 can correct it. In the switching area, security is normally provided by giving any termination access to multiple paths through the network. A particu- lar example of this is the use of sectionalisation where the switching network is divided into planes of switching, operating in parallel State Class and all accessible to all terminations. Thus a fault affecting any one Routing records plane of switching will not cause loss of service to any terminations, service the traffic being carried by the remaining planes, albeit at a slightly TABLES reduced grade of service. In the control area, security is normally provided by replication. Figure 31.20 SPC software structure In its simplest form this would mean a single duplicated control unit. The two units can operate: Bibliography 31/17 1. In parallel with a fault cutting out the defective unit. Whichever method is employed to provide the operational se- 2. As worker and spare with automatic changeover in the event of curity, it is essential that adequate check circuits and routines are a fault. provided to detect the faults together with appropriate means for 3. Alternatively under the control of a time cycle. reporting them (e.g. lamp display, audible alarm or print out) such that the fault can be remedied within the mean time between failures However, except in very small systems, a single duplicated con- (MTBF) for the equipment concerned. trol is unlikely to be adequate and a number of alternative options are available: 31.8 Bibliography 1. Triplication, where three identical units are used in parallel with majority decision equipment on the outputs. Atkinson, J. (1950) Telephony, Pitman. 2. One-in-N Sparing, where a number of similar equipments are Bear, D. (1988) Principles of Telecommunication Traffic Engineer- involved and one (or more) spares are provided such that, in the ing, Peter Peregrinus. event of a fault, one of the spares may be switched into service Berkeley, G.S. (1949) Traffic and Trunking Principles in Automatic in place of the faulty equipment