rts of the building of a public exchange. The E&M amounts of data end-to-end from user to user. signalling circuits use one lead for each direction of transmission 2. Closed user group facility enables a logical grouping of users and a common earth return which makes them more prone to noise within the public network. Connected party identities can be than a balanced 2-wire system. displayed on the PABX feature phones. Each group is identified The E-lead has an earth potential applied from a contact and the by a number and the network places restrictions upon calls into receiving equipment has a battery connected relay as the receiving and out of the group. Most of the functionality is provided by circuit. The same circuit is used in the opposite direction on the the network and the PABX has to provide the appropriate call M-lead but in reverse, as in Figure 34.6. The impedance between set up and acknowledgement messages. sending and receiving ends is thus limited to about 140 ohms. Calling line identification provides the PABX with the identity In the UK the E&M signalling system is called SSDC5 and is now of the calling customer and apart from displaying the calling only used between equipment in the same building. The relay line number on the operator's position on a PABX, it can also contact is pulsed at 10pps from the PABX control system to send be made use of in ACD applications where the customer's line digits into the private network. It is possible to use the E&M circuit 34/8 PABX and key systems Tie line signalling PABX circuit Signalling directions Figure 34.6 E&M signalling only for line seizure and release signals and use MF inter register In the UK the signalling system is SSAC15 and is similar to CEPT signalling for address signalling. L1 which makes it suitable for international use. The system oper- ates on a forward and backward signalling basis, as in Figure 34.8, 34.4.2 Double current signalling with both directions of signalling separated. Signals are sent as either the application of the 228