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Contents 1. Introduction 1.1 The Fundamental Problem of Communications 51.2 The Transmission Medium-Attenuation Constraints 91.3 The Transmission Medium- Interference Constraints 121.4 The Transmission Medium- Bandwidth Constraints 131.5 DSL Keeps Unshielded Twisted Pair (UTP) Copper Cable Attractive as a Premises Transmission Medium 181.6 A Brief History of DSL 211.7 Program 222. xDSL Modems: Fundamentals and Flavors 2.1 The Simple DSL Transceiver 242.2 The Many Flavors of DSL 28 2.2. 1 IDSL 28 2.2. 2 The HDSL Family: HDSL, SDSL, MSDSL and HDSL 2 28 2.2. 3 The ADSL Family: ADSL, MDSL, RADSL and Splitterless DSL 33 2.2.

4 VDSL 363. The Role of DSLAM sers 374. Virtual DSL: The Role of the DSL Simulator 395. Standards 466. Digital Subscriber Line - DSL Glossary 48 Bibliography 95 Index of Illustrations Figure 1-1 Source, User pair with information 5 Figure 1-2 Representations of information 6 Figure 1-3 Examples of sources and users generating / desiring "data" 6 Figure 1-4 Source, transmission medium, user 7 Figure 1-5 Disturbance travelling in transmission medium 7 Figure 1-6 The model which represents the fundamental problem of Communications 8 Figure 1-7 Input data signal attenuating as it propagates down a transmission medium 10 Figure 1-8 Regenerating and repeating an attenuated signal in order to reach user 11 Figure 1-9 Example transfer function of a transmission medium 14 Figure 1-10 Binary data from source represented by impulse train put into transmission medium by transmitter. Impulses are T seconds apart 15 Figure 1-11 Input signal is positive impulse.

Resulting output signal shows time dispersion 16 Figure 1-12 Cost trends of common transmission media 19 Figure 2-1 A typical DSL Transceiver block diagram 25 Figure 2-2 Transmitter of digital transmission system 26 Figure 2-3 Generic DSL Reference Model 27 Figure 2-4 T 1 Components 29 Figure 2-5 The HDSL Architecture 30 Figure 2-6 Photo of Model 681/682 HDSL Modem 31 Figure 2-7 ADSL reference model 34 Figure 2-8 Conventional ADSL configuration with splitter 34 Figure 2-9 Photo of Model 684 MDSL Modem 35 Figure 2-10 The VDSL Architecture 37 Figure 3-1 DSL-based services reference diagram 39 Figure 4-1 Diagram of modem testing on local loop connection 41 Figure 4-2 Diagram of modem testing on coil of twisted pair cable 41 Figure 4-3 Diagram of modem testing on DSL Simulator 42 Figure 4-4 Photo of Model 454 - Local Loop Simulator 43 Figure 4-5 Photo of Model 455 - Local Loop Simulator 43 Figure 4-6 Photo of Model 457 - Automated Local Loop Simulator 44 Figure 4-7 Photo of Model 456 - Loop Interference Simulator 45 Figure 4-8 Diagram of Models 454 and 456 451. Introduction 1.1 The Fundamental Problem of Communications The subject of interest in this book is the use of Digital Subscriber Line (DSL) technology to increase the rate and improve the quality of data communications over copper cable. It is an important topic both within the context of data communications today and into the future. All, or almost all, aspects of this subject will be explored.

However, it seems rather forbidding just to jump into this topic. Rather, it is more appropriate to take a step back and talk about the nature of communications first, in order to introduce some needed terminology. Such a step back will also provide us with a broader perspective on the subject of DSL technology as a transmission facilitator. In short, it will help us to answer the question, 'Why should we be interested in DSL?' The reader well-versed in data communications may, of course, choose to skip this introduction and suffer no real penalty. The subject of communications really begins with the situation shown in Figure 1-1. Here is an entity called the Source and one called the User - located remotely from the Source.

The Source generates Information, and the User desires to learn what this Information is. Figure 1-1: Source, User pair with information Examples of this situation abound. However, let us focus our attention on the case illustrated in Figure 1-2. Here, the Information is a sequence of binary digits - 0's and 1's, commonly called 'bits.

' Information in this case is termed 'data. ' Information of this type is generally associated with computers, computing-type devices, and peripherals - equipment shown in Figure 1-3. Limiting Information to data presents no real limitation. Voices, images, indeed most other types of Information can be processed to look like data by sampling and Analog-to-Digital conversion. Figure 1-2: Representations of information Figure 1-3: Examples of sources and users generating / desiring "data " In practice, it is impossible for the User to obtain the Information without the chance of error. Such errors may spring from a variety of deleterious effects, which we will examine, in greater detail later in this chapter.

The possibility of error means that the User seeking the Information - that is, the binary sequence - must be content in learning it to within a given fidelity. The fidelity measure usually employed is the Bit Error Rate (BER). The BER is the probability that a specific generated binary digit at the Source, a bit, is received in error, opposite to what it is, at the User. There are some real questions as to how appropriate this fidelity measure is in certain applications. Nonetheless, it is so widely employed in practice that further discussion is not warranted. The question then arises as to how to send the binary data stream from the Source to the User.

We refer to any physical entity used for this purpose as a Transmission Medium. As shown in Figure 1-4, the Transmission Medium is located between the Source and the User, accessible to both. The Transmission medium has a set of properties described by physical parameters. This set of properties exists in a quiescent state; however, at least one of these properties can be stressed or disturbed at the Source end.

This is accomplished by imparting energy in order to stress the property. The disturbance affects the parts of the Transmission Medium around it, then travels from the Source end to the User end. Once the disturbance or stressed property reaches the User end, it can be sensed and measured. This propagation of a disturbance by the Transmission Medium is illustrated in Figure 1-5.

Figure 1-4: Source, transmission medium, user Figure 1-5: Disturbance travelling in transmission medium There are many types of transmission media. The Transmission Medium could be air, with the stressed property being the air pressure put on sound waves. It could be an electromagnetic field set up in space by the current put on an antenna - a radio or wireless system. It could be a pair of electrical conductors, with the stressed property being the potential difference (the voltage) between the conductors - an electrical transmission line. It could be a cylindrical glass tube with the stressed property being the intensity of light in the tube - a fiber optic cable. Even written communication can be interpreted in this fashion: a sheet of writing paper provides the Transmission Medium, with the stressed property being the light-dark pattern on the paper.

The Source can have a disturbance to the Transmission Medium generated in sympathy to the Information - that is, it can generate a disturbance which varies in time exactly as the Information. This encoded disturbance will propagate to the User. The User can then sense the disturbance and decide the identity of the Information that it represents. The process of the Source generating a disturbance in sympathy with the Information and launching it into the Transmission Medium is referred to as 'modulation and transmission. ' The process of the User sensing the received disturbance and deciding what Information it represents is referred to as 'reception and demodulation. ' In this work, we will refer to the device that carries out modulation and transmission as the Transmitter.

