Another desirable property of interconnection networks is node symmetry. A node symmetric network has no distinguished node, that is, the ``view'' of the rest of the network is the same from any node. Rings, fully connected networks, and hypercubes are all node symmetric. Trees and stars, shown in Figure 12, are not. A tree has three different types of nodes, namely a root node, interior nodes, and leaf nodes, each with a different degree. A star has a distinguished node in the center which is connected to every other node. When a topology is node asymmetric a distinguished node can become a communications bottleneck.
A more formal definition of a communication bottleneck is based on a property known as the bisection width, which is the minimum
Figure 12: Tree and Star.
number of links that must be cut in order to divide the topology into two independent networks of the same size (plus or minus one node). The bisection width of a tree is 1, since if either link connected to the root is removed the tree is split into two subtrees. The bisection bandwidth of a parallel system is the communication bandwidth across the links that are cut in defining the bisection width. This bandwidth is useful in defining worst- case performance of algorithms on a particular network, since it is related to the cost of moving data from one side of the system to the other.