Figure 7-5 A 4×4 crossbar switch.
The routing operation in switching networks can be controlled centrally or distributed over the entire switching network, i.e., each cross-point in the network participates in the routing operation as in the
case of self-routing. Typically, the central routing control is employed in the crossbar switching network whereas the Banyan network is inherently self-routing by design. The central routing requires a very fast
routing control logic which is capable of setting N connections simultaneously per time slot. Self-routing due to its distributed nature is required to set up a single connection at each cross-point per time slot. Its
routing speed is thus determined by the connection request arrival rate per input port. Therefore, self- routing is highly desirable for high-speed switching networks.
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Copyr ight © CRC Pr ess LLC
by Abhijit S. Pandya; Ercan Sen CRC Press, CRC Press LLC
ISBN: 0849331390 Pub Date: 110198
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III. Multistage Interconnection Networks MINs for ATM Switching Networks
One class of switching networks is multistage interconnection networks. A typical multistage interconnection network consists of a large number of elementary N×N cross-point switches N
≤ 32
connected through external links in a certain fashion to provide a larger switching network structure.
A. Nonblocking MINs
A large non-blocking MIN can be constructed using nonblocking switching elements such as crossbar switches. For example, a 16×16 nonblocking MIN can be constructed using 8 4×4 crossbar switching
elements in a two-stage formation as shown in Figure 7-6. This particular configuration provides a single internal path for each inputoutput port pair. It is also possible to provide more than one
concurrent internal path for each inputoutput port pair using additional stages and different switch sizes. For example, the number of concurrent internal paths for each inputoutput pair can be increased from
one to four using a 3-stage configuration as shown in Figure 7-7.
B. Blocking MINs
One of the best known switching network models in this class is the Banyan network in which the elementary switch is a 2×2 crossbar switch. Figure 7-8 illustrates an 8×8 Banyan switch which is
constructed from these 2×2 crossbar switches. For the 2×2 switch only 1 bit address is required. For the 8×8 Banyan switch, 3 bit addressing is required to route a cell through the switch. Each input and output
port of the 8×8 Banyan switch is identified by a 3-bit address.
The cell at input port 100 shows the data field as well as the destination address field which is 010 in this case. In order to route this cell from input port 100 to output port 010, one bit is used for each stage.
There is one unique path for each input-output pair. The bold line indicates the path through which the cell is routed through the switch. In the worst case situation, in an N×N Banyan switching network,
during a single connection interval, out of N disjoint input-output routing requests cannot be
serviced due to contentions at the connection links between stages.
Figure 7-6 Two-stage MIN configuration for a 16×16 nonblocking switch with a single internal path
for each inputoutput pair.
