41.3.3.5 Routing topology is determined by the means of providing the interconnect between the ports. Some examples of topologies are: Self-routing switches require only one translation at the matrix input and this results in the appending of a header prefix which is used for 1. Bus, ring: the simplest switching arrangement where a number routing purposes by each of the matrix switching elements and discarded on exit from the switch. In translation table controlled of ports are 'hung' on a single bus and communicate using a switches the path is marked locally in translation tables within each defined protocol which may use slots or some form of conten- switching stage, permitting routing to proceed on the basis of the tion resolution. Simple and easily expanded but limited in original external header. overall capacity by the throughput of the bus. Aring is in effect Routing through the switch matrix may be connection oriented i.e. a closed bus offering the possibility of different access proto- cells always follow the same route through the switch and therefore cols but with generally similar limitations. remain in sequence. Alternatively, a connectionless approach to m matrix: an array of simple 'on/off' switch elements routing within the switch may be taken, the output port address providing the possibility of many co-existent paths between n being carried in the cell and individual cells being routed toward inputs and m outputs (analogous to the electro-mechanical that port according to the local routing algorithm at the particular crossbar switch). Avoids the capacity limitations of the bus and point in the switch matrix. With connectionless routing, cell se- ring by spreading the traffic over a number of paths but expan- quence integrity across the matrix may not be maintained. sion of the number of ports can result in very large cross-point matrices. 3. Multi-stage matrices: If switching elements with a pxq capa- 41.3.3.6 Buffering bility are used instead of the simple 'on/off' function, then more complex a