Chapter 1 Assigned Problems

Answer Key

 

2. An alternative to a LAN is simply a big timesharing system with terminals for all users. Give two advantages of a client-server system using a LAN.

The LAN model can be grown incrementally. If the LAN is just a long cable, it cannot be brought down by a single failure (if the servers are replicated) It is probably cheaper. It provides more computing power and better interactive interfaces.

3. The performance of a client-server system is influenced by 2 network factors: the bandwidth of the network and the latency. Give an example of a network that exhibits high bandwidth and high latency. Then give an example of one with low bandwidth and low latency.

A transcontinental fiber link might have many gigabits/sec of bandwidth, but the latency will also be high due to the speed of light propagation over thousands of kilometers. In contrast, a 56-kbps modem calling a computer in the same building has low bandwidth and low latency.

4. Besides bandwidth and latency, what other parameters is needed to give a good characterization of the quality of service offered by a network used for digitized voice traffic?

A uniform delivery time is needed for voice, so the amount of jitter in the network is important. This could be expressed as the standard deviation of the delivery time. Having short delay but large variability is actually worse than a somewhat longer delay and low variability.

6. A client-server system uses a satellite network, with the satellite at a height of 40,000 Km. What is the best-case delay in response to a request?

The request has to go up and down, and the response has to go up and down. The total path length traversed is thus 160,000 km. The speed of light in air and vacuum is 300,000 km/sec, so the propagation delay alone is 160,000/300,000 sec or about 533 msec.

8. A collection of five routers is to be connected in a point-to-point subnet. Between each pair of routers, the designers may put a high-speed line, a medium-speed line, a low-speed line, or no line. If it takes 100 ms. of computer time to generate and inspect each topology, how long will it take to inspect all of them?

Call the routers A, B, C, D, and E. There are ten potential lines: AB, AC, AD, AE, BC, BD, BE, CD, CE, and DE. Each of these has four possibilities (three speeds or no line), so the total number of topologies is 4**10 = 1,048,576. At 100 ms each, it takes 104,857.6 sec, or slightly more than 29 hours to inspect them all.

10. A disadvantage of a broadcast subnet is the capacity wasted when multiple hosts attempt to access the channel at the same time. As simplistic example, suppose that time is divided into discrete slots, with each of the n hosts attempting to use the channel with probability p during each slot. What fraction of the slots are wasted due to collisions?

Distinguish n + 2 events. Events 1 through n consist of the corresponding host successfully attempting to use the channel, i.e., without a collision. The probability of each of these events is p(1 - p)**n - 1. Event n + 1 is an idle channel, with probability (1 - p)**n. Event n + 2 is a collision. Since these n + 2 events are exhaustive, their probabilities must sum to unity. The probability of a collision, which is equal to the fraction of slots wasted, is then just 1 - np(1 - p)**n - 1 - (1 - p)**n.

11. What are two reasons for using layered protocols?

Reasons for using layered protocols; using them leads to breaking up the design problem into smaller, more manageable pieces, and layering means that protocols can be changed without affecting higher or lower ones.

13. What is the principal difference between connectionless communication and connection-oriented communication?

Connection-oriented communication has three phases. In the establishment phase a request is made to set up a connection. Only after this phase has been successfully completed can the data transfer phase be started and data transported. Then comes the release phase. Connectionless communication does not have these phases. It just sends the data.

18. Which of the OSI layers handles each of the following: (a) Dividing the transmitted bit stream into frames. (b) Determining which route through the subnet to use.

(a) Data link layer. (b) Network layer.

19. If the unit exchanged at the data link level is called a frame and the unit exchanged at the network level is called a packet, do frames encapsulate packets or do packets encapuslate frames? Explain your answer.

Frames encapsulate packets. When a packet arrives at the data link layer, the entire thing, header, data, and all, is used as the data field of a frame. The entire packet is put in an envelope (the frame), so to speak (assuming it fits).

21. List two ways in which the OSI reference model and the TCP/IP reference model are the same. Now list two ways in which they differ.

Both models are based on layered protocols. Both have a network, transport, and application layer. In both models, the transport service can provide a reliable end-to-end byte stream. On the other hand, they differ in several ways. The number of layers is different, the TCP/IP does not have session or presentation layers, OSI does not support internetworking, and OSI has both connection-oriented and connectionless service in the network layer.

22. What is the main difference between TCP and UDP?

TCP is reliable connection oriented protocol that allows data originating on one host to be delivered on a second host without errors, whereas UDP is a connectionless service that does not guarantee reliability.With UDP the programmer has to provide sequencing and control flow.

25. When a file is transferred between two computers, two acknowledgement 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 ins not acknowledged. In the second one, the packets are not acknowledged individually , but the entire file is acknowledged when it arrives. Discuss these 2 approaches.

If the network tends to lose packets, it is better to acknowledge each one separately, so the lost packets can be retransmitted. On the other hand, if the network is highly reliable, sending one acknowledgement at the end of the entire transfer saves bandwidth in the normal case (but requires the entire file to be retransmitted if even a single packet is lost).

29. Ethernet and wireless networks have some similarities and some differences. One property of the Ethernet is that only one frame at a time can be transmitted on the Ethernet. Does 802.11 share this property with Ethernet? Discuss your answer.

Think about the hidden terminal problem. Imagine a wireless network of five stations, A through E, such that each one is in range of only its immediate neighbors. Then A can talk to B at the same time D is talking to E. Wireless networks have potential parallelism, and in this way differ from Ethernet.