to-noise margin. For example, each time providers double the number of channels or the bit rate, 3 dB of additional signal-to-noise margin is needed.

Does the system offer standards-compliant maintenance interfaces?

Standard transaction language 1 interfaces are widely available for DWDM systems. Interfaces should readily fit into a service provider's typical maintenance scheme.

8. Conclusion

Optical networking provides the backbone to support existing and emerging technologies with almost limitless amounts of bandwidth capacity. All-optical networking (not just point-to-point transport) enabled by optical cross-connects, optical programmable add/drop multiplexers, and optical switches provides a unified infrastructure capable of meeting the telecommunications demands of today and tomorrow. Transparently moving trillions of bits of information efficiently and cost-effectively will enable service providers to maximize their embedded infrastructure and position themselves for the capacity demand of the next millennium.

Glossary

ATM
asynchronous transfer mode

DWDM
dense wavelength division multiplexing

IEC
interexchange carrier

IP
Internet protocol

LEC
local exchange carrier

OAM&P
operations, administration, maintenance, and provisioning

OWAD
optical wavelength add/drop

OXC
optical cross-connect

SDH
synchronous digital hierarchy

SONET
synchronous optical network

TDM
time division multiplexing

Global System for Mobile Communication (GSM)

Definition and Overview

Definition
Global system for mobile communication (GSM) is a globally accepted standard for digital cellular communication. GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz. It is estimated that many countries outside of Europe will join the GSM partnership.

Overview
This tutorial provides an introduction to basic GSM concepts, specifications, networks, and services. A short history of network evolution is provided in order set the context for understanding GSM.

1. Introduction: The Evolution of Mobile Telephone Systems

Cellular is one of the fastest growing and most demanding telecommunications applications. Today, it represents a continuously increasing percentage of all new telephone subscriptions around the world. Currently there are more than 45 million cellular subscribers worldwide, and nearly 50 percent of those subscribers are located in the United States. It is forecasted that cellular systems using a digital technology will become the universal method of telecommunications. By the year 2005, forecasters predict that there will be more than 100 million cellular subscribers worldwide. It has even been estimated that some countries may have more mobile phones than fixed phones by the year 2000 (see Figure 1).

Figure 1. Cellular Subscriber Growth Worldwide

The concept of cellular service is the use of low-power transmitters where frequencies can be reused within a geographic area. The idea of cell-based mobile radio service was formulated in the United States at Bell Labs in the early 1970s. However, the Nordic countries were the first to introduce cellular services for commercial use with the introduction of the Nordic Mobile Telephone (NMT) in 1981.

Cellular systems began in the United States with the release of the advanced mobile phone service (AMPS) system in 1983. The AMPS standard was adopted by Asia, Latin America, and Oceanic countries, creating the largest potential market in the world for cellular.

In the early 1980s, most mobile telephone systems were analog rather than digital, like today's newer systems. One challenge facing analog systems was the inability to handle the growing capacity needs in a cost-efficient manner. As a result, digital technology was welcomed. The advantages of digital systems over analog systems include ease of signaling, lower levels of interference, integration of transmission and switching, and increased ability to meet capacity demands. Table 1 charts the worldwide development of mobile telephone systems.

Year	Mobile System
1981	Nordic Mobile Telephone (NMT) 450
1983	American Mobile Phone System (AMPS)
1985	Total Access Communication System (TACS)
1986	Nordic Mobile Telephony (NMT) 900
1991	American Digital Cellular (ADC)
1991	Global System for Mobile Communication (GSM)
1992	Digital Cellular System (DCS) 1800
1994	Personal Digital Cellular (PDC)
1995	PCS 1900—Canada
1996	PCS—United States

Table 1. The Development of Mobile Telephone Systems

2. GSM

Throughout the evolution of cellular telecommunications, various systems have been developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. The GSM standard is intended to address these problems.

From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen. A summary of GSM milestones is given in Table 2.

Year	Milestone
1982	GSM formed
1986	field test
1987	TDMA chosen as access method
1988	memorandum of understanding signed
1989	validation of GSM system
1990	preoperation system
1991	commercial system start-up
1992	coverage of larger cities/airports
1993	coverage of main roads
1995	coverage of rural areas

Table 2. GSM Milestone

3. The GSM Network

GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to limit the designers as little as possible but still to make it possible for the operators to buy equipment from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system (OSS). The basic GSM network elements are shown in Figure 2.

Figure 2. GSM Network Elements

The Switching System

The switching system (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units.

