itched network also has an SS7 gateway, which is capable of communicating with the underlying ILEC's SS7 network SO that it can map the appropriate IP SS7 and Next-Generation Networks D Links D Links SS7 Network SS7 Network STP Signal Transfer Point SCP Service Control Point A Links A Links Gateway Class 5 Switch Packet-Switched Network Gateway Gateway Circuit-Switched Network Switch Switch Class 5 Switch Figure 4.16 Next-generation gateway switches addresses associated with the destination telephone number, which is served by a given destination media gateway. The next-generation gateway switch, therefore, provides a means to seamlessly interoperate between two very important and exist- ing infrastructures. Signaling gateways allow both legacy and new equipment to seamlessly operate over high-bandwidth, scalable, and available IP-based core net- works instead of taxing the TDM-based legacy SS7 network. Figure 4.17 is an end-to-end view of a next-generation network. It shows an interconnected environment between the legacy circuit-switched network and the emerging packet-based networks. A subscriber at the customer premises (e.g., a residence, a business site) is connected to the local exchange, known as the end office, by access lines. From there, trunks link to a media gateway switch, which, through SS7 interfaces, can reach into the underlying intelligence within the SS7 network and further add the necessary information to process the call as requested. The call then goes out on a packet basis throughout a series of switches or routers (depending on what the provider is using as the backbone) and reaches a destina- tion media gateway switch that unpackages the voice, undigitizes it, and delivers it to the destination phone. Chapter 4 The PSTN SS7 Network Softswitch Class 5 IP/ATM/MPLS Media Class 5 Networks Media Office Gateway Gateway Office Unified Translation Single- Other Messaging Services Number IP-Based Applications Service Servers Figure 4.17 An end-to-end next-generation network Next-Generation Networ