Cost - Effective Use of Networks Employing Dynamic

Dynamic Alternative Routing (DAR) is a simple but highly efficient decentralised routing scheme developed for the UK trunk network. This paper examines how to dimension the UK network given known multiple traffic patterns together with unpredictable variations in traffic. Previous methods are first examined. We then present the results of a cost comparison of DAR and AAR using simple sizing techniques. Finally, more powerful techniques are examined based on the implied cost method of Kelly. DAR was developed for application to the UK trunk network [1]. The development and motivation for it are described fully by Stacey and Songhurst [2]. The UK trunk network comprises a top tier of 55 fully-interconnected Digital Main Switching Units (DMSUs), and a lower tier of local exchanges, each of which is connected to two DMSUs. Fig 1 illustrates part of the network available to connect two local exchanges L1 and · L2. local network main network E L1 L2 B 0 Figure 1 DAR r.outing Calls originating at local L1, and bound for distant local L2, are offered first to DMSU B, overflowing to DMSU A if that link is busy. From B the call is offered in turn to each of the target local's two DMSUs, and there are therefore two single-link paths available through the main network. If these are unavailable an alternative two-link path is chosen via a currently recommended tandem DMSU (E in Fig 1). If the lInk (BE) to the tandem exceeds its trunk reservation threshold then the call is blocked and a new tandem is chosen by B from the available set, for use by the next call to L2 from B which overflows both links BD and BC.