The Reliability of Telephone Traffic Load Measurements by Switch Counts

Of prime importance to the telephone traffic engineer is the determination of the busy season busy hour load carried by groups of trunks or other circuits of a telephone switching system. Three direct methods of measuring such loads are found in the field today. These are: a. Peg Count and Holding Time Method The number of calls carried by the circuit group during the observation period is counted. This number multiplied by the average holding time per call (in hundreds of seconds) and divided by the length of the observation period (in hours) gives an estimate of the group load in units of hundred-call-seconds per hour (CCS). The major drawback to this peg count method is that it requires a separate determination of the average holding time per call for the group under observation. R. I. Wilkinson 1 has analyzed the sources of errors of holding time measurements. In addition, correlation between load and holding time introduces an error which has not been studied. b. Switch Count Method At fixed intervals the circuit group is scanned and the number of busy circuits is counted. The total number of busy conditions counted divided by the number of scans is, then, an estimate of the load on the group in units of average simultaneous calls or erlangs*. This estimate is generally converted to CCS (1 erlang = 3f> CCS) by traffic engineers since the * The name " e r l a n g " for average simultaneous call was adopted at a plenary meeting of the C C I F at Montreux in October, 19-1G.