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SHDSL [Symmetric High-bitrate Digital Subscriber Loop] is an ITU-T [International Telecommunications Union - Telecommunications] standardized multi-rate symmetric DSL.

SHDSL is designed to transport rate-adaptive 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 (or 4-wire mode copper bonded circuit). 4-wire circuits are those traditionally provisioned by HDSL, SDSL, T1, E1, and services beyond E1. SHDSL adheres to the following recommendations: ITU G.991.2 G.SHDSL, ETSI TS 101-524 SDSL and the ANSI T1E1.4/2001-174 G.SHDSL. One of the key benefits of SHDSL is “reach” distance capabilities. Symmetric rates of 2.3 Mbps can be achieved out to >10kft (3 km) scaling down to 192 kbps at > 20 kft (6 km) on a single AWG26 [Average Wire Gauge] cable pair. The optional 4-wire mode can be used to extend the reach for specific target rates such as 2.3 Mbps at >16kft (5 km). The ITU standard supports up to eight repeaters per pair allowing for extremely long distance (CO [Central Office] to customer) reach scenarios. SHDSL uses TC-PAM [Trellis Coded Pulse Amplitude Modulation] (shown next) for transmission. Applications can be split on separate pairs or shared “big pipe” mode.

TC-PAM [Trellis Code Pulse Amplitude Modulation] (often associated with modem transmission) is to form a trellis (used in a grapevine), grid, plane or constellation. Then partition the constellation into subsets such as 64 subsets shown here set aside in Euclidean space (points on a plane can be located with coordinates). Then determine the number of bits based on the number of signal levels such as 5 to determine the bit rate. SHDSL is a Physical Layer 1 protocol. While ATM [Asynchronous Transfer Mode] and Ethernet are the two principal Datalink Layer 2 protocols, Frame Relay and IP could also be provisioned. User applications can be split on separate pairs (4-wire mode) or shared “big pipe” mode. There are discussions related to bonding up to 32 or more pairs. In addition, there are discussions related to “packet bonding” of ethernet packets. However, this may not be necessary as GigE [Gigabit Ethernet] emerges or allow for gigabit data rates over old plant.

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[tags]dsl,shdsl,superwan,itu-t,tc-pam[/tags]