TABLE OF CONTENTS This chapter has presented evidence to show that overloads occur frequently in telephony networks, and can greatly exceed the capacity of the terminating lines (and of the network) to handle the surge of calls. The typical causes of overloads are media-stimulated events, emergencies and equipment failures. Their impact on the network is to reduce effective switch throughput (ultimately leading to switch failure) and to generate high levels of repeat attempts most of which will fail to complete successfully, but nevertheless consume network resources, thereby reducing the capacity available to other, non-event, call streams. It is not economic to provide sufficient network or terminating line capacity to handle such events; consequently overload controls are necessary to ensure that switches are protected and ineffective traffic is minimised.
Overload controls need to be automatic, fast-acting and adaptive. As this chapter has shown, it is straightforward to define, in a generic way, how such controls should behave, without defining how they should be implemented to achieve that behaviour. Thus, for example, a key requirement is that the control should adaptively cause the admitted calling rate offered to an overloaded resource to converge close to the maximum rate it can handle, subject to keeping response times small enough to avoid customers abandoning calls in set-up.
It has been argued that such behavioural requirements should be a part of the specification of any control appearing in an international or UK telecommunications standard. Otherwise, there is no assurance that an implementation of a control...
