Engineering Measures .1 Screening (Does Not Work)

From time to time, the suggestion is raised that cables with individually screened pairs should be used to reduce crosstalk. Although this can be dismissed on economic grounds, at least for DSL,¹³there are also technical objections.

• Screening increases the attenuation of wanted signals [Hughes 1997].

• Screening makes the cables heavier, larger, and harder to bend.

• Effective screening requires great care with earthing.

• Screening does eliminate capacitive coupling, but not magnetic coupling at DSL frequencies.

To summarize, screening attenuates legitimate signals, is expensive, and does not elimi-nate crosstalk. There are other changes to construction that would be beneficial for DSL use,

11Mandelbrot took it further and invented fractals.

12The period in which Vbis observed to find one peak value.

13If network operators could afford to re-cable the local access network, they would do it in glass!

for example, the tighter twisting now used in CAT5 cables. However, the crosstalk proper-ties of existing cables are not a mistake by the manufacturers: these cables were designed for voice use and are fit for purpose. Control of voice crosstalk was once a major design problem that was effectively solved in the 1930s by twisting neighboring pairs (or quads) with different pitches. This solution is, in reality, so good that DSL is viable even though it uses the cables at frequencies two or three orders of magnitude above the cables’ design frequencies.

3.4.2 Enforced Continence

In a classical telecommunications channel (e.g., the deep space channel), one way of im-proving the SNR at the receiver is to transmit more power; a system designer will use the most powerful transmitter that is affordable. However, the access network channel is dom-inated by crosstalk, so increasing the transmit power would increase the noise environment correspondingly, with no gain in SNR.¹⁴

Superficially, one might expect a newcomer to see the status quo as an opportunity to be exploited. In the past (the days of monopoly telcos), this did not happen because the existing environment was investment by the same people as the newcomer: big loss+ small gain = no action. In the brave new world of local loop unbundling (LLU), in which incumbent network operators must make lines available for lease by competitive access carriers, even an aggressive newcomer realizes the “gain” only exists until he becomes an investor and is as vulnerable to crosstalk as the rest of the potential victims. The result is that anyone who wants to do real business wants his or her neighbors’ behavior constrained. The local access network being crosstalk limited leads directly to the need for spectral management, which is discussed in detail in Volume 2 of this series.

3.4.3 Deployment Discipline

In many LLU regimes, a system is permitted to be connected on lines on which it will work and on some lines on which it will not.¹⁵Then it is the operator’s problem to choose those lines on which they will offer service. This decision will be conditioned by, among other things, the expectation of the noise environment. It should be noted that this expectation can be different for different operators in the same network: they may, for example, take different views on how widespread DSL will be in five years time. This is discussed in more detail in Volume 2 of this series.

3.4.4 Band Duplexing

Because NEXT is a worse impairment than FEXT, a given frequency band (when used in one direction) will have higher capacity if all the neighbors use the same frequency band in that one direction only. At low frequencies, however, the most efficient use of telephone pairs is to send signals bidirectionally, whereas at high frequencies, the best overall capac-ity is achieved if separate bands are dedicated to each direction. In a virgin network, one might imagine engineering the regime change to best advantage, but in a real (“heritage”) network, this band dedication can only happen for bands not already in use. Thus in prac-tise, unidirectional working is attractive for the bandwidth above the present bidirectional

14Decreasing the transmit power is also possible and is attractive until other sources of noise become significant.

In practise, the standard transmit powers are chosen so crosstalk will dominate comfortably, but not beyond that.

15A major exception is the United States, where there are performance guarantees; there the discipline is on the guarantor.

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systems. The usual form of duplexing is frequency division, dedicating different nonover-lapping bands to each direction. This is the case for VDSL and for the higher frequencies in the downstream channel of ADSL. Time division duplexing (“ping-pong”) is also possible, where nonoverlapping periods of time are dedicated to each direction. This is used in Japan, originally for ISDN and now for ADSL too.

3.4.5 Interleaving

ADSL has two countermeasures against impulsive noise: interleaving with a forward error correction code, and, for DMT ADSL, a judicious choice of symbol length.¹⁶

Interleaving works against burst errors by breaking the data stream into blocks, each of which is protected using a Reed–Solomon code. The blocks are then further broken up into small pieces that are shuffled by an interleaving process. Following this procedure, the pieces of any one of the original blocks are widely separated. The intention is to ensure a noise burst will only damage a correctable number of the pieces of each block. At the receiver, the pieces are reassembled into blocks, each of which is then corrected by the code.

The detailed schemes used are discussed further in Chapter 9.

3.5 Modelling

This section discusses how to implement a noise model. Eventually a noise model becomes a specification, perhaps a formula for a power spectral density (PSD) in a simulation, or perhaps a table of numbers for an arbitrary waveform generator on a laboratory bench.

This section is concerned with how and why the model is constructed.

3.5.1 Use of Noise Models

Noise models must be implemented for simulation work and for laboratory testing. In the former case, the “model” is a means of generating numbers in a computer program. In the latter case the “model” is a means of generating electrical waveforms in physical equipment.

Simulation provides predictions of system performance and indirectly supports a variety of engineering and managerial processes, including:

• Optimizing the design of DSL modems (typically by manufacturers)

• Deciding whether performance is sufficiently promising to warrant continued development (manufacturers, standards bodies)

• Determining the performance to test for in a standard (standards bodies)

• Optimizing the DSL population in a network (part of spectrum management)

• Determining the service limits for the modem (operators)

Laboratory testing is used to verify the conformance of real equipment to standards and to calibrate subsequent simulations. Despite the obvious differences in form of the models for simulation and laboratory use, the logic of construction is similar,¹⁷and the differences will be left as an exercise for the reader.

