ard Njj sections of the network operation. as the offered traffic between nodes i and j. Then the linear pro- Dynamic routeing strategies 5/11 gramme has as its objective function the total flow through the optimisation procedure to predetermine a routeing pattern. This network, and has as constraints the limits imposed by the levels of pattern may be changed hourly, typically in relation to time zone offered traffic and the capacities of the links. differences. It may be shown that the bound of expression 5.52 holds for the In Canada, Bell Northern Research has proposed a scheme called overall network loss probability, L, where A is given by Equation Dynamically Controlled Routeing (DCR), based on a central con- 5.53. troller which receives information on the current state of all links at regular intervals of about 5-10 seconds. This information is used by L 21-F(2) (5.52) the controller to determine a routeing pattern which is then dis- tributed back to the nodes. More recently further dynamic routeing strategies have been pro- (5.53) posed such as Dynamic Alternative Routeing (DAR) for the British Telecom national network (see Stacey, 1987). Key, 1990 gives a recent survey of distributed dynamic routeing strategies. In this equation A is just the total offered traffic to the network and F(A) is the maximum carried load obtained by solving the above 5.4.1 Dynamic Non-Hierarchical Routeing linear programme. (DNHR) Note also that if the arrival streams are in fact deterministic, then the bound can be attained by a dynamic routeing strategy which The DNHR strategy has been in operation throughout the entire routes traffic according to a solution to the linear programme. AT&T trunk network since 1987 following initial operation within a sub network since 1984. The strategy maintains fixed sequences of alternative routes for each source destination node pair but where 5.3.4.2 Erlang bound the sequences are changed during the day to follow changes in the offered traffic patterns. It is designed to take advantage of t