Service Availability: Principles and Practice

Chapter 1

Definitions, Concepts, and Principles

Francis Tam

Nokia Research Center, Helsinki, Finland

1.1 Introduction

As our society increasingly depends on computer-based systems, the need for making sure that services are provided to end-users continuously has become more urgent. In order to build such a computer system upon which people can depend, a system designer must first of all have a clear idea of all the potential causes that may bring down a system. One should have an understanding of the possible solutions to counter the causes of a system failure. In particular, the costs of candidate solutions in terms of their resource requirements must also be known. Finally, the limits of the eventual system solution that is put in place must be well understood.

Dependability can be defined as the quality of service provided by a system. This definition encompasses different concepts, such as reliability and availability, as attributes of the service provided by a system. Each of these attribute can therefore be used to quantify aspects of the dependability of the overall system. For example, reliability is a measure of the time to failure from an initial reference instant, whereas availability is the probability of obtaining a service at an instant of time. Complex computer systems such as those deployed in telecommunications infrastructure today require a high level of availability, typically 99.999% (five nines) of the time, which amounts to just over five minutes of downtime ...