TABLE OF CONTENTS In telephone networks, overloads can be caused by the following (either singly or in combination):
media-stimulated mass calling events such as televotes, charity appeals, competitions and marketing campaigns;
emergencies;
network equipment failures;
auto-scheduled calling.
In the absence of effective controls, such overloads would threaten the stability of network systems, and cause a severe reduction in successful call completions. Ultimately, systems would fail due to lost tasks, and services would not be available to customers.
Overloads peak at calling rates much greater than the predictable daily profile peak to which the network can be economically dimensioned. Figure 7.1 shows the relative calling rates for a particular service taken over consecutive 15-min periods for a month.
The overloads can be 4 to 5 times higher than the systematic daily peak quarter hour rate (normalised to one in Fig 7.1), or even worse than that: the vertical axis has been truncated at 5 times the systematic peak quarter hour rate, but in fact peak levels have reached (albeit much less often) as high as 64 times the systematic peak quarter hour rate. Table 7.1 quantifies in how many quarter hours per year the calling rate exceeds increasing multiples of the systematic quarter hour peak.
| Calling rate expressed as a multiple of systematic quarter hour peak | Number of quarter hours per year calling rate is exceeded (% of quarter hours... |
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