计算机网路基础第三章习题

1、2.The following character encoding is used in a data link protocol:A: B: FLAG: ESC: Show the bit sequence transmitted(in binary) for the four-character frame A B ESC FLAG when each of the following framing methods is used:(a) Byte count.(b) Flag bytes with byte stuffing.(c) Starting and ending flag

2、bytes with bit stuffing.(a) (b) (c) 7. In the textbook, the authors show that for a channel with error rate 10-6, error detecting based retransmission is more efficient than error correcting (see 27th slide). Please give ranges of the channel error rate in which error correcting is more efficient, c

3、onsidering only for blocks (1000 bits) with at most 1 bit error.Suppose the error rate is x. N=1/(x*1000);For error correcting, it need 10*N bit of checkbits.For error detecting it need 2001bit10*N2001, 10/(x*1000)Arrive：18+0.3=18.3ms Acknowledgement： 18 ms Time=18+18.3=36.3ms36.3/0.3=121 frames to

4、fill the pipe So 7-bit sequence numbers are needed.32.Frames of 1000 bits are sent over a 1-Mbps channel using a geostationary satellite whose propagation time from the earth is 270 msec. Acknowledgements are always piggybacked onto data frames. The headers are very short. Three-bit sequence numbers

5、 are used. What is the maximum achievable channel utilization for(a) Stop-and-wait?(b) Protocol 5?(c) Protocol 6?1000*1mbps=1s.start : t=0msecthe first frame has been fully transmitted : t = 1 msec,the first frame has fully arrived : t = 271 msec the first frames acknowledgement has been fully sent

6、: t = 272 msecacknowledgement has fully arrived : t = 542 msecSo the cycle is 542 msec. (a) k = 1, efficiency = 1/542 = 0.18%.(b) k = 7, efficiency = 7/542 = 1.29%.(c) k = 4, efficiency = 4/542 = 0.74%33.Compute the fraction of the useful data bandwidth for protocol 6 on a heavily loaded 50-kbps sat

7、ellite channel with data frames consisting of 40 header and 3960 data bits. Assume that the signal propagation time from the earth to the satellite is 270 msec. ACK frames never occur. NAK frames are 40 bits. The error rate for data frames is 1%, and the error rate for NAK frames is negligible. The sequence numbers are 3 bits.With a 50-kbps 8-bit sequence numbers, the pipe is always full. Retransmissions per frame = 0.01. Each good frame: 40 header bits, 1% of 4000 bits, a 4