We will refer to the device that carries out reception and demodulation as the Receiver. The whole of data communications then devolves to the model illustrated in Figure 1.6. Here, the Source generates bits as Information. The User wants to learn the identity of this Information, these bits.

The entities used to get the Information from the Source to the User are the Transmitter, the Transmission Medium and the Receiver. The fundamental problem of communications is to choose the terminal equipment - the Transmitter and Receiver - and to choose the Transmission Medium so as to satisfy the requirements for a given Source-User pair. Figure 1-6: The model which represents the fundamental problem of communications The fundamental problem of communications is one of design. Collectively, the combination of Transmitter, Transmission Medium and Receiver is known as the 'communication link' or 'data link' - the latter term deriving from the limitation placed on the Information to the form of a sequence of bits. The disturbance launched into the Transmission Medium by the Transmitter is usually referred to as the 'input data signal. ' The resulting disturbance at the Receiver is termed the 'output data signal.

' In the context of our discussion, the fundamental problem is to design a data link appropriate for connecting a given Source-User pair. There is no cookbook method to solve this design problem and come up with the best unique solution. While there is science here, there is also art. There are always alternative solutions. Each solution has its own particular twist, which in turn provides some additional attractive feature to the solution. However, the feature is peripheral to Source-User requirements.

Most exercises in obtaining the design solution usually begin with choosing a Transmission Medium to meet the general requirements of the Source-User pair. In other words, the data link design process pivots on choosing the Transmission Medium. Every Transmission Medium has constraints on its operation, on its performance. It is these constraints that truly decide which Transmission Medium will be employed for the data link design. 1.2 The Transmission Medium - Attenuation Constraints Have a Transmitter launch a disturbance, an input data signal, into a Transmission Medium. As the disturbance propagates down the Transmission Medium to the Receiver, its amplitude will decrease, growing weaker and weaker.

The disturbance is said to suffer attenuation, a situation illustrated in Figure 1-7. One immediate question that arises is why does attenuation occur? There are several reasons. It would be worthwhile to point out and describe two of them: spatial dispersion and loss due to heat.

Spatial dispersion can best be considered by revisiting Figure 1-7, which illustrates a one-dimensional propagation of the disturbance. However, often, this disturbance may propagate in two or even three dimensions. The User / Receiver may be located in a small solid angle relative to the Source / Transmitter. The received disturbance, the output data signal, appears attenuated relative to the transmitted disturbance because, in fact, it represents only a small fraction of the overall energy imparted in the disturbance when it was launched. This is exactly the situation with free space propagation of waves through an electromagnetic field transmission medium, such as that which occurs in any sort of radio transmission. Figure 1-7: Input data signal attenuating as it propagates down a transmission medium Loss due to heat refers to the basic interaction of the disturbance with the material from which the Transmission Medium is comprised.

As the disturbance propagates, a portion of the energy is transferred into the Transmission Medium and heats it. For a mechanical analogy, consider rolling a ball down a cement lane. The ball is the disturbance launched into the lane, which represents the Transmission Medium. As the ball rolls along, it encounters friction. It loses part of its kinetic energy to heating the cement lane and begins to slow down. The disturbance becomes attenuated.

This is the situation with using the potential difference between a pair of electrical conductors as the Transmission Medium. Attenuation increases with the distance through the Transmission Medium. In fact, the amplitude attenuation is measured in dB / km. As propagation continues, attenuation increases. Ultimately, the propagating signal is attenuated to a minimal detectable level. That is, the signal is attenuated until it can just be sensed by the Receiver - in the presence of whatever interference is expected.

The distance at which the signal reaches this minimal level could be quite significant. The Transmission Medium has to be able to deliver at least the minimal detectable level of output signal to the Receiver by the User. If it cannot, communications between the Source and User cannot take place. There are some tricks to getting around this. Suppose the disturbance has been attenuated to the minimal detectable level, yet it has still not arrived at the Receiver / User. The output signal at this location can then be regenerated.

The signal can be boosted back up to its original energy level. It can be repeated and continue to propagate on its way to the Receiver / User. This is shown in Figure 1-8. Figure 1-8: Regenerating and repeating an attenuated signal in order to reach the user Nevertheless, the attenuation characteristics are an item of significance.

The Transmission Medium selected in the design must have its attenuation characteristics matched to the Source-User separation. The lower the attenuation in dB / km, the greater advantage a Transmission Medium has. 1.3 The Transmission Medium - Interference Constraints Have a Transmitter launch an input data signal into a Transmission Medium. As it propagates down the Transmission Medium, the disturbance will encounter all sorts of deleterious effects, which are termed 'noise' or 'interference. ' In the simplest example, that of one person speaking to another person, what we refer to as noise really is what we commonly understand noise to be. What is noise / interference ?

It is some extraneous signal that is usually generated outside of the Transmission Medium. Somehow, it gets inside of the Transmission Medium and realizes its effect - usually by adding itself to the propagating signal, but sometimes by multiplying the propagating signal. The term noise is generally used when this extraneous signal appears to have random amplitude parameters, like background static in AM radio. The term interference is used when this extraneous signal has a more deterministic structure, like 60-cycle hum on a TV set.

In any case, when the Receiver obtains the output data signal, it must make its decision about what Information it represents - and demodulate the signal - in the presence of this noise / interference. Noise / interference may originate from a variety of sources. It may come from the signals generated by equipment located near the Transmitter / Transmission Medium / Receiver. This may be equipment that has nothing at all to do with the data link, such as motors on air conditioners or automated tools.

Noise / interference may also come from atmospheric effects or from the use of multiple electric grounds. It may be generated by active circuitry in the Transmitter or the Receiver, or it may come from the operation of other data links. In obtaining the design solution, noise / interference makes its effect best known through the BER. The level of noise / interference drives the BER. Of course, this can be countered by having the Transmitter inject a stronger input signal.

It can also be countered by making the Receiver capable of detecting lower minimal output signals. However, this comes with greater expense. Neither of these solutions hides the fact that there is concern with noise / interference because of its impact on the BER. The susceptibility to noise / interference varies from Transmission Medium to Transmission Medium.

Consequently, during the design process, the designer must pay attention to the application underlying the communication needed by the Source-User pair and to the BER required by this application. The designer must then select the Transmission Medium that has a noise / interference level capable of delivering the required BER. 1.4 The Transmission Medium - Bandwidth Constraints Consider again the model illustrated in Figure 1-6. Suppose the input signal the Transmitter sends to the Transmission Medium is the simple co sinusoidal signal of amplitude '1' at frequency 'f 0' Hz. The output response to this at the Receiver is designated 'T (f 0) '.

