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The idea for writing an introductory book on Digital Subscriber Line technology was first brought to me by Gail Nelson. Her initial proposal was just to concentrate on the subject of test equipment and in particular, wireline simulators. However, Michael Breneisen suggested that the work go beyond this focus and address the subjects of signaling devices and DSLAMs.
The detailed technical material in this work was obtained by researching many sources. A complete and particular acknowledgement of every source would have led to a plethora of footnotes. The reader often finds diversion to these an unwelcome distraction. As a result, I avoided their use. However, even with this intent I would be at fault if I did not give specific credit to the references given in the 'Bibliography.' I leaned in these heavily.
Ken Krechmer (Action Consulting, Palo Alto, California) was particularly helpful in directing me to needed references. His encyclopedic knowledge of Digital Subscriber Line technology is truly impressive.
Pat O'Hara served as both editor and graphic artist. She literally performed magic in taking my typed manuscript and turning it into this finished document.
Tony Horber took time from a busy product development schedule and checked the work for technical accuracy. I am indebted to him.
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 - 0s and 1s, 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 carrying 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 traveling 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.
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
Nonetheless, 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.
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.
Consider again the model illustrated in Figure 1-6. Suppose the input signal the Transmitter sends to the Transmission Medium is the simple cosinusoidal signal of amplitude '1' at frequency 'f0' Hz. The output response to this at the Receiver is designated 'T(f0)'. Now consider the cosinusoidal test input signal frequency f0 to be varied from 0 Hz on up to ∞. The resulting output signal as a function of frequency is T(f0) - 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 cosinusoidal 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 "intersymbol 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 intersymbol interference problem grows worse. Output pulses with time dispersion get squeezed next to one another. The growing level of intersymbol interference makes it increasingly harder for the Receiver to demodulate these signals.
To some extent, the intersymbol 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 intersymbol 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. Intersymbol interference will not disappear. Rather, it will be lessened and made more tolerable as the bandwidth grows larger. In particular, to lessen the intersymbol interference the bandwidth of the Transmission Medium must get larger in relation to the Source's generated bit rate, R BPS.In considering the data link design task, the first line of defense against time dispersion and intersymbol 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. Intersymbol interference will not disappear. Rather, it will be lessened and made more tolerable as the bandwidth grows larger. In particular, to lessen the intersymbol 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 intersymbol 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.
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 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 millenium 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 - TBPS). 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 1-15: 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 lightwaves 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.
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 1960s, this led to the development of the T-carrier system - the basis of which was a local loop digital system known as T1 (T-carrier, level 1 multiplexing). The T-carrier system led to the development of digital trunk lines. By the mid-1970s, digital trunk lines had become commonplace and digital switches made their first appearances. Through this period, T1 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 1970s, 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, T1 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), 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 Information Superhighway.
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 chapter to another, the information can be gleaned easily without going back to the very beginning.
A brief summary of the succeeding chapters follows:
Chapter 2 - We examine first the basic technological architecture underlying the
DSL modem. With this foundation, we shall follow with a study of the various different flavors of xDSL modems that have appeared over the past two decades, along with their specifications and uses. These flavors include: ISDN DSL (IDSL), Asymmetric DSL (ADSL), Single-line or Symmetric DSL (SDSL), Rate Adaptive DSL (RADSL), Universal ADSL (UADSL, also known as
G. Lite or DSL Lite), Consumer DSL (CDSL), Moderate Speed DSL (MSDSL), High-bit-rate DSL and High-bit-rate DSL 2 (HDSL/HDSL2), and Very-high-bit-rate DSL (VDSL).
Chapter 3 - The DSL modem is only half the system that allows you to convert your copper
local loop into a high-speed data transmission channel. The other half lies at the CO, the platform that gathers together signals from DSL and traditional POTS, combines them in a digital signal and dispatches them down the line to the destination CO. This is the Multiservices Digital Subscriber Line Access Multiplexer (DSLAM), which we shall discuss here.
Chapter 4 - This section deals with the testing of DSL modems and, as such, should be of special interest to manufacturers. Theoretically, there are several ways to test your modem's specifications: testing on the local loop itself, on a wire coil, and on a DSL
simulator. The simulator overcomes difficulties of space and distance constraints, crosstalk and mutual inductance - difficulties that afflict the first two methods of testing - by combining real world operational factors with
laboratory convenience. As such, it presents the manufacturer with the most realistic testing environment for a DSL product.
Chapter 5 - This section enumerates standards that cover the use of DSL technology, providing the names and contact information of organizations from which these standards can be ordered.
Chapter 6 - A glossary that covers the subject of digital subscriber line technology. It provides terminology specifically covered in this book, as well as terminology that may be encountered in the broader realm of communications in general.
In analog data communication along the PSTN, a voice-band modem converts data from a piece of terminal equipment into electronic signals in the 200 Hz to 3.4 kHz frequency band. This allows the existing public network to transmit electronic data in the same way it traditionally would a human voice. In the early decades of data communication, this was not so much of a problem. However, as modems have evolved to transmit and receive data at ever-higher speeds, and as software has evolved to carry ever more complex forms of information, data communication presses up against the physical limitations of the copper medium. The bandwidth of 200-3400 Hz is simply too narrow to fit this data comfortably. The result is a communications bottleneck. Downloading a web page becomes an increasingly cumbersome process the more detailed its graphics are. Try to connect to a web page that features animation - or worse yet, video footage - and your computer will slow to a crawl.
DSL frees the end-user from the limitations of voice bandwidth, providing bandwidth measured in the hundreds of kilohertz and enabling communications at least 100 times faster than that available over pure POTS, while still allowing you to make phone calls while your PC or fax is transmitting or receiving. Let us examine a typical DSL modem to see how it accomplishes this.
Figure 2-1: A Typical DSL Transceiver Block Diagram
The DSL chip set includes both analog and digital components. Among the analog components are analog transmit and receiver filters, the Digital to Analog Converter (DAC), the Analog to Digital Converter (ADC), and the automatic gain device (to adjust the received signal level to that which is suitable to the input of the ADC).
