COMMON-CHANNEL SIGNALING In channel-associated signaling (CAS) systems, the signaling information for a trunk is carried by the trunk itself. In common-channel signaling (CCS), a common signaling link (SL) carries signaling messages for a number of trunks. Just as multi-frequency (MF) signaling became feasible with the introduction of the second-generation (common-control) switching systems, CCS was developed for the third-generation (stored program controlled, SPC) exchanges that were introduce