1118 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



of calls presented to the system exceeds the capacity of the common 

 control elements provided, the excess calls are delayed. The things which 

 customers, operators and connecting switching machines do when they 

 encounter delays tend to aggravate the overload. The reactions of oper- 

 ators and customers to delays can be illustrated by two examples. 



The first is taken from the operation of a network of No. 4 toll crossbar 

 systems when one of the No. 4's is heavily overloaded. Operators placing 

 calls through the overloaded system encounter, let us say, an abnormal 

 number of "no circuit" conditions in the outgoing trunks. This causes 

 them to make additional attempts to get circuits. These additional at- 

 tempts plus the excessive number of fii'st attempts overload the markers. 

 Sender holding time is then increased because of delays in connecting to 

 the markers and this, added to the abnormal number of sender usages, 

 results in a further shortage of senders. Operators trying to place calls 

 through the system are therefore slowed down because of slow "sender 

 attached" signals. (These are the signals which tell the operators that 

 they can start keying or dialing.) Senders in connecting systems are also 

 delayed w^aiting for senders to become idle in the overloaded office. The 

 overload therefore tends to spread to all connecting systems. 



However, it is possible to provide remedies which limit the reaction 

 to the overloaded system. These remedies are arrangements to rapidly 

 clear out senders waiting for senders ahead. Automatic alternate routing 

 is also useful in routing traffic around overloaded systems. 



The second example is taken from local systems. Here the reaction of 

 customers to delays compounds the overload. A severe overload results 

 in a shortage of senders, much as described above. A shortage of senders 

 in a local system causes dial tone delays. There are always some custom- 

 ers who either do not listen for dial tone or who will not wait very long 

 for it, and who start to dial before senders are attached to their lines. 

 The result of such dialing is either a partial digits condition under which 

 the sender waits for a considerable interval for a full complement of 

 digits, or a wrong number when the first digit is clipped. The delays 

 reduce sender capacitj^ still further and the wrong numbers further in- 

 crease the attempts. The load "snowballs" and the ability of the system 

 to handle calls degenerates. 



Here again arrangements are available to control the overload. These 

 include features for blocking calls before they reach the senders and 

 markers, and for returning paths busy signals with a minimum of com- 

 mon circuit holding time. 



While there is, then, a somewhat greater capacity for overloads in 

 step-by-step because of less efficient use of equipment, common control 



