TRAFFIC INTENSITY DEFINES THE SIZE OF SWITCHES AND THE CAPACITY OF TRANSMISSION LINKS
Nevertheless, suitable forecasts of BH traffic can be made. However, before proceeding
further in this discussion, consider the following definitions of the busy hour.
1. Busy Hour. The busy hour refers to the traffic volume or number of call attempts,
and is that continuous one-hour period being wholly in the time interval concerned
for which this quantity (i.e., traffic volume or call attempts) is greatest.
2. The Average Busy Season Busy Hour (ABSBH). This is used for trunk groups and
always has a grade of service
criterion applied. For example, for the ABSBH load,
a call requiring a circuit in a trunk group should encounter all trunks busy (ATB)
no more than 1% of the time.
Other definitions of the busy hour may be found in Ref. 1.
When dimensioning telephone exchanges and transmission routes, we shall be working
with BH traffic levels and care must be used in the definition of the busy hour.
Peak traffic loads are of greater concern than average loads for the system planner
when dimensioning switching equipment.
Another concern in modern digital switching systems is call attempts. We could say
that call attempts is synonymous with offered traffic. Even though a call is not carried and
is turned away, the switch's processor or computer is still exercised. In many instances
a switch's capability to route traffic is limited by the peak number of call attempts its
processor can handle.
Measurement of Telephone Traffic
. If we define telephone traffic as the
aggregate of telephone calls over a group of circuits or trunks with regard to the duration
of calls as well as their number, we can say that traffic flow (
A) is expressed as
A = C × T ,
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 × 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,
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.
Grade 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
The first letter c in ccs stands for the Roman number 100.