Single-pair high-speed digital subscriber line (SHDSL) is a form of

symmetric digital subscriber line A symmetric digital subscriber line (SDSL) is a digital subscriber line (DSL) that transmits digital data over the copper wires of the telephone network, where the bandwidth in the downstream direction, from the network to the subscriber, is id ...

(SDSL), a data communications technology for equal transmit and receive (i.e. symmetric) data rate over

copper Copper is a chemical element; it has symbol Cu (from Latin ) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish-orang ...

telephone line A telephone line or telephone circuit (or just line or circuit industrywide) is a single-user circuit on a telephone communication system. It is designed to reproduce speech of a quality that is understandable. It is the physical wire or oth ...

s, faster than a conventional voiceband

modem The Democratic Movement (, ; MoDem ) is a centre to centre-right political party in France, whose main ideological trends are liberalism and Christian democracy, and that is characterised by a strong pro-Europeanist stance. MoDem was establis ...

can provide. As opposed to other DSL technologies, SHDSL employs trellis-coded pulse-amplitude modulation (TC-PAM). As a

baseband In telecommunications and signal processing, baseband is the range of frequencies occupied by a signal that has not been modulated to higher frequencies. Baseband signals typically originate from transducers, converting some other variable into ...

transmission scheme, TC-PAM operates at frequencies that include those used by the analog voice

plain old telephone service Plain old telephone service (POTS), or publicly offered telephone service, is basic Voice band, voice-grade telephone service. Historically, POTS has been delivered by Analog signal, analog signal transmission over copper loops, but the term also d ...

(POTS). As such, a frequency splitter, or

DSL filter A DSL filter (also DSL splitter or microfilter) is an analog low-pass filter installed between analog devices (such as telephones or analog modems) and a plain old telephone service (POTS) line. The DSL filter prevents interference between su ...

, 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 made SHDSL a popular choice by businesses for

private branch exchange A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone system differs from ...

(PBX),

virtual private network Virtual private network (VPN) is a network architecture for virtually extending a private network (i.e. any computer network which is not the public Internet) across one or multiple other networks which are either untrusted (as they are not con ...

(VPN),

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and other data services. SHDSL features symmetrical data rates in both the upstream and downstream directions, from 192 kbit/s to 2,312 kbit/s of payload in 8 kbit/s increments for one pair and 384 kbit/s to 4,624 kbit/s in 16 kbit/s increments for two pairs of wires. The reach varies according to the loop rate and

noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

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. With the 32-TC-PAM modulation scheme described in Annexes F and G, symmetric data rates of up to 5,696 kbit/s are permitted on one pair in the optional extended SHDSL mode. Up to four pairs of wires may be bonded using M-pair bonding to produce data rates as high as M5,696 kbit/s. By using four bonded wire pairs, a single SHDSL interface can transmit up to 22,784 kbit/s. The SHDSL payload may be either 'clear channel' (unstructured), T1 or E1 (full rate or fractional), multiple

ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. ...

Basic Rate Interface (BRI),

Asynchronous Transfer Mode Asynchronous Transfer Mode (ATM) is a telecommunications standard defined by the American National Standards Institute and International Telecommunication Union Telecommunication Standardization Sector (ITU-T, formerly CCITT) for digital trans ...

(ATM) cells or

Ethernet Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

packets. A 'dual bearer' mode allows a mixture of two separate streams (e.g. T1 and ATM) to share the SHDSL bandwidth.

SHDSL standards

The industry standard for SHDSL is defined by

ITU-T The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) is one of the three Sectors (branches) of the International Telecommunication Union (ITU). It is responsible for coordinating Standardization, standards fo ...

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 (M-pair) * Optional extensions to allow user data rates up to 5696 kbit/s per pair, described in Annexes F and G * Optional support for dynamic rate repartitioning, allowing flexible change of the SHDSL data rate without service interruption, described in Annex E.10.3 * New payload definitions including Ethernet packet transfer mode (PTM), described in Annex E.11 SHDSL supersedes the older HDSL symmetric DSL technology defined in ITU-T G.991.1. SHDSL has largely been replaced by VDSL2 because of greater bandwidth, less interference and greater utilization. In

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, a variant of SHDSL was standardized by the

European Telecommunications Standards Institute The European Telecommunications Standards Institute (ETSI) is an independent, not-for-profit, standardization organization operating in the field of information and communications. ETSI supports the development and testing of global technical ...

(ETSI) using the name 'SDSL'. This ETSI variant is compatible with the

SHDSL standardized regional variant for Europe and must not be confused with the usage of the term SDSL in North America to refer to

External links

ITU-T Recommendation G.991.2: Single-pair high-speed digital subscriber line (SHDSL) transceivers
{{DSL technologies Digital subscriber line ITU-T recommendations ITU-T G Series Recommendations Telecommunications-related introductions in 2001