The modulation/demodulation function of the DSL transceiver, the modem proper, is digital. Modulation defines the process of converting each successive data symbol vector - in this case, a DSL input bit - into a continuous time analog signal that represents the message corresponding to each successive group of bits. At the far end of the transmission, the receiving DSL unit converts these analog signals back into bit form, hence "demodulation." Subsumed within the modulation/demodulation function are such aspects of digital signal processing as echo cancellation, adaptive channel equalizing, symbol/bit conversion, timing recovery, constellation mapping. In the cases of Carrierless AM/PM (CAP) or Quadrature Amplitude Modulation (QAM) line codes, the modem also provides the digital shaping filter, while in the case of Discrete MultiTone (DMT) line code, the modem includes Fast Fourier Transform (FFT) and Inverse Fast Fourier Transform (IFFT).
This brings us to the other major digital function of the DSL chip set, coding/decoding - performed by a part of the transceiver prosaically-known as the encoder. The task of the encoder is to map data bits from a digital bit stream prior to modulation and transmission. The importance of coding varies depending on the flavor of xDSL in use. Earlier DSLs, such as IDSL and HDSL, require no coding at all. Later DSLs, ADSL for example, can use Reed-Solomon codes, trellis codes or both. In the most recent generation of DSL systems, HDSL2 being the prime example, coding forms a critical part of the DSL transceiver. The relationship of the encoder to the modulator in transmission appears below in Figure 2-2.
Figure 2-2: Transmitter of a digital transmission system
Besides the DSL chip set, but there are two other components our hypothetical DSL transceiver may contain. The first element is the hybrid circuit, an interface converter for conversion from four-wire, dual half-duplex to two-wire full-duplex. The second element is the POTS splitter, a low-pass filter that separates the voice channel out from the DSL communication spectrum. The POTS splitter thus allows you to use your phone line as a phone line while simultaneously using it for data communication via modem, fax machine, or other terminal equipment.
These, then, are the bare bones of an average DSL modem. The modem connects the customer premises to the local loop, the actual digital subscriber line. The digital signal may require regeneration while traveling along the local loop, a process carried out by repeaters. At the far end of the local loop lies the central office, the CO, where another DSL modem will pick up the digital transmission. A bird's eye view of this generic DSL architecture appears below in Figure 2-3.
| Central Office - Building where local loops connect to transmission links. |
| LT: Line Termination - Building where local loops connect to transmission and switching equipment. |
| Local Loop - Telephone wire connecting the CO to the customer premises. |
| MDF: Main Distributing Frame - Wire cross-connection field used to any loop to any Central Office equipment. |
| NID: Network Interface Device - Point of demarcation between the customer installation and the telephone company outside wire. |
| NT: Network Termination - DSL Modem at the customer end of the local loop. |
| Repeater - Signal regeneration device located near the midpoint of a cable. |
| TE: Terminal Equipment - End user equipment such as a personal computer or a telephone. |
Figure 2-3: Generic DSL Reference Model
As we will discover, however, terms like "average" and "generic" must be applied very carefully when referring to DSL technology. There exists a veritable alphabet soup of DSL modems, with more combinations of letters clumping together all the time. Let's grab our spoons and taste a few.
There are nearly a dozen different types or "flavors" of DSL modem currently in existence or in development. We will now examine each type with respect to its transmission capabilities and limitations.
Between 1982 and 1988, the American National Standards Institute, better known as ANSI, developed the standards that defined IDSL (ISDN DSL) communications. IDSL functions on the Basic Rate Interface (BRI) model of ISDN - also known as 2B+D - which provides an overall data transmission rate of 144 KBPS. The two B (bearer) channels are circuit switched and can carry 64 KBPS of either voice or data in either direction. The D (data) channel carries control signals and customer call data in a packet-switched mode, operating at 16 KBPS. Remaining throughput is absorbed by Operational, Administrative, Maintenance and Provisioning (OAM&P) channels.
IDSL runs on a single pair of wires at a maximum distance of 18 kilofeet (kft) - roughly 3.4 miles/5.4 km. What differentiates IDSL from traditional ISDN is that ISDN requires connection through a CO voice switch. IDSL runs directly through xDSL equipment, rendering unnecessary the expensive upgrades to CO switches which ISDN otherwise demands. For this reason, IDSL is sometimes known as "BRI without the switch."
A variant on IDSL relies on the Primary Rate Interface (PRI) model, using a single D channel at 64 KBPS and some 23 to 30 B channels. In this variant, the channels are bonded to form higher bit rates, providing a theoretical capacity of up to 768 KBPS.
2.2.2 The HDSL Family: HDSL, SDSL, MSDSL and HDSL2
The search for a more cost-effective route to provide PRI bandwidths on local loops also led to the development of an entirely new flavor of DSL. AT&T Bell Laboratories and Bellcore developed the concept High-bit-rate DSL (HDSL) in late 1986. Prototype HDSL systems first appeared in 1989 and became commercially available three years later. In technical terms, HDSL is a dual-duplex repeaterless T1 technology. This means that HDSL transmits data symmetrically over two pairs of wire at the standard T1 data rate of 1.544 MBPS (each pair carrying 784 KBPS), an order of magnitude faster than IDSL. By itself, T1 transmission requires a repeater every 6000 feet to "clean up" and relay the digital signal to its destination, a requirement that often made such transmission too expensive for the average customer.
Figure 2-4: T1 Components
HDSL overcomes this limitation using a line code adapted from IDSL called 2B1Q (Two Binary, one Quaternary). The 2B1Q code compresses two binary bits of data into one time state as a four-level code. This doubles the effective range of T1 transmission from 6,000 to 12,000 feet without repetition, slicing the cost of T1 communication, with a side benefit of reducing crosstalk. In its original form, HDSL required addition of a third wire pair to bring the data rate up to E1 levels (2.048 MBPS).
