36 BELL SYSTEM TECHNICAL JOURNAL 



been one of the important ways in which increased speed of service 

 has been brought about. 



The handling of this suburban telephone trafific adds greatly to 

 the complexity of the transmission, trunking and operating problem 

 of the larger cities. This is illustrated by Fig. 27 showing the sub- 

 urban toll area surrounding New York City. It will be noted that 

 the city itself includes 168 central offices and in the suburban areas 

 in the metropolitan district there are in addition 159 central offices. 



In many cases of the shorter haul toll service, the volume of traffic 

 between two offices is sufficient to warrant direct trunks and the calls 

 are completed over these direct trunks by the usual local traffic meth- 

 ods. In order to provide an efficient trunking arrangement for the 

 smaller volume of traffic between widely separated offices, however, 

 tandem trunking arrangements are provided, by which the calls are 

 routed through a central switching point and from that point dis- 

 tributed to the terminating offices. Either manual or dial central 

 office equipment is used as outlined below, each type having its field 

 of application depending upon the amount of traffic and the portion 

 of traffic to and from manual and dial central offices. The trunks 

 to the central switching point, or tandem office, are in general of 

 somewhat larger gauge than interoffice trunks because of the greater 

 distances involved and the correspondingly more severe transmission 

 requirements. In some of the longer trunks telephone repeaters are 

 used. It has not been found generally economical to use repeaters 

 at the tandem switching point although this is done in certain in- 

 stances and it is possible that in the near future the more general 

 use of repeaters in this way may become an economical means of 

 meeting the transmission requirements. 



Manual Straightforward Tandem 



The manual straightforward tandem is used in those medium sized 

 areas in which most of the suburban calls are between manual switch- 

 boards. The arrangement of the equipment is shown in Fig. 28. 

 The tandem trunks from the originating office terminate on the plugs 

 located at the rear of the keyboard and the tandem completing 

 trunks to the various terminating offices appear in jacks in the face 

 of the switchboard. The completing trunks are provided with lamp 

 signals indicating idle trunks. When a call comes in on a tandem 

 trunk the operator is advised by a flashing lamp signal on that trunk 

 and her telephone set is automatically connected to it. The work of 

 the tandem operator is limited to making the connection at the tandem 

 board and making the disconnection when advised by lamp signal 