• home location register (HLR)—The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.
• mobile services switching center (MSC)—The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.
• visitor location register (VLR)—The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.
• authentication center (AUC)—A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.
• equipment identity register (EIR)—The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

The Base Station System (BSS)

All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

• BSC—The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.
• BTS—The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

The Operation and Support System

The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

Additional Functional Elements

Other functional elements shown in Figure 2 are as follows:

• message center (MXE)—The MXE is a node that provides integrated voice, fax, and data messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail, e-mail, and notification.
• mobile service node (MSN)—The MSN is the node that handles the mobile intelligent network (IN) services.
• gateway mobile services switching center (GMSC)—A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC.
• GSM interworking unit (GIWU)—The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware equipment is physically located at the MSC/VLR.

4. GSM Network Areas

The GSM network is made up of geographic areas. As shown in Figure 3, these areas include cells, location areas (LAs), MSC/VLR service areas, and public land mobile network (PLMN) areas.

Figure 3. Network Areas

The cell is the area given radio coverage by one base transceiver station. The GSM network identifies each cell via the cell global identity (CGI) number assigned to each cell. The location area is a group of cells. It is the area in which the subscriber is paged. Each LA is served by one or more base station controllers, yet only by a single MSC (see Figure 4). Each LA is assigned a location area identity (LAI) number.

Figure 4. Location Areas

An MSC/VLR service area represents the part of the GSM network that is covered by one MSC and which is reachable, as it is registered in the VLR of the MSC (see Figure 5).

Figure 5. MSC/VLR Service Areas

The PLMN service area is an area served by one network operator (see Figure 6).

Figure 6. PLMN Network Areas

5. GSM Specifications

Before looking at the GSM specifications, it is important to understand the following basic terms:

• bandwidth—the range of a channel's limits; the broader the bandwidth, the faster data can be sent
• bits per second (bps)—a single on-off pulse of data; eight bits are equivalent to one byte
• frequency—the number of cycles per unit of time; frequency is measured in hertz (Hz)
• kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents 1,000 bits per second
• megahertz (MHz)—1,000,000 hertz (cycles per second)
• milliseconds (ms)—one-thousandth of a second
• watt (W)—a measure of power of a transmitter

Specifications for different personal communication services (PCS) systems vary among the different PCS networks. Listed below is a description of the specifications and characteristics for GSM.

• frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).
• duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart.
• channel separation—The separation between adjacent carrier frequencies. In GSM, this is 200 kHz.
• modulation—Modulation is the process of sending a signal by changing the characteristics of a carrier frequency. This is done in GSM via Gaussian minimum shift keying (GMSK).
• transmission rate—GSM is a digital system with an over-the-air bit rate of 270 kbps.
• access method—GSM utilizes the time division multiple access (TDMA) concept. TDMA is a technique in which several different calls may share the same carrier. Each call is assigned a particular time slot.
• speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.

6. GSM Subscriber Services

There are two basic types of services offered through GSM: telephony (also referred to as teleservices) and data (also referred to as bearer services). Telephony services are mainly voice services that provide subscribers with the complete capability (including necessary terminal equipment) to communicate with other subscribers. Data services provide the capacity necessary to transmit appropriate data signals between two access points creating an interface to the network. In addition to normal telephony and emergency calling, the following subscriber services are supported by GSM:

• dual-tone multifrequency (DTMF)—DTMF is a tone signaling scheme often used for various control purposes via the telephone network, such as remote control of an answering machine. GSM supports full-originating DTMF.
• facsimile group III—GSM supports CCITT Group 3 facsimile. As standard fax machines are designed to be connected to a telephone using analog signals, a special fax converter connected to the exchange is used in the GSM system. This enables a GSM–connected fax to communicate with any analog fax in the network.
• short message services—A convenient facility of the GSM network is the short message service. A message consisting of a maximum of 160 alphanumeric characters can be sent to or from a mobile station. This service can be viewed as an advanced form of alphanumeric paging with a number of advantages. If the subscriber's mobile unit is powered off or has left the coverage area, the message is stored and offered back to the subscriber when the mobile is powered on or has reentered the coverage area of the network. This function ensures that the message will be received.
• cell broadcast—A variation of the short message service is the cell broadcast facility. A message of a maximum of 93 characters can be broadcast to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and reports on accidents.
• voice mail—This service is actually an answering machine within the network, which is controlled by the subscriber. Calls can be forwarded to the subscriber's voice-mail box and the subscriber checks for messages via a personal security code.
• fax mail—With this service, the subscriber can receive fax messages at any fax machine. The messages are stored in a service center from which they can be retrieved by the subscriber via a personal security code to the desired fax number.

Supplementary Services

GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary services are defined by GSM and are characterized as revenue-generating features. A partial listing of supplementary services follows.

