- 2075 Chaitra · 3+5 marks
In-channel vs CCS; SS7 architecture
Short answer
In-channel signaling shares the speech path, is slow, limited and fraud-prone; common channel signaling uses a separate data link, is fast, flexible and supports database access. SS7 architecture consists of SSPs, mated STP pairs, SCPs and A-F links. The protocol stack has MTP levels 1-3, SCCP, TCAP, ISUP/TUP and MAP/INAP/OMAP.
Differentiate between in-channel signaling and common channel signaling. Also explain the Architecture of Common channel signaling system 7 with proper block diagram and Protocol stack.
In-channel vs common channel signaling
| Point | In-channel signaling | Common channel signaling (CCS) |
|---|---|---|
| Path | Same channel as the speech | Separate dedicated data link |
| Examples | DC loop, 2600 Hz SF, MF R1/R2 | CCITT No. 6, SS7 |
| Speed | Slow (seconds) | Fast (well under 1 s) |
| Signal set | Small, fixed | Large, easily extended |
| Signaling during call | Limited | Possible at any time |
| Fraud risk | Tones can be imitated | Not reachable by user |
| Trunk use | Trunk held during set-up | Trunk seized only when needed |
| Database access | Not possible | Possible (TCAP/SCP) |
SS7 network architecture
SS7 is a separate packet network that carries signaling for the voice network.
mated pair mated pair
SSP-A --A-- STP1 ---B--- STP3 --A-- SSP-B
\ | \ / | /
\ C B B C /
\--A-- | / \ | --A--/
\ STP2 ---B--- STP4 /
|
SCP (A link to STP pair)
SSP-A ==========F (direct)========= SSP-B
- SSP (Service Switching Point): telephone exchange that originates/terminates SS7 messages.
- STP (Signal Transfer Point): packet switch that routes SS7 messages; deployed in mated pairs for reliability.
- SCP (Service Control Point): database for 800/free-phone, number portability, HLR, prepaid.
- Links: A (SSP–STP), B (STP pair to STP pair), C (between mated STPs), D (local to regional STP), E (SSP to alternate STP), F (direct SSP–SSP).
SS7 protocol stack
OSI layer SS7 stack
+-----------+ +----------------+------+-----+
| 7 Applic. | | MAP/INAP/OMAP | | |
+-----------+ +----------------+ | |
| 4 - 6 | | TCAP | ISUP | TUP |
+-----------+ +----------------+ | |
| 3 Network | | SCCP | | |
| | +----------------+------+-----+
| | | MTP Level 3 (network) |
+-----------+ +-----------------------------+
| 2 Link | | MTP Level 2 (link) |
+-----------+ +-----------------------------+
| 1 Physical| | MTP Level 1 (data link) |
+-----------+ +-----------------------------+
- MTP-1: 64 kb/s physical signaling data link.
- MTP-2: reliable link — flags, CRC, sequence numbers, retransmission; units MSU, LSSU, FISU.
- MTP-3: message routing by point codes and signaling network management.
- SCCP: global title translation, connectionless and connection-oriented classes.
- TCAP: dialogues with databases (query/response).
- ISUP/TUP: circuit-related call control (IAM, ACM, ANM, REL, RLC).
- MAP/INAP/OMAP: mobile, intelligent network and maintenance applications.
✓ Answer checked by Claude Opus 5.5 (high effort)
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