6.4 HDSL4 | DSL Advances

An example of a system that uses the two wire pair option of SHDSL technology is HDSL4.

HDSL4 may seem to be an odd technology to develop following the success of HDSL2 because HDSL4 uses four wires for DS1 transmission while the primary goal of HDSL2 was to reduce cost by reducing the number of wires from four to two. Each technology has its place. HDSL2 is the technology of choice for DS1 (1.544 Mb/s) transmission for lines within the carrier serving area design rules (CSA, up to 9 kft of 26 AWG wire). HDSL2's use of only two wires reduces copper -line costs and also reduces the cost of the transceiver because a two-wire system requires one-half as many line interfaces. However, extending HDSL2 beyond CSA length lines is not attractive due to the high cost of midspan repeaters and spectral incompatibility with other systems.

HDSL4's role is to serve lines beyond HDSL2's reach. The use of four wires permits the transmitted signal PSD to avoid the higher frequencies used by HDSL2. This enables nonrepeatered HDSL4 to reach 11 kft of 26 AWG, whereas HDSL2 can reach only 9 kft. Also, the lower frequency characteristic of HDSL4 enables HDSL4 to remain spectrally compatible with other systems in the same cable even when HDSL4 is repeatered to virtually any distance. HDSL4 makes the trade-off of using an additional pair of wires to reduce the number of required repeaters, and provided that spare pairs are available, this is generally a very cost-effective trade-off. The combination of HDSL2 for shorter lines and HDSL4 for longer lines makes T1 and HDSL technology obsolete. For example, a 20 kft all 26 AWG loop would require two repeaters using first-generation HDSL technology, but HDSL4 would require only one repeater and the HDSL4 would have much better spectral compatibility.

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