• call forwarding—This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.
• barring of outgoing calls—This service makes it possible for a mobile subscriber to prevent all outgoing calls.
• barring of incoming calls—This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.
• advice of charge (AoC)—The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.
• call hold—This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.
• call waiting—This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.
• multiparty service—The multiparty service enables a mobile subscriber to establish a multiparty conversation—that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.
• calling line identification presentation/restriction—These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.
• closed user groups (CUGs)—CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

Glossary

ADC
American Digital Cellular

AMPS
advanced mobile phone service

AoC
advice of charge

AUC
authentication center

bps
bits per second

BSC
base station controller

BSS
base station system

BTS
base transceiver station

CGI
cell global identity

CUG
closed user group

DCS
digital cellular system

DTMF
dual-tone multifrequency

EIR
equipment identity register

GIWU
GSM interworking unit

GMSC
gateway mobile services switching center

GMSK
Gaussian minimum shift keying

GSM
global system for mobile communication

HLR
home location register

Hz
hertz

ISDN
integrated services digital network

k
kilo

kbps
kilobits per second

LA
location area

LAI
location-area identity

LPC
linear predictive coding

MHz
megahertz

MSC
mobile services switching center

MSN
mobile service node

MXE
message center

NMT
Nordic Mobile Telephone

OMC
operations and maintenance center

OSS
operation and support system

PCS
personal communications services

PDC
personal digital cellular

PLMN
public land mobile network

SS
switching system

TACS
total access communication system

TDMA
time division multiple access

VLR
visitor location register

Signaling System 7 (SS7)

Definition and Overview

Definition
Signaling System 7 (SS7) is an architecture for performing out-of-band signaling in support of the call-establishment, billing, routing, and information-exchange functions of the public switched telephone network (PSTN). It identifies functions to be performed by a signaling-system network and a protocol to enable their performance.

This tutorial was authored by Art Doskow, Senior Member of Technical Staff–Signaling and Control Architecture Evolution, Bell Atlantic.

1. What is Signaling?

Signaling refers to the exchange of information between call components required to provide and maintain service.

As users of the PSTN, we exchange signaling with network elements all the time. Examples of signaling between a telephone user and the telephone network include: dialing digits, providing dial tone, accessing a voice mailbox, sending a call-waiting tone, dialing *66 (to retry a busy number), etc.

SS7 is a means by which elements of the telephone network exchange information. Information is conveyed in the form of messages. SS7 messages can convey information such as:

• I’m forwarding to you a call placed from 212-555-1234 to 718-555-5678. Look for it on trunk 067.
• Someone just dialed 800-555-1212. Where do I route the call?
• The called subscriber for the call on trunk 11 is busy. Release the call and play a busy tone.
• The route to XXX is congested. Please don’t send any messages to XXX unless they are of priority 2 or higher.
• I’m taking trunk 143 out of service for maintenance.

SS7 is characterized by high-speed packet data and out-of-band signaling.

2. What is Out-of-Band Signaling?

Out-of-band signaling is signaling that does not take place over the same path as the conversation.

We are used to thinking of signaling as being in-band. We hear dial tone, dial digits, and hear ringing over the same channel on the same pair of wires. When the call completes, we talk over the same path that was used for the signaling. Traditional telephony used to work in this way as well. The signals to set up a call between one switch and another always took place over the same trunk that would eventually carry the call. Signaling took the form of a series of multifrequency (MF) tones, much like touch tone dialing between switches.

Out-of-band signaling establishes a separate digital channel for the exchange of signaling information. This channel is called a signaling link. Signaling links are used to carry all the necessary signaling messages between nodes. Thus, when a call is placed, the dialed digits, trunk selected, and other pertinent information are sent between switches using their signaling links, rather than the trunks which will ultimately carry the conversation. Today, signaling links carry information at a rate of 56 or 64 kbps. It is interesting to note that while SS7 is used only for signaling between network elements, the ISDN D channel extends the concept of out-of-band signaling to the interface between the subscriber and the switch. With ISDN service, signaling that must be conveyed between the user station and the local switch is carried on a separate digital channel called the D channel. The voice or data which comprise the call is carried on one or more B channels.

Why Out-of-Band Signaling?

Out-of-band signaling has several advantages that make it more desirable than traditional in-band signaling.

• It allows for the transport of more data at higher speeds (56 kbps can carry data much faster than MF outpulsing).
• It allows for signaling at any time in the entire duration of the call, not only at the beginning.
• It enables signaling to network elements to which there is no direct trunk connection.

3. Signaling Network Architecture

If signaling is to be carried on a different path from the voice and data traffic it supports, then what should that path look like? The simplest design would be to allocate one of the paths between each interconnected pair of switches as the signaling link. Subject to capacity constraints, all signaling traffic between the two switches could traverse this link. This type of signaling is known as associated signaling, and is shown below in Figure 1.

