What is Sub System Number – SSN?
Signalling Connection Control Part (SCCP) sub system numbers are used to identify applications within network entities which use SCCP signalling.
Sub System Number is a number which is assign to every entities and it is used to communicate between own entities.
Entities in the GSM network have a subsystem number (SSN). The SSN is used to address a particular subsystem within an SS7 node. One node in a GSM network may contain, for example, MSC and HLR, or MSC, HLR and gsmSCF. Such a node would have one SPC in the SS7 network, but its internal subsystems would have different SSN.
The SSN may also be used in the STP, when performing GT translation, to derive the SPC of the destination entity. The CAP SSN is not allocated to a node but to a protocol. This SSN is used by any entity that talks CAP, such as gsmSCF, gsmSSF (in MSC, GMSC), gprsSSF (in SGSN), assisting gsmSSF, etc. When MSC/gsmSSF and gsmSCF reside in the same node in the SS7 network, then the SSN of an incoming SCCP message cannot be used to select the entity for which the message is destined. Instead, the node may have to analyse, for example, the application context of the protocol or the operation code. The SSN is used during dialogue establishment only. When MSC/gsmSSF and gsmSCF have established a CAMEL relationship, the two entities have exchanged a TC transaction identifier. The transaction identifier is unique within an SS7 node and relates to a specific (CAMEL) dialogue. The SS7 node allocates a new TC transaction identifier for each TC dialogue it initiates or accepts. When CAMEL was first released (1996), SSN 5 was allocated to CAP. It turned out later that this SSN was already allocated to another entity in the SS7 network. For that reason, the SSN was changed to 146. At the same time, a separate SSN for MAP messaging with the SCP (SSN 147) was introduced.
For Example :
We have assigned below sub system numbers to our nodes to communicate with each other. These subsystem numbers are configured in STP.
Below Table contains some SSNs that are relevant for CAMEL.
| Entity | Protocol | SSN |
| HLR | MAP | 6 |
| VLR | MAP | 7 |
| MSC | MAP | 8 |
| CAP | 146 | |
| SCP | MAP | 147 |
| IM-SSF | MAP | 147 |
| SGSN | MAP | 149 |
| GGSN | MAP | 150 |
GT (Global Title) – is used to connect from our entities to others entities.