alling Signalling path Voice path Chapter 1 connections. The capability to signal between dedicated network nodes that do not handle voice is in fact the basis of all the advanced services provided by the current PSTN. By implementing service logic and switching logic separately, we encourage service creation and user plane manipulation to evolve independently. For instance, when someone dials a toll-free number, the system must be able to look up the phone number dialed by the user (it'll be a 1-800 num- ber in the United States) and translate it into the (geographically deter- mined) number of the company the user wants to reach. This database look up is performed entirely through signalling messages (see Figure 1-17). Toll-free and other services can be implemented thanks to the separation between the signalling and the voice path. Otherwise, it would be necessary to route the voice path towards the database and then back from it in order to perform the number look-up, leading to a severe decrease in voice qual- ity experienced by the caller. Today, two main CCS systems are in use: SS6 and SS7. SS6 was stan- dardized in 1968 and was meant to be used in international connections. SS7 was standardized in 1980 and remains the dominant signalling system at present. Database Figure 1-17 (just signalling) User calling a toll free number. 1-800-55-5555 1-212-555-5544 Voice and Voice and signalling signalling Voice and signalling Signalling path Voice path Signalling in the Circuit-Switched Network SS7 provides both circuit-related and non-circuit-related signalling. Circuit-related signalling is, as its name suggests, associated with the establishment of a voice channel. But frequently no voice channel has to be established-as when the system consults a routing table-and this kind of query can be handled by non-circuit-related signalling. As an enabler, non- circuit signalling was a vital step toward mobility. Let's first look at circuit related signalling. ISDN User Part (ISUP) and Telephone User Part (TUP) are the 