Consider a stream of calls flowing across a group of trunks from one population of endpoints to another. The number of simultaneous calls traversing the trunks generally varies over time (that is, it increments by one every time a new call arrives on an available trunk, and it decrements by one every time an existing call terminates). The corresponding carried load (or usage), expressed in Erlangs, is defined as the average number of simultaneous calls that are traversing the trunks during a given time period (for example, during the busy hour). Note that in this example, the number of active calls always equals the number of busy trunks (since each active call requires exactly one trunk). Therefore, the call usage (that is, the average number of simultaneous active calls) equals the trunk usage (that is, the average number of simultaneous busy trunks) in this example.
If a call arrives while all trunks are busy, it is said to be blocked at the trunk group. In other words, not all calls that are offered to the trunks are actually carried by the trunks. Accordingly, the corresponding offered load, expressed in Erlangs, is defined as the average number of simultaneous calls that would have been traversing the trunks during a given time period (for example, during the busy hour), had there been enough trunks to prevent blocking. Note that in this example, the offered call load (that is, the average number of simultaneous active calls had there been enough trunks to carry all call attempts) equals the offered trunk load (that is, the average number of simultaneous busy trunks had there been enough trunks to carry all call attempts) in this example.
To summarize so far, the traffic load, expressed in Erlangs, represents the average number of simultaneous active calls or busy resources, during a given time period (for example, the busy hour).
Also note that the usage of a single station, when expressed in Erlangs, represents the fraction of time the station is in use. For example, a station that carries 0.1 Erlang of usage is busy 10% of the time (during the time interval of interest; for example, the busy hour).
Two fundamental characteristics of a stream of call traffic are the call rate (usually expressed in calls per hour) and the average call duration (usually expressed in seconds). The corresponding call usage can be defined as follows:
Usage (in Erlangs) = [(calls per hour)(seconds per call)]/3600
Note that in some traffic reports, the call rates are termed as call counts. If a particular report is associated with a period of time other than one hour, care must be taken not to mistakenly apply the call count as the calls per hour in the preceding formula. Be careful to convert call counts to calls per hour before applying the formula.
The term ccs stands for centum call seconds, which is a period of time 100 s in duration. To minimize confusion, although ccs is technically a unit of time and could be used as such, in this case it is only used to designate traffic loads.
Recall that a traffic load expressed in Erlangs is tacitly associated with a given time period (typically one hour). If that is the case, the relationship between a traffic load expressed in Erlangs and that same load expressed in ccs is:
Usage (in Erlangs) = Usage (in ccs)/36
However, consider a case in which a particular load, expressed in Erlangs, represents the average number of simultaneous active calls or busy resources, during a given time period other than one hour. In such a case, the denominator in the preceding expression should be set to equal the number of 100 s intervals in the time period of interest.
Finally, since a single station carrying one ccs of traffic is busy for 100 s during the busy hour, the maximum traffic that can be carried by a single station or trunk is 36 ccs.