Now consider the co sinusoidal test input signal frequency f 0 to be varied from 0 Hz on up to yen. The resulting output signal as a function of frequency is T (f 0) - or, suppressing the subscript, T (f). This is generally referred to as the transfer function of the Transmission Medium. Generally, the ordinate target value 'T (f) ' for a given frequency 'f' is referred to as the transfer function gain - although, in fact, it is a loss - and is expressed logarithmically in dB relative to the amplitude '1' of the input signal.

One example transfer function is illustrated in Figure 1-9. Though it is just an example, not to be taken as typical in any sense, it illustrates a feature common to the transfer function of any Transmission Medium that obtainable in the real, physical world. The transfer function rolls off with frequency. The transfer function shown here oscillates, but the maximum value of its oscillation becomes less and less. However, the transfer function itself never rolls off completely to become dead flat zero beyond a certain frequency.

This roll off with frequency means that the Transmission Medium attenuates the co sinusoidal signals of the higher frequencies that are given to it as inputs. The energy of these higher frequency signals is somehow lost, usually as heat, in traversing the Transmission Medium. The greater the distance through the Transmission Medium, the more high frequency signals get attenuated. This is a consequence of the greater interaction between the propagating signals and the material comprising the Transmission Medium. Figure 1-9: Example transfer function of a transmission medium This roll off feature of the transfer function is present in every Transmission Medium regardless of how it is derived. It is present in sound waves, in electrical conductors, in fiber optic cables, in CDs, in audio or videotapes, and even in a sheet of writing paper.

The transfer function shown rolls off with frequency. However, most of its activity, most of its area, most of its mass, most of its spread, seems to be below a given frequency. In this example, it looks like the frequency 'F. ' The frequency spread of the transfer function is referred to as its bandwidth. As mentioned above, bandwidth decreases with the propagation distance through the Transmission Medium. As frequency spread is very subjective, so too is the measure of bandwidth. When you discuss communications with someone and they mention bandwidth, it would be wise to ask exactly how they are defining it.

There is a definition in the glossary at the back of this book, but this is only one such definition. There are many. For example, there is the 3 dB bandwidth, mean square bandwidth, first lobe bandwidth, brick wall bandwidth and on and on. In a study carried out seventeen years ago, Dr. Kenneth S. Schneider identified over twenty-five separate definitions of bandwidth. All have validity. Whether one definition is meaningful or not depends on the context in which it is applied.

One definition may be appropriate for describing satellite communication links and another more appropriate for an FCC official considering the request for a broadcast AM radio license. In any case, a Transmission Medium has a transfer function, and the frequency spread of this transfer function is measured by the bandwidth. The bandwidth parameter has implications with respect to the performance of the data link being designed. Consider the illustration shown in Figure 1-10. Here, the Source is generating data, '0s' and '1s', every T seconds. Let T = 1/R, in which case the Source generates data at R bits per second (BPS).

To send this data to the User, the Transmitter generates either a positive or a negative impulse every T seconds. What is an impulse? It is an infinitesimally narrow pulse that is also infinitely high, so that it has energy of '1. ' Figure 1-10: Binary data from source represented by impulse train put into transmission medium by transmitter.

Impulses are T seconds apart. Now what comes out at the Receiver in response to the positive impulse sent at time zero to represent the binary data bit '1?' An example result is illustrated in Figure 1-11. Notice that this response out of the Transmission Medium to the input impulse is a pulse spread out in time with its center at t seconds, when t is not equal to 0 seconds. While this example output cannot be called typical, it does indicate a property typical of all output signals received from the Transmission Medium: the time spreading of the output pulse, called 'time dispersion.

' Time dispersion is a result of the finite bandwidth of the Transmission Medium. To be exact, it is due to the fact that the transfer function of the Transmission Medium - indeed, of any Transmission Medium - attenuates the higher signals. Figure 1-11: Input signal is positive impulse. Resulting output signal shows time dispersion Look closely at the output signal pulse shown in Figure 1-11. Because it is spread in time, it will interfere with the output pulses, due to input data signals which will come after it. These do not appear in the illustration, but the implication should be clear.

Likewise, these subsequent data signals will generate output pulses that will also be spread in time. Each will also interfere with both the pulses coming before it and after it. This type of interference is called 'inter symbol interference. ' It is not just a consequence of the input signals being impulses.

An input signal, of finite duration and of any shape, will generate an output signal with time dispersion. As the data rate from the Source increases, the inter symbol interference problem grows worse. Output pulses with time dispersion get squeezed next to one another. The growing level of inter symbol interference makes it increasingly harder for the Receiver to demodulate these signals. To some extent, the inter symbol interference can be undone by sophisticated signal processing in the Receiver. This usually goes under the name of 'equalization.

' However, in many cases equalization still cannot deliver the data from the Receiver with the BER required by the Source-User pair. In other cases, the data being generated by the Source, say R BPS, is so high that an equalizer cannot be obtained fast enough to keep up with the output signals. In considering the data link design task, the first line of defense against time dispersion and inter symbol interference lies in the proper selection of the Transmission Medium. The larger the bandwidth of the Transmission Medium, the fewer high frequency components will be attenuated during propagation and the smaller the time dispersion. As a result, there will be less interference between different output pulses. Make no mistake.

Inter symbol interference will not disappear. Rather, it will be lessened and made more tolerable as the bandwidth grows larger. In particular, to lessen the inter symbol interference the bandwidth of the Transmission Medium must get larger in relation to the Source's generated bit rate, R BPS. The Transmission Medium must be selected to accommodate the bit rate generated by the Source. It must have sufficient bandwidth so that it will generate tolerable inter symbol interference at the Receiver. This means selecting a Transmission Medium that has a bandwidth that is some multiple of the bit rate, R. A number of rules of thumb are often used to do this.

However, they are too specific and not worth discussing at this point, particularly as the measure of bandwidth is subjective. The important point is that the selection of Transmission Medium candidates is limited to those matched to the data rate requirement, R. This means that as R increases, the selection of Transmission Medium candidates becomes more limited. The information technology explosion in the world has made this selection task ever more challenging. Continuously, PCs are becoming more powerful. More complex applications programs can be run and are finding their way into easily usable software. As a result, the Source bit rate requirement is growing geometrically every few years.

To put this in perspective, consider that just ten years ago a Transmission Medium would have been quite acceptable if it had a bandwidth matched to a Source bit rate of 9,600 BPS. This source rate was typical of that generated by most data equipment applications. Today, with the growing demand for video services and the plethora of graphics in computer applications, the demand more often than not is for a Transmission Medium with a bandwidth requirement matched to Source bit rates well upwards of 1 MBPS, possibly 1 GBPS. 1.5 DSL Keeps Unshielded Twisted Pair (UTP) Copper Cable Attractive As a Premises Transmission Medium You may be able to find the ideal Transmission Medium relative to attenuation, interference and bandwidth. Yet you still may not be able to select it as part of the solution to the data link design problem for the simple reason that it costs too much. It presents an expense beyond the budget allowed for the Source-User communications.

This is nothing new or revolutionary. Money alone does not drive the world, but it does have a tremendous influence on the ultimate choice of solution to any problem based in technology. This is as true today at the turn of the millennium as it was at the turn of the twentieth century. As a case in point, let us examine briefly the fiber optic solution to the problems of attenuation, interference and bandwidth. Fiber optic cable - at least, that of the pure glass-silica variety (glass core with glass cladding) - has a far lower attenuation rate than coaxial cable. Whether it is fabricated fully from glass or uses plastic cladding, fiber optic cable can carry signals with full immunity from electromagnetic-based forms of noise and interference.

In terms of bandwidth, fiber optic cable has superiority over copper of several orders of magnitude - transmitting well above 10 MHz for up to 4 km. In some cases, dependent on distance and repeaters, it can transmit data at rates measurable in gigabits per second (1 billion bits per second - GBPS) or even terabits per second (1 trillion bits per second - MBPS). To put this in perspective, unshielded twisted pair copper cable transmitting over a distance of 4 km can support 0-to-100 MBPS, while coaxial cable can support about 20 MBPS over the same distance. Thus in terms of attenuation, interference and bandwidth, fiber optic cable beats copper, hands down. Fiber optic cable, however, has problems of its own, and cost ranks chief among them. As illustrated in Figure 1-12, fiber is far more expensive than Unshielded Twisted Pair (UTP) copper.

If you are starting from scratch, building your premises and its communications infrastructure from the ground up, fiber presents a worthwhile investment - a large investment, to be sure, but one that will eventually pay for itself. Figure 1-12: Cost trends of common transmission media Suppose, however, that you are not starting from scratch. In this case, you would have to rip out the old copper infrastructure before you could lay down your new fiber optic cable. Herein lies the problem. UTP copper cable has been the Transmission Medium of choice for nearly one hundred twenty years. There is a tremendous amount of copper infrastructure already in place at every level, from the home office to the global communications network.

The local loop connecting business premises and the telephone central office (CO) runs on copper pairs. For the same reason, copper provides the most common Transmission Medium for Internet access. Simply put: copper is everywhere. As a result, the cost replacing copper with fiber is often prohibitively high. Another drawback of fiber lies in one of its strengths.

Fiber transmits data via light waves rather than electrical signals. This cuts down on interference, but it also eliminates one of the benefits that copper grants: the ability to transmit DC voltage along with the signal. This additional voltage allows telephones to continue functioning during a power outage. Without the additional voltage, you risk losing your phones as well as your PCs and peripherals when the lights go out. For this reason alone, it is unlikely fiber will ever replace copper entirely for desktop communications. This appears to place us at an impasse.

Traditional copper is too slow and too vulnerable to cope with the increasingly steep demands of data transmission. Fiber can be too expensive to make a shift practicable, even without its own vulnerability. If only there were a way to marry the cost benefits of copper to the technological advantages of fiber, we would have a really attractive Transmission Medium. Thankfully, there is and we do. It is called Digital Subscriber Line (DSL) technology.

1.6 A Brief History of DSL Necessity is the mother of invention. In the case of DSL, that necessity took the form of the need to eliminate interference, particularly in the form of noise generated by inclement weather, to which analog signals transmitted along copper wire are so vulnerable. Shortly before World War II, a British engineer working for ITT in France grew so annoyed by this analog line noise that he set to work on the problem of how to digitize analog voice signals. The war soon put an end to these experiments, but the increasing globalization of the economy that followed the war led to a demand for constant improvement in telecommunications quality. AT&T, in conjunction with IBM, carried out much of the basic postwar research on digital telephone technology. These experiments came to focus on a technique of sharing bandwidth in time slots known as 'time division multiplexing' (TDM) - a method long-considered too expensive and technically impractical for analog transmission.

By the early 1960's, this led to the development of the T-carrier system - the basis of which was a local loop digital system known as T 1 (T-carrier, level 1 multiplexing). The T-carrier system led to the development of digital trunk lines. By the mid-1970's, digital trunk lines had become commonplace and digital switches made their first appearances. Through this period, T 1 remained under the control of the sole public switched telephone network (PSTN), AT&T. Early digital telecommunications enthusiasts predicted the growth of an integrated digital network, a technology that later came to be called Integrated Services Digital Network (ISDN). Skeptics, noting the failure of the digital promise to produce through the 1970's, joked that the acronym really stood for 'It Still Does Nothing. ' By 1981, however, ISDN began meeting initial expectations, and 1982 saw ISDN form the core of the original DSL technology: IDSL (ISDN DSL).

Two years later, the US Government ordered the divestiture of AT&T. With the breakup of the PSTN, T 1 first became available for customer installation and DSL technological development exploded. This explosion fed and was itself nourished by the rapid advances in computer technology and the development of the Internet over the next decade, both of which demanded increasingly higher rates of data transmission. ISDN, so long in coming, soon found itself surpassed by newer flavors of DSL, particularly High-bit-rate DSL (HDSL, developed between 1988-91 and HDSL 2, developed in 1998-1999), Asymmetric DSL (ADSL, developed between 1991-95), and Very-high-bit-rate DSL (VDSL, under development since 1995). The universe of DSL technology referred to collectively as xDSL, now forms a key ingredient of the asphalt that makes up the In format ion Superhighway. 1.7 Program This book has been written so that each chapter stands on its own. There is no need to read the chapters in order.

While there may occasionally be cross-references from one chaptMultiple pairs of wires can prove troublesome when it comes to digitizing the analog local loop for residential service, as opposed to commercial premises. Using a single pair proves less troublesome. The result has been an offshoot or little brother of HDSL, the basic version of which runs at 784 KBPS, full-duplex on a single pair of wires. This flavor is known as SDSL, standing either for Symmetric DSL or Single-pair DSL, depending on the source. Since its introduction, SDSL has developed various incarnations, with the data rate varying inversely to the maximum distance.

One proprietary form of SDSL is Multirate Symmetric DSL (MSDSL). To confuse matters further, MSDSL is also sometimes referred to simply as 'MDSL,' for Multirate DSL, an acronym shared by a form of Asymmetric DSL technology. Table 2-1: SDSL Speeds and Distances Another variation of HDSL, recently standardized, is HDSL 2. Like SDSL, it functions on a single, full-duplex twisted pair.

