Question

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Consider a virtual circuit (VC) network with a 3-bit field for the VC number. Suppose that the network wants to set up a virtual circuit that traverses four links: Link A, Link B, Link C, and Link D. Suppose that each of these links is currently carrying other virtual circuits, and the VC numbers of these other VCs are as follows:

Link A	Link B	Link C	Link D
000	101	001	011
100	111	110

a) If each VC is required to use the same VC number on all links along its path, what VC number could be assigned to the new VC.

b) If each VC is permitted to have different VC numbers on the different links along the path (so that the forwarding table must perform VC number translation), how many different combinations of eight VC numbers (one for each of the four links) could be used given the above assignments already exist?

Answer 1

a)

Notice that there is no one common VC number that is not being used on all the four links. As a result, there is no VC number, which could be assigned to the new VC. This example illustrates a problem associated with having to pick a common VC number along the end-to-end of a virtual circuit.

b)

In this case, a VC number can be assigned independently on each of the four links. Since there are two vacant VC numbers on each of these four links, the possible combinations are One example combination is (10,00,00,10).