The primary use of HDSL is to provide companies and individuals with Internet access to servers, but not just from clients. Additional applications include providing links on private campus networks with installed copper cable plant, video conferencing and distance learning applications, providing PRI for ISDN, extending central PBX to other office park locations, providing LAN extensions and connections to fiber rings and providing wireless system base station connections.
Figure 2-5: The HDSL Architecture
Multiple 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.
| SDSL Data Rate (KBPS) |
Maximum (feet) |
Distance (km) |
|---|---|---|
| 128 | 22,000 | 6.71 |
| 256 | 21,500 | 6.56 |
| 384 | 14,500 | 4.42 |
| 768 | 13,000 | 3.97 |
| 1,024 | 11,500 | 3.51 |
Table 2-1: SDSL Speeds and Distances
Another variation of HDSL, recently standardized, is HDSL2. Like SDSL, it functions on a single, full-duplex twisted pair. Unlike SDSL, it can transmit the full T1 (1.544 MBPS) or E1 (2.048 MBPS) at a distance of up to 12 kft without repeaters. The downside is that HDSL2 - designed for the T1/E1 leased line business market, rather than the residential market - does not include voice circuit support.
Subsequent to the printing of this book, a new flavor of HDSL, called SHDSL, has emerged. It is the first standardized multi-rate symmetric DSL and is designed to transport symmetrical data across a single copper pair at data rates from 192 KBPS to 2.3 MBPS or 384 KBPS to 4.6 MBPS over two pairs. Refer to the chart in Figure 2-9.
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. Lechleider 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 kft. 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 wideband 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.
Subsequent to the printing of this book, new flavors of ADSL have emerged, including ADSL2+ and ADSL2++, capable of doubling the transmission speed of typical ADSL connections to 2.2 MHz and 4.5 MHz, respectively. Both are backwards compatible to ADSL and provide improved reach. Refer to the chart in Figure 2-9 for a comparison.
Figure 2-6: ADSL Reference Model
Figure 2-7: 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 tradeoff is speed. The downstream data transmission rate for MDSL is only 800 KBPS to 1 MBPS; its upstream rate, a mere 100 KBPS.
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 kft 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 (CDSL), Universal ADSL (UDSL or UADSL), DSL Lite and G.Lite (for ITU Recommendation G.992.2, which governs this flavor). To call this flavor "splitterless" 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 splitterless 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.
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 tradeoff for these fantastic transmission speeds is in distance. VDSL only has a service range of 1.5 to 4.5 kilofeet, restricting its usefulness. For this reason, VDSL technology is targeted for use as the last link in fiber in the loop (FITL), fiber to the curb (FTTC) and fiber to the neighborhood (FTTN) 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 2-8: The VDSL Architecture
Figure 2-9: Sampling technologies and related bandwidths
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.
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, traveling 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 telco 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 multiservices 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
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 telco. 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
A second method is to test your modem on a coil of twisted pair cable, with the cable mimicking a local loop. This testing method has 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: Example of modem testing on DSL Simulator, featuring Telebyte's Model 458-2SL and Model 4801
Figure 4-4: Model 459-A High-Density/Production Local Loop Simulator
The sleek design, reliability and low cost of the Telebyte Model 459-A makes it the right choice for the production test environment. In addition, loop lengths are programmable from 4 kft to 22 kft, in 2-kft increments (plus 26 kft). The Model 459-A is ideally suited to test DMT DLSAM channels and modems. The Model 459-A simulates 26 AWG PIC and offers the bandwidth requirements for xDSL technologies such as ADSL, G.Lite, HDSL, HDSL2, G.SHDSL (Annex 1) and SDSL.
Figure 4-5: Model 458 Multi-Channel Local Loop Simulator
The Telebyte Model 458 Multi-Channel Local Loop Simulator provides a wide variety of configurations through use of up to 16 plug-in Line Modules. It is ideally suited for testing DSL modems and other bandwidth-compliant telecom devices in a high-volume production line environment. The 458 Control Module interfaces with a controlling PC or terminal via IEEE-488 or RS-232 to control loop-length settings. A user-friendly interface or Common user-command language may be used for control. The Model 458 simulates 26 AWG PIC and offers the bandwidth requirements for xDSL technologies such as ADSL, G.Lite, HDSL, HDSL2, G.SHDSL (Annex 1) and SDSL.
Figure 4-6: Model 458-2SL Multi-Channel Local Loop Simulator
This compact unit is extremely easy to use and a perfect fit when only one or two-channel operation is required. Telebyte Model 458-2SL Multi-Channel Local Loop Simulator is ideally suited for testing DSL modems and other bandwidth-compliant telecom devices in a high-volume production line environment. When used in conjunction with one to two 458 Line Modules, it becomes a two-channel local loop simulator. The built-in Control Module interfaces with a PC or terminal via IEEE-488 or RS-232 to control loop-length settings or it can be used as a stand-alone unit. User-command language may be used for control. In addition, line length is displayed on the front of the unit and can be set via a rotary encoder.
Additional Features:
Figure 4-7: A wide variety of Model 458 Line Modules plug into the 458 and 458-2SL card cages.
| Line Module | Technology | Cable | Lengths/Increments | Bandwidth |
| 458-LM-20 | ADSL, ADSL2, ADSL Lite ISDN, G.lite | 26 AWG PIC Insulation | 0-20.5 kft/500 ft | DC to 1.5 MHz |
| 458-LM-J20 | ADSL Japan | 0.4 mm Paper Insulation | 0-6.15 km/150 m | DC to 1.1 MHz |
| 458-LM-E20 | ADSL, ISDN, SHDSL/G.SHDSL (Annex A) | 0.4 mm PE Insulation | 0-6.15 km/150 m | DC to 1.5 MHz |
| 458-LM-HD | ADSL2++ | 26 AWG PIC |
Channel 1: 0 - 31,750 ft/250 ft Channel 2-8: 0-30,000 ft/2,000 ft |
DC to 4.5 MHz |
| 458-LM-HDE | ADSL2++ | 0.4 mm PE |
Channel 1: 0 - 9,450/150 m Channel 2-8: 0-9,000/600 m |
DC to 4.5 MHz |
| 458-LM-HDJ | ADSL2++ | 0.4 mm Paper | 0 km to 7.5 km/500-m | DC to 4.0 MHz |
| 458-LM-JSM | ADSL | 0.4 mm Paper Insulation | 0-1.55 km/50 m | DC to 1.1 MHz |
Figure 4-8: Model 460-V High-Density/Production VDSL Local Loop Simulator
Introducing the industry's first VDSL Local Loop Simulator. Ideally suited for the functional production testing of VDSL devices, this compact and reliable unit accurately simulates attenuation and impedance characteristics of 26 AWG and provides up to 24 channels in a 2-U high rack-mountable chassis. Loop lengths are programmable from 0 to 5,500 ft in 500-ft increments, bandwidth ranges from 100 kHz to 12 MHz and the 460-V can be remotely controlled via RS-232.
