is still no voice transfer, they do not interfere with the user plane. Exchange Exchange Figure 1-15 Channels in a PCM link. Synchronization channel 0 Voice channel 1 Voice channel 2 [...] Voice channel 14 Voice channel 15 SIGNALLING CHANNEL 16 Voice channel 17 Voice channel 18 [...] Voice channel 30 Voice channel 31 Signalling in the Circuit-Switched Network Common Channel Signalling (CCS) In CCS, voice and signalling, a rule, traverse different paths through the network (although their paths are still related to a certain extent). All the nodes handling media also han- dle signalling, so that, if a voice path traverses two exchanges, these two exchanges will also receive the associated signalling. How signalling is routed between them, however, is different in CCS than other systems. After leaving one exchange in the voice path, signalling can traverse a set of intermediate nodes, none of which handle voice before reaching the next one. We call these intermediate nodes Signalling Transfer Points (STPs). STPs are a special feature of CSS, necessitating the implementation of a dedicated network optimized for signalling transport, and thus capable of better performance and reliability than shared voice/signal networks. Dedicated Signalling Network Signalling traffic associated with a call is bursty in nature. It is very intense in certain phases of the call, like call establishment or call release, but otherwise quite low once the call is estab- lished. Bursty is good in that it enables a single signalling channel to con- trol several voice channels. In practice, one signalling channel can handle several thousands of voice time slots. Implementing a signalling network independent from its voice counter- part enables service creation and makes interworking different networks easier. SS7 is the most widespread CCS system at present, and it is SS7 in particular that allows the creation of services that go beyond basic CAS Figure 1-16 Signalling Transfer Points. Voice and Voice and signalling signalling Voice and sign