e most recent implementation of a concept called common channel in- teroffice signaling (CCIS). Previous chapters described how signaling for a call could be carried on the pair or pairs of wires that transported the call. We also briefly touched on the concept of Inte- grated Services Digital Network (ISDN), in which all signaling for a group of calls could be carried in a separate channel on the same DS-1, a technique referred to as associ- ated signaling. More generally, associated signaling refers to the use of a signaling path that parallels the trunks between two switches and which signals for those trunks. CCIS offers a more powerful approach. Rather than using signaling paths that par- alleled the trunks, CCIS sets up a separate data network for signaling. The SS7 net- work makes use of the 64-kbps DS-0 for the physical layer connections between net- work elements. These connections are called signaling links. The SS7 network does not replace the trunks that are used to carry voice and data traffic between switches. It is a separate data network established for the purpose of signaling for those trunks. The architecture of the SS7 network provides for high reliability through the use of redun- dant signaling links and redundant nodes in the signaling network. 14.2 SS7 Network Architecture The SS7 network is designed to be highly physically redundant for reliability. It is maintained as a physical network separate from the facilities and switches that carry voice and data traffic. The two networks touch only at the switches themselves, as re- quired to communicate the signaling functions for which SS7 was designed from the SS7 signaling network to the switches it controls. The local exchange office at the telephone company is a service switching point (SSP) for SS7. The SSP is a separate logical entity connected to the local telephone switch, but the entire local exchange office is sometimes referred to as the SSP. The Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary