计算机网络第一章作业

第一章

P96-5: This elementary problem begins to explore propagation delay and transmission delay, two central concepts in data networking. Consider two hosts, A and B, connected by a single link of rate R bps. Suppose that the two hosts are separated by m meters, and suppose the propagation speed along the link is s meters/sec. Host A is to send a packet of size L bits to Host B.

a. Express the propagation delay, d prop , in terms of m and s .

b. Determine the transmission time of the packet, d trans , in term 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= d trans , where is the last bit of the packet?

e. Suppose d prop is greater than d trans . At time t=d trans , where is the first bit of the packet?

f. Suppose d prop is less than d trans . At time t= d trans , where is the first bit of the packet?

g. Suppose s=2.5*108, L=100 bits, and R=28 kbps. Find the distance m so that d prop equals d trans .

Answer

Problem 5

a) s m d prop /= seconds.

b) R L d trans /= seconds.

c) )//(R L s m d end to end +=-- seconds.

d) The bit is just leaving Host A.

e) The first bit is in the link and has not reached Host B.(or s*d trans =s*L/R ) f) The first bit has reached Host B.

g) Want

()893105.210

2810083=⨯⨯==S R L m km.

2. Consider sending a packet of 3000bits over a path of 5 links. Each link transmits at 1000bps. Queuing delays, propagation delay and processing delay are negligible.

(1).Suppose the network is a packet-switched virtual circuit network. VC setup time is 0.1 seconds. Suppose the sending layers add a total of 500 bits of header to each packet. How long does it take to send the file from source to destination?

(2).Suppose the network is a packet-switched datagram network and a connectionless service is used. Now suppose each packet has 200 bits of header. How long does it take to send the file? (3).Suppose that the network is a circuit-switched network. Further suppose that the transmission rate of the circuit between source and destination is 200bps. Assuming 0.02s setup time and 200 bits of header appended to the packet, how long does it take to send the packet?

Solution:

(1). t=5*(3000+500)/1000+0.1=17.6s

(2). t=5*(3000+200)/1000=16s

(3). t=(3000+200)/200+0.02=16.02s