the 193rd position. The resulting pattern of the 12-bit word is which involves indicating such states as idle, busy, ringing, no 100011011100. This entire process repeats every 1.5 milliseconds. ringing, loop open, etc. The BFf bit alternates (1, 0, 1, 0, 1, 0, ...) every other frame. The For data applications the A/B signalling has no relevance. How- receiving multiplexer can identify this unique sequence in the in- ever, when transferring data, the impact of losing the LSB every six coming digital data stream to maintain or re-establish frame boun- frames is not acceptable. There are two ways to get around this daries. Frame 6 is identified by the fact that it occurs when the BFs inherent difficulty. Instead of transmitting data in 8-bit quantities, is a 1 preceded by three BFs which were Os. Frame 12 is identified they are transferred in 7-bit quantities. Therefore, the fact that the by the fact that it occurs when the BFs is 0, preceded by three BFs LSB is robbed does not affect the overall performance because it is which were 1s. The receiver can easily identify this sequence, in the not used. To achieve this, data must be transmitted 7 bits at a time absence of severe line errors or impairments, and thus identify the every 125 usec. In practical terms this involves either using a device desired frames which contain signalling information. that is capable of outputting at 7-bit increments or providing a The 193rd bit signalling described above involves the manage- special timing pulse to enable the data output. ment of the DS1 facility itself. As indicated in Section 21.3.2, each Many communications controller devices can be operated in the individual voice channel requires its own signalling information latter mode but only a few in the former. As only 7 bits are (call set-up, call completion, etc.). The signalling information must transmitted every 125 usec, the data rate is reduced from the be transmitted along with the PCM samples. To achieve this the 64kbit/s available to 56kbit/s. With this method, 12%