CS 118 Spring 2011 : Homework 1Problem 1Consider two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts areseparated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to senda packet of size L bits to Host B.a. Express the propagation delay, dprop, in terms of m and s.b. Determine the transmission time of the packet, dtrans, in terms of L and R.c. Ignoring processing and queuing delays, obtain an expression for the end-to-end delay.d. Suppose Host A begins to transmit the packet at time t = 0. At time t = dtrans, where is the last bitof the packet?e. Suppose dpropis greater than dtrans. At time t = dtrans, where is the first bit of the packet?f. Suppose dpropis less than dtrans. At time t = dtrans, where is the first bit of the packet?a. dprop= m/s secondsb. dtrans= L/R secondsc. dend−to−end= dprop+ dtrans= (m/s + L/R) secondsd. The last bit is just leaving Host A.e. The first bit is in the link and has not reached Host B.f. The first bit has reached Host B.Problem 2Using the same scenario as in Problem 1, suppose m = 10, 000 kilometers, R = 1 Mbps, and s = 2.0 × 108meters/sec. (Please show your work. Hint: Each part of this question builds on the previous part.)a. Calculate the bandwidth-delay product, R × dprop.b. Consider sending a file of 200,000 bits from Host A to Host B. Suppose the file is sent continuouslyas one big message. What is the maximum number of bits that will be in the link at any time? Howabout sending a smaller file of 10,000 bits?c. Provide an interpretation of the bandwidth-delay product – what does it represent?d. What is the width (in meters) of a bit in the link? Is it longer than a football field (which is about100 meters long)?Problem 2 continued on next page. . . Page 1 of 2CS 118 Spring 2011 : Homework 1a. R = 1 Mbs = 1,000,000 bpsm = 10,000 km = 10,000,000 mdprop= m/sdprop= 10, 000, 000 m / (2.0 × 108) m/sec = 0.05 secondsR × dprop= 1, 000, 000 bps × 0.05 secs = 50,000 bitsb. Each bit will be in the link for dpropseconds. The link can send R bits at once. Thus, a maximumof R × dprop= 50, 000 bits can be in the link. 200, 000 > 50, 000, so the link will reach its maximumcapacity of 50,000 bits. For the 10,000 bits file, the maximum number of bits in the link is 10,000.c. The bandwidth-delay product represents the maximum number of bits that can fit in the link.d. We can find the width of a bit by dividing the length of the link by the maximum number of bits thatit can fit:width = m/(R × dprop) = 10,000,000 m / 50,000 bits = 200 meters per bitYes, the width of a bit is longer than a football field.Problem 3Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associatedURL is not cached in your local host, so a DNS look-up is necessary to obtain the IP address. Suppose thatyour host must contact n DNS servers before receiving the needed IP address from DNS; the successive visitsincur RTTs of RT T1, ..., RT Tn(don’t worry about the details of how DNS works). Further suppose thatthe Web page associated with the link is a small HTML file, consisting only of references to three very smallobjects on the same server. Let RT T0denote the RTT between the local host and the server containing theobject. Neglecting transmission times, how much time elapses (in terms of RT T0, RT T1, ..., RT Tn) fromwhen you click on the link until your host receives all of the objects, if you are using:a. Nonpersistent HTTP with no parallel TCP connections?b. Nonpersistent HTTP with parallel connections?c. Persistent HTTP with pipelining?a. DNS RTTs + HTML RTTs (with TCP handshake) + 3 object RTTs (with TCP handshakes)= (RT T1+ ... + RT Tn) + 2RT T0+ 3 × 2RT T0= 8RT T0+ (RT T1+ ... + RT Tn)b. DNS RTTs + HTML RTTs (with TCP handshake) + parallel object RTTs (with TCP handshakes)= (RT T1+ ... + RT Tn) + 2RT T0+ 2RT T0= 4RT T0+ (RT T1+ ... + RT Tn)c. DNS RTTs + HTML RTTs (with TCP handshake) + RTT for all referenced objects= (RT T1+ ... + RT Tn) + 2RT T0+ RT T0= 3RT T0+ (RT T1+ ... + RT Tn)Page 2 of
View Full Document
Unlocking...