课程名称︰ 高等计算机网路 课程性质︰ 选修课程 课程教师︰ 逄爱君 开课学院： 电机资讯学院 开课系所︰ 资讯工程研究所 考试日期（年月日）︰ 98/11/16 考试时限（分钟）： 180 是否需发放奖励金： 是 （如未明确表示，则不予发放） 试题 : 1. Wireless Link Characterice:(10%) A. Please describe three "unique" wireless link characteristics.(5%) B. Please elaborate on the relationship among SNR, BER, and PHT.(5%) 2. Hidden Terminal:(10%) A. What is "hidden terminal" problem?(5%) B. How does 802.11 MAC solve the hidden terminal Problem? (5%) 3. Multimedia Networking Applications: (20%) A. Please give three different kinds of multimedia networking applications.(5%) B. Please compare the QoS requirements of the three kinds of applicatoin. C. What are the challenges for multimedia applications over the Internet? Also, please describe possible solutions to overcome the challenges.(10%) 4. Please compare the public-key and symmetric-key (as detailed as possible) for network security.(7%) 5. Message Confidentiality, Message integrity and Endpoint Authentication.(13%) A. Why do we need "message integrity" and "endpoint authentication" if we already have "message confidentially"?(5%) B. What is "Digital Signature"? Why do we CA for "Diginal Signature"?(8%) 6. Suppose the correspondent in Figure 6.21 were mobile. Sketch the additinal network- layer infrastructure that would be needed to route the datagram from the original mobile user to the (now mobile) correspondent. Show the structure of the datagram(s) between the original mobile user and the (now mobile) correspondent, as in Figure 6.22.(10%) 7. Consider the leaky bucket policer that polices the average rate and burst size of a packet flow. We now want to police the peak rate, p as well. Show how the output of this leaky bucket policer can be fed into a second leaky bucket policer so that the two leaky buckets in series police the average rate, peak rate, and burst size. Be sure to give the bucket size and token generation rate for the second policer.(10%) 8. Consider the figure below. A sendere begins sending packetized audio peridically at t=1. The first packet arrives at the receiver at t=8.(10%) (a) What are the delays (from sender to receiver, ignoring any playout delays) of packets 2 through 8? Note that each vertical and horizontal line segment in the figure has a length of 1,2,or 3 time units. (b) If audio playout begins as soon as the first packet arrives at the receiver at t=8, which of the first eight packets sent will not arrive in time for playout? (c) If audio playout begin at t=9, which of the first eight packets sent will not arrive in time for playout? (d) What is the minimum playout delay at the receiver that results in all of the first seven packets arriving in time for their playout? 9. Consider the following pseudo-WEP protocol. The key is 4 bits and the IV is 2 bits. The IV is appended to the end of the key when generating the keystream. Suppose that the shared secret key is 1101. The keystreams for the four possible inputs are as follows: 110100:0010101101010101001011010100100... 110101:1010011011001010110100100101101... 110110:0001101000111100010100101001111... 110111:1111101010000000101010100010111... Suppose all message are 8 bits long.Suppose the ICV (integrity check) is 4 bits long, and is calculated by XOR-ing the firsdt 4 bits of data with the lst 4 bits of data. Suppose the pseudo-WEP packet consists of three fields: first the IV field , then the message field, and last the ICV field, with some of there field encrypted. (a) We want to send the message m=1010000 using IV=11 and using WEP. What will be the values in the three WEP fields?(3%) (b) Show that when the receive decrypts the WEP packet, it recovers the message and the ICV.(3%) (c) Suppose Trudy intercepts a WEP packet (not necessarily with the IV=11) and wants to mudify it before forwaring it to the receiver. Suppose Trudy flips the first ICV bit. Assuming that Trudy does not know the keystreams for any of the IVs, what other bit(s) must Trudy also flip so that the received packet passes the ICV check?(4%) .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.- -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. .-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. -.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-. -- 没有不可能的事, 只有不愿做的事 --

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