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BELL SYSTEM TECHNICAL JOURNAL 



connections between the district junctors and the outgoing trunks are 

 used in series and are chosen simultaneously. Generally in other 

 systems the establishment of a connection involving three such paths, 

 is made in three successive stages with a possibility that after a 

 selection has been made at one stage it will be found that the paths 

 accessible to it are all busy and, therefore, the connection cannot be 

 completed 



Before describing the next stage in the establishment of a call, 

 it is desirable to point out other features and functions of the origi- 

 nating marker. 



1. The marker permits wide variations in the sizes of trunk groups, 

 permitting trunk groups as small as two and as large groups as may 

 be required. This makes for an efficient use of the office link frame 

 terminals and thereby tends to reduce the office link frame equipment. 



2. The marker makes a second trial to establish connections over 

 alternate trunk routes in case calls cannot be completed over the 

 normally used groups because of busy conditions. 



3. The marker makes a continuity test of the circuits over which 

 the switches are controlled and tests them for short-circuits, crosses, 

 opens and grounds which would interfere with the proper establish- 

 ment of a call and where troubles are detected, it signals this condition 

 to a common "trouble indicator" where an indication of the trouble 

 and its location is recorded and a maintenance alarm given. The call 

 is then completed over another group of circuits. 



The first stage in the progress of the call through the terminating 

 end of a crossbar office is illustrated in Fig. 13. It consists of con- 

 necting the incoming end of the selected trunk to a terminating sender 

 for the purpose of receiving the number of the called line from the 

 subscriber sender. 



When the incoming trunk is selected by the originating end of the 

 office equipment, the "sender link control" circuit associated with the 

 terminating sender link frame on which the incoming trunk appears, 

 is called into action. The control circuit then proceeds with the 

 following functions: 



1. To locate the incoming trunk circuit, which appears on one of 

 the ten horizontal paths of a primary switch. 



2. It selects an idle sender link between this primary switch and a 

 secondary switch on which there is an idle terminating sender. 



3. The control circuit selects one of the idle terminating senders 

 reached through the secondary switch and then operates the selecting 

 and holding magnets associated with the selected circuits, which will 

 establish the connection from the incoming trunk to the terminating 

 sender. 



