Crossbar Networks

Figure 2.3: An $8 \times 8$ crossbar network (a) straight switch setting; and (b) diagonal switch setting.

• While the single bus can provide only a single connection, the crossbar can provide simultaneous connections among all its inputs and all its outputs.
• The crossbar contains a switching element (SE, also called a cross-point) at the intersection of any two lines extended horizontally or vertically inside the switch (see Fig. 2.3).
• Simultaneous connections between $P_i$ and $M_{8-i+1}$ are made.
• The number of SEs (switching points) required is 64 and the message delay to traverse from the input to the output is constant, regardless of which input/output are communicating.
• In general for an $N \times N$ crossbar, the network complexity, measured in terms of the number of switching points, is $O(N^ 2)$ while the time complexity, measured in terms of the input to output delay, is $O(1)$.
• Notice also that the crossbar is a nonblocking network that allows a multiple input/output connection pattern (permutation) to be achieved simultaneously. However, for a large multiprocessor system the complexity of the crossbar can become a dominant financial factor.