ITU-T Rec. Q.521 (1988)
1 General
2 Exchange functions - Introduction and framework
2.1 General
2.2 Exchange model
3 Utilization of exchange functions for services
3.1 General
4 General functions required for operation of an exchange in the IDN, ISDN or mixed analogue/digital environment
4.1 Timing and synchronization
4.2 Signalling