DS1

A {DS level} and {framing specification} for

synchronous digital streams, over circuits in the North

American {digital transmission hierarchy}, at the {T1}

transmission rate of 1544000 bits per second ({baud}).

In the US a DS1 "frame" is composed of 24 eight-bit {bytes}

(commonly from 24 {DS0} sources, originally digitised

voice-grade telephone signals), plus one {framing bit} (193

bits). 8000 bytes per second come from each source, and thus

8000 frames per second are transported by the DS1 signal. The

result is 193▫8000 ◦ 1,544,000 bits per second.

In the original standard, the framing bits continuously

repeated the sequence 110111001000, and such a 12-frame unit

is called a super-frame. In voice telephony, errors are

acceptable (early standards allowed as much as one frame in

six to be missing entirely), so the least significant bit in

two of the 24 streams was used for signaling between network

equipments. This is called {robbed-bit signaling}.

To promote error-free transmission, an alternative called the

extended super-frame (ESF) of 24 frames was developed. In

this standard, six of the 24 framing bits provide a six bit

{cyclic redundancy check} (CRC-6), and six provide the actual

framing. The other 12 form a virtual circuit of 4000 bits per

second for use by the transmission equipment, for {call

progress signals} such as busy, idle and ringing. DS1 signals

using ESF equipment are nearly error-free, because the CRC

detects errors and allows automatic re-routing of connections.

Compare {T-carrier systems}.

[Kenneth Sherman, "Data Communications : a user's guide",

third edition (1990), Reston/Prentice-Hall/Simon & Schuster].

(1996-03-30)