The electrical energy transmitted across the copper wire-line as a modulated signal also radiates energy onto adjacent copper wire loops that are located in the same cable bundle. This cross coupling of electromagnetic energy is called crosstalk.

In the telephone network, multiple insulated copper pairs are bundled together into a cable called cable binder. Adjacent systems within a cable binder that transmit or receive information in the same range of frequencies can create significant crosstalk interference. This is because crosstalk-induced signals combine with the signals that were originally intended for transmission over the copper wire loop. The result is a waveform shaped differently than the one originally transmitted.

Crosstalk can be categorized in one of two forms. Near End Crosstalk, commonly referred to as NEXT, is the most significant because the high-energy signal from an adjacent system can induce relatively significant crosstalk into the primary signal. The other form is Far End Crosstalk or FEXT, which is typically less of an issue because the Far End interfering signal is attenuated as it traverses the loop.

Crosstalk is a dominant factor in the performance of many systems. As a result, DSL system performance is often stated relative to the presence of other systems, which may introduce crosstalk. For example, the loop reach of a DSL system may be stated as being in the presence of 49 ISDN disturbers or 24 HDSL disturbers. As you can imagine, it is rather unlikely that you will deploy a DSL service in a 50-pair cable that happens to have 49 (two-wire) ISDN circuits or 24 (four-wire) HDSL circuits concurrently running in the same bundle. Therefore, these performance parameters typically represent a conservative performance outlook.

Transmitting and receiving information using the same frequency spectrum creates interference within the single loop system itself. This interference differs from Crosstalk because the offending transmit waveform is known to the receiver and can effectively be subtracted from the attenuated receive signals. Eliminating the effects of the transmitter is referred to as echo cancellation.

Minimizing Crosstalk

If the effects of the attenuation and Crosstalk are not too significant, the DSL system can accurately reconstruct the signal back into a digital format. However, when the effect of these phenomena becomes too significant, the signals are misinterpreted at the far end and bit errors occur.

Some DSL systems use different frequency spectra for the transmit and receive signals. This frequency-separated implementation is referred to as Frequency Division Multiplexing (FDM). The advantage of FDM-based systems over echo-canceled systems is that NEXT is eliminated. This is because the system is not receiving in the same range of frequencies in which the adjacent system is transmitting. FEXT is present, and the FEXT signal is substantially attenuated and less of an interferer because the origin of the FEXT signal is at the distant end of the loop. Therefore, FDM-based systems often provide better performance than echo-canceled systems, in terms of crosstalk from similar adjacent systems.

One interesting phenomenon that should be considered is that echo-canceled systems of a like type, introduce what is called Self Next. Self Next introduces significant interference to other like-type echo-canceled systems in the same cable binder. As a result, the deployment of multiple like-type echo-canceled systems will degrade the performance of all other like-type systems within the cable binder. For example, a single CAP or 2B1Q-based T1 HDSL system may achieve the targeted 12 kft (kilofeet) loop reach. However, as additional CAP or eB1Q-based systems are added to the cable bundle, the loop reach of the first system and the subsequent systems may be reduced to 9 kft or less. This same phenomenon is true of nearly all echo-canceled systems, such as 2B1Q in general, echo-canceled CAP HDSL and SDSL, and echo-canceled DMT ADSL systems. Therefore, when selecting a DSL technology, service providers should examine the system performance in the presence of Self Next, which is certain to exist as more services are deployed.

The engineering compromise of FDM systems is that the separated upstream and downstream signals occupy a greater range of frequencies than echo-canceled systems, which overlap the transmit and receive signals resulting in less reach. In some cases, attenuation becomes the most significant factor in performance. In other cases, Crosstalk is the most significant factor in performance. Therefore, the optimal implementation varies as a function of the environment. In deployments where Crosstalking systems are expected to be limited and NEXT is moderate to low, an echo-canceled system may perform better. In other cases, where deployments of Crosstalking systems are expected to be significant and NEXT is likely to be more dominant, an FDM system may perform better.

About the only sure way to manage the issues of Crosstalk, is to first research the services that are deployed within a given cable bundle and avoid those services that will provide substantial Crosstalk. One example of this is the traditional T1 or E1 services. The spectral placement of T1 AMI and similarly the E1 HDB3-based services provides extensive Crosstalk to almost all DSL-based services. As a result, most service providers follow design rules that do not allow the use of T1 or E1 services in the same cable bundles with DSL-based services. You should expect reductions in loop reach in scenarios where T1 or E1 is provisioned in the same cable bundle as DSL-based services.

The DSL Series is brought to you in association with Paradyne Corp.

Next month: Basics—The Varieties of DSL