 depend on k. Therefore (i,j) may be determined by the relation when it is subject to a general overload of traffic. Accepting calls of Equation 5.66 in terms of L(i,j) and hence B(i,j) by Equation over long routes in these circumstances just adds to the levels of 5.64. congestion felt by all calls. The 'sticky random' or Dynamic Alternative Routeing (DAR) Pk(i,j)L(i,j) (5.65) strategy handles this situation by remembering a current tandem trunk exchange (labelled node 5) through which to send calls at- (5.66) tempting to overflow to longer routes. First, it attempts the route (1,5,2,3) and then overflows to the alternative destination trunk exchange along route (1,5,4,3). For each of these two routes the The proportions p (i, j) may then be used to calculate new esti- trunk reservation is used to give priority to the more directly routed mates for the offered traffics and hence for the link blockings B. (i, calls in preference to the indirect calls using longer routes. If none j) and B(i, j). Using the method of direct repeated substitution has of these routes is prepared to accept the call then it is at this stage been found to lead to oscillatory behaviour in some cases. In finally rejected from the network. The sticky random strategy then practice, this may be readily overcome by damping the iteration in again comes into play to reset the current tandem trunk exchange by the sense that, if solving the fixed point Equation 5.67 for some choosing at random from amongst a pre-defined set (nodes function f, then one should update X by Equation 5.68 where a (0 < 5,6,7,8,...). In this way, the sticky random strategy is able to shift the a the damping factor. A value around a = 0.8 has been found patterns of offered traffic to match the presence of spare capacity to be satisfactory in practice. that it finds to form these longer paths. It should be pointed out that this strategy applies in parallel, in a decentralised fashion, throug- x = f(x) (5.67) hout the network. In an example such as the British Telecom nation