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In telecommunication, a call-second is a unit used to measure communications traffic density.

Note 1: A call-second is equivalent to 1 call with a duration of 1 second.

Note 2: One user making two 75-second calls is equivalent to two users each making one 75-second call. Each case produces 150 call-seconds of traffic.

Note 3: The acronym CCS (Centum Call Seconds) is often used to describe 100 call-seconds.

Note 4: 3600 call-seconds = 36 CCS = 1 call-hour.

Note 5: 3600 call-seconds per hour = 36 CCS per hour = 1 call-hour per hour = 1 erlang = 1 traffic unit.

In a communication network, a trunk (link) can carry numerous concurrent calls by means of multiplexing. Hence a particular number of CCS can be carried in infinitely many ways as calls are established and cleared over time. For example 3600 could be one call for an hour, or 2 (possibly concurrent) calls for half an hour each. CCS gives a measure of the average number of concurrent calls (i.e. Erlangs) over a time period of one hour.

Hence, in a one-hour period:

1 CCS = 100 Call-Seconds = 1/36 erlangs.