 the 8kbit/s that have long been used to manage the DS1 channels and combining it into a 24th channel. Often referred to as facilities. 2000 bits are used for framing (6 bits distributed over 24 the "23B + D" format, this framing and signalling scheme allows 23 framing bits and there are 333 such 24-bit words per second). 2000 T1 channels to carry up to 64kbit/s of voice, data, video, or other bits are used for error detection and performance determination (6 information, while the D channel transmits all of the control data, bits distributed over 24 framing bits). Finally, 4000 bits are used for including sophisticated signalling techniques described in CCITT telemetry and facility management and/or reconfiguration (12 bits Recommendation I.431/Z.921/Q.931. (The "B" in this designation distributed over 24 framing bits). stands for bearer; the "D" stands for delta, i.e. the channel that transmits change information). 21.4.3 Clear channels for data applications As already mentioned, users of T1 facilities are restricted from 21.5 The CEPT PCM-30 framing format accessing the full 64kbit/s bandwidth of the PCM channel on any one circuit of the DS1, whether for voice or data applications, The CEPT PCM-30 is a PCM format used for time division multi- because the eighth bit of each word is unavailable. In voice trans- plexing of 30 voice or data circuits onto a single twisted pair cable mission, the eighth bit has been used for in-band signalling (remem- using digital repeaters. As already mentioned, the delay with which ber that only the sixth and twelfth frames are really affected). In CEPT has undertaken the definition of a primary digital PCM speech, this generally does not cause any problem, but the same system has allowed it to profit from the experience of the American condition cannot be guaranteed for data. The implication has been systems. the deployment of 56kbit/s facilities and a loss of 8kbit/s of usable The basic characteristics of this primary PCM multiplex equip- bandwidth. The maximum data rate in a D