302 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



the marker is unable to set up a connection to a tone trunk, the originat- 

 ing register is able to return the tone signal. 



Number Groups 



Each number group is a translating circuit which provides terminating 

 information for a consecutive block of 1000 directory numbers. When 

 a marker transmits the called line number to a number group, it re- 

 ceives back the line location on a line link frame, the type of ringing 

 required, whether or not the line is in a PBX group, and certain minor 

 information. The number group also includes facilities for selecting an 

 idle line in a PBX group. As with link frames, only one marker can 

 connect to a number group at a time, but markers can connect to dif- 

 ferent number groups simultaneously without interference. 



Connectors 



The marker connector, providing access from line link frames and 

 register to markers, include circuits which assign preference of all line 

 link frames and registers for specific markers. This helps distribute 

 marker usage. 



In addition, when several line link frames and registers are com- 

 peting simultaneously for busy markers, a gating arrangement allocates 

 the order of service to reduce excessive delays. Although shown as one 

 block on Fig. 2, the marker connector circuits are provided one per fine 

 link frame and one per sub-group of ten or less registers. 



The frame connector, which provides access from markers to link 

 frames, includes preference and lockout features since only one of several 

 competing markers can connect to a given frame at one time. Although 

 shown as one circuit on Fig. 2, the frame connectors are actually dis- 

 tributed over the frames. 



Outgoing senders of a particular type are divided into two groups 

 and the sender connector permits connection from two markers, each 

 to one sender in each group simultaneously. Preference and lockout 

 features obtain. 



The number group connectors, provided one per number group, 

 are similar to frame connectors and give access from markers to number 

 group. 



Handling a Typical Call 



An understanding of the operational intricacies of a telephone switch- 

 ing system cannot be gained by a discussion of components and can only 



