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Authors:
James F. Kurose, Keith W. Ross
Chapter:
The Network Layer
Exercise:
Problems
Question:5 | ISBN:9780132856201 | Edition: 6

Question

Consider a VC network with a 2-bit field for the VC number. Suppose that the network wants to set up a virtual circuit over four links: link A, link B,link C, and link D. Suppose that each of these links is currently carrying two other virtual circuits, and the VC numbers of these other VCs are as follows:

 

 

 

 

 

 

 

In answering the following questions, keep in mind that each of the existing VCs may only be traversing one of the four links.

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 in the different links along its path (so that forwarding tables must perform VC number transla- tion), how many different combinations of four VC numbers (one for each of the four links) could be used?

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Answer

The VC (virtual circuit) number contains two digits. So, the size of VC number is 2 bits. To step the network, the VC uses four links A, B, C and D respectively. Each link carries two links.

a.    

It is not possible to establish a new VC over the network. The VC is using a number with 2 bits size.

Only four numbers can be created using 2 bits number. There are four links in total and there is no room for the new VC.

 

b.

The number of links is 4, and size of VC numbers is 2.

The number of combinations can be calculated as 24=16.

Therefore, each VC can have one of these combinations.

Example combination is (00,10,00,01).

 

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