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 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
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)
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