rol almost everything happening in the network. The interface between terminals and the network is called user-to- network interface (UNI), and the interface between exchanges is knows as network-to-network interface (NNI). Protocols used in the latter are more complex because more information flows between switches than flows to the terminals. Terminals are only informed about what is essential in order to keep the user updated about the status of a call. This paradigm, consist- ing of an intelligent network and dumb terminals, opposes the IP paradigm in general, and the SIP paradigm in particular. Access Signalling What kind of control information is signalled in analoge subscriber lines? It consists of off/on hook signals, dialed numbers, and various tones that inform the user about call status. Off/on hook signals are used to request dial tone for a call attempt and to accept and terminate calls by picking up the phone and by hanging up respectively. The dialed numbers are trans- mitted from the terminal to the local exchange through pulses or tones. (Old systems employ decadic pulses, incidentally, while newer terminals implement Dual Tone Multi-Frequency [DTMF] tones.) Tones also convey status information to the user plane. Standard tones have been defined (such as busy tone or alerting), but they sometimes vary from country to country. Local Local Figure 1-13 Terminal exchange exchange Terminal Access and trunk signalling. Access Trunk Access signalling signalling signalling Signalling in the Circuit-Switched Network The implementation of digital terminals made it necessary to develop digital access signalling systems. Digital Subscriber Line No. 1 (DSS-1) is the most widespread. It is used in the ISDN and also in mobile networks; signalling between Global System for Mobile (GSM) communications mobile terminals and Base Transceiver Stations (BTS) is based on DSS-1. Figure 1-14 is an example of a DSS-1 call flow. It shows the message exchange between a terminal receiving a call and its local exchange. DSS-1