rect route. 5.4.3 Dynamic Alternative Routeing (DAR) DAR is a simple, decentralised dynamic routeing strategy that was (5.62) originally designed for the British Telecom trunk network consist- ing of between 50 and 60 main switches or nodes which are fully connected. The strategy operates as follows (see Gibbens, 1990). Nowsuppose that p (i,j)(whereki,j)is thelong run proportion Calls arriving between a source destination node pair first attempt of (i, overflow traffic offered node k. Then Equation 5.27 the direct route and are accepted so long as there is at least one free generalises to the asymmetric case of Equation 5.63 where B (i,j) circuit available. If there are no free circuits on the direct route then and B (i, j) are the (i, j) link blocking probabilities for fresh and the call attempts a currently nominated two-link alternative route overflow calls respectively. Dynamic routeing strategies 5/13 requires a route from its trunk exchange (labelled as node 1) to its kmi,j destination local exchange (node 3). There are two trunk exchanges (5.63) that connect to node 3 - namely, nodes labelled 2 and 4. This situation, where a local exchange connects to two trunk exchanges, kxi,j is often repeated throughout the network and is called 'dual-parent- Let L(i,j) be the blocking on the two-link alternative route via ing'. First, the call attempts the route (1,2,3) and if a free circuit can not be found on each of these two links it overflows to the route tandem node k given by Equation 5.64. (1,4,3) via the alternative destination trunk exchange (node 4). If this second route does not have a free circuit available on each link 1-L(i,j)[1-B2(i,k)][1-B2(k,j)] (5.64) then the call is either lost or further, but necessarily longer, routes are attempted. In the case of DAR, it may be shown that the route blocking rates A situation where a network may implicitly judge it better to lose are approximately equalised. Hence expression 5.65, which does the call rather than accept it using routes containing many links is not