Other Products Related to DSL Simulation:
Figure 4-9: Model 4801 Universal xDSL Noise/Interference Simulator (8-slot version shown)
The innovative Model 4801 Universal xDSL
Noise/Interference Simulator features the lowest noise floor
of any noise simulator on the market today. This
highly-advanced system allows manufacturers of today’s xDSL
technologies to test signaling devices such as transceivers
and DSLAMs with ease and accuracy. Our design uses unique
technology to achieve real-life noise simulation, allowing
for accurate and repeatable testing. Reduce clutter with
this all-in-one unit that includes a powerful built-in PC,
eliminating the need to add a PC to the test configuration.
The Universal xDSL Noise/Interference Simulator seamlessly
integrates with our outstanding line of wire line (local
loop) simulators for the ultimate in affordability, accuracy
and ease of use. Modular design allows for a system that can
be expanded as needed.
Figure 4-10: Model 4101-J Japanese ADSL+ Test Loop Simulator
The 4101-J supports TCM loops 2 and 5 and is fully compliant with NTT’s proposed amendment to ITU-T for ADSL+. The system also meets NTT Laboratory’s expanded requirements for attenuation accuracy and loop length increments. This device will operate from DC to 4 MHz. The 4101-J can be controlled by a PC or terminal via IEEE-488 or RS-232 to control loop-length settings. Control can be accomplished by using the supplied GUI or through the use of scripts.
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 (T1, 1.544 MBPS) and Europe (E1, 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: www.adsl.com | |
| Tel: (510) 608-5905 | |
| Fax: (510) 608-5917 |
| ANSI | American National Standards Institute (ANSI) |
|---|---|
| 11 West 42nd Street | |
| New York, N Y 10036 | |
| Internet: www.ansi.org | |
| Tel: (212) 642-4900 | |
| Fax: (212) 398-0023 |
| Bellcore | Direct Sales |
|---|---|
| Telcordia Technologies, Inc. [Formerly Bellcore] | |
| 8 Corporate Place, PYA 3A-184 | |
| Piscataway, NJ 08854-4156 | |
| Internet: www.telcordia.com | |
| 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 Lucioles | |
| 06921 Sophia Antipolis Cedex | |
| France | |
| Internet: www.etsi.org | |
| Tel: +33 (0) 4 92 94 43 95/43 64 | |
| Fax: +33 (0) 4 93 65 47 16 |
| IEEE | IEEE Customer Service |
|---|---|
| 445 Hoes Lane | |
| PO Box 1331 | |
| Piscataway, NJ 08855-1331 | |
| Internet: www.ieee.com | |
| 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: www.itu.int | |
| Tel: +41 22 730 5857 or +41 22 730 5859 | |
| Fax: +41 22 730 5853 |
| VDSL | The VDSL Coalition |
|---|---|
| The VDSL Coalition is a group of leading semi-conductor, modem, and communications-network equipment firms and network operators who are committed to support the development of Very-High-Speed Digital Subscriber Line (VDSL) standards for the next generation of telecommunications applications. | |
| Internet: http://www.vdslcoalition.net |
# | A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X
2B + 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, 2B + D is carried on one or two pairs of wires (depending on the interface). See also BRI.
2B1Q - Two Binary, one Quaternary. A line coding technique that compresses two binary bits of data into one time state as a four-level code.
3B2T - A baseband line code where three binary bits are encoded into two ternary symbols.
4B3T - A baseband line code where four binary bits are encoded into three ternary symbols.
5ESS - A digital central office switching system made by AT&T.
10Base-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.
100Base-T - A 100 MBPS LAN that maintains backward compatibility with10Base-T networks running at 10 MBPS. Competitor to 100VGAnyLAN.
23B + 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).
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AAL5 (ATM Adaption Layer 5) - AAL5 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) - BellCore 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 E1 or T1 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, PBXs, 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 T1 for telecommunications. Member of the ISO.
API - Application Programming Interface.
APON (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 transmittals 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 T1.
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 ITUT in 1988 to create a BISDN). 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 Adaption 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 http://www.atmforum.com for more information.
ATM25 - ATM Forum-defined 25.6 MBPS cell-based user interface based on IBM token ring network.
Attenuation - Signal loss resulting from transversing 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 Transceiver 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 Transceiver 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 bursted 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 from18 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.
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B8ZS (Bipolar with 8 Zero Substitution) - Line code for T1 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 RBOC (Regional Bell Operating Company).
Bellcore (Bell Communications Research) - The research arm of the regional telephone companies. Bellcore was part of Bell Laboratories before the breakup of AT&T. Bellcore plays a leading role in developing ISDN standards and other ISDN activities among its member telephone companies.
BER (Bit Error Rate) - Measure of transmission quality indicating the number of bits incorrectly transmitted in a given bit stream compared to the total number of bits transmitted in a given duration of time.
BERT (Bit Error Rate Test) - A test that reflects the ratio of errored bits to the total number transmitted. Usually shown in exponential form (106) to indicate that one out of a certain number of bits are in error.
