Discusión y conclusiones

July 22, 2005 10:44 CRC-AU1913 AU1913˙Book

72 Fundamentals of DSL Technology

ABSTRACT In the DSL channel, the dominant type of noise is crosstalk from other DSL systems; it typically sets the maximum data rate one may expect. Impulsive noise is also significant, being the cause of most errors in the delivered payload. This chapter discusses crosstalk in detail and practical noise modelling in general.

3.1 Crosstalk

Crosstalk is the leakage into one channel of signal power from another channel. For DSL, this means coupling between pairs in the same cable. The coupling mechanism is a consequence of the cable’s construction. It increases both with cable length and frequency. It is worst between adjacent pairs. For any two pairs in a cable section, the coupling function does not usually change appreciably over time, and it is symmetrical in that the same coupling function is observed between two ends when measured in either direction. Having said that, the coupling functions appear random: any one coupling function shows dramatic nulls in arbitrary positions between peaks that follow a trend, but not particularly closely.

The coupling functions between different pairs of wire-pairs are unrelated. As an example, see Figure 3.1, which shows the measured NEXT coupling function between a pair and its three strongest coupling neighbors, in a sample of 10-pair 0.5 mm cable 149 meters long. All the pair ends were terminated (in 100). The models discussed later are plotted as “trends”

for comparison. The spacing between the dips in each curve is often 525 kHz, suggesting that some of this dynamic behavior is resonance effects between the ends of the cable section.

Indeed, in real lines with bridged taps, the resonance nulls are prominent. However, in Figure 3.1, the nulls don’t line up and are not regular in any of the curves, so resonance is not a full explanation. The complicated and unpredictable forms of the individual coupling functions mean that most practical work uses simplified models of the trends. However, an installed system that uses multiple pairs could theoretically measure the couplings relating

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model of eqn (3.3)

model of eqn (3.21)

FIGURE 3.1

Typical NEXT coupling functions.

Cable Section

Tx

Source pair

Victim pair NEXT

coupling Rx

FIGURE 3.2 NEXT coupling.

to its own pairs and equalize out coupling from itself. Such systems are being proposed [Ginis 2003].

Crosstalk coupling follows different trends, depending on network configuration: it all has the same physical cause, but different configurations produce different mixes of the crosstalk leakage and cable attenuation.

Near-end crosstalk (NEXT) is the coupling between transmitters and receivers at the same end of a cable, as shown in Figure 3.2. NEXT typically imposes the limit to DSL system performance when the co-located transmitters and receivers use the same band-width. Considering crosstalk between like systems provides the “self-NEXT” bounds on symmetric technologies (for example, ISDN, HDSL, and SHDSL). Between unlike systems, the consideration of who disturbs whom is the subject of spectrum management.

Far-end crosstalk (FEXT) is the coupling between transmitters and receivers at the op-posite ends of a cable, as shown in Figure 3.3. The self-FEXT bound shows higher capacity than the self-NEXT bound, and it is important to note that capacity diminishes more slowly with distance. However, FEXT is negligible¹when NEXT is also present,²and so the higher capacity is only available when NEXT is managed away. An example is ADSL deployed from the exchange. Considering the part of the downstream band not shared with the up-stream band, crosstalk from all technologies must be controlled for ADSL to have its best performance. Where the transmitters are not co-located, for example in VDSL’s upstream channels, there are extra complications [Kirkby 1995].

NEXT and FEXT are exhibited by cables with two ends. A real cable network can have a more complicated structure, with branches and with access points along its length.

This gives rise to some extra variants of crosstalk which are of importance in special cases:

secondary NEXT and third-circuit crosstalk.

Secondary NEXT is NEXT where the noise sources are separated from the receiver under study, joining the cable some distance away and then transmitting away from the receiver, as illustrated in Figure 3.4. The coupling function is the simple combination of ordinary NEXT and attenuation. Ordinary NEXT would be more powerful than this, so secondary NEXT is only significant when circumstances force the spatial separation of the interfer-ing transmitters from the victim receiver. One example is where the victim is the ADSL downstream channel, and the interferer is a symmetrical system. ADSL typically has a longer reach³than the high-rate symmetrical systems, making secondary NEXT significant in standardization work. Also, some administrations limit the deployment of SDSL in order to protect ADSL, making secondary NEXT significant in spectral management work.

1For a qualitative comparison see Section 3.6.

2NEXT coupling is always present. It is common usage to use “crosstalk” for both the coupling and the power coupled.

3Reach here means viable deployment distance.

July 22, 2005 10:44 CRC-AU1913 AU1913˙Book

74 Fundamentals of DSL Technology

Cable Section

Tx

Rx Source pair

Victim pair

FEXT coupling

FIGURE 3.3 FEXT coupling.

Tx

Rx

Source pair

Victim pair

FIGURE 3.4

Secondary NEXT coupling.

Tx Rx

FIGURE 3.5 3CXT coupling.

Third-circuit crosstalk (3CXT) is crosstalk where the transmitter and receiver are co-located at an interior point of the cable network, their signals are flowing in the same direction, and there are other lines in the cable that are unbroken at this connection point.

Figure 3.5 illustrates the mechanism that results in 3CXT. This configuration is typical for in-terference between repeaters of asymmetric systems.⁴The reader is cautioned that although the diagram may suggest that 3CXT is NEXT squared, real 3CXT is more powerful than this.⁵