On the Addressing Problem for Loop Switching

The methods used to perform the switching functions of the Bell System have been developed under the fundamental assumption that the holding time of the completed call is long compared to the time needed to set up the call. In considering certain forms of communication with and among computers the possibility arises that a message, with its destination at its head might thread its way through a communication network without awaiting the physical realization of a complete dedicated path before beginning on its journey. One such scheme has been proposed by J. R. Pierce and may be called “loop switching.” We imagine subscribers, perhaps best thought of as computer terminals or other data generating devices, on one-way loops. These “local” loops are connected by various switching points to one another as well as to other “regional” loops which are in turn connected to one another as well as to a “national” loop. If a message from one loop is destined for a subscriber on another loop it proceeds around the originating loop to a suitable switching point where it may choose to enter a different loop, this process continuing until the message reaches its destination. The question naturally comes up, how the message is to know which sequence of loops to follow. It would be desirable for the equipment at each junction to be able to apply a simple test to the destination addressing the loops which has several attractive features: (i) It permits an extremely simple routing strategy to be used by the messages in reaching their destmations. (ii) By using this strategy, a message will always take the shortest possible path between any two local loops in the same region. {iii) The method of addressing applies to any collection of loops, no matter hoio complex their interconnections. The addressing scheme we propose will be applied primarily to local loops where the mutual interconnections may be quite varied. If a certain amount of hierarchical structure is introduced into the regional and national loop structure, as suggested by J. R. Pierce 1 it is possible to achieve addressings which are both compact and quite efficient.