Binary Eight Zero Suppression - A technique in T-1 that modifies the AMI encoding to ensure minimum pulse density without altering customer data. When eight zeros in a row are detected, a pattern with intentional bipolar violations is substituted. These violations enable the receiving end to detect the pattern and replace the zeros.
Binder Group - Cable pairs are typically arranged under the cable sheath in binder groups. The binder is a spirally wound colored thread or plastic ribbon used to separate and identify cable pairs by means of color-coding. The enclosed pair group is called a binder group. The groups are composed of insulated twisted copper pairs that are also twisted within each binder. Typically they are wrapped in 25 pair bundles. For example, pairs 1-25 might be in one binder group and pairs 26-50 in another. In xDSL, one often hears discussions of signal interference between adjacent pairs within a binder group. The best of all worlds is to keep a data pair separated from another data pair by assigning it to an adjacent binder group. If the data pairs are too close to each other they create what telcos call "disturbers" (i.e., crosstalk). If a "disturber" exists in the binder group serving your SNI, NID, MPOE, etc., you may not "qualify" for DSL service.
Biphase - A baseband line code, also known as the Manchester line code.
Bipolar Return to Zero - A bipolar signal in which each pulse returns to zero amplitude before its time period ends. This prevents the buildup of DC current on the signal line.
Bipolar Violation - The occurrence of two successive pulses of the same polarity in a bipolar signal. In T-1 it is detected as an error.
BISDN (Broadband Integrated Services Digital Network) - A technology suite designed for high-bandwidth multimedia applications and the integration of voice, data, and video. The two transmission types are ATM and STM.
Bit - A contraction of "binary digit." A bit is the smallest element of information in the digital system.
Bit Robbing - A technique in T-1 multiplexing in which the least significant bit (bit 8) of each byte in selected frames is robbed from being used to carry message information and instead is used to carry signaling information.
Bit Stuffing - Extra bit(s) that are conditionally inserted into the frame to adjust the transmitted bit rate.
Blocking - Whenever bits cannot make their way from an input port to an output port in a network node, they are considered to be blocked. In the voice network, the call will not go through. In a data network, the bits may be stored in a buffer or discarded, depending on the situation.
BONDing (Bandwidth ON Demand; sometimes written as BOND-ING or BONDING) - The automatic combining of both B channels into a 128 channel for faster data transfer.
BPS (Bits Per Second) - The raw data rate of a system. Indicates the speed at which bits are transmitted across a data connection.
BRA (Basic Rate Access) - See BRI (Basic Rate Interface).
BRI (Basic Rate Interface) - An ISDN interface typically used by smaller sites and customers. This interface consists of a single 16 KBPS data (or "D") channel plus two 64 KBPS bearer (or "B") channels for voice and/or data for a rate of 144 KBPS. Also known as Basic Rate Access, or BRA.
Bridge - A device that connects two networks of the same type.
Bridge Tap (or Bridged Tap) - A connection of another local loop to the primary local loop. Generally it behaves as an open circuit at DC, but becomes a transmission line stub with adverse effects at high frequency. It is generally harmful to DSL connections and should be removed. Extra phone wiring within one's house is a combination of short bridge taps. A POTS splitter isolates the house wiring and provides a direct path for the DSL signal to pass unimpaired to the ATU-R modem.
Broadband - Sharing the bandwidth of a medium such as copper or fiber optic cable, to carry more than one signal - allowing the integration of voice, video, and data over a single transmission medium. Strictly speaking, a telecommunications link that runs at more than 1.5 MBPS in the US and more than 2 MBPS everywhere else. This is the primary rate of ISDN. Today, most people consider broadband speeds to be much higher, perhaps as high as 5 or 10 MBPS. Includes elements of bandwidth and delay, neither of which can be ignored.
Brouter - A device that can provide the functions of a bridge, router, or both concurrently; a brouter can route one or more protocols, such as TCP/IP and/or XNS, and bridge all other traffic.
Brownout - In the context of DSL, a situation that occurs when a CO (or local exchange) cannot handle all of the calls attempted and even disrupts calls in progress. Also called a "brown down."
Browser - A universal client for accessing information on the World Wide Web portion of the Internet. The development of the browser directly led to the explosion of interest in the Web and indirectly to the current crisis in local access speeds that DSL addresses.
Buffer - A storage area in a computer or other processor's memory dedicated for telecommunications purposes. The whole art of network design is a balancing of the need to buffer bits in order to store and process them and the need for adequate bandwidth and delay to actually send the bits somewhere. Larger buffers can compensate for slower links and network nodes that experience blocking, but at the risk of offending waiting users.
Bus Networks - A bus network is a multiple access medium for small networks and usually only consists of one cable and the devices that are attached to it.
Byte - A group of eight bits.
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Cable Binder - A cable binder is used to bundle multiple insulated copper pairs together in the telephone network.
Cable Modem - A modem designed to operate over cable TV lines; used to achieve extremely fast access to the Internet. Capable of speeds up to 10 MBPS for downloading.
CACH EKTS (Call Appearance Call Handling Electronic Key Telephone Service) - Supplements EKTS to allow more than one directory number and multiple call appearances on each directory number. See EKTS (Electronic Key Telephone Service).
Call Appearances - A supplementary ISDN service that allows multiple incoming calls. Each directory number can have multiple call appearances, depending on the switch type.
Caller ID (Calling Number Identification) - A telephone company service that delivers the calling party's telephone number to the called party. The number can appear on an ISDN telephone, an LCD screen, a computer screen, or another device
CAN (Campus Area Network) - A network that involves interconnectivity between buildings in a set geographic area, such as a campus, an industrial park, or other such private environment.
CAP (Carrierless Amplitude & Phase Modulation, or Competitive Access Provider) - A transmission technology for implementing a DSL connection. Transmit and receive signals are modulated into two wide-frequency bands using passband modulation techniques. Licensed by Globe Span Technologies, Inc., this modulation is based on QAM and is used in ADSL modems. CAP is a competitor to DMT modulation. Regardless of its standards status, it is used in many telephone companies' ADSL trials.
