DS1
DS1
(communications)DS1 is commonly used to multiplex 24 DS0 channels. Each DS0channel, originally a digitised voice-grade telephone signal,carries 8000 bytes per second (64,000 bits per second). A DS1frame includes one byte from each of the 24 DS0 channels andadds one framing bit, making a total of 193 bits per frameat 8000 frames per second. The result is 193*8000 = 1,544,000bits per second.
In the original standard, the successive framing bitscontinuously repeated the 12-bit sequence 110111001000, andsuch a 12-frame unit is called a super-frame. In voicetelephony, errors are acceptable (early standards allowed asmuch as one frame in six to be missing entirely), so the leastsignificant bit in two of the 24 streams was used forsignaling between network equipments. This is calledrobbed-bit signaling.
To promote error-free transmission, an alternative called theextended super-frame (ESF) of 24 frames was developed. Inthis standard, six of the 24 framing bits provide a six bitcyclic redundancy check (CRC-6), and six provide the actualframing. The other 12 form a virtual circuit of 4000 bits persecond for use by the transmission equipment, for call progress signals such as busy, idle and ringing. DS1 signalsusing ESF equipment are nearly error-free, because the CRCdetects 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].