Figure 1. Associated Signaling

Associated signaling works well as long as a switch’s only signaling requirements are between itself and other switches to which it has trunks. If call setup and management was the only application of SS7, associated signaling would meet that need simply and efficiently. In fact, much of the out-of-band signaling deployed in Europe today uses associated mode.

The North American implementers of SS7, however, wanted to design a signaling network that would enable any node to exchange signaling with any other SS7–capable node. Clearly, associated signaling becomes much more complicated when it is used to exchange signaling between nodes which do not have a direct connection. From this need, the North American SS7 architecture was born.

4. The North American Signaling Architecture

The North American signaling architecture defines a completely new and separate signaling network. The network is built out of the following three essential components, interconnected by signaling links:

• signal switching points (SSPs)—SSPs are telephone switches (end offices or tandems) equipped with SS7-capable software and terminating signaling links. They generally originate, terminate, or switch calls.
• signal transfer points (STPs)—STPs are the packet switches of the SS7 network. They receive and route incoming signaling messages towards the proper destination. They also perform specialized routing functions.
• signal control points (SCPs)—SCPs are databases that provide information necessary for advanced call-processing capabilities.

Once deployed, the availability of SS7 network is critical to call processing. Unless SSPs can exchange signaling, they cannot complete any interswitch calls. For this reason, the SS7 network is built using a highly redundant architecture. Each individual element also must meet exacting requirements for availability. Finally, protocol has been defined between interconnected elements to facilitate the routing of signaling traffic around any difficulties that may arise in the signaling network.

To enable signaling network architectures to be easily communicated and understood, a standard set of symbols was adopted for depicting SS7 networks. Figure 2 shows the symbols that are used to depict these three key elements of any SS7 network.

Figure 2. Signaling Network Elements

STPs and SCPs are customarily deployed in pairs. While elements of a pair are not generally co-located, they work redundantly to perform the same logical function. When drawing complex network diagrams, these pairs may be depicted as a single element for simplicity, as shown in Figure 3.

Figure 3. STP and SCP Pairs

5. Basic Signaling Architecture

Figure 4 shows a small example of how the basic elements of an SS7 network are deployed to form two interconnected networks.

Figure 4. Sample Network

The following points should be noted:

1. STPs W and X perform identical functions. They are redundant. Together, they are referred to as a mated pair of STPs. Similarly, STPs Y and Z form a mated pair.
2. Each SSP has two links (or sets of links), one to each STP of a mated pair. All SS7 signaling to the rest of the world is sent out over these links. Because the STPs of a mated pair are redundant, messages sent over either link (to either STP) will be treated equivalently.
3. The STPs of a mated pair are joined by a link (or set of links).
4. Two mated pairs of STPs are interconnected by four links (or sets of links). These links are referred to as a quad.
5. SCPs are usually (though not always) deployed in pairs. As with STPs, the SCPs of a pair are intended to function identically. Pairs of SCPs are also referred to as mated pairs of SCPs. Note that they are not directly joined by a pair of links.
6. Signaling architectures such as this, which provide indirect signaling paths between network elements, are referred to as providing quasi-associated signaling.

6. SS7 Link Types

SS7 signaling links are characterized according to their use in the signaling network. Virtually all links are identical in that they are 56–kbps or 64–kbps bidirectional data links that support the same lower layers of the protocol; what is different is their use within a signaling network. The defined link types are shown in Figure 5 and defined as follows:

Figure 5. Link Types

A Links

A links interconnect an STP and either an SSP or an SCP, which are collectively referred to as signaling end points ("A" stands for access). A links are used for the sole purpose of delivering signaling to or from the signaling end points (they could just as well be referred to as signaling beginning points). Examples of A links are 2–8, 3–7, and 5–12 in Figure 5.

Signaling that an SSP or SCP wishes to send to any other node is sent on either of its A links to its home STP, which, in turn, processes or routes the messages. Similarly, messages intended for an SSP or SCP will be routed to one of its home STPs, which will forward them to the addressed node over its A links.

C Links

C links are links that interconnect mated STPs. As will be seen later, they are used to enhance the reliability of the signaling network in instances where one or several links are unavailable. "C" stands for cross (7–8, 9–10, and 11–12 are C links). B links, D links, and B/D links interconnecting two mated pairs of STPs are referred to as either B links, D links, or B/D links. Regardless of their name, their function is to carry signaling messages beyond their initial point of entry to the signaling network towards their intended destination. The "B" stands for bridge and describes the quad of links interconnecting peer pairs of STPs. The "D" denotes diagonal and describes the quad of links interconnecting mated pairs of STPs at different hierarchical levels. Because there is no clear hierarchy associated with a connection between networks, interconnecting links are referred to as either B, D, or B/D links (7–11 and 7–12 are examples of B links; 8–9 and 7–10 are examples of D links; 10–13 and 9–14 are examples of interconnecting links and can be referred to as B, D, or B/D links).

