Blockage in Multistage Interconnection Networks

• A number of classification criteria exist for MINs. Among these criteria is the criterion of blockage.
• Blocking Networks; Blocking networks possess the property that in the presence of a currently established interconnection between a pair of input/output, the arrival of a request for a new interconnection between two arbitrary unused input and output may or may not be possible.
• Examples of blocking networks include Omega, Banyan, Shuffle-Exchange, and Baseline. Consider, for example the SEN shown in Figure 2.5b. In the presence of a connection between input 101 and output 011, a connection between input 100 and output 001 is not possible. This is because the connection 101 to 011 uses the upper output of the third switch from the top in the first stage. This same output will be needed by the requested connection 100 to 001.
• This contention will lead to the inability to satisfy the connection 100 to 001, that is, blocking. Notice however that while connection 101 to 011 is established, the arrival of a request for a connection such as 100 to 110 can be satisfied.
• Nonblocking Networks; Nonblocking networks are characterized by the property that in the presence of a currently established connection between any pair of input/output, it will always be possible to establish a connection between any arbitrary unused pair of input/output. The Clos is a well-known example of nonblocking networks.