Solutions to Homework #1

Chapter 1, exercises 6,7,9,15

Homework 1 Grade Distribution:

Points	Frequency
9	1
10	0
11	2
12	3
13	19
14	13

6. (4 points)

• circuit switching: There is a dedicated circuit between the two nodes, over which they exchange bit streams. Once a circuit is established, resources are dedicated for the communication. This means the nodes can count on the level of service (e.g., bit rate) they will receive.
• packet switching: Nodes send/receive messages (rather than bit streams) without establishing a circuit through the network. Resources (e.g., physical links) are statistically multiplexed, meaning that they can be more efficiently utilized.

Additional possibilities: packet switching avoids set-up overhead and lowers initial latency, packet switching allows fault-tolerance since packets may be able to take different paths through the network.

(1 point for each definition and 1 point for an advantage or disadvantage of each)

7. ( 2 points)

• Addresses might be absolute, meaning that all source nodes can use the same address to identify a particular destination node. (in contrast, different source nodes might use different relative addresses.) Addresses might also be hierarchical. Such addresses usually provide hints as how data should be routed towards the destination.

9. (4 points)

• Computer communications are bursty and the links are idle for much of the time. In contrast, telephone and cable signals are smooth and well-behaved. This also means that it is possible to determine how many phone connections and TV channels can share a given link (i.e., how many frequencies and timeslots to multiplex), whereas the number of computer communications that can share is single link varies over a large range.

(1 point each for recognizing bursty versus well-behaved traffic, and 1 point each for its implication on link utilization)

15.(4 points)

• Jitter is the variation in the latency of packets. In voice applications, this variation would distort the audio: packets arrive early or late. This would not happen in a circuit switched network, which delivers a constant bit rate. Jitter can be managed in a packet switched network by putting buffers on the receiver; early packets are buffered until needed, and late packets simply cause the buffer to be drained. (See also Section 9.3 of the text.)

(1 point for describing jitter, 1 point for what's the problem with jitter, 1 point for why circuit switching is used, and 1 for how to use packet-switched )