E Links

While an SSP is connected to its home STP pair by a set of A links, enhanced reliability can be provided by deploying an additional set of links to a second STP pair. These links, called E (extended) links provide backup connectivity to the SS7 network in the event that the home STPs cannot be reached via the A links. While all SS7 networks include A, B/D, and C links, E links may or may not be deployed at the discretion of the network provider. The decision of whether or not to deploy E links can be made by comparing the cost of deployment with the improvement in reliability. (1–11 and 1–12 are E links.)

F Links

F (fully associated) links are links which directly connect two signaling end points. F links allow associated signaling only. Because they bypass the security features provided by an STP, F links are not generally deployed between networks. Their use within an individual network is at the discretion of the network provider. (1–2 is an F link.)

7. Basic Call Setup Example

Before going into much more detail, it might be helpful to look at several basic calls and the way in which they use SS7 signaling (see Figure 6).

Figure 6. Call Setup Example

In this example, a subscriber on switch A places a call to a subscriber on switch B.

1. Switch A analyzes the dialed digits and determines that it needs to send the call to switch B.
2. Switch A selects an idle trunk between itself and switch B and formulates an initial address message (IAM), the basic message necessary to initiate a call. The IAM is addressed to switch B. It identifies the initiating switch (switch A), the destination switch (switch B), the trunk selected, the calling and called numbers, as well as other information beyond the scope of this example.
3. Switch A picks one of its A links (e.g., AW) and transmits the message over the link for routing to switch B.
4. STP W receives a message, inspects its routing label, and determines that it is to be routed to switch B. It transmits the message on link BW.
5. Switch B receives the message. On analyzing the message, it determines that it serves the called number and that the called number is idle.
6. Switch B formulates an address complete message (ACM), which indicates that the IAM has reached its proper destination. The message identifies the recipient switch (A), the sending switch (B), and the selected trunk.
7. Switch B picks one of its A links (e.g., BX) and transmits the ACM over the link for routing to switch A. At the same time, it completes the call path in the backwards direction (towards switch A), sends a ringing tone over that trunk towards switch A, and rings the line of the called subscriber.
8. STP X receives the message, inspects its routing label, and determines that it is to be routed to switch A. It transmits the message on link AX.
9. On receiving the ACM, switch A connects the calling subscriber line to the selected trunk in the backwards direction (so that the caller can hear the ringing sent by switch B).
10. When the called subscriber picks up the phone, switch B formulates an answer message (ANM), identifying the intended recipient switch (A), the sending switch (B), and the selected trunk.
11. Switch B selects the same A link it used to transmit the ACM (link BX) and sends the ANM. By this time, the trunk also must be connected to the called line in both directions (to allow conversation).
12. STP X recognizes that the ANM is addressed to switch A and forwards it over link AX.
13. Switch A ensures that the calling subscriber is connected to the outgoing trunk (in both directions) and that conversation can take place.
14. If the calling subscriber hangs up first (following the conversation), switch A will generate a release message (REL) addressed to switch B, identifying the trunk associated with the call. It sends the message on link AW.
15. STP W receives the REL, determines that it is addressed to switch B, and forwards it using link WB.
16. Switch B receives the REL, disconnects the trunk from the subscriber line, returns the trunk to idle status, generates a release complete message (RLC) addressed back to switch A, and transmits it on link BX. The RLC identifies the trunk used to carry the call.
17. STP X receives the RLC, determines that it is addressed to switch A, and forwards it over link AX.
18. On receiving the RLC, switch A idles the identified trunk.

8. Database Query Example

People generally are familiar with the toll-free aspect of 800 (or 888) numbers, but these numbers have significant additional capabilities made possible by the SS7 network. 800 numbers are virtual telephone numbers. Although they are used to point to real telephone numbers, they are not assigned to the subscriber line itself.

When a subscriber dials an 800 number, it is a signal to the switch to suspend the call and seek further instructions from a database. The database will provide either a real phone number to which the call should be directed, or it will identify another network (e.g., a long-distance carrier) to which the call should be routed for further processing. While the response from the database could be the same for every call (as, for example, if you have a personal 800 number), it can be made to vary based on the calling number, the time of day, the day of the week, or a number of other factors.

The following example shows how an 800 call is routed (see Figure 7).

