IS250 Computer Based Communications Networks and Systems

Spring 2010

Assignment 6

Assignment 6 is due at 2pm (before start of class) on Tuesday 4/27. Please see grading policy on course homepage for additional details regarding early/late submissions.

Please submit your answers in plain text (with any images as attachments) to i250hw@ischool.berkeley.edu.

. Flow Control

Consider a GEO satellite channel that has a throughput capacity of 2Mbps. Assume that the satellite orbits at 20,000 miles above the earth, and that radio transmissions propagate at the speed of light. Assume that processing delays are negligible.

(a) (1 point) What is the round-trip-time (RTT) of this channel?

(1 point) If the stop-and-go technique is used for flow control, and the data packets are 1500 bytes in size, what is the maximum achievable data rate (in bits per second)?

(1 point) Based on your answer to part b, what is the utilization of the channel capacity (in percent)?

(1 point) If the flow control technique is changed to sliding window, what should the window size be so that the channel utilization is increased to at least 50%?

Note: This question is modified from Comer Exercise 26.7 (page 446).

2. TCP Congestion Control

A host transmits a VERY large file using TCP. 

(2 points) Assume that slow start and congestion avoidance are in force, but not fast retransmit, fast recovery, or flow control. Plot the size of the congestion window (cwnd) over time, starting with cwnd=1 at the beginning of the session.  Each horizontal grid represents one RTT, and each vertical grid represents one segment size.  Assume that the sender can send multiple segments instantaneously as allowed by the congestion window size. Further assume that, whenever cwnd reaches or exceeds 8 segments, the very next segment sent is lost (but not the subsequent segments), and the sender times out after 2 RTTs.

cwnd vs. time

(2 points) Assume each segment is 1000 bytes, and the average RTT is 100ms.  For the transfer of a very large file, what is the average throughput of this session (in bits per second)?


Web browsers employ caching to improve access times of frequently requested pages. Section 4.8 of Comer describes caching in web browsers.

(a) (2 points) Describe the steps a browser takes to determine whether to use an item from its cache.

(b) (2 points) Look up the "conditional GET" method in the specification of HTTP/1.1. What advantage does this method offer over the HEAD and GET methods used in Algorithm 4.1 (page 58 of Comer)?

4. Multimedia Networking

(2 points) Explain how a jitter buffer permits the playback of an audio stream even if the Internet introduces jitter. [Comer Exercise 29.2, page 506]