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4CHANNEL-ASSOCIATEDINTEREXCHANGE SIGNALINGChannel-associated interexchange signaling (CAS)—also known as per-trunksignaling—has been in existence from the beginning of automatic telephonyand was the only form of interexchange signaling until 1976. It is still used intelecommunication networks but in many countries has largely been replaced bycommon-channel signaling.Early CAS systems were developed independently by individual equipment manufacturersand exist in many varieties. Later CAS systems, notably those developedafter the Second World War, show the increasing influence of national and internationalstandards.This section describes three important CAS systems and their use on frequencydivisionmultiplexing (FDM) analog trunks and time-division multiplexing (TDM)digital trunks (Sections 1.4.5 and 1.5.2).The acronyms in the figures of this chapter are explained in Section 4.5.4.1 INTRODUCTION4.1.1 Interexchange Signaling ExampleFigure 4.1-1 shows a typical interexchange signaling sequence for a call from subscriberS 1 to subscriber S 2 . The subscriber signaling for the call is not shown.After exchange A has received the called number from S l , it decides to route thecall via intermediate exchange B. Exchange A seizes an available trunk T 1 and sendsa seizure signal on the trunk. Exchange B responds with a proceed-to-send (or wink)signal, indicating that it is ready to receive the digits of the called number. ExchangeA sends the digits and then cuts through (sets up a path in its switchblock betweenthe subscriber line of S 1 and T 1 ).Signaling in Telecommunication Networks, Second Edition, by John G. van Bosse and Fabrizio U. DevetakCopyright # 2007 John Wiley & Sons, Inc.75