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1.1 TELECOMMUNICATION NETWORKS 3Figure 1.1-2. Connections involving subscriber S p .SL p , a temporary path across exchange A, trunk T 1 , a temporary path acrossexchange B, and SL r . The connections of Fig. 1.1-2 are set up (switched “on”) atthe start of a call and released (switched “off”) when the call ends.Setup and Release. The setup and release of connections in telecommunicationnetworks are triggered by signals. Starting and ending a call involve signalingbetween the subscribers and their local exchanges and, for interexchange calls,signaling between the exchanges along the connection.Figure 1.1-3 shows the signaling for the setup of the connection of Fig 1.1-2(b).Subscriber S p sends a request-for-service signal to exchange A (by lifting thehandset of a telephone) and then signals the digits of the telephone number of S r(with the dial or keyset of the telephone).From the received number, exchange A determines that S r is served by exchangeB, and that the call is to be routed out on a trunk in group TG 1 (Fig. 1.1-1). It thensearches for an idle trunk in this group and finds trunk T 1 . Exchange A now seizesthe trunk and sends a seizure signal, followed by signals that represent digits of thecalled number, to exchange B. It then sets up a path between SL p and T 1 .Figure 1.1-3. Setup of a connection.