 send any other signaling information over that trunk; for instance, passing a call-waiting tone would not be feasible. We have moved away from the per-trunk signaling environment to what we use today-common-channel signaling (CCS; see Figure 4.12). You can think of CCS as being a separate subnetwork over which the signaling message flows between intel- ligent networking components that assist in the call completion and in the delivery of the service logic needed to deliver the requested feature. Today, we predomi- nantly use the ITU-T standard SS7 for CCS. SS7 refers to a group of telephony sig- naling protocols used to set up the majority of the world's PSTN telephone calls. Switching Switching Office A Office B Per-trunk Per-trunk Signaling Trunk Signaling Per-trunk Per-trunk Signaling Trunk Signaling Per-trunk Per-trunk Signaling Trunk Signaling Figure 4.11 Per-trunk signaling Chapter 4 The PSTN Switch A Switch B Interoffice Trunk Voice Path Signal Path Note: Voice path and signal path are separate Database Figure 4.12 Common-channe signaling While it is generally referred to as SS7, in North America it is also sometimes called CCS7 for Common Channel Signaling System 7, and in Europe, particularly the United Kingdom, it is referred to as C7 (for CCITT 7). SS7 Architecture SS7 is critical to the functioning and operation of the modern network. With SS7, a packet data network overlays and controls the operation of the underlying voice networks; signaling information is carried on an entirely different path than voice and data traffic. Signaling does not take a great deal of time, SO it is possible to mul- tiplex many signaling messages over one channel-and that's why the signaling system is a packet network. The signaling system takes advantage of the efficien- cies of statistical multiplexing for what is essentially bursty data. The SS7 signaling data link is a full-duplex digital transmission channel that operates at either 56Kbps or 64Kbps, depending on the standards under which the network is oper- ating (