Unlike SDSL, it can transmit the full T 1 (1.544 MBPS) or E 1 (2.048 MBPS) at a distance of up to 12 kit without repeaters. The downside is that HDSL 2 - designed for the T 1/E 1 leased line business market, rather than the residential market - does not include voice circuit support. 2.2. 3 The ADSL Family: ADSL, MDSL, RADSL and Splitterless DSL All the flavors of DSL we have examined thus far have one facet in common: they all have the same rate of data transmission downstream (from service provider to customer) as upstream (from customer to service provider). There are a number of applications for DSL, however, in which the data traffic downstream tends to be much heavier than requests for data sent back upstream. This is especially true for Video-on-Demand (VOD), but also holds true for Internet access (particularly on the World Wide Web) and LAN bridging.

It follows that a DSL used for these applications could allocate bandwidth more efficiently were it able to transmit data asymmetrically, accelerating data transmission downstream at the cost of upstream transmission speed. This would have the additional benefit of reducing near-end crosstalk (NEXT). This is the guiding concept behind Asymmetric DSL (ADSL). The early concept for ADSL originated in 1989, while HDSL was still in the prototype phase, under J.W. Lech leider and others at Bellcore.

Stanford University and AT&T Bell Labs developed ADSL from concept to prototype between 1990 and 1992, with field technology trials beginning three years later. The International Telecommunications Union (ITU) gave determination to a set of ADSL recommendations in October 1998. ADSL employs one of two modulation techniques, CAP and DMT. CAP is "Combined Amplitude Phase Modulation". DMT is "Discrete Multi-Tone Modulation".

DMT has recently been adopted as the ADSL standard. ADSL has a downstream transmission rate of between 1 and 9 MBPS, with an upstream transmission rate of between 64 KBPS and 1 MBPS and can operate at distances up to 18 kit. ADSL also allows the use of standard voice telephony in addition to data transmission, by the use of a POTS splitter. With the splitter, data transmission and POTS flow through the same line, with the digital transmission restricted to a frequency band above that of voice telephony. In physical terms, voice-band signals are attached to the red and green inside wires to the telephone, while wide band signals attach to the yellow and black inside wires to the customer's ADSL. A low-pass filter (LPF) for the voice wiring is placed at or near the customer premises' Network Interface Device (NID), while a high-pass filter is installed in the customer's ADSL modem-proper (ATU-R) for higher frequency data.

Figure 2-7: ADSL reference model Figure 2-8: Conventional ADSL configuration with splitter ADSL has already generated a number of offshoots, such as Medium-bit-rate DSL - MDSL, not to be confused with the symmetric DSL that sometimes goes by that acronym. MDSL evolved as a way to provide a less complex, less expensive ADSL modem. The trade off is speed. The downstream data transmission rate for MDSL is only 800 KBPS to 1 MBPS; its upstream rate, a mere 100 KBPS. The Model 684 provides high-speed transmission of synchronous data over distances up to 22,000 feet, with an exceptionally high degree of data reliability.

Since no conditioning of the transmission lines is necessary when MDSL is used, the Model 684 represents a plug-and-play solution for using copper lines for high-speed data transmission and Internet access. The Model 684 incorporates intelligence that provides for maximizing the data rate transmission over a given line. The Model 684 operates at 768,512,384 and 256 KBPS. A pair of Model 684's will attempt to operate over the highest speed for a given line. Figure 2-9: Photo of Model 684 MDSL Modem Another version of ADSL is Rate-Adaptive DSL (RADSL).

In some situations, line conditions or sensitivity to environmental changes may interfere with operation at the assumed optimum speed. RADSL compensates for such hazards, adjusting the operating rate to the highest possible for the local loop. On average local loops, RADSL may have a downstream rate of 7 to 10 MBPS and an upstream rate of 512 to 900 KBPS. On long loops (18 kit or more), RADSL operates downstream at about 512 KBPS and 128 KBPS upstream. RADSL, like ADSL, makes use of a POTS splitter to separate ADSL from voice-band transmission. For all the advantages the POTS splitter grants, however, it also carries the disadvantage of requiring the setup of extra premises wiring, as existing substandard wiring will degrade ADSL performance.

Further, the shifting of ADSL transmission to higher frequency bands to accommodate POTS reduces ADSL data rates and loop reach. Splitterless DSL, recently standardized, solves this problem. Splitterless DSL goes by a plethora of different trade names, including Commercial DSL (DSL), Universal ADSL (DSL or UADSL), DSL Lite and G. Lite (for ITU Recommendation G. 992.2, which governs this flavor). To call this flavor 'splitter less' is actually something of a misnomer. Rather than eliminating the need for a splitter altogether, it allows the line to be split at the CO end of the connection. This takes much of the burden off the customer, who can now have ADSL service merely by plugging an ADSL modem into a phone jack, without the need for extensive premises rewiring or splitter installation.

This makes splitter less DSL both simpler and less expensive than earlier versions of ADSL. Now that it has become standardized, it is expected to become the dominant version. Splitterless DSL carries downstream data transmissions at 1 MBPS to 6 MBPS and upstream transmissions at 128 KBPS to 384 KBPS. 2.2. 4 VDSL Very-high-bit-rate DSL (VDSL) is the newest flavor of DSL technology and has been in development since late 1995. Unlike its various elder siblings, VDSL has the option of either symmetric or asymmetric transmission. The highest symmetric rate proposed would leave current HDSL modems in the dust, zipping data transmission along at 26 MBPS.

The asymmetric rates currently under consideration vary from 13 MBPS downstream/1.6 MBPS upstream, to 26 MBPS downstream/3.2 MBPS upstream, to an incredible 52 MBPS downstream/6.4 MBPS upstream. The trade off for these fantastic transmission speeds is in distance. VDSL only has a service range of 1.5 to 4.5 kilo feet, restricting its usefulness. For this reason, VDSL technology is targeted for use as the last link in fiber in the loop (FIT), fiber to the curb (F TTC) and fiber to the neighborhood (F TTN) networks. Like ADSL, VDSL allows for the coexistence of digital and POTS transmission on the same twisted pair by use of a POTS splitter. Figure 2-10: The VDSL Architecture This accounts for all the major, and quite a few of the minor, flavors of DSL modem currently on the market.

However, there is one more device we require before our digital transmissions can make the leap from the CO to the end-user destination: the Digital Subscriber Line Access Multiplexer, or DSLAM. 3. The Role of the DSLAM We have now followed the path of data along the digital subscriber line from its commercial or residential source, via the local loop, to the CO. The local loop here terminates at the Main Distribution Frame (MDF), to be picked up by one of the CO's many DSL modems. If the form of DSL allows for the carrying of both analog and digital signals, a POTS splitter will separate out the signals. The analog signal will follow its time-honored path along the copper-wire infrastructure.