16Other DSL technologies, such as SDSL, do not include impulse suppression because of strict latency requirements.

17At least one laboratory generates its noise via its simulation suite.

3.5.2 Production

To predict the performance of DSL, its environment must be represented in some detail:

perhaps a particular configuration of cables with a particular population of systems running in it, and afflicted with particular sources of noise. The noise model will represent the noise as experienced at the receiver of interest. Therefore, different noise models will result from different assumptions about:

• The physical size of the network (for example, different countries, whether in town or rural).

• The cables used (varies by country).

• Where the receiver is (for example, in the local exchange, in a cabinet, in a house, in an office).

• The neighboring systems(varies by locale, with time, and with simulator’s optimism¹⁸).

In keeping with industry practice, this chapter shall pursue models in the frequency domain, eventually producing a PSD for the noise as experienced at a modem receiver. The development shall start with the noise sources and their spectra, then take into consideration the coupling between these sources and the receiver, and finally determine how to combine the noise components.

3.5.2.1 Network Model

Each simulator will have to take a view on what the network reality is. Much guidance can be obtained from published materials, especially the normative tests in standards (for example, [ETSI ADSL 2002]), where the operators involved try to ensure the required performance is tested in an environment representative of their network. However, other aspects, such as how full a network will be in five years’ time, are guesswork; companies take commercial risks based on these guesses and usually regard the details as proprietary. To obtain an initial assessment of a proposed DSL system, one common assumption is that the cable is a simple single section with two ends, 100 percent filled with instances of the modem under consideration. The initial objective of the simulations is to find the longest cable in which the modem still works. This is called “self-limited reach.”

An assumption popular with naive proponents of a new system is that a single instance of the system operates in cables without any other system present. This unsurprisingly predicts operation over great distances. Although such an analysis might tell of the quality of the engineering in the modem, and is certainly easy to check in a laboratory, it says nothing about operational performance in a real network. Hence, such a simulation is valueless to a network operator, who is only concerned with providing services that can be guaranteed under even worst-case noise conditions.

3.5.2.2 Sources

Because crosstalk is the dominant impairment for DSL, the most important sources are the other DSL systems in the network.

3.5.2.3 Standards

Standards define spectral masks, so a standards-conformant modem has an upper bound on the transmitted PSD. In recent standards, these masks have become fairly tight in the

18Different operators in the same unbundled network may take different commercial views of risk.

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signal bandwidth. Out of the transmitted band, the modems typically cut off faster than the mask. Older standards specified more generous masks, so the true signal cannot really be inferred from the mask. (Really ancient standards, such as G703 30 channel PCM, only included time domain pulse masks; as the systems did not include spectral scrambling they cannot be said to have a “spectrum,” because the spectrum changes depending on the traffic being carried.)

3.5.2.4 The Experts

In order to get consistent results between the participants of international debates, the experts have published nominal spectra (“templates”) for most systems of interest. These are derived from measurements and or detailed understanding of the systems’ modulations and are generally the best models available [ETSI Spect. Manag. 2002].

3.5.2.5 Measurement

If a new system is to be included in a simulation (or the effects of a fault are to be studied), it is necessary for the simulator to determine the spectrum for himself, typically by mea-surement. It is also common to include other forms of noise, for example, a−140 dBm/Hz

“background” noise almost by default,¹⁹and sometimes radio-frequency (RF) tones to rep-resent RF interference (RFI). (See Chapter 13 for details of RFI.)

3.5.2.6 Coupling

Coupling is by a combination of crosstalk and line attenuation. Crosstalk models are dis-cussed above in Section 3.1, and line attenuation is disdis-cussed in Section 2.3.5. In general, there will be a coupling path from every transmitter in the network to every receiver.²⁰ The noise sources other than crosstalk are usually provided in the form in which they are expected at the receiver, so they are not modified by coupling.

3.5.2.7 Summation

At first sight one would expect that, because the various noise sources are independent physical processes, the power of their sum would be the sum of their powers (and at each frequency independently, so the PSD of their sum is the sum of their PSDs). However, this conflicts with the FSAN models of crosstalk, and crosstalk contributions should be added by the FSAN sum method.

3.5.3 Impulsive Noise

At present, it is not possible to predict the effect of impulsive noise on system performance either by simulation or by laboratory tests, because it has no adequate statistical characteri-zation. Statistics that have been gathered by field tests (for example, in [Cook 1993]) simply show so much variation that predictions cannot be made with confidence.

As a result, mainstream modelling of performance considers stationary noise only. Im-pulsive noise is “modelled” in system design usually by a consensus of experts that a level

19A value apparently set by agreement of the experts. Johnson noise is an insignificant part of this value. Johnson noise, the classical noise due to thermal movement of the electrons in a conductor, is−174 dBm/Hz at 20 degrees C.

20It is possible to save computer time by omitting insignificant terms, although these days it is usually more efficient to implement all terms rather than spend human effort in deciding what to leave out.

of protection is appropriate²¹and in testing to verify that the protection built in is, in fact, operating properly. A typical standards conformance test²² is to exercise the modem in a typical stationary noise environment, to stress it realistically but so it should work properly.

Next, the modems are disturbed with bursts of white noise of a specified duration, where each burst has an amplitude sufficient to ruin the modem’s data carriage but not to damage the electronics, and where the bursts are sufficiently separated that they are independent events to the modem. To pass, a modem must deliver undamaged payload.