The SSN (SubSystem Number) is a numeric identifier used in the Signaling Connection Control Part (SCCP) of the SS7 (Signaling System No. 7) protocol suite. SSNs uniquely identify applications or subsystems within a node (or Signaling Point) that uses SCCP.
SCCP allows communication between these applications over an SS7 network. An SSN is used along with Point Code (PC) to route messages to the correct software component or service inside a network node. It’s essentially like an “application port” for the SS7 protocol stack.
Why is SSN important?
In SCCP, you don’t only send a message to a node (via the Point Code), but to a specific application or service on that node. SSN provides this ability — enabling multiple services (like HLR, MSC, MAP, ISUP, etc.) to run on the same node while being distinguishable.
Without SSN, the destination node wouldn’t know which internal service should handle the incoming signaling message.
SSN use in Mobile and Telecom Networks
Different telecom applications are identified by specific SSN values. For example:
- SSN 6 is for HLR, handling subscriber information.
- SSN 8 is for MSC, managing call control and mobility.
- SSN 146 is for CAP, part of CAMEL for IN services like prepaid billing.
SSN Table (SubSystem Numbers)
| SSN Value | SSN SCCP User | Description |
|---|---|---|
| 0 | Unknown | SSN not known or not used. |
| 1 | SCCP management | SCCP internal management functions. |
| 3 | ISUP | ISDN User Part – voice call setup, teardown, etc. |
| 4 | OMAP | Operation, Maintenance and Administration Part |
| 5 | MAP | Mobile Application Part |
| 6 | HLR | Home Location Register (MAP) |
| 7 | VLR | Visitor Location Register (MAP) |
| 8 | MSC | Mobile Switching Center (MAP) |
| 9 | EIR | Equipment Identity Register (MAP) |
| 10 | AUC | Authentication Center |
| 11 | ISDN supplementary | ISDN Supplementary services |
| 13 | BISDNE2EAPP | Broadband ISDN Edge-to-Edge Application |
| 14 | TCTR | TC Test Responder |
| 142 | RANAP | Radio Access Network Application Part |
| 143 | RNSAP | Radio Network Subsystem Application Part |
| 145 | GMLC | Gateway Mobile Location Centre (MAP) |
| 146 | CAP | CAMEL Application Part – for intelligent/prepaid network services |
| 147 | gsmSCF | GSM Service Control Function (MAP) |
| 148 | SIWF | Serving Interworking Function (MAP for GPRS) |
| 149 | SGSN | Serving GPRS Support Node (MAP) |
| 150 | GGSN | Gateway GPRS Support Node (MAP) |
| 249 | PCAP | Positioning Calculation Application Part |
| 250 | BSC BSSAP | BSC Base Station System Application Part (BSSAP-LE) |
| 251 | MSC BSSAP | MSC Base Station System Application Part (BSSAP-LE) |
| 252 | SMLC BSSAP | Serving Mobile Location Center (BSSAP-LE) |
| 253 | BSSOM | Base Station System O&M (A-interface) |
| 254 | BSSAP | Base Station System Application Part (A-interface) |
yREMORA’s Signal Transfer Points (STPs), such as the ySTP and oSTP products, support advanced SCCP-level routing capabilities – including routing based on SubSystem Number (SSN), Global Title (GT), or Translation Type (TT).