For the digital signal, however, one step before the signal can be shot along to its destination. The CO must now collect all the disparate digital signals from its modems and combine them into a single signal, via multiplexing. The aggregate signal then loads onto backbone switching equipment, travelling through an access network (AN) - also known as a Network Service Provider (NSP) - at speeds of up to 1 GBPS and emerging at a destination CO. At this point, the signal is then fragmented into its component parts and transmitted via tel co modems to its final residential or commercial receivers. The device that performs these functions of signal combination and fragmentation is called the Digital Subscriber Line Access Multiplexer, or DSLAM. The average DSL customer will never have to purchase a DSLAM.

For the CO looking to make itself DSL-compatible, there are a number of features to consider in selecting which DSLAM best suits your needs and the needs of your subscribers. Alternatively, if you plan to enter the market of DSLAM manufacturers, these are features you should consider in constructing your product. Chiefly, there is the question of multi services support. As mentioned in the previous chapter, DSL technology is evolving at lightning speed. A DSLAM is a massive investment. To obtain the best value for your dollar, you should seek (or design) a system that allows for adaptation in the face of increasing application diversity.

A similar concern is that of DSL code support: make sure your prospective DSLAM is flexible in the matter of line code and line protocol deployment. Remember that the newer flavors of DSL are particularly dependent on coding for proper transceiver functioning, and a good DSLAM should reflect this. Your DSLAM should also meet compatibility requirements for the various Network Management Systems (NMS) platforms, for better control and monitoring of performance. Apart from these internal concerns, there are also two external, hardware-related issues that bear on DSLAM selection. The first is DSLAM line aggregation.

The more DSL lines you can aggregate on a single output for network connection to a DSLAM, the greater the economy of space and scale. The greater the savings, the more cheaply you can supply your customers with DSL services and the more potential DSL subscribers you will have. Second is maintainability. The fastest, most flexible DSLAM in the world will fall to pieces rapidly if it is denied proper upkeep. To protect yourself against this, make sure your prospective DSLAM meets Network Equipment Building System (NEBS) standards of compliance.

Figure 3-1: DSL-based Services Reference Diagram 4 Virtual DSL: The Role of the DSL Simulator You are in the process of developing a new line of xDSL modems. The modem has gone from the drawing board to the prototype stage. Before you put the new modem into production, however - before you commit to a massive investment of man-hours and parts - it would help to know whether the unit performs as it was designed to or whether it still has some kinks to be eliminated. You will want to know just how well the modem performs on the local loop.

Consider another scenario. You have just purchased a new DSL modem. It has been touted as the most advanced of its kind on the market. Possibly it represents a new flavor of xDSL modem entirely. In any case, you take a look at the new modem's spec sheet and are skeptical as to whether it can perform as well as its manufacturers claim.

You want to put it through its paces and see how it measures up. In either case, there are three methods by which you can test your DSL modem's performance. All three methods of testing boil down to an examination of the BER performance of the modem over the local loop at a specific distance and in a specific interference environment. The first method is to use the local loop itself, set up with your local tel co. This is a real world test, throwing the modem into the deep end and watching whether it will sink or swim. Unfortunately, it does not provide the most reliable measure of performance.

The 'live' local loop you use for the test is not necessarily representative of the parameters within which the modem must function 'in the field. ' You will have no control over, and in some cases no knowledge of, the loop length. The level and type of interference on the line will also be outside your capability to gauge. These operating factors apart, a local loop for testing is physically awkward in a manufacturing environment. Figure 4-1: Diagram of modem testing on local loop connection second method is to test your modem on a coil of twisted pair cable, with the cable mimicking a local loop. This testing method as the advantage of being inexpensive.

It allows you to simulate the attenuation and delay of the local loop, further allowing you to measure accurately the loop length. What it cannot do is simulate interference. It ignores the effects of crosstalk. And it too is physically awkward in the manufacturing environment. Coiled cable is bulky and difficult to fit on a laboratory bench. Multiple coils can take up considerable workspace.

Figure 4-2: Diagram of modem testing on coil of twisted pair cable The third method is to use test equipment specifically designed to measure the capabilities of your DSL modem, a tool known as the 'DSL simulator. ' As its name implies, the simulator can do what the local loop and cable coil tests cannot: accurately represent attenuation, delay and multiple types of interference over a variety of loop lengths. Like the communications media they are meant to mimic, simulators come in both digital and analog. The digital simulators rely on a format known as Digital Signal Processing (DSP). The DSP-based simulator performs its tests via chips and microprocessors, running programs that provide a digital filter approximation. DSP-based simulation, unfortunately, shares the problem of space constraint with local loop and wire coil testing.

The current generation of digital simulators, while suitable for laboratory use, are too large for bench-level use. Furthermore, DSP-based simulators are costly, running into the tens of thousands of dollars, a prohibitive expense in a factory space demanding multiple test devices. As such, the DSP-based simulator tends to be impractical for use in the manufacturing environment. This brings us to the analog alternative. Analog DSL simulation relies on lumped, linear, bilateral, passive electrical filters composed of resistors, capacitors and inductors. These tried-and-true electrical components provide an analog approximation of loop conditions.

Ironically, analog DSL simulation is much cheaper and more compact than digital DSL simulation. Figure 4-3: Diagram of modem testing on DSL Simulator Telebyte offers four varieties of passive, analog DSL simulators. The Model 454 Local Loop Simulator mimics the amplitude characteristic and delay (or phase) of the 26 AWG copper cable pair generally used by Telcos for local loops. Designed for testing ADSL, HDSL, T 1 and E 1 modems, the Model 454 has a frequency range from DC to 1500 KHz. It is capable of simulating characteristics of cable lengths of 500 feet, 1000 feet, 2000 feet and 3000 feet - lengths which may be connected to act in tandem, up to a maximum simulation length of 18,500 feet (18.5 kit). Its compact size and mass (6'H x 12.7' W x 13.5' D; 12 lbs.) make it an ideal testing device for both the laboratory and the production line.

Figure 4-4: Photo of Model 454 - Local Loop Simulator variant of the Local Loop Simulator is the Model 455 Rack Mountable Local Loop Simulator, similarly designed for testing ADSL, HDSL, T 1 and E 1 modems. A slimmer, rack-mountable model - measuring 1.75' H x 19' W x 16' D - the Model 455 is capable of simulating greater loop lengths than its cousin, the Model 454. Beginning with length increments of 500 feet, it can provide accurate simulation for local loop lengths of 500, 1000 feet, 2000 feet, 4000 feet and 8000 feet. Acting in tandem, these loop lengths provide a maximum simulated loop length of 23,500 feet (23.5 kit). While the prospective range is greater, the range of frequency simulation remains the same for the Model 455 as for the Model 454, with accurate simulation from DC to 1500 KHz. Figure 4-5: Photo of Model 455 - Local Loop Simulator For both the Model 454 and the Model 455, the tandem connection necessary for maximum loop length simulation is accomplished by control via front panel switches - 'off-line' control.

