Single-pair high-speed digital subscriber line Information & Single-pair high-speed digital subscriber line Links at HealthHaven.com

DSL technologies
	Standard
ADSL	ANSI T1.413 Issue 2 ITU G.992.1 (G.DMT) ITU G.992.2 (G.Lite)
ADSL2	ITU G.992.3/4 ITU G.992.3 Annex J ITU G.992.3 Annex L
ADSL2+	ITU G.992.5 ITU G.992.5 Annex M
HDSL	ITU G.991.1
HDSL2
IDSL
MSDSL
PDSL
RADSL
SDSL
SHDSL	ITU G.991.2
UDSL
VDSL	ITU G.993.1
VDSL2	ITU G.993.2

Single-Pair High-speed Digital Subscriber Line (SHDSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. Compared to ADSL, SHDSL employs frequencies that include those used by traditional POTS telephone services to provide equal transmit and receive (i.e. symmetric) data rates. As such, a frequency splitter, or microfilter, cannot be used to allow a telephone line to be shared by both an SHDSL service and a POTS service at the same time. Support of symmetric data rates has made SHDSL a popular choice by businesses for PBX, VPN, web hosting and other data services.

SHDSL features symmetrical data rates from 192 kbit/s to 2,304 kbit/s of payload in 64 kbit/s increments for one pair and 384 kbit/s to 4,608 kbit/s in 128 kbit/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 3,000 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 similar speeds to single pair lines while increasing the error/noise tolerance.

An optional extended SHDSL mode allows symmetric data rates up to 5696 kbit/s on one pair. Higher data rates may be achieved using two or up to four copper pairs.

The SHDSL payload may be either 'clear channel' (unstructured), T1 or E1 (full rate or fractional), multiple ISDN Basic Rate Access (BRA), Asynchronous Transfer Mode (ATM) cells or Ethernet packets. A 'dual bearer' mode allows a mixture of two separate streams (e.g. T1 and ATM) to share the SHDSL bandwidth.

[edit] SHDSL Standards

The industry standard for SHDSL is defined by ITU-T recommendation G.991.2. This was first published in February 2001. SHDSL equipment is also known by the standard's draft name of G.SHDSL. Major updates to G.991.2 were released in December 2003. Equipment conforming to the 2003 version of G.991.2 is often referred to by the standard's draft name of G.SHDSL.bis or just SHDSL.bis. The updated G.991.2 features:

Optional support for up to four copper pair connections
Optional extensions to allow user data rates up to 5696 kbit/s, described in Appendix F.
Optional support for Dynamic Rate Repartitioning, allowing flexible change of the SHDSL data rate without service interruption, described in Appendix E.10.3.
New payload definitions including Ethernet Packet Transfer Mode (PTM), described in Appendix E.11

SHDSL supersedes the older HDSL symmetric DSL technology defined in ITU-T G.991.1.

In Europe, a variant of SHDSL was standardized by ETSI using the name 'SDSL'. This ETSI variant is compatible with the ITU-T SHDSL standardized regional variant for Europe and must not be confused with the usage of the term 'SDSL' in North America.

[edit] See also

Digital Subscriber Line (DSL)
Asymmetric Digital Subscriber Line (ADSL)

[edit] External links