traffic flow (A) is expressed as A=C x T, (4.1) where C designates the number of calls originated during the period of one hour, and T is the average holding time, usually given in hours. A is a dimensionless unit because we are multiplying calls/hour by hour/call. Suppose that the average holding time is 2.5 minutes and the calling rate in the BH for a particular day is 237. The traffic flow (A) would then be 237 X 2.5, or 592.5 call-minutes (Cm) or 593.5/60, or about 9.87 call-hours (Ch). The preferred unit of traffic intensity is the erlang, named after the Danish mathemati- cian A.K. Erlang (Copenhagen Telephone Company, 1928). The erlang is a dimensionless unit. One erlang represents a circuit occupied for one hour. Considering a group of cir- cuits, traffic intensity in erlangs is the number of call-seconds per second or the number of call-hours per hour. If we knew that a group of 10 circuits had a call intensity of 5 erlangs, we would expect half of the circuits to be busy at the time of measurement. In the United States the term unit call (UC), or its synonymous term, hundred call- second, abbreviated ccs, 3 generally is used. These terms express the sum of the number of busy circuits, provided that the busy trunks were observed once every 100 seconds (36 observations in 1 hour) (Ref. 2). The following simple relationship should be kept in mind: 1 erlang = 36 ccs, assuming a 1-hour time-unit interval. 2Grade of service refers to the planned value criterion of probability of blockage of an exchange. This is the point where an exchange just reaches its full capacity to carry traffic. This usually happens during the busy hour. 3 The first letter C in CCS stands for the Roman number 100. TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK Extensive traffic measurements are made on switching systems because of their numer- ous traffic-sensitive components. Usual measurements for a component such as a service circuit include call attempts, calls carried, and usage. The typical holding time for a common-contr