he network to be exploited for facilities. The 2/6mf signals may be pulse or continuous (compelled). In 43.1.5.1 30-channel PCM compelled, a continuous acknowledgement backward signal ceases the forward information signal, which cessation ceases the acknow- Time slot 16 is used for signalling for the 30 speech channels in each ledgement. direction. A 16-frame multiframe technique gives 16 (0 to 15) A number of different detail 2/6mf signalling systems exist, but appearances of time slot 16. The 8 bits of time slot 16 of frames 1 are the same in principle. The CCITT R2 system is based on: to 15 of the multiframe are divided into two groups of 4 bits each. Thus, in every 2ms (the multiframe time) 4 out-slot signalling bits 1. Continuous compelled signalling. are available for each speech channel. The coding gives 15 signal Forward signals 120Hz spaced in the band 1380Hz to 1980Hz, possibilities for each speech channel (code pattern 0000 is not used). backward signals 120Hz spaced in the band 1140Hz to 540Hz. This is ample for line signalling requirement. Should inband inter- 3. To increase the signal repertoire if required, both the forward register m.f. signalling apply, the PCM system deals with this as for and backward 2/6 frequency combinations have primary and speech. secondary meanings by a shift condition. The logic behind the approach was that time slot 16 would be the 64kbit/s bearer for common channel signalling in the future inte- grated digital networks. The PCM system would be retained in service, but with signalling change. 43.2 Common channel signalling Stored programme control (s.p.c.) of switching prompted a reap- 43.1.5.2 24-channel PCM praisal of the network signalling technique. Processor control makes possible the concentration of signalling logic for a large number of traffic circuits. With s.p.c. it is inefficient for the proces- This is the N. American system. In the pioneer D1 version, bit 8 of sor, which works in the digital mode, to deal with on-traffic-circuit the octet in each time slot