TABLE OF CONTENTS Over the last few years, BT has been active in the development of network overload controls. The motivation for this work was, and is, the need for effective, automatic controls to protect a range of service platforms from demand levels that are becoming increasingly volatile and unpredictable. Some of the control techniques developed have been patented [1] and are now implemented and running on several of BT's service platforms (see Chapter 8).
The expertise gained from this work has enabled industry to improve the way overload controls are defined in various UK and international standards. Notable examples are the UK adaptive ISDN user part automatic congestion control (ISUP ACC) [2] and the Asynchronous Transfer Mode (ATM) Forum's Signalling Congestion Control [3].
The ideas discussed in this chapter were first developed for controlling overloads in telephony networks; consequently, much of it draws upon that experience directly. However, many of the basic ideas apply across a wide range of services and technologies with minor variations, and can be fruitfully applied to the newer services and technologies. Examples of this are discussed throughout this chapter, which is structured as follows.
Section 7.2 describes the causes of overloads. This is largely tutorial material and covers, for example, overloads caused by equipment failures, by media-stimulated call surges to specific numbers, and by emergencies.
Section 7.3 discusses the impact of overloads on customer behaviour (specifically customer abandons and reattempts) and on node throughput.
Section 7.4 discusses the variations...
