SS7 signaling points: STP, SSP, SCP

Main signaling points in an SS7 network

A signaling point (SP) is any node that can send, receive or forward SS7 messages. Each has a unique point code (14 bits in ITU). There are three main types: SSP, STP and SCP.

   [SCP]         [SCP]       databases
     |A            |A
  [STP]=====C====[STP]       mated pair
    |  \          /  |
    A   \--B/D--/    A
    |                |
  [SSP] ======trunk===== [SSP]   exchanges
   |                       |
 phones                  phones

Roles in the signaling process:

• SSPs start and end call signaling (they are the "users").
• STPs relay messages (they are the "routers").
• SCPs answer queries (they are the "databases").

Signal Switching Point (SSP)

(Standard ITU-T/ANSI name: Service Switching Point; similarly the SCP is the Service Control Point.)

• A telephone exchange (local, tandem or mobile switch) with SS7 software.
• Converts subscriber events (off-hook, digits, answer, on-hook) into ISUP messages (IAM, ACM, ANM, REL, RLC) for trunk set-up and release.
• Detects calls needing special handling (e.g. 1660/800 numbers, prepaid) and sends TCAP queries to an SCP, then routes the call using the reply.
• Is the origin or end point of signaling messages; it does not normally relay messages for others.

Signal Transfer Point (STP)

• A high-capacity packet switch/router for SS7 messages; it does not switch speech.
• Receives messages on one link and forwards them on another using the destination point code (MTP level 3 routing).
• Performs global title translation (SCCP) to find which SCP should get a query (e.g. translate a toll-free number to the SCP point code).
• Provides screening (gateway security), measurement of traffic, and network management such as rerouting around failed links.
• Deployed in mated pairs joined by C-links so that one failure does not cut the network.

Signal Control Point (SCP)

• A database server connected to STPs by A-links.
• Answers TCAP queries from SSPs and gives instructions for the call.
• Supports Intelligent Network services: toll-free and premium-rate number translation, calling card and prepaid validation, number portability, virtual private networks.
• In mobile networks, the HLR/VLR databases play the SCP role through MAP.
• Usually duplicated (mated SCPs) for reliability.

Example: toll-free call

1. The caller dials a toll-free number; the SSP sends a TCAP query.
2. The STP performs global title translation and forwards it to the right SCP.
3. The SCP returns the real routing number.
4. The SSP sends an ISUP IAM through the STP to the far SSP to set up the trunk.