.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Signaling with SS7 Key Telephony Concepts SSP converts voice-switch signaling into SS7 messages. Most of the SSP SS7 traffic is related to voice circuits. In addition to voice-circuit signaling, the SSP uses SS7 for database access. This started with toll-free 1-800 number lookups and added 1-900 number lookups, but lo- cal number portability is changing the SS7 traffic mix. Now that landline subscribers can take their telephone numbers with them when they move to a new location, almost every call will require checking the local number portability (LNP) database to deter- mine which network provides services to the called number. Once the routing number that identifies the call's actual destination has been retrieved, then the SSP can begin circuit connections for a call. The entry point to the SS7 network is a signal transfer point (STP). Physically, the STP can be attached to a voice switch, so tandem switches provide voice switching and SS7 STP services using a colocated computer. Standalone STP equipment allows com- panies to centralize their SS7 operations. STPs come in redundant pairs because all SS7 network components must be implemented in redundant pairs. The service control point (SCP) is the SS7 interface to telephone company databases. These databases have routing numbers for toll-free, area code 900, and LNP, as well as credit-card validation data, fraud protection, and Advanced Intelligent Network (AIN) services used for creating subscriber services. The SCP itself may not store the data but merely provides SS7 access to a computer database. The basic flow of SS7 signaling messages from one SSP to another SSP is shown in Fig. 14.1. There are six types of SS7 links (A through F): Access links (A) connect the SSP to the STP. There are at least two for redundancy in case one link or one STP fails. The maximum number of A links to one STP pair is 16 to each STP. Bridge l