1、Homework #1 1. How long does it take to send a file of 640,000 bits from host A to host B over circuit- switched networks when all links are 1.536 Mbps, each link uses TDM with 24 slots/sec, and there is 500msec to establish end-to-end circuit. 2. Suppose users share a 1Mbps link. Also suppose each
2、user requires 100 kbps when transmitting, but each user transmits only 10 percent of the time. Suppose there are 40 users. a. When circuit switching is used, how many users can be supported? b. For the remainder of this problem, suppose packet switching is used. Find the probability that a given use
3、r is transmitting. c. Find the probability that at any given time, exactly users are transmitting simultaneously. d. Find the probability that there are 11 or more users transmitting simultaneously. 3. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three link
4、s, of rates = 500 kbps, = 2 Mbps, and = 1 Mbps. a. Assuming no other traffic in the network, what is the throughput for the file transfer. b. Suppose the file is 4 million bytes. Roughly, how long will it take to transfer the file to Host B? c. Repeat (a) and (b), but now with reduced to 100 kbps. 9
5、 13 . Homework #1() 1. How long does it take to send a file of 640,000 bits from host A to host B over circuit- switched networks when all links are 1.536 Mbps, each link uses TDM with 24 slots/sec, and 500msec to establish end-to-end circuit. Bandwidth: 1.536 Mbps / 24 = 64Kbits Transmission time:
6、640,000bits / 64Kbits = 10 sec Overall time = circuit establishment time + transmission time = 10.5 sec 2. Suppose users share a 1Mbps link. Also suppose each user requires 100 kbps when transmitting, but each user transmits only 10 percent of the time. Suppose there are 40 users. a. When circuit sw
7、itching is used, how many users can be supported? 1Mbps / 100kbps = 10 users b. For the remainder of this problem, suppose packet switching is used. Find the probability that a given user is transmitting. 10 % c. Find the probability that at any given time, exactly users are transmitting simultaneou
8、sly. d. Find the probability that there are 11 or more users transmitting simultaneously. 3. Suppose Host A wants to send a large file to Host B. The path from Host A to Host B has three links, of rates = 500 kbps, = 2 Mbps, and = 1 Mbps. a. Assuming no other traffic in the network, what is the thro
9、ughput for the file transfer. min(500kbps, 2Mbps, 1Mbps) = 500kbps b. Suppose the file is 4 million bytes. Roughly, how long will it take to transfer the file to Host B? (4 million bytes) / (500kbps) = 64 secs c. Repeat (a) and (b), but now with reduced to 100 kbps. min(500kbps, 100kbps, 1Mbps) = 100kbps (4 million bytes) / (100kbps) = 320 secs
