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116 INTRODUCTION TO COMMON-CHANNEL SIGNALINGFigure 5.1-5. Signaling network with two routes for each signaling relation (quasiassociatedsignaling).A third alternative is shown in Fig. 5.1-6. This arrangement differs fromFig. 5.1-3(a) in that exchanges A, B, and C are now signaling transfer and endpoints (STEPs). Under normal conditions, all signaling is associated. However,when, for example, SL 1 fails, the messages related to trunks of group TG 1 aresent via C, which then acts as the STP for the signaling traffic of relation (A,B).We conclude this section by exploring two well known signaling networks.5.1.3 The (Former) Bell System Signaling NetworkFigure 5.1-7 shows part of the quasiassociated signaling network originallydeployed by the Bell System for CCIS [2,4]. The basic structure has been retainedfor SS7 signaling in AT&T’s present long-distance network [5].The territory of the United States is divided into a number of regions, and eachregion is equipped with a pair of STPs (only two regions are shown inFig. 5.1-7). Each exchange with CCS trunks is a signaling end point and has anFigure 5.1-6. Signaling network with combined signal transfer and end points (STEPs).