CAPI (Common ISDN API) - A collection of functions for handling ISDN communications at the messaging level. A more powerful set of functions than WinISDN.
Carrier Service Area - Area served by a LEC, RBOC, or telco, often using DLC technology.
CAT3 (Category 3 Cabling) - A rating for twisted pair copper cabling that is tested to handle 16 MHz of communications. Handles 10 MBPS of LAN traffic and is commonly used as telephone wiring.
CAT5 (Category 5 Cabling) - A rating for twisted pair copper cabling that is tested to handle 100 MHz of communications. CAT-5 cable is generally required for higher-speed data communications, such as Ethernet LANs and possibly low-speed ATM.
CBR - Committed Bit Rate.
CCITT (Comité Consultatif International de Télégraphie, or International Telephone and Telegraph Consultative Committee) - The former international standards body that developed telecommunications standards; now called the International Telecommunications Union (ITU).
CCS (Common Channel Signaling) - An integral part of ISDN known as signaling system 7, CCS is a method for sending call-related information between switching systems by means of a dedicated signaling channel. This signaling channel is separate from the bearer or B channels. CCS allows services such as call forwarding and call waiting to be provided anywhere in the network. Other acronyms for common channel signaling are CCSS, CCSS7, and SS7.
CDSL (Consumer DSL) - A trademarked version of DSL that is somewhat slower than ADSL (1 MBPS downstream, probably less upstream) but has the advantage that a "splitter" does not need to be installed at the user's end. Rockwell, which owns the technology and makes a chipset for it, believes that phone companies should be able to deliver it in the $40-45 a month price range. CDSL uses its own carrier technology rather than DMT or CAP ADSL technology.
Cell - A fixed-length protocol unit used in a data link layer protocol. Since it is fixed-length, a cell requires no special delimiting symbols. ATM is the international standard way of building networks that employ cells. Contrast with Packet and Frame.
Centrex - A type of business telephone service that is like having a private branch exchange (PBX) located in your local central office. A single-line telephone service delivered to individual desks (the same as you get at your house) with additional features.
CEU (Commercial End-user) - See SU, Service User.
Channel - A generic term for a communications path on a given medium; multiplexing techniques allow providers to put multiple channels over a single medium. See also Multiplexer.
CHAP (Challenge Handshake Authentication Protocol) - A security protocol that arranges an exchange of random numbers between computers. The machine receiving the number from the first computer performs calculations on that number using a previously agreed-upon string of characters as a secret encryption key.
Circuit - A path through a network from source to destination (and usually back).
Circuit Switching - A switching system that establishes a dedicated physical communications connection between endpoints, through the network, for the duration of the communications session; this is most often contrasted with packet switching in data communications transmissions. See also Packet Switching.
CLEC (Competitive Local Exchange Carrier) - An LEC which, when competition begins, has the less dominant position in the market; the carrier entering the market, challenging the ILEC market position.
Client/server - A model for computer interactions. One half of the typical interaction between user and information content on the Internet and Web is called the client. The other half is the server. Usually, a user runs a client process to obtain and process information present on a remote server. That is, the client "talks" and the server "listens" and responds to client requests. The interaction between client and server is usually asymmetrical, with more bits flowing from server to client than vice versa. Client/server architecture is generally opposed to centralized mainframe computing architecture.
Cloud - A commonly used term that describes any large network.
CO (Central office/Central site) - In North America, a circuit switch that terminates all the local access lines in a particular geographic serving area; a physical building where the local switching equipment is found. xDSL lines running from a subscriber's home connect at their serving central office. Known as a public exchange elsewhere. See also Serving Central Office.
CODEC (Coder/Decoder) - A hardware device or software program that converts analog information streams into digital signals, and vice versa; generally used in audio and video communications where compression and other functions may be necessary and provided by the CODEC as well.
Commercial End-user - See Service user.
Common Carrier - Telephone companies that provide long-distance telecommunications services. Also known in US as XC (IntereXchange Carrier).
Community Antenna Television - Also known as Cable TV.
Companding - Compressing the dynamic range of a signal prior to transmission, with matching expansion at the receiver to regain the original signal.
Compatibility Packages - A standardized method for offering ISDN service. Not all telephone companies use this service.
Compression - The act of sampling and reducing a signal for the purposes of saving storage or transmission capacity; MPEG1 and MPEG2 are two key encoding and compression algorithms that enable full-motion video over smaller bandwidth circuits, such as those offered by ADSL, SDSL, and HDSL.
Concentrator - A device that serves as a point of consolidation of network links so that multiple circuits may share common limited network resources.
Connection Oriented - A term applied to network architecture and services which require the establishment of an end-to-end, predefined circuit prior to the start of a communications session. Frame relay circuits are examples of connection-oriented sessions. See Connectionless.
Connectionless - A term applied to network architecture and services which do not involve the establishment of an end-to-end, predefined circuit prior to the start of a communications session. Cells or packets are sent into the connectionless network, and are sent to their destination based on addresses contained within their headers. The Internet and SMDS are two examples of connectionless networks. See Connection Oriented.
Constant Bit Rate - Specifies a fixed bit rate so that data is sent in a steady stream. This is analogous to a leased line.
Convergence - The concept that at some unspecified time in the future, all information will be digital, all networks will become one (no more separate TV, voice, or data networks), all user devices will just be various forms of computers.
Core Network - Combination of switching offices and transmission plants connecting switching offices together. In the US, local exchange core networks are linked by several competing interexchange networks; in the rest of the world, the core network extends to national boundaries.
COS (Corporation for Open Systems) - Member-based organization that promotes open systems and connectivity. COS is instrumental in developing ISDN Ordering Codes for streamlining the acquisition of ISDN service from the telephone companies. Also instrumental in getting industry support for ISDN-1 standards.
CPE (Customer premises equipment) - A wide range of customer premises-terminating equipment which is connected to the local telecommunications network. This includes telephones, modems, terminals, routers, set top boxes, etc.
