94 BELL SYSTEM TECHNICAL JOURNAL 



The switches are arranged in two vertical files of ten primary switches 

 and ten secondary switches. There are twenty subscriber lines 

 connected to the verticals of each of the ten primary switches and 

 twenty trunk circuits are connected to the twenty verticals on each 

 secondary switch. The horizontal multiples on the primary switches 

 are connected to the horizontal terminals of the secondary switches, 

 each primary switch having one horizontal path connected to each of 

 the ten secondary switches. With this arrangement, the twenty lines 

 of any primary switch have access to all 200 trunks connected to the 

 secondary switches. Since all of the primary switches are wired in 

 this manner, that is, with their ten horizontal paths distributed over 

 the ten secondary switches, then all of the 200 lines on the primary 

 switches have access to the 200 trunks on the secondary switches. 

 It is evident that another vertical file of ten primary switches may be 

 added with twenty subscriber lines connected to the verticals of each 

 switch, and with the horizontal paths strapped and connected to the 

 horizontal paths of the primary switches shown. This would give 

 400 lines access to the 200 trunks on the secondary switches. In 

 actual practice on a line link frame, several files of primary switches 

 may be connected together in this manner depending upon the traffic 

 volume of the subscriber lines. The circuit paths connecting the 

 horizontal rows of terminals of the primary switches to the horizontal 

 rows of terminals of the secondary switches are called "line links." 



To establish a path from a line circuit on a primary switch to a 

 trunk circuit on a secondary switch, the common "control" circuit 

 serving this line link frame locates the subscriber line to be served and 

 then simultaneously selects an idle "line link" on the primary switch 

 on which the subscriber line appears and a group of trunks wired to a 

 secondary switch in which there are one or more idle trunks. Thus 

 the selection of the line link is made contingent upon the availability 

 of trunks, and by means of this together with the primary-secondary 

 distribution of the links a very efficient usage of the links and trunks 

 is obtained. 



In the "line link" frame shown in Fig. 9, it will be seen that the 

 trunks on the verticals of the secondary switches are split into groups 

 of 100 trunks each, one group being connected to the "district junc- 

 tors" and used for originating traffic and the other group of 100 

 trunks being connected to "line junctors" and used for terminating 

 traffic. 



It will be noticed that there is but one crossbar switch appearance 

 of a subscriber line in the office. This is on a vertical unit of a primary 

 crossbar switch where both the originating and terminating calls are 