Figure 7. Database Query Example

1. A subscriber served by switch A wants to reserve a rental car at a company's nearest location. She dials the company's advertised 800 number.
2. When the subscriber has finished dialing, switch A recognizes that this is an 800 call and that it requires assistance to handle it properly.
3. Switch A formulates an 800 query message including the calling and called number and forwards it to either of its STPs (e.g., X) over its A link to that STP (AX).
4. STP X determines that the received query is an 800 query and selects a database suitable to respond to the query (e.g., M).
5. STP X forwards the query to SCP M over the appropriate A link (MX). SCP M receives the query, extracts the passed information, and (based on its stored records) selects either a real telephone number or a network (or both) to which the call should be routed.
6. SCP M formulates a response message with the information necessary to properly process the call, addresses it to switch A, picks an STP and an A link to use (e.g., MW), and routes the response.
7. STP W receives the response message, recognizes that it is addressed to switch A, and routes it to A over AW.
8. Switch A receives the response and uses the information to determine where the call should be routed. It then picks a trunk to that destination, generates an IAM, and proceeds (as it did in the previous example) to set up the call.

9. Layers of the SS7 Protocol

As the call-flow examples show, the SS7 network is an interconnected set of network elements that is used to exchange messages in support of telecommunications functions. The SS7 protocol is designed to both facilitate these functions and to maintain the network over which they are provided. Like most modern protocols, the SS7 protocol is layered.

Physical Layer

This defines the physical and electrical characteristics of the signaling links of the SS7 network. Signaling links utilize DS–0 channels and carry raw signaling data at a rate of 56 kbps or 64 kbps (56 kbps is the more common implementation).

Message Transfer Part—Level 2

The level 2 portion of the message transfer part (MTP Level 2) provides link-layer functionality. It ensures that the two end points of a signaling link can reliably exchange signaling messages. It incorporates such capabilities as error checking, flow control, and sequence checking.

Message Transfer Part—Level 3

The level 3 portion of the message transfer part (MTP Level 3) extends the functionality provided by MTP level 2 to provide network layer functionality. It ensures that messages can be delivered between signaling points across the SS7 network regardless of whether they are directly connected. It includes such capabilities as node addressing, routing, alternate routing, and congestion control.

Collectively, MTP levels 2 and 3 are referred to as the message transfer part (MTP).

Signaling Connection Control Part

The signaling connection control part (SCCP) provides two major functions that are lacking in the MTP. The first of these is the capability to address applications within a signaling point. The MTP can only receive and deliver messages from a node as a whole; it does not deal with software applications within a node.

While MTP network-management messages and basic call-setup messages are addressed to a node as a whole, other messages are used by separate applications (referred to as subsystems) within a node. Examples of subsystems are 800 call processing, calling-card processing, advanced intelligent network (AIN), and custom local-area signaling services (CLASS) services (e.g., repeat dialing and call return). The SCCP allows these subsystems to be addressed explicitly.

Global Title Translation

The second function provided by the SCCP is the ability to perform incremental routing using a capability called global title translation (GTT). GTT frees originating signaling points from the burden of having to know every potential destination to which they might have to route a message. A switch can originate a query, for example, and address it to an STP along with a request for GTT. The receiving STP can then examine a portion of the message, make a determination as to where the message should be routed, and then route it.

For example, calling-card queries (used to verify that a call can be properly billed to a calling card) must be routed to an SCP designated by the company that issued the calling card. Rather than maintaining a nationwide database of where such queries should be routed (based on the calling-card number), switches generate queries addressed to their local STPs, which, using GTT, select the correct destination to which the message should be routed. Note that there is no magic here; STPs must maintain a database that enables them to determine where a query should be routed. GTT effectively centralizes the problem and places it in a node (the STP) that has been designed to perform this function.

In performing GTT, an STP does not need to know the exact final destination of a message. It can, instead, perform intermediate GTT, in which it uses its tables to find another STP further along the route to the destination. That STP, in turn, can perform final GTT, routing the message to its actual destination.

Intermediate GTT minimizes the need for STPs to maintain extensive information about nodes that are far removed from them. GTT also is used at the STP to share load among mated SCPs in both normal and failure scenarios. In these instances, when messages arrive at an STP for final GTT and routing to a database, the STP can select from among available redundant SCPs. It can select an SCP on either a priority basis (referred to as primary backup) or so as to equalize the load across all available SCPs (referred to as load sharing).

ISDN User Part (ISUP)

ISUP user part defines the messages and protocol used in the establishment and tear down of voice and data calls over the public switched network (PSN), and to manage the trunk network on which they rely. Despite its name, ISUP is used for both ISDN and non–ISDN calls. In the North American version of SS7, ISUP messages rely exclusively on MTP to transport messages between concerned nodes.

Transaction Capabilities Application Part (TCAP)

TCAP defines the messages and protocol used to communicate between applications (deployed as subsystems) in nodes. It is used for database services such as calling card, 800, and AIN as well as switch-to-switch services including repeat dialing and call return. Because TCAP messages must be delivered to individual applications within the nodes they address, they use the SCCP for transport.

