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].