382 BELL SYSTEM TECHNICAL JOURNAL 



c) Toll office tandems, for completing connections from manual 



trunks to ringdown toll circuits. 



d) Straightforward toll line tandems, for completing connections 



from straightforward toll circuits to toll switching trunks. 



e) Toll switching trunk tandems, for completing connections from 



manual trunks to manual toll switching trunks. 

 II. Dial tandems, at which the connections are made wholly by means 

 of switch mechanisms controlled either at the tandem office 

 or at a distant office. These include — 



a) Operator tandems for completing connections from manual 



trunks to dial (or manual) trunks. 



b) Full selector tandems, for completing connections from dial 



trunks to dial (or manual) trunks. 



c) Trunk concentrating tandems, for automatically concentrating 



or collecting traffic which is to be completed over either manual 

 or dial trunks. 



Manual trunks include all types of trunks over which the order is 

 passed orally by an operator or by a machine as in the case of call 

 announcer trunks. Dial trunks include those over which the order is 

 transmitted in the form of electrical impulses. 



Traffic normally routed over direct straightforward trunks frequently 

 is handled through a tandem system during the night and other hours 

 of light traffic. This is sometimes an economical arrangement since it 

 makes it unnecessary to provide incoming "B" operators during such 

 hours except on positions handling the tandem completing trunks. 

 The speed of connection at such times is substantially as fast as over 

 direct trunks because of the number of "B" positions which it would 

 be necessary otherwise to cover with a small number of operators. 

 Also, a tandem system may be used as an emergency routing during 

 periods when direct trunk groups are out of service because of cable or 

 other failure. Frequently tandem systems are used as overflow 

 routings for traffic normally handled over small direct trunk groups. 



Table I indicates the number of the different types of tandem systems 

 in use in the Bell System. In addition to systems of the types shown, 

 tandem operation is obtained through the use of regular local central 

 office equipment in a number of cities where the volume of eligible 

 traffic is very small. 



Manual Tandems 

 Manual Straightforward Tandems 

 In these tandem systems the incoming and outgoing trunks are of 

 the straightforward type. The incoming trunks are terminated on 



