PERFORMANCE ENHANCEMENT IN RECURSIVE COPY NETWORKS FOR MULTICAST ATM SWITCHING - A SIMPLE FLOW-CONTROL SCHEME

As promising copy networks of very large multicast switching networks for Broadband ISDN, multi-stage Recursive Copy Networks (RCN) have bee n proposed recently. In the multicast switch structure, the RCN preced es a point-to-point switch. At an RCN, all the copies of a master cell are generated recursively, i.e., a few copies of the master cell are made initially, and by considering each of these copies to be master c ells, more copies are made which, in turn, are again considered to be master cells to make still more copies, the process thus progressing r ecursively till all the required copies are made. By this principle of recursive generation of copies, the number of copies that can be gene rated is independent of the hardware size of the RCN. A limitation of RCNs is that buffer sizes at all stages except the first stage have to be large so as to keep the cell loss due to buffer overflow within de sired limits. This paper inspects a flow control scheme by which the p robability of buffer overflow can be kept low, even though the buffer sizes at later stages are not large. Under this flow control procedure , a cell is not transmitted from a stage to the succeeding stage, if t he occupancy level of the buffer of the succeeding stage exceeds a thr eshold. We study by simulation the performance aspects of such a flow control scheme in RCNs under cut-through switching scheme and under st ore-and-forward switching scheme. At high load intensities, the overfl ow probability can be reduced by an order of magnitude in 2-stage RCNs and by two orders of magnitude in 3-stage RCNs. To restrict the overf low probability within a given limit, the required buffer size is less under flow control than under no flow control. The implementation of the flow control is simple and the control overhead is small, thereby making the scheme attractive for implementation in high speed switchin g environments. Further, the proposed flow control scheme does not dis turb the cell sequence.