CRC (Cyclic Redundancy Check) - A test used to confirm that data has been delivered without error. In a data packet, the CRC character is calculated by assigning binary values to blocks of data. If the block of data does not match its assigned CRC value upon delivery, the data is errored.
Crosstalk - The interference caused by signals on adjacent circuits in a network. Annoying enough in the analog voice network, crosstalk is a hazard that limits distance and speed on digital networks.
CSA (Carrier Serving Area) - CSA loop design rules specify the characteristics of loops served by Digital Loop Carrier sites.
CSD (Circuit-Switched Data) - A circuit-switched call for data in which a transmission path between two users is assigned for the duration of a call at a constant, fixed rate.
CSN (Circuit-Switched Network) - A network that establishes a physical circuit temporarily on demand (typically when a telephone or other connected device goes off hook) and keeps that circuit reserved for the user until it receives a disconnect signal.
CSU (Channel Service Unit) - See DSU/CSU (Data Service Unit/ Channel Service Unit).
CSV (Circuit-Switched Voice) - A circuit-switched call for voice in which a transmission path between two users is assigned for the duration of a call at a constant, fixed rate.
CSV/CSD (Alternate Circuit-Switched Voice/Circuit-Switched Data) - A B channel configuration that allows either circuit-switched voice or circuit-switched data communication.
Cyclic Code - An error correcting code implemented with a feedback shift register.
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D Channel - In an ISDN interface, the "data" or D channel is used to carry control signals and customer call data in a packet-switched mode. In the BRI (basic rate interface) the D channel operates at 16, part of which will handle setup, teardown, and other characteristics of the call. Also, 9600 BPS will be free for a separate conversion by the user. In the PRI (primary rate interface), the D channel runs at 64. The D channel is sometimes referred to as the delta channel. See also BRI, PRI, and ISDN.
DA (Distribution area) - A loop serving area for a Feeder Distribution Interface (FDI).
DACS (Digital Access & Cross-Connect System) - A device that allows DS0 channels to be individually routed and reconfigured.
DBrnc - The logarithmic power ratio of a C-message weighted filtered signal with respect to 1 nanowatt.
DCE (Data Circuit [terminating] Equipment) - Any device that is connected to the subscriber end of a transmission circuit and provides the appropriate termination functions for that connection. A modem or DSU/CSU are considered DCE. Also called Data Communications Equipment.
DDS (Digital Data Service) - Private line digital service that provides digital communication circuits with data rates of 56/64.
DECNET - Digital Equipment Corporation's proprietary network architecture.
Dedicated Line - A transmission circuit that is reserved by the provider for the full-time use of the subscriber. Also called a Private Line.
Delay - A contributing measure of the carrying capacity of a link, delay indicates how long it takes bits to find their way through a network, but says nothing about the bandwidth through the network. Delay can be zero, but the network can be useless if it only delivers one bit per hour. This is important for bandwidth bound applications such as bulk data transfers, which need adequate bandwidth to function properly.
Delay Bound - An application, which will not necessarily benefit from more bandwidth in a network, but can only run properly with a minimum and stable delay at their disposal. A voice telephone call is a good example of a delay bound application. Adding more bandwidth beyond what it needs will not make the voice call any better.
Demarcation Point - The point at the customer premises where the line from the telephone company meets the premises wiring. From the demarcation point, the end-user is responsible for the wiring.
Demodulation - Conversion of a carrier signal or waveform (analog) into an electrical signal (digital).
Desktop Video Conferencing - A PC-based video conferencing system that allows people to conduct video conferencing in real time from their desks. The basic desktop video conferencing system includes a video camera, a video card, and an adapter card.
DFE (Decision Feedback Equalizer) - An adaptive filter used to compensate for the frequency response of the channel.
DFT (Discrete Fourier Transform) - A signal transformation that is often implemented as a fast Fourier transformation on a digital signal processor.
DHCP (Dynamic Host Configuration Protocol) - A TCP/IP protocol that provides static and dynamic address management.
Dial up - The process of initiating a switched connection through the network; when used as an adjective, this is a type of communication that is established by a switched-circuit connection.
Digital - Having only discrete values, such as 0 or 1. Opposite of analog, which is continuously varying over time. A text file on a computer is a good example of digital information and voice is the prime example of analog information. However, either can be sent over a telecommunications link with an analog or digital signal.
Digital Hierarchy - The progression of digital transmission standards typically starting with DS-0 (64) and going up through at least DS-3.Twenty-four DS-0s make up a DS-1; 28 DS-1s make up a DS-3. There are other links (including a DS-2), but these are less common.
Discrete Time Domain - Signal values that are defined at periodic time intervals.
Distribution - Portion of the telephone cabling plant that connects subscribers to feeder cables from the CO.
Distribution Cable - The portion of the telephone loop plant that connects the feeder cable to the drop wires.
DLC (Digital Loop Carrier) - Network transmission equipment, consisting of a CO terminal and a remote terminal, used to provide a pair gain function. Concentrates many local loop pairs onto a few high-speed digital pairs or one fiber optic pair for transport back to the CO.
DLCI (Data Link Connection Identifier) - The frame relay virtual circuit number used in internetworking to denote the port to which the destination LAN is attached.
DMS100 - A digital central office switching system made by Northern Telecom.
DMT (Discrete MultiTone) - DSL technology using digital signal processors to divide the signal into 256 subchannels.
DN (Directory Number) - A telephone number for ISDN. A BRI line can have up to eight directory numbers, depending on the switch type used by the telephone company.
Downstream - Refers to the transmission direction from the CO to the customer premises.
DRAM (Dynamic Random Access Memory) - Memory used to store data in PCs and other devices.
Drop Wire - The section of the local loop connecting the distribution cable to the customer premises.
DS-0 (Digital Signal 0) - In the digital hierarchy, this signaling standard defines a transmission speed of 64.
DS-1 (Digital Signal 1) - In the digital hierarchy, this signaling standard defines a transmission speed of 1.544 MBPS; a DS-1 is composed of 24 DS-0 signals; this term is often used interchangeably with T-1.
