- 2070 Asar · 5+3 marks
SS7 building blocks and links
Short answer
SS7 building blocks are Service Switching Points (SSPs), Signal Transfer Points (STPs) in mated pairs, Service Control Points (SCPs), and signaling link sets. Links are A (access), B (bridge), C (cross), D (diagonal), E (extended), and F (fully associated), plus link sets between nodes.
Explain the major building blocks or parts used in SS7 network. Show the different links used in SS7 network also.
Major building blocks of an SS7 network
SS7 is a packet-switched signaling network separate from the voice network. Its nodes are called signaling points, each identified by a point code.
- Service Switching Point (SSP): a telephone exchange (local, tandem or MSC) with SS7 software. It originates and terminates ISUP call-control messages, and sends TCAP queries to SCPs when a call needs database information (e.g. toll-free number).
- Signal Transfer Point (STP): a packet switch that receives SS7 messages and routes them to the correct destination by point code or global title translation. STPs are installed in mated pairs so that the failure of one does not stop signaling. They may also screen messages (gateway STP between operators).
- Service Control Point (SCP): a database server that answers TCAP queries: 800/free-phone translation, local number portability, calling-card validation, prepaid, HLR in mobile networks. Also usually deployed in pairs.
- Signaling links and link sets: 64 kb/s bidirectional channels connecting the nodes; parallel links between two nodes form a link set (up to 16 links) and share load.
Links used in SS7
A A
SSP1 ---> STP-a <==C==> STP-b <--- SSP1
|| \ / ||
B/D|| B B ||B/D
|| / \ ||
STP-c <==C==> STP-d ---A--- SSP2
SSP1 -------------F------------------ SSP2
SSP1 ----E---- STP-c (alternate pair)
SCP ----A---- STP-a / STP-b
| Link | Connects | Purpose |
|---|---|---|
| A (Access) | SSP/SCP to its home STP pair | Normal entry to the network |
| B (Bridge) | Two mated STP pairs at the same level | Carry traffic between regions |
| C (Cross) | The two STPs of one mated pair | Used only when a link fails |
| D (Diagonal) | Local STP pair to higher-level (regional) pair | Hierarchical routing |
| E (Extended) | SSP to a remote (non-home) STP pair | Backup route |
| F (Fully associated) | Directly between two SSPs | Heavy traffic between two switches, no STP |
The "quad" arrangement of B/D links between two mated pairs gives each message several alternative paths, giving very high availability.
✓ Answer checked by Claude Opus 5.5 (high effort)
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