96 BELL SYSTEM TECHNICAL JOURNAL 



completed by means of the same line link circuits. Thus all originating 

 traffic from any of the twenty lines on any primary switch flows 

 through the associated ten line links to the 100 district junctors and 

 all terminating traffic to these twenty lines flows through the same 

 ten line links from the 100 line junctors. 



This single "primary and secondary" trunking arrangement also is 

 used at other points in the system, such as in the originating and 

 terminating sender link switch frames, where the circuits reached are 

 non-directional, that is, where any one of the selectable circuits wired 

 to the frame can be used for setting up a connection. 



For the switch frames where the circuits reached are directional, 

 that is, where a particular called line or a particular group of trunks 

 must be used in order to complete a connection, the problem of trunking 

 becomes more complex and it is necessary to provide a trunking 

 arrangement using two "primary and secondary" switch frames 

 arranged in tandem. 



Figure 10 shows a typical arrangement of this kind which is necessary 

 to secure the required trunking flexibility and efficiency. This figure 

 shows an "incoming link" frame to which incoming trunks are con- 

 nected and a "line link" frame to which subscriber lines are connected 

 as described above. These two frames are used in tandem for estab- 

 lishing the terminating connections between the incoming trunks and 

 the called subscriber lines. As is indicated, 100 incoming trunks are 

 connected to the 100 horizontal paths of the ten incoming link frame 

 primary switches, there being ten incoming trunks connected to each 

 of the primary switches. A total of 150 to 700 subscriber lines may 

 appear on the verticals of the primary switches of the line link frame; 

 however, only 200 lines or twenty on the verticals of each of the ten 

 primary switches are shown in the figure. 



In order to connect a particular incoming trunk to a particular 

 called line, an idle channel is selected through these two switch frames, 

 consisting of an "incoming link" on the incoming link frame, a "line 

 junctor" between the two frames and a "line link" on the line link 

 frame, and all are connected in series as shown in the figure. It will 

 be noted that the incoming trunks on each of the primary switches 

 have access to twenty incoming links appearing on the twenty verticals 

 of the switch. These twenty incoming links are distributed over the 

 ten secondary switches of the frame, two links being connected to 

 each switch, one to each half switch. It will be observed that in 

 order to provide for the distribution of the twenty incoming links 

 over the ten secondary switches, the horizontal paths of the secondary 

 switches are separated between the tenth and eleventh verticals, 



