- 2080 Chaitra · 4+4 marks
In-channel vs CCS; SS7 link types
Short answer
In-channel signaling uses the same channel as speech (tones, pulses, CAS bits), giving slow setup and limited capacity. Common channel signaling (SS7) uses a separate shared data link with labelled messages, giving fast setup, high capacity and database services. SS7 link types: A access, B bridge, C cross, D diagonal, E extended, F fully associated.
Compare and contrast between in-channel and common channel signaling. Explain different types of signalling links available in SS7 network.
In-channel signaling vs common channel signaling
In-channel signaling sends the signals of a call over the same channel (or a channel tied to it) as the speech, e.g. DC loop pulses, in-band tones (SF 2600 Hz, MF), or out-of-band/CAS bits in TS16 of E1. Common channel signaling (CCS) sends signals for many circuits as messages over a separate signaling link shared by all of them (SS7).
| Point | In-channel signaling | Common channel signaling |
|---|---|---|
| Path of signals | Same channel as speech (or tied to it) | Separate shared data link |
| Signal form | Tones, pulses, bits | Labelled digital messages |
| Speed of call set-up | Slow (seconds) | Fast (≈ 1 s or less) |
| Signals during speech | Limited (may disturb speech) | Any time, no interference |
| Information capacity | Few signals | Large (caller ID, services) |
| Trunk holding time | Trunk held during set-up | Trunk seized only when needed |
| Fraud risk | High (tones can be imitated) | Low |
| Database / IN services | Not possible | Possible (TCAP, SCP) |
| Equipment | Signaling unit per trunk | Shared signaling terminals |
| Reliability | Failure affects one circuit | Link failure affects many; links duplicated |
Types of signaling links in SS7
SS7 links connect signaling points (SSP, STP, SCP). They are named by their role:
SSP1 SSP2
| \ A A / |
| STPa----STPb | C = mated pair link
F | | \ D / | |
| | STPc | | B/D = between pairs
SSP3 -- E --> STPd
- A-link (Access): connects an SSP or SCP to its home STP pair. Each SSP has A-links to both STPs of the pair for redundancy.
- B-link (Bridge): connects two mated STP pairs at the same level (peer pairs); usually a quad of links.
- C-link (Cross): connects the two STPs of a mated pair; used only when other links fail (for recovery).
- D-link (Diagonal): connects STP pairs at different hierarchy levels (e.g. local STP to regional/gateway STP).
- E-link (Extended): connects an SSP to a remote STP pair other than its home pair; an alternative route for reliability.
- F-link (Fully associated): connects two SSPs directly, without STPs, when there is heavy traffic between them (associated mode).
Links between the same two points form a link set (up to 16 links); MTP level 3 shares load among links and reroutes traffic on failure.
✓ Answer checked by Claude Opus 5.5 (high effort)
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