However, the Model 457 Automated Local Extended Wireline Simulator provides for an additional method. The Model 457 bears a strong resemblance to the Model 455 in terms of its simulation length range, frequency range, physical dimensions, rack-mount ability and the option of off-line control through front panel switches. However, the Model 457 has the benefit of a second method of control - 'on-line' control, through either an I -488 interface or an RS-232 interface. With on-line control, the user may set the desired tandem length using a software program residing in a PC or other master device. The front panel switches remain inoperable from the moment the software is engaged to the moment the program releases control.

Figure 4-6: Photo of Model 457 - Automated Local Extended Wireline Simulator An excellent companion unit to the Telebyte's Local Loop Simulator is Telebyte's Model 456 Loop Interference Simulator, designed for testing ISDN, HDSL, ADSL and T 1 modems. The Model 456 generates shaped narrow band and broadband white noise, along with two types of crosstalk spectrum's. Using front panel selection switches, the user may select one of eight different noise shapes - including flat noise at 3 KHz, 50 KHz, or 100 KHz, T 1, ISDN, HDSL and ADSL - with four additional switches allowing a choice of interference levels from -20 dB to -60 dB. Figure 4-7: Photo of Model 456 - Loop Interference Simulator The Model 456 thus can act in conjunction with any of Telebyte's Local Loop Simulators to provide a complete test equipment suite for modems as illustrated below. Figure 4-8: Diagram of Models 454 and 4565. Standards Any network architecture must follow some set of protocols.

On the one hand, the set of protocols may be homegrown - that is, specified by the designer of the network. On the other hand, the set of protocols may conform to a recognized, published set of standards. Each flavor of DSL has its own set of standards to which it conforms, a situation made all the more complex in symmetric DSL by the difference in standard transmission speed between North America (T 1, 1.544 MBPS) and Europe (E 1, 2.048 MBPS). A discussion of these published standards is well beyond the scope of the present work. However, the interested reader may order them from several sources. Several are listed below.

When calling these organizations, it will be worth your while both to order a catalog and to request inclusion on their update services. This will allow you to keep informed of new standards and of supplements to existing ones as they are approved. ADSL Forum The ADSL Forum 39355 California Street, Suite 307 Fremont, CA 94538 Internet: web Tel: (510) 608-5905 Fax: (510) 608-5917 ANSI American National Standards Institute (ANSI) 11 West 42nd Street New York, N Y 10036 Internet: web Tel: (212) 642-4900 Fax: (212) 398-0023 Bellcore Direct Sales Telcordia Technologies, Inc. [Formerly Bellcore] 8 Corporate Place, PYA 3 A-184 Piscataway, NJ 08854-4156 Internet: web Tel: (800) 521-2673 (U.S. and Canada) Tel: (732) 699-5800 (International) Fax: (732) 336-2559 ETSI European Telecommunications Standards Institute (ETSI) 650 route des Lucio les 06921 Sophia Antipolis Cedex France Internet: web Tel: +33 (0) 4 92 94 43 95/43 64 Fax: +33 (0) 4 93 65 47 16 I I Customer Service 445 Hoes Lane PO Box 1331 Piscataway, NJ 08855-1331 Internet: web Tel: (800) 678-4333 Tel: (908) 562-1393 Fax: (908) 981-9667 ITU International Telecommunication Union (ITU) Telecommunications Standardization Bureau (TSB) Place des Nations CH-1211 Geneva 20 Switzerland Internet: web Tel: +41 22 730 5857 or +41 22 730 5859 Fax: +41 22 730 58536. Digital Subscriber Line-DSL-Glossary 2 B + D - The basic rate interface (BRI) in ISDN. A single ISDN circuit divided into two 64 digital B channels for voice and data and one 16 D channel for low-speed data and signaling.

Either one or both of the 64 channels may be used for voice or data. In ISDN, 2 B + D is carried on one or two pairs of wires (depending on the interface). See also BRI. 2 B 1 Q - Two Binary, one Quaternary. A line coding technique that compresses two binary bits of data into one time state as a four-level code.

3 B 2 T - A base band line code where three binary bits are encoded into two ternary symbols. 4 B 3 T - A base band line code where four binary bits are encoded into three ternary symbols. 5 ESS - A digital central office switching system made by AT&T. 10 Base-T - A 10 MBPS Ethernet LAN that runs over twisted pair wiring. This network interface was originally designed to run over ordinary twisted pair (phone wiring) but is predominantly used with Category 3 or 5 cabling. 100 Base-T - A 100 MBPS LAN that maintains backward compatibility with 10 Base-T networks running at 10 MBPS. Competitor to 100 VGAnyLAN.

23 B + D - The primary rate interface (PRI) in ISDN. A circuit with a wide range of frequencies that is divided into twenty-three 64 'bearer' channels for carrying voice, data, video, or other information simultaneously and one D 'delta' 16 for telephony data. See also PRI (primary rate interface) AAL 5 (ATM Adaptation Layer 5) - AAL 5 has been adapted by the ATM Forum for a Class of Service called High Speed Data Transfer. ABCD Parameters - The transfer characteristics of a two-port network describing the input voltage and current to the output voltage and current.

Access line - The physical telecommunications circuit connecting an end user location with the serving central office in a local network environment. Also called the local loop or 'last mile. ' See also Local Loop. Access network - That portion of a public-switched network that connects access nodes to individual subscribers. The access network today is predominantly passive twisted pair copper wiring. Access rate - The transmission speed of the physical access circuit between the end-user location and the local network.

This is generally measured in bits per second; also called Access speed. Adapter - See Adapter Card. Adapter card - Circuit board or other hardware that provides the physical interface to a communications network; an electronics board installed in a computer which provides network communication capabilities to and from that computer; a card that connects the DTE to the network. Also called a Network Interface Card. See also Data Termination Equipment and Network Interface Card. ADSL (Asymmetric Digital Subscriber Line) - Bell Core term for delivery of digital information over ordinary copper phone lines.

