56
TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK
route or connection between exchanges. We could say we are dimensioning the route. To
dimension the route correctly we must have some idea of its usage--that is, how many
people will wish to talk at once over the route. The usage of a transmission route or
switch brings us into the realm of traffic engineering; and usage may be defined by two
parameters: (1) calling rate, or the number of times a route or traffic path is used per unit
time period; or more properly defined, "the call intensity per traffic path during the busy
hour (BH)"; and (2) holding time, or "the average duration of occupancy of one or more
paths by calls." A traffic path is a "channel, time slot, frequency band, line, trunk switch,
or circuit over which individual communications pass in sequence." Carried traffic is the
volume of traffic actually carried by a switch, and offered traffic is the volume of traffic
offered to a switch. Offered traffic minus carried traffic equals lost calls. A lost call is
one that does not make it through a switch. A call is "lost" usually because it meets
congestion or blockage at that switch.
To dimension a traffic path or size a telephone exchange, we must know the traffic
intensity representative of the normal busy season. There are weekly and daily variations
in traffic within the busy season. Traffic is random in nature. However, there is a certain
consistency we can look for. For one thing, there is usually more traffic on Mondays
and Fridays, and there is a lower volume on Wednesdays. A certain consistency can also
be found in the normal workday variation. Across a typical day the variation is such
that a one-hour period shows greater usage than any other one-hour period. From the
hour of the day with least traffic intensity to the hour of greatest traffic, the variation
can exceed 100:1. Figure 4.1 shows a typical hour-to-hour traffic variation for a serving
switch in the United States. It can be seen that the busiest period, the busy hour (BH), is
between 10
A.M.
and 11
A.M.
(The busy hour from the viewpoint of grade of service was
introduced in Section 1.3.4). From one workday to the next, originating BH calls can vary
as much as 25%. To these fairly "regular" variations, there are also unpredictable peaks
caused by stock market or money market activity, weather, natural disaster, international
events, sporting events, and so on. Normal traffic growth must also be taken into account.
Figure 4.1
Bar chart of traffic intensity over a typical working day. (US, mixed business and residential).