Operations, Maintenance, and Administration Part (OMAP)

OMAP defines messages and protocol designed to assist administrators of the SS7 network. To date, the most fully developed and deployed of these capabilities are procedures for validating network routing tables and for diagnosing link troubles. OMAP includes messages that use both the MTP and SCCP for routing.

10. What Goes Over the Signaling Link

Signaling information is passed over the signaling link in messages, which are called signal units (SUs).

Three types of SUs are defined in the SS7 protocol.

1. message signal units (MSUs)
2. link status signal units (LSSUs)
3. fill-in signal units (FISUs)

SUs are transmitted continuously in both directions on any link that is in service. A signaling point that does not have MSUs or LSSUs to send will send FISUs over the link. The FISUs perform the function suggested by their name; they fill up the signaling link until there is a need to send purposeful signaling. They also facilitate link transmission monitoring and the acknowledgment of other SUs.

All transmission on the signaling link is broken up into 8-bit bytes, referred to as octets. SUs on a link are delimited by a unique 8-bit pattern known as a flag. The flag is defined as the 8-bit pattern "01111110". Because of the possibility that data within an SU would contain this pattern, bit manipulation techniques are used to ensure that the pattern does not occur within the message as it is transmitted over the link. (The SU is reconstructed once it has been taken off the link, and any bit manipulation is reversed.) Thus, any occurrence of the flag on the link indicates the end of one SU and the beginning of another. While in theory two flags could be placed between SUs (one to mark the end of the current message and one to mark the start of the next message), in practice a single flag is used for both purposes.

11. Addressing in the SS7 Network

Every network must have an addressing scheme, and the SS7 network is no different. Network addresses are required so that a node can exchange signaling nodes to which it does not have a physical signaling link. In SS7, addresses are assigned using a three-level hierarchy. Individual signaling points are identified as belonging to a cluster of signaling points. Within that cluster, each signaling point is assigned a member number. Similarly, a cluster is defined as being part of a network. Any node in the American SS7 network can be addressed by a three-level number defined by its network, cluster, and member numbers. Each of these numbers is an 8-bit number and can assume values from 0 to 255. This three-level address is known as the point code of the signaling point. A point code uniquely identifies a signaling point within the American SS7 network and is used whenever it is necessary to address that signaling point.

Network numbers are assigned on a nationwide basis by a neutral party. Regional Bell operating companies (RBOCs), major independent telephone companies, and interexchange carriers (IXCs) already have network numbers assigned. Because network numbers are a relatively scarce resource, companies' networks are expected to meet certain size requirements in order to be assigned a network number. Smaller networks can be assigned one or more cluster numbers within network numbers 1, 2, 3, and 4. The smallest networks are assigned point codes within network number 5. The cluster to which they are assigned is determined by the state in which they are located. The network number 0 is not available for assignment and network number 255 is reserved for future use.

12. Signal Unit Structure

SUs of each type follow a format unique to that type. A high-level view of those formats is shown in Figure 8.

Figure 8. Signaling Unit Formats

All three SU types have a set of common fields that are used by MTP Level 2. They are as follows:

Flag

Flags delimit SUs. A flag marks the end of one SU and the start of the next.

Checksum

The checksum is an 8-bit sum intended to verify that the SU has passed across the link error-free. The checksum is calculated from the transmitted message by the transmitting signaling point and inserted in the message. On receipt, it is recalculated by the receiving signaling point. If the calculated result differs from the received checksum, the received SU has been corrupted. A retransmission is requested.

Length Indicator

The length indicator indicates the number of octets between itself and the checksum. It serves both as a check on the integrity of the SU and as a means of discriminating between different types of SUs at level 2. As can be inferred from Figure 8, FISUs have a length indicator of 0; LSSUs have a length indicator of 1 or 2 (currently all LSSUs have a length indicator of 1), and MSUs have a length-indicator greater than 2. According to the protocol, only 6 of the 8 bits in the length indicator field are actually used to store this length; thus the largest value that can be accommodated in the length indicator is 63. For MSUs with more than 63 octets following the length indicator, the value of 63 is used.

BSN/BIB FSN/FIB

These octets hold the backwards sequence number (BSN), the backwards indicator bit (BIB), the forward sequence number (FSN), and the forward indicator bit (FIB). These fields are used to confirm receipt of SUs and to ensure that they are received in the order in which they were transmitted. They also are used to provide flow control. MSUs and LSSUs, when transmitted, are assigned a sequence number that is placed in the forward sequence number field of the outgoing SU. This SU is stored by the transmitting signaling point until it is acknowledged by the receiving signaling point.

