Method of routing calls for a network of digital time-switched exchanges

A method of routing calls for digital time-switched exchanges interconnected by trunk groups each of which interconnects two exchanges over one or more PCM trunks, characterized in that each exchange determines in real time a utilization cost for each of the trunk groups connected thereto on the basis of the busy fraction of the least busy trunk in the trunk group under consideration as seen from the exchange, and broadcasts an operating update by contagion over the trunk groups to the other exchanges in the network whenever there is a significant modification in the utilization cost of a trunk group connected thereto, said update specifying at least the identity of the originating exchange and the identity of the trunk group concerned, together with the order number of said update and the new utilization cost to be taken into account, each exchange storing all of the operating updates produced thereby or received from other exchanges, and in that the routing of a call to be set up through the network is selected by the network exchange from which said call is initially requested, which exchange itself determining the minimum utilization cost routing by means of a shortest path search algorithm known per se, and on the basis of the stored updates, and producing a routing set-up message specifying the network exchange(s) through which the call is to pass, said set-up message being transmitted over the trunk groups concerned as the chosen routing is set up and being reduced at each exchange by the portion concerning said exchange itself.