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CHAPTER 18
Virtual Circuit Switching:
Frame Relay and ATM
Solutions to Odd-Numbered Review Questions and Exercises
Review Questions
1. Frame Relay does not use flow or error control, which means it does not use the
sliding window protocol. Therefore, there is no need for sequence numbers.
3. T-lines provide point-to-point connections, not many-to-many. In order to connect
several LANs together using T-lines, we need a mesh with many lines. Using
Frame Relay we need only one line for each LAN to get connected to the Frame
Relay network.
5. Frame Relay does not define a specific protocol for the physical layer. Any proto-
col recognized by ANSI is acceptable.
7. A UNI (user network interface) connects a user access device to a switch inside
the ATM network, while an NNI (network to network interface) connects two
switches or two ATM networks.
9. An ATM virtual connection is defined by two numbers: a virtual path identifier
(VPI) and a virtual circuit identifier (VCI).
11. In an UNI, the total length of VPI+VCI is 24 bits. This means that we can define
224 virtual circuits in an UNI. In an NNI, the total length of VPI+VCI is 28 bits.
This means that we can define 228 virtual circuits in an NNI.
Exercises
13. We first need to look at the EA bits. In each byte, the EA bit is the last bit (the eight
bit from the left). If EA bit is 0, the address ends at the current byte; if it 1, the
address continues to the next byte.
Address โ 10110000 00010111
The first EA bit is 0 and the second is 1. This means that the address is only two
bytes (no address extension). DLCI is only 10 bits, bits 1 to 6 and 9 to 12 (from
left).
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