课程名称︰ 无线多媒体系统研究 课程性质︰ 多媒体 课程教师︰ 吴晓光 开课学院： 电资 开课系所︰ 资工 考试日期（年月日）︰ 2010.12.03 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. (Needless Transmission Deferement) IEEE 802.11 DCF employs RTS/CTS (Request to Send/ Clear to Send) mechanism for mitigating effect of hidden terminals. The RTS/CTS mechanism sets transmission deferral timer to neighbor terminals by exchanging RTS/CTS between transmitter and receiver. Then, in the case that RTS/CTS exchange succeeds, effect of hidden terminals could be suppressed, otherwise,any nieghbor which received RTS and/or CTS defers its new transmission needlessly although DATA packet corresponding to the previous RTS/CTS exchange will not be transmitted. In DCF, a transmitter S sends RTS to a receiver R in order to prevent transmissions from hidden terminals. Howeverm the RTS may collide with a packet from a hidden terminal at the R and R will not return CTS to the terminal S when the hidden terminal I transmits any packet in parallel with the RTS transmission of terminal S. Then, terminals Xs set needless NAV(RTS) although RTS/CTS handshake among the terminal S and the terminal R is missed. (A) Can you offer a scenario which will cause needless transmission deferment due to RTS. (B) Do you have a solution to solve this needless transmission due to RTS. (C) Offer a scenario which will cause needless transmission