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2.2 STANDARDS FOR SIGNALING SYSTEMS 49generations of exchanges. Some old-technology exchanges can handle only olderinterexchange signaling systems and cannot be upgraded to accommodate newerforms of signaling. Modern exchanges using SPC technology (Section 1.7) by andlarge can be retrofitted to handle newer signaling systems but, depending on theirvintage, the process can require a nontrivial investment and long lead times. Insome countries, such as the United States, telecoms have shifted a significantportion of the capital investment previously earmarked for new exchanges to equipmentbased on packet technology (Chapter 20), which requires a whole new set ofsignaling systems.As a consequence, a connection routed via one or more intermediate exchanges ofdifferent vintages may involve trunks with different interexchange signaling systems.Call-control at such exchanges has to include procedures for signaling interworkingbetween the different signaling systems. One aspect of interworking is the conversionof the formats of individual signals and/or messages. More difficult problems arisewhen a signal, or an information element in a common-channel signaling message,exists in a new system but not in an older system. Signaling interworking functionscan be quite complex and have to be designed with care.2.2 STANDARDS FOR SIGNALING SYSTEMSThe equipment in a telecommunication network is usually purchased from severalmanufacturers. In order to ensure that equipment from different suppliers can beinterconnected without problems, the telecoms have developed (and continue todevelop) standards that are documented in specifications and that have to be metby the manufacturer’s equipment. Specifications cover signaling and many otheraspects of telecommunications. The information in this book is based largely on signalingstandards published by the organizations mentioned below.2.2.1 North American OrganizationsAlliance for Telecommunications Industry Solutions (ATIS). ATIS, originallycalled the Exchange Carrier Standards Association (ECSA), was created afterdivestiture to ensure that the North American network would continue to operate asan integrated entity [11]. With a membership that includes all the major telecomsand equipment suppliers, ATIS manages requirements through an entity called theT1 Committee, responsible for generating drafts of telecommunication standards.Proposed standards are submitted to the American National Standards Institute(ANSI) and published as American National Standards for Telecommunications.Telcordia. Telcordia Technologies is an independent company that had its originsas the requirements coordination arm of the Bell System, when it was calledBellcore. Prior to divestiture in 1984, the Bell System was the dominant telecomand equipment manufacturer and set the de facto standards for the North Americantelecommunication network [12].