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2INTRODUCTION TO SIGNALING2.1 OVERVIEWThis section presents a brief historical outline of signaling. The earliest telephoneexchanges were “manual” switchboards, in which all calls were set up and takendown by operators. Signaling between subscribers and operators was limited toringing. To make a call, the subscriber would send a ringing signal. This alertedan operator, who would connect her telephone to the calling line and ask for thecalled number. The operator then would connect her telephone to the called lineand ring the line. After answer by the called party, the operator would establishthe connection.Signaling as we know it today started around 1890, with the invention, by AlmonB. Strowger (a Kansas City undertaker), of an automatic switchboard that couldreceive the called number dialed by the calling subscriber and would then automaticallyset up the connection. During the past 100 years, signaling applications andtechnology have evolved in parallel with the developments in telecommunications.2.1.1 Early SignalingSignaling in the period from 1890 to 1976 had three main characteristics. In the firstplace, its application was limited to plain old telephone service (POTS): the setupand release of connections between two subscribers. In the second place, thesignals were carried by the same circuit (subscriber line, trunk) that carried thespeech during the call. This type of signaling is known as channel-associated signaling(CAS) [l,2]. Finally, signaling took place only between a subscriber and the localexchange (subscriber signaling), and between the exchanges at the two ends of atrunk (interexchange signaling).Initially, automatic telephony was possible only for calls between subscribersserved by the same exchange, which required subscriber signaling only. Later on,Signaling in Telecommunication Networks, Second Edition, by John G. van Bosse and Fabrizio U. DevetakCopyright # 2007 John Wiley & Sons, Inc.43