CS4514 Practice Mid-Term Exam

1. When transferring a file between two computers, (at least) two acknowledgements strategies are possible. In the first one, the file is chopped up into packets, which are individually acknowledged by the receiver, but the file transfer as a whole is not acknowledged. In the second one, the packets are not acknowledged individually, but the entire file is acknowledged when it arrives. Which one would probably perform better under conditions of high packet loss? Which one would probably perform better under conditions of low load? Explain your answer.

2. A system has a n-layer protocol hierarchy. Applications generate messages of length M bytes. At each of the layers, an h-byte header is added. What fraction of the network bandwidth is filled headers?

3. Television channels are 6 MHz wide. How many bits/sec can be sent if four-level digital signals are used? Assume a noiseless channel.

4. A simple telephone system consists of two end offices and a single toll office to which each end office is connected by a 1-MHz full-duplex trunk. The average telephone is used to make four calls per 8-hour workday. The mean call duration is 6min. Ten percent of the calls are long-distance (i.e., pass through the toll office). What is the maximum number of telephones an end office can support? Assume 4 kHz per circuit.

5. To provide more reliability than a single parity bit can give, an error-detecting coding scheme uses one parity bit for checking all the odd numbered bits and a second parity bit for all the even numbered bits. What is the Hamming distance of this code?

6. A channel has a bit rate of 4kbps and a propagation delay of 20 msec. For what range of frame sizes does stop-and-wait give an efficiency of at least 50 percent?

7. Frames of 1000 bits are sent over a 1-Mbps satellite channel. Acknowledgements are always piggy backed onto data frames. The headers are very short. Three-bit sequence numbers are used. What is the maximum achievable channel utilization for:

   • Stop-and-Wait
   • Protocol 5
   • Protocol 6

8. Ten thousand airline reservation stations are competing for the use of a single slotted ALOHA channel. The average station makes 18 requests/hour. A slot is 125 microsec. What is the approximate total channel load?

9. Measurements of a slotted ALOHA channel with an infinite number of users show that 10 percent of the slots are idle.

   • a) What is the channel load, G?
   • b) What is the throughput?
   • c) Is the channel underloaded or overloaded?

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