G.SHDSL
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'Single-pair high-speed digital subscriber line' (SHDSL) is a telecommunications technology for DSL subscriber lines. It describes a transmission method for signals on copper pair lines, being mostly used in Access networks to connect subscribers to Telephone exchanges or POP Access Points.
G.shdsl was standardized in 02/2001 internationally by ITU-T with recommendation G.991.
It features symmetrical data rates from 192 kBit/s to 2304 kBit/s of payload in 64 kBit/s increments for one pair and 384 kBit/s to 4608 kBit/s in 128kBit/s increments for two pair applications. The reach varies according to the loop rate and noise conditions (more noise or higher rate means decreased reach) and may be up to 3000 meters. The two pair feature may alternatively be used for increased reach applications by keeping the data rate low (halving the data rate per pair will provide similiar speeds to single pair lines while increasing the error/noise tolerance)
The payload may be either 'clear channel' (unstructured), T1 or E1 (full rate or fractional), n x ISDN Basic Rate Access (BRA), Asynchronous Transfer Mode (ATM) or 'dual bearer' mode (i.e. a mixture of two separate streams (e.g. T1 and 'packet based') sharing the payload bandwidth of the G.shdsl loop).
The latest standardisation efforts (G.shdsl.bis) tend to allow for flexibly changing the amount of bandwidth dedicated to each transport unit to provide 'dynamic rate repartitioning' of bandwidth demands douring the uptime of the interface and optionally provides for 'extended data rates' by using a different modulation method (32-TCPAM instead of 16-TCPAM). Also a new payload type is introduced: packet based, e.g. to allow for Ethernet-frames to be transported natively (currently, they may only be framed in ATM or T1/E1/...).
In Europe G.SHDSL was standarized by ETSI using the name 'SDSL'. This ETSI variant is not compatible with the ITU-T G.shdsl standardized regional variant for Europe and must not be confused with the usage of the term 'SDSL' in North America.