计算机网络英文版习题答案

P63 #5 Consider sending a packet of F bits over a path of Q links. Each link transmits at R bps. The network is lightly loaded so that there are no queuing delays. Propagation delay is negligible.

a.Suppose the network is a packet-switched virtual-circuit network. Denote the VC setup time by t s seconds. Suppose the sending layers add a total of h bits of header to the packet. How long does it take to send the file from source to destination?

t s+[(F+h)/R]Q

b.Suppose the network is a packet-switched datagram network and a connectionless service is used. Now suppose each packet has 2h bits of header. How long does it take to send the packet?

[(F+2h)/R]Q

c.Finally, suppose that the network is a circuit-switched network. Further suppose that the transmission rate of the circuit between source and destination is R bps. Assuming ts setup time and h bits of header appended to the packet, how long does it take to send the packet?

t s+(F+h)/R

P64 #6 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 that the propagation speed along the link is s meters/sec. Host A sends 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 terms of L and R.

[c] Ignoring processing and queueing delays, obtain an expression for the end-to-end delay.

[d] Suppose Host A begins to transmit the packets 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 x 108, L=100 bits and R=28kbps. Find the distance m so that d prop = d trans.

[a] d prop = m/s

[b] d trans = L/R

[c] end-to-end delay = d prop + d trans=m/s+L/R

[d] The beginning position of the link.

[e] On the channel between A and B.

[f] On the host B.

[g] m/s = L/R = > m = sL/R = > m = 892.86 km

P65 #10 Consider the queueing delay in a router buffer. Suppose that all packets are L bits, the transmission rate is R bps, and that N packets simultaneously arrive at the buffer every LN/R seconds. Find the average queueing delay of a packet (in terms of L, R and N). (Hint: The queueing delay for the first packet is zero; for the second packet L/R; for the third packet 2L/R. The Nth packet has already been transmitted when the second batch of packets arrives.)

As the Nth packet has already been transmitted when the next batch of packets arrive, we only need to consider the delay for a single batch of packets.

Average delay = Total delay / Number of packets

Delay for 1st packet = 0

Delay for 2nd packet = L/R

Delay for 3rd packet = 2L/R

...

...

Delay for Nth packet = (N-1)L/R

Total delay for N packets = (0 + 1 + 2 ... +(N-1) ) * (L/R)

Using the formulas for sum of integer series, this can be written as: Total delay for N packets = (N-1) * (N/2) * (L/R)

Therefore, average delay for N packets = ((N-1) * L) / 2R

P170 #12 What is the difference between persistent HTTP with pipelining and persistent HTTP without pipelinning? Which of the two is used by HTTP/1.1?

For the persistent connection without pipelining, the client issues a new request only when the previous has been received. In this case, the client experiences one RTT in order to request and receive each of the referenced objects.

For the persistent connection with pipelining, the client issues a request as soon as it encounters a reference. It is possible for only RTT to be expended for all the referenced objects.