Symmetric Digital Subscriber Line (SDSL)

Symmetric Digital Subscriber Line (SDSL) is a type of Digital Subscriber Line (DSL) technology in which upstream and downstream speeds are equal.

Symmetric Digital Subscriber Line (SDSL) technology can provide subscribers with permanent, high-speed data connections for Internet access and other uses much more cheaply than T1 lines can.

SDSL - Symmetric Digital Subscriber Line
Symmetric Digital Subscriber Line

SDSL is sometimes interpreted to stand for “Single-line DSL” instead of “Symmetric DSL” because it uses a single twisted-pair copper wire.

How It Works

SDSL is based on the same DSL technology that is used in High-bit-rate Digital Subscriber Line (HDSL) and Asymmetric Digital Subscriber Line (ADSL) implementations of DSL. SDSL can deliver data at speeds of up to 2 Mbps, which is comparable to speeds achieved by HDSL circuits, although typical SDSL speeds are equal to those of a T1 line (1.544 Mbps). While HDSL needs two pairs of copper wires (four wires), SDSL requires only one pair (two wires).

And unlike ADSL technology, where downstream speed greatly exceeds upstream speed, SDSL transmission speeds are the same in both directions. However, this is true only if the length of the local loop connection does not exceed 3000 meters and the quality of the phone circuit is good. Also, while ADSL allows subscribers to connect both voice (phone) and data (computers) at their customer premises, SDSL allows only a data connection over a given pair of wires.


SDSL uses the same line coding algorithm (2B1Q) that is used in HDSL and in Integrated Services Digital Network (ISDN) circuits.