<communications> 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 1,544,000 bits per second (baud).
DS1 is commonly used to multiplex 24 DS0 channels. Each DS0 channel, originally
a digitised voice-grade telephone signal, carries 8000 bytes per second (64,000
bits per second). A DS1 frame includes one byte from each of the 24 DS0 channels
and adds one framing bit, making a total of 193 bits per frame at 8000 frames
per second. The result is 193*8000 = 1,544,000 bits per second.
In the original standard, the successive framing bits continuously repeated the
12-bit 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
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].
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