Single-pair High-speed Digital Subscriber Line

Single-pair High-speed Digital Subscriber Line

Symmetric High-speed Digital Subscriber Line (SHDSL) is a form of digital subscriber line (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 TC-PAM modulation and frequencies that include those used by analog plain old telephone service (POTS) to provide equal transmit and receive (i.e. symmetric) data rates. As such, a frequency splitter, or DSL filter, 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 (PBX), virtual private network (VPN), web hosting 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 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 Interface (BRI), 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.