History of inter-exchange signalling

SS6 = CCIS (common channel interoffice signaling) was widely deployed in North America, but not in Europe (=> concentrating on SS7 instead).

Starting from 1980 (mainly in Europe), CAS was being replaced by SS7. The use of stored program control (SPC) exchanges made this possible. Like CCIS, signalling messages are transmitted over separate signalling channels. Unlike CCIS, SS7 technology is based on protocol stacks.

Before 1970, only channel-associated signalling (CAS) was used. In CAS systems, signalling always occurs in-band (i.e. over voice channels).

CAS

CCIS

SS7