346 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1953 



tone delay, probability, and whether or not all registers are busy. Where 

 dial tone delay is concerned, it is determined by comparing the origina- 

 ting time of a dial tone call slip with three dial tone delay time counters. 

 These counters give present time minus 1, 2 and 9 seconds respectively 

 so that matching the time of origination against them indicates whether 

 the delay was < 1 sec, 1-2 sec, 2-9 sec. or > 9 sec 



The probability factor is obtained from a circuit w^hich is capable of 

 lighting one lamp out of ten, one out of three, etc on a random basis 

 when a key is depressed. By means of the circuit, calls can be assigned 

 to various categories in correct proportion in accordance with the best 

 available traffic information. Whether or not all registers are busy is 

 indicated by the all register busy circuit controlled by the assignment 

 operator. 



If the assignment for a trunk or a register is that it be held for a period 

 of time and then released, the assignment operator makes use of the 

 holding time book as described later. If the assignment is for a register 

 to return with a bid for a marker, or a new call to return into the system 

 (after encountering busy, for example), the time is noted on the call 

 slip and the latter is passed to the originating operator for subsequent 

 action. For new call return time, a letter designation associated with 

 the signal lamp is also entered on the call slip. The letter is carried for- 

 ward on the new call slip. If subsequent attempts of this same line meet 

 busy or overflow, the letter designation is used to identify the same 

 category of return time instead of using the random circuit. 



When a trunk call is set up, the trunk and one or two lines must be 

 kept out of ser\'ice for one of several fixed holding times. There are ten 

 different assigned holding times with an equal likelihood of an established 

 call falling into any one of them as determined by the traffic assign- 

 ment circuit. False start, tone trunk and don't answer connections 

 provide four additional holding times. 



The holding time counters provided at the assignment position in- 

 dicate present time plus a fixed holding time. Thus each counter gives 

 the time at which a connection, set up at present time and assigned to 

 that particular holding time, will release its elements back into ser\dce. 



Holding time starts for a given call at the time the release lamp 

 lights at the assignment position marker unit associated with that call. 

 At such a time, the assignment operator obtains the one or two line pegs 

 of the call and plugs them in the trunk jack or jacks (identified by the 

 frame connector cord), noting at the same time the trunk number. The 

 operator then depresses a key which causes the traffic assignment circuit 

 to light a lamp under one of the holding time counters, thereby assign- 



