The Service Switching Point (SSP) is the local exchange in the telephone network. An SSP can be a combination voice switch and SS7 switch, or an adjunct computer connected to the local exchange's voice switch. The SSP provides the functionality of communicating with the voice switch via the use of primitives and creating the packets, or signal units, needed for transmission in the SS7 network (Fig. 2.2).

Figure 2.2

The Service Switching Point (SSP) must convert signaling from the voice switch into SS7 signaling messages, which can then be sent to other exchanges through the SS7 network. The exchange will typically send messages related to its voice circuits to the exchanges with a direct connection to it.

In the case of database access, the SSP will be sending database queries through the SS7 network to computer systems located centrally to the network. This was the first usage of the SS7 network, as the need for 800 number lookup became necessary.

The traffic mix found in most SS7 networks is still primarily circuit-related messages. With the implementation of applications such as Local Number Portability (LNP), the traffic mix is changing significantly, becoming predominately non-circuit-related messages. These messages originate from SSPs and are used to connect voice circuits from one exchange to another exchange. The SSP does not use circuit-related messages exclusively, however.

Before a switch can route a call, it must first be able to access information regarding the destination of the call. For most Plain Old Telephone Service (POTS) systems, the telephone number dialed is sufficient for routing. However, with 800 and 900 numbers, routing is impossible, because the dialed digits do not provide enough information about the destination.

Even POTS may now require a database query. If the switch determines that the called number has been "ported," it will generate a primitive to the SSP requesting the SSP send a query to an LNP database to determine to which exchange the ported number has been reassigned. Porting is the movement of a number from one switch to another (regardless of geography) allowing subscribers to move or change primary telephone companies without changing their telephone numbers.

For this reason, the Service Switching Point (SSP) must access a remote database to learn the routing number assigned to the 800 or 900 number or the new location of a ported number. Once this information has been retrieved, the SSP can then begin circuit connections based on the new routing number information.

The SSP function is to use the information provided by the calling party (such as dialed digits) and determine how to connect the call. A routing table will identify which trunk circuit to use to connect the call, and which exchange this trunk terminates at. An SS7 message must be sent to this adjacent exchange requesting a circuit connection on the specified trunk.

The adjacent exchange grants permission to connect this trunk by sending back an acknowledgement to the originating exchange. Using the called party information in the setup message, the adjacent exchange can determine how to connect the call to its final destination. This may require several trunk connections between several adjacent exchanges. The SSP function manages these connections until the final destination is reached.

Many SSP functions are accomplished by adding a computer adjunct to existing switches. This computer receives signals from the voice switch which are used to trigger the transmission of specific SS7 messages. The called and calling party address must be passed from the voice switch to the SSP for transfer across the network.

Using adjuncts allows telephone companies to upgrade their SS7 signaling points without replacing expensive switches, providing a modular approach to networking. Upgrades are typically limited to software loads, since these computers require very little hardware.

There are very few features required of an SSP. The ability to send messages using the ISDN User Part (ISUP) protocol and the Transaction Capabilities Application Part (TCAP) protocol is the only requirement, other than the network management defined in the Bellcore publications. Specific Bellcore requirements for an SSP can be found in Bellcore publication TR-TSY-000024, Service Switching Points (SSPs) Generic Requirements (this has since been updated, and the new document is known as GR-024-CORE).

Copied with permission,McGraw-Hill Telecommunications from the book Signaling System #7, Second Edition by Travis Russell, 1998 McGraw-Hill Telecommunications, pages 63-65.