Because the seven bits allocated to the forward sequence number can store 128 distinct values, it follows that a signaling point is restricted to sending 128 unacknowledged SUs before it must await an acknowledgment. By acknowledging an SU, the receiving node frees that SU's sequence number at the transmitting node, making it available for a new outgoing SU. Signaling points acknowledge receipt of SUs by placing the sequence number of the last correctly received and in-sequence SU in the backwards sequence number of every SU they transmit. In that way, they acknowledge all previously received SUs as well. The forward and backwards indicator bits are used to indicate sequencing or data-corruption errors and to request retransmission.

13. What are the Functions of the Different Signaling Units?

FISUs themselves have no information payload. Their purpose is to occupy the link at those times when there are no LSSUs or MSUs to send. Because they undergo error checking, FISUs facilitate the constant monitoring of link quality in the absence of signaling traffic. FISUs also can be used to acknowledge the receipt of messages using the backwards sequence number and backwards indicator bit.

LSSUs are used to communicate information about the signaling link between the nodes on either end of the link. This information is contained in the status field of the SU (see Figure 8). Because the two ends of a link are controlled by independent processors, there is a need to provide a means for them to communicate. LSSUs provide the means for performing this function. LSSUs are used primarily to signal the initiation of link alignment, the quality of received signaling traffic, and the status of the processors at either end of the link. Because they are sent only between the signaling points at either end of the link, LSSUs do not require any addressing information.

MSUs are the workhorses of the SS7 network. All signaling associated with call setup and tear down, database query and response, and SS7 network management takes place using MSUs. MSUs are the basic envelope within which all addressed signaling information is placed. As will be shown below, there are several different types of MSUs. All MSUs have certain fields in common. Other fields differ according to the type of message. The type of MSU is indicated in the service-information octet shown in Figure 8; the addressing and informational content of the MSU is contained in the signaling information field.

14. Message Signal Unit Structure

The functionality of the message signal unit lies in the actual content of the service information octet and the signaling information field (see Figure 8).

The service information octet is an 8-bit field (as might be inferred from its name) that contains three types of information as follows:

1. four bits are used to indicate the type of information contained in the signaling information field; they are referred to as the service indicator; the values most commonly used in American networks are outlined in Table 1

   Value	Function
   0	signaling network management
   1	signaling network testing and maintenance
   3	signaling connection control part (SCCP)
   5	ISDN user part (ISUP)

   
   Table 1. Common Signaling Indicator Values

2. two bits are used to indicate whether the message is intended (and coded) for use in a national or international network; they are generally coded with a value of 2, national network
3. the remaining 2 bits are used (in American networks) to identify a message priority, from 0 to 3, with 3 being the highest priority; message priorities do not control the order in which messages are transmitted; they are only used in cases of signaling network congestion; in that case, they indicate whether a message has sufficient priority to merit transmission during an instance of congestion or whether it can be discarded en route to a destination

The format of the contents of the signaling information field is determined by the service indicator. (Within user parts, there are further distinctions in message formats, but the service indicator provides the first piece of information necessary for routing or decoding the message.)

The first portion of the signaling information field is identical for all MSUs currently in use. It is referred to as the routing label. Simply stated, the routing label identifies the message originator, the intended destination of the message, and a field referred to as the signaling-link selection field which is used to distribute message traffic over the set of possible links and routes. The routing label consists of 7 octets that are outlined below in Table 2 (in order of transmission).

Octet Group	Function	Number of Octets Involved
destination point code (DPC)	contains the address of the node to which the message is being sent	3 octets
originating point code (OPC)	contains the address of message originator	3 octets
signaling link selection (SLS)	distributes load among redundant routes	1 octet

Table 2. Routing Label

Point codes consist of the three-part identifier (network number, cluster number, and member number), which uniquely identifies a signaling point.

Glossary

A links
access links

ACM
address complete message

ANM
answer message

B links
bridge links

BIB
backward indicator bit

BSN
backward sequence number

D links
diagonal links

DPC
destination point code

E link
extended link

F link
fully associated link

FIB
forward indicator bit

FISU
fill in signal unit

FSN
forward sequence number

IAM
initial address message

ISDN
integrated services digital network

ISUP
ISDN user part

kbps
kilobits per second

LSSU
link status signal unit

MF
multifrequency

MSU
message signal unit

MTP
message transfer part

OMAP
operations, maintenance, and administration part

OPC
originating point code

PSTN
public switched telephone network

RBOC
regional Bell operating company

RCL
release complete message

REL
release message

RSP
route set prohibited test message

RSR
restricted test message

SCCP
signaling connection control part

SCP
signal control point

SLS
signaling link selection

SS7
signaling system 7

SSP
signal switching Point

STP
signal transfer point

SU
signal unit

TCAP
transaction capabilities application part

TFA
transfer allowed message

TFP
transfer prohibited message

TFR
transfer restricted message