 is divided into a speech channel (300Hz to overload the transmission system. 3400Hz) and a signalling channel by filtration. One frequency only 3. Semi-continuous, tone-off idle and off during speech (not is used, located approximately midway between two adjacent 4kHz preferred). channels. The forward and backward signalling paths are separate 4. Pulse. on the 4-wire circuit, the duplex signalling being achieved by the two simplex signalling paths. Signalling frequency 3700, 3825, or With the 2-state continuous signalling mode in each direction, 3850Hz is typical; the CCITT recommends 3825Hz. E-lead and outband signalling enables the basic loop d.c. signalling condition M-lead control applies for outband signalling. to be simulated far more easily compared with inband v.f. signall- Outgoing Incoming gates Forward Directional signals high pass 2/6 m.f. (2-wire) filter (2-wire) As transmit channel f Transmit Receive Receive Transmit Backward signals stage Directional (2 wire) 2/6 m.f. lowpass filter f 9 + (2 - wire) As receive channel f f 11 + Figure 43.5 General arrangement of interregister i.f. signalling system Common channel signalling 43/7 ing, making for much simpler signalling terminals. For the general The address signalling may be decadic d.c. pulsing, but is more application, tone-on idle (off during speech) is preferred. usually 2-out-of-6 multifrequency (2/6mf), giving potential for 15 signals in each direction (Figure 43.5). The signalling is non-de- 43.1.5 PCM signalling cadic, the 2/6mf combination giving the address digit value. This contributes to fast signalling. On 2-wire lines, the two simplex signalling paths are obtained by filtered bandwidth separation. Applied when a PCM system is applied point-to-point in an anal- ogue network. The CCITT specifies for two PCM systems: 30-chan- Subscriber and system facilities often require network signalling. nel (32 time slots) and 24-channel (24 time slots). The increased signal repertoire of 2/6mf compared with decadic interregister signalling enables t