---
title: "CAS block diagram; CCS advantages"
subject: "Telecommunications"
chapter: 5
key: "semester_8/telecommunications:cas-vs-ccs-block-diagram"
source: https://www.bctengineer.com/q/semester_8/telecommunications/cas-vs-ccs-block-diagram
site: BCT Engineering
---

# CAS block diagram; CCS advantages

Telecommunications · Chapter 5: Signaling Systems

- Asked in: 2079 Bhadra, 4 marks
- Video: [Sinalling Techniques](https://www.youtube.com/watch?v=G9mQzYg2HfY) — @Ahmad Sir 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/telecommunications/cas-vs-ccs-block-diagram
- Whole chapter: https://www.bctengineer.com/md/semester_8/telecommunications/chapter-5

## Question

Provide the block diagram of channel associated signaling. Describe how common channel signaling achieves faster call set-up and greater trunking efficiency as compared to channel associated signaling.

## Short answer

In common channel signaling, call setup is faster because all digits travel in a single high-speed IAM message over a 64-kbps link, avoiding slow MF tone exchanges. Trunk efficiency is higher because the trunk is only seized after answer; busy calls release immediately, reducing holding time and freeing timeslots for speech.

## Answer

**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).

```text
 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).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is channel associated signaling?
   A. Signals of all circuits sent over one shared link
   B. Signals of each speech circuit sent on that circuit or a channel permanently tied to it
   C. Signals sent only by an STP
   D. Signals sent as packets with point codes
   Answer: B. CAS uses signaling tied to each trunk, e.g. abcd bits in TS16 of E1 or in-band tones, with one signaling unit per trunk. (Key point: CAS definition)

2. Why is call set-up faster with CCS?
   A. Digits are sent one by one as MF tones
   B. All digits and call data go in one message at 64 kbps without register handshakes
   C. A register is used at each exchange
   D. The trunk is tested several times
   Answer: B. CCS messages take milliseconds, while CAS sends digits one by one with handshakes. Set-up falls from about 10-20 s to 1-3 s. (Key point: Why CCS is faster)

3. Why does CCS give greater trunking efficiency?
   A. The trunk is seized only for the conversation, and a busy B returns REL without wasting trunk time
   B. The trunk is seized at the start of set-up
   C. Each trunk has its own signaling unit
   D. TS16 is reserved for signaling in E1
   Answer: A. In CAS the trunk is held during set-up even if B is busy. In CCS holding time is shorter, so fewer trunks are needed for the same traffic. (Key point: Trunking efficiency)

## Related in Telecommunications

- [Common channel signaling and SS7 call set-up](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Accs-ss7-call-setup) — Telecommunications, chapter 5, asked 2×
- [Signaling; CAS vs common channel](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Asignaling-cas-vs-ccs) — Telecommunications, chapter 5
- [SS7 call set-up and release](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Ass7-call-setup-release) — Telecommunications, chapter 5

## Related in other subjects

- [PCM block diagram; T1 signaling rate and frame](https://www.bctengineer.com/question.md?key=semester_6%2Fcommunication_systems%3Apcm-block-t1-rate-frame) — Communication Systems, chapter 5