Broadband Packet Switching Technologies: A Practical Guide to ATM Switches and IP Routers

CHAPTER 10

CLOS-NETWORK SWITCHES

In Chapter 6 and Chapter 7, we have studied how to recursively construct a large switch from smaller switch modules based on the channel grouping principle. In such a construction, every input is broadcast to each first-stage module, and the input size of first-stage modules is still the same as that of the entire switch.

In this chapter we consider a different approach to build modular switches. The architecture is based on the Clos network (see Fig. 10.1). Switch modules are arranged in three stages, and every module is interconnected with every module in the adjacent stage via a unique link. In this book, the three stages are referred as input stage, middle stage, and output stage, respectively. The modules in those stages are accordingly called input modules, central modules, and output modules. Each module is assumed to be nonblocking and could be, for example, one of the crossbar switches described previously. Inputs are partitioned into groups, and only one group of inputs are connected to each input module, thereby reducing the size of each module.

One may wonder why we are not just considering a two-stage interconnection network in which every pair of modules of adjacent stages are interconnected with a dedicated link. In that case, no two ...