ADSL uses a system of frequency division whereby lower frequency POTS signals are delivered to the home unaltered while digital signals traverse the phone line at higher frequencies for delivery to end stations such as a video CODEC or PC. Asymmetric refers to the fact that the downstream (to the user) channels can outweigh the upstream (to the network) channels by a ratio as high as 20: 1. This asymmetry is a good fit for video on demand and Internet access applications where the paradigm is a small request up to the network and a large delivery to the user. ADSL Forum - The organization developing and defining xDSL standards, including those affecting ADSL, SDSL, HDSL, and VDSL. Members participate in committees to vote on ADSL specifications; auditing members receive marketing and technical documentation. AFE (Analog Front End) - Functions including the analog-digital conversation, analog filter, and line driver.

AGC (Automatic Gain Control) - Receiver adaptation to the received signal level so as to reduce dynamic range of the signal input to the analog-to-digital converter. AIX (Advanced Interactive Executive) - IBM's implementation of UNIX. Always On - Current dial-up services require the user to 'make a call' to the ISP. The connection is only active during the duration of the call. Most xDSL implementations (including ADSL, UADSL, and SDSL) enable the connection to be always on in a fashion similar to a LAN. AMI (Alternate Mark Inversion) - Line code, also known as 'Bipolar.

' Used to accommodate the ones density requirements of E 1 or T 1 lines. Binary information is represented by pulses with three possible amplitudes. AN (Access Node) - A point on the edge of the access network that concentrates individual access lines into a smaller number of feeder lines. Access nodes may also perform various forms of protocol conversion. Typical access nodes are DLC systems concentrating individual voice lines to T-1 lines, cellular antenna sites, P BXs, and ONUs. Analog - An electrical signal or wave form in which the amplitude and / or frequency vary continuously.

The current basis for most residential telephone service. Analog front end - The analog front ends are responsible for converting the digital signal to analog and force the signal onto the twisted pair line. ANSI (American National Standards Institute) - The primary standards organization for the US. Accredits standards bodies, such as Committee T 1 for telecommunications. Member of the ISO. API - Application Programming Interface.

A PON (ATM Passive Optical Network) - A passive optical network running ATM. ASIC (Application Specific Integrated Circuit) - A chip designed for a specific application. Examples of an ASIC application can be SDSL or other broadband solutions. Asynchronous transmission - Data transmission one character at a time to the receiving device, with intervals of varying lengths between transmittal's and with start bits at the beginning and stop bits at the end of each character, to control the transmission. In xDSL and in most dial-up modem communications, asynchronous communications are often found in Internet access and remote office applications. See Synchronous transmission.

ATIS (Alliance for Telecommunications Industry Solutions) - Sponsors Standards Committee T 1. ATM (Asynchronous Transfer Mode) - A protocol that packs digital information into 53-byte cells (5-byte header and 48 bytes of payload) that is switched throughout a network over virtual circuits (standardized by the ITU in 1988 to create a ISDN). Its ability to accommodate multiple types of media (voice, video, and data) makes it a likely player for full service networks based on ADSL and VDSL. ATM Adaptation Layer 5 - A standard adopted by the ATM Forum for a class of service called High Speed Data Transfer. ATM cell - An ATM cell is 53 bytes long containing a 5-byte header and a 48-byte payload. The header of an ATM cell contains all necessary information for data to reach the appropriate endpoint.

The payload portion of an ATM cell can contain any type of information, be it voice, video, or data. ATM connection - An ATM connection is actually one physical connection between two endpoints, that contains multiple VCs. Furthermore, multiple VCs can be grouped to traverse a VP. See also Permanent Virtual Circuit, Switched Virtual Circuit, Virtual Channel Identifier, and Virtual Path Identifier. ATM Forum - The organization tasked with developing and defining ATM standards. See web for more information.

ATM 25 - ATM Forum-defined 25.6 MBPS cell-based user interface based on IBM token ring network. Attenuation - Signal loss resulting from trans versing the transmission medium. ATU (ADSL Transceiver Unit) - The ADSL Forum uses terminology for DSL equipment based on the ADSL model for which the Forum was originally created. The DSL endpoint is known as the ATU-R and the CO unit is known as the ATU-C. These terms have since come to be used for other types of DSL services, including RADSL, MSDSL and SDSL. ATU now represents xDSL services. ATU-C (ADSL Transmission Unit - Central Office) - The ADSL modem or line card that physically terminates an ADSL connection at the telephone service provider's serving central office.

ATU-R (ADSL Transmission Unit - Remote) - The ADSL modem or PC card that physically terminates an ADSL connection at the end-user's location. Available bit rate - Provides a guaranteed minimum capacity but allows data to be busted at higher capacities when the network is free. AWG (American Wire Gauge) - A measure of the thickness of copper, aluminum, and other wiring in the US and elsewhere. Copper cabling typically varies from 18 to 26 AWG, the higher the number, the thinner the wire. The thicker the wire, the less susceptible it is to interference. In general, thin wire cannot carry the same amount of electrical current the same distance that thicker wire can.

B 8 ZS (Bipolar with 8 Zero Substitution) - Line code for T 1 transmission. Ones are encoded as pulses of alternating polarity, and eight consecutive zeros are represented by a pulse of the same polarity as the previous pulse. B channel - A 'bearer' channel is a fundamental component of ISDN interfaces. Carries 64 in either direction, is circuit switched, and can carry either voice or data. See also BRI (basic rate interface), PRI (primary rate interface), and ISDN (Integrated Services Digital Network). Backbone - Equipment that provides connectivity for users of distributed network and includes the network infrastructure.

Backbone LAN - A transmission facility designed to connect two or more LANs. Bandwidth - The difference between the highest and lowest frequencies of a band that can be passed by a transmission medium without undo distortion. As a measure of carrying capacity, bandwidth indicates how many bits per second a link can carry, but says nothing about the delay through the network. Bandwidth bound - An application, which will not necessarily benefit from lower delay in a network, but can only run properly with a minimum amount of bandwidth at their disposal.

A bulk transfer file is a good example of a bandwidth bound application. Baseband - Using the entire bandwidth of a transmission medium, such as copper cable, to carry a single digital data signal. Note that this limits such transmission to a single form of data transmission, since digital signals are not modulated. See also Broadband.

Basic encoding rate - Bit error rate, or the ratio of received bits that are in error; also, a rule for encoding data units described in ANSI. See Bit Error Rate Test. Baud - Transmission rate of a multilevel-coded system when symbols replace multiple bits. Baud rate is always less than bit rate in such systems. Bearer services - A communication connection's capacity to carry voice, circuit, or packet data.

The two B channels in a BRI connection are bearer channels. See B Channel, BRI (Basic Rate Interface). Bell system - Before 1984, the local telephone companies that belonged to AT&T were commonly grouped together as the Bell system. All others were independents. After 1984, it became common to speak of the entire telephone network as the public switched telephone network (PTSN). See ITC (Independent telephone company), PTSN, and R BOC (Regional Bell Operating Compa.