face between circuit-switched networks (with their powerful SS7 infrastructure) and the emerging packet-switched networks that need to be capable of handling the more tra- ditional type of voice communications on a more cost-effective basis. As Figure 4.13 shows, there are three prerequisite components in the SS7 network: Service-switching points (SSPs)-SSPs are the switches that originate and terminate calls. They receive signals from the CPE and perform call pro- cessing on behalf of a user. The user, by dialing particular digits, triggers the network to request certain services. For instance, if you preface a num- ber with a toll-free prefix, that toll-free arrangement triggers the local exchange, or SSP, to initiate a database lookup to determine the physical address of that toll-free number (i.e., where it resides in the network). The SSP reaches into the network to find the database that can translate the toll- free number into a physical address in order to then complete the toll-free call. The SSP does this by interacting with the SCP, as discussed shortly. SSPs are typically implemented at local exchanges, access tandem offices, or toll centers that contain the network-signaling protocols. The SSP serves as the source and destination point for the SS7 messages. Service control points (SCPs)-The SCP is the network element that interfaces with the SSP as well as the STP. Most importantly, the SCP is Chapter 4 The PSTN Subscriber Line Figure 4.13 An SS7 network the network element that contains the network configuration and call- completion database; in other words, it contains the service logic to act on the types of calls and features the users are requesting. SCPs are centralized nodes that contain service logic-basically software and databases-for the management of the call. They provide functions such as digit translation, call routing, and verification of credit cards. The SCPs receive traffic from the SSP via the STP and return responses, based on that query, via the STP. Signal transfer points (STPs