CAS block diagram; CCS advantages

Channel associated signaling (CAS) sends the signals of each speech circuit on that circuit or on a channel permanently tied to it (e.g. the abcd bits of each channel in TS16 of an E1 multiframe, or in-band MF tones).

 Exchange A                         Exchange B
 +--------+  +------+  trunk 1  +------+  +--------+
 |        |--|Sig.  |===========|Sig.  |--|        |
 | Switch |  |unit 1| speech+sig|unit 1|  | Switch |
 |        |--|Sig.  |===========|Sig.  |--|        |
 |        |  |unit 2|  trunk 2  |unit 2|  |        |
 +--------+  +------+           +------+  +--------+
     ^  one signaling unit per trunk  ^
     +--------- control ---------------+

How CCS gives faster call set-up

• High-speed messages: in CCS (SS7) all digits and call data go in one IAM message over a 64 kbps link, taking a few milliseconds. In CAS, digits are sent one by one as MF tones or pulses, with handshakes at each exchange, taking several seconds.
• No register handshakes: CAS needs a register at each exchange to receive and resend digits; CCS passes messages directly between processors.
• Overlap of tasks: the next exchange can start processing as soon as the message arrives.
• Typical set-up time falls from about 10–20 s (CAS, multi-link calls) to 1–3 s (CCS).

How CCS gives greater trunking efficiency

• In CAS, the speech trunk is seized at the start of set-up and held while digits are sent and the called line is tested, even if B turns out to be busy or does not answer.
• In CCS, signaling is separate, so the trunk is used only for the conversation; for a busy called party, a REL comes back immediately and no trunk time is wasted.
• Shorter holding time per call means fewer trunks are needed for the same traffic (in erlangs) and grade of service.
• No per-trunk signaling units are needed, and on E1 systems that do not carry the signaling link, TS16 is free for speech (31 channels instead of 30).