The Intelligent Network

In its simplest form, a SSP communicating with a Service Control Point (SCP) to retrieve information about processing a phone call demonstrates an IN. This communication is triggered in different ways, but most often occurs in response to dialing phone numbers that have special significance—such as Service Access Codes (SAC), numbers that have been "ported" by the Local Number Portability (LNP) act, or numbers that have special services subscribed to them, such as the O Called Party Busy feature (described with trigger types in the "IN CS-2/AIN 0.2" section of this chapter). In the existing telephony network, this exchange of IN messages happens millions of times each day and is transparent to the phone user. Figure 11-1 shows a simple IN message exchange between an SSP and an SCP.

The communication between the SSP and the SCP takes place over the SS7 network using the TCAP layer of SS7. As the SSP handles calls, the SCP is queried for information about how to process the call. It does not happen for every call but only for those that require IN services, such as those mentioned previously. While a complete view of the IN architecture includes a number of other nodes with additional functions, these two nodes are at the core of IN processing. We begin with this minimal view to gain an understanding of how the IN model works and why it is needed.