THROWDOWN MACHINE FOR TRAFFIC STUDIES 299 



metropolitan business exchanges. Although the system includes facilities 

 for toll and tandem switching, these were not included in the throwdown 

 studies and ^\dll not be discussed. 



The Switching Network 



The No. 5 system is built around an interconnecting network con- 

 sisting of two types of switching frames utilizing crossbar switches and 

 known as line link frames and trunk link frames. This is illustrated in 

 block diagram form on Fig. 2. Each frame is double-ended and provides 

 means for connecting any point on one side of the frame to any point on 

 the other side. The connecting paths are known as links. 



All subscriber lines in the office connect to one side of the line link 

 frames, each of which can serve, roughly, 300 to 500 lines; and all trunks 

 connect to one side of the trunk link frames, each of which has 160 trunk 

 appearances. The other sides of line and trunk link frames are connected 

 together in such a manner that each line link frame has access to all 

 trunk link frames over several paths. 



These interconnecting paths are known as junctors. The basic maxi- 

 mum number of line link and trunk hnk frames is 20 and 10 respectively, 

 and this is the size embodied in the throwdown machine. However, in 

 practice, multipling arrangements can be employed to double the number 

 of frames to give greater subscriber and traffic capacity. 



With the system described above, any subscriber line can be con- 

 nected to any trunk over one of several paths, each consisting of two 

 links and a junctor and known as a channel. On connections to outgoing 

 or incoming trunks, a single channel is required; on connections through 

 intraoffice trunks (connection between two local subscribers), two chan- 

 nels, one from each end of the trunk, are required. The method of com- 

 bining links and junctors to form channels will be described later. 



Dial Registration — Originating Registers 



The circuits that receive and store the dialed signals from the sub- 

 scriber are known as originating registers. These circuits, in quantity 

 as determined by desired quality or grade of service, are distributed over 

 the trunk link frames as equally as possible. A connection between 

 subscriber and register is set up through the switching network just as 

 between subscriber and trunk. The registers call in control equipment 

 for setting up the subscriber's connection when dialing is completed. 