DS-3 (Digital Signal 3) - In the digital hierarchy, this signaling standard defines a transmission speed of 44.736 MBPS; a DS-3 is composed of 28 DS-1 signals; this term is often used interchangeably with T-3.
DSL (Digital Subscriber Line) - The non-loaded, local-loop copper connection between the NSP and the customer premises. DSL can provide simultaneous high-speed digital data access and POTS service over the same twisted-pair wiring. Technically, DSL equates to ISDN, but this is decreasingly enforced terminology. See ADSL (Asymmetrical DSL), CDSL (Consumer DSL), HDSL (High Bit Rate DSL), IDSL (ISDN DSL), MDSL (Moderate Speed Digital Subscriber Line), SDSL (Symmetric DSL), UADSL (Universal ADSL or G.Lite), VDSL (Very High Data Rate DSL), and xDSL.
DSLAM (Digital Subscriber Line Access Multiplexer) - A CO platform for DSL modems that provides high-speed data transmission and optional POTS service simultaneously over traditional twisted-pair wiring.
DSP (Digital Signal Processor) - The microprocessor that handles line signaling in a modem. Designed to perform speedy, complex operations on digitized waveforms.
DSS1 (Digital Subscriber Signaling System No. 1) - The network access signaling protocol for users connecting to ISDN. It includes the CCITT Q.931 and Q.932 standards.
DSU/CSU (Digital Service Unit/Channel Service Unit) - The interface required to change one form of digital signal to another. Many of the devices used in xDSL technologies are basically advanced forms of DSU/CSU, such as the HTU (HTML Termination Unit). Contrast with Modem.
DSX-1 - DS1 Cross-connect. A 1.544 Mbps AMI signal used for short distances to interconnect equipment within a CO.
DTE (Data Terminal Equipment) - The equipment, such as a computer or terminal, that provides data in the form of digital signals for transmission.
DWDM (Dense Wave Division Multiplexing) - A SONET term. High-speed versions of WDM, which is a means of increasing the capacity of SONET fiber optic transmission systems through the multiplexing of multiple wavelengths of light. Each wavelength channel typically supports OC-48 transmission at 2.5 GBPS. A 32-channel system will support an aggregate 80 GBPS.
DWMT (Discrete Wavelet Multi-Tone) - A multicarrier modulation system pioneered by Aware Inc. that, according to the vendor, isolates its subchannels in a method that is superior to conventional DMT modulation. In the vendor's own words, "DWMT is able to maintain near optimum throughput in the narrow band noise environments typical of ADSL, VDSL, and Hybrid Fiber Coax, while DMT systems may be catastrophically impaired."
Dynamic Bandwidth Allocation - A key feature of ISDN remote-access devices that allows automatic adjustment of the number of B channels in use depending on the volume of data being sent or received. This feature saves you money because each B channel is billed as a separate charge. Automatically adjusting bandwidth up or down depending on your data volume means you use only what you need.
Dynamic IP Addressing - An IP address is assigned to the client for the current session only. After the session ends, the IP address returns to a pool of IP addresses. See IP (Internet Provider).
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E1 (or E-1) - European equivalent of a T1 circuit. It is a term for a wide band digital interface used for transmitting data over a telephone network at 2.048 MBPS.
E3 (or E-3) - European equivalent of a T3 circuit. It is a term for a wide band digital interface used for transmitting data over a telephone network at 34MBPS.
EC (Echo Cancellation) - A DSP time domain technique for removing echoes. See Echo supressor/Echo canceller.
ECH (Echo-Cancelled Hybrid) - A 2-to-4 wire conversion with Echo Cancellation. A hybrid transformer is often used to interface to the line.
Echo - The reflecting of a signal back to its source due to a variety of reasons. Whenever the same bandwidth is used for transmission in both directions, echo is a concern. In all cases, some form of echo control must be used to compensate for these effects, which can be annoying for voice but devastating for data. Both the voice network and simple modems employ echo cancellation techniques. Also known as "positive feedback" or "singing."
Echo Suppressor/Echo Canceller - These are active devices used by the phone company to suppress positive feedback on the phone network. They work by predicting and subtracting a locally generated replica of the echo based on the signal propagating in the forward direction. Modems deactivate these devices by sending the 2100 Hz answer tone with 180-phasereversals every 450 msec at the beginning of the connection.
EIA/TIA (Electronic Industries Association/Telecommunications Industry Association) - This organization provides standards for the data communications industry to ensure uniformity of the interface between DTEs and DCEs.
EKTS (Electronic Key Telephone Service) - A service that provides PBX-like capabilities using ISDN add-on features. It ties these add-on features to keys on your ISDN telephone, allowing you to have a hold button or forward button, for example.
EMC (Electromagnetic Compatibility) - Prevents unintended radio frequency interference (see RFI).
EMS (Element Management System) - A management system that provides functions at the element management layer.
Enterprise Network - A widely dispersed, multifaceted telecommunications network for a particular purpose or organization; a term for all of an organization's telecommunications networking services and equipment.
Ethernet - A type of network used to connect devices within a single building or campus at speeds up to 10/100 MBPS. Within the OSI model, Ethernet is defined at layer one (physical) and layer two (data link). Based on Carrier Sense Multiple Access/Collision Detection (CSMA/CD), Ethernet works by simply checking the wire before sending data. Sometimes two stations send at precisely the same time in which case a collision is detected and retransmission is attempted. Ethernet is a widely-implemented standard for LANs. See also 10Base-T or 100Base-T.
ETSI (European Telecommunications Standardization Institute) - An organization that produces technical standards in the area of telecommunications.
EU - European Union. Formerly known as EC, European Commission.
Exchange Area - A geographical area in which a single, uniform set of tariffs for telephone service is in place. A call between any two points in an exchange area is considered a local call. See also LATA (local access and transport area).
EZ-ISDN - A standardized set of ISDN line configurations developed by the NIUF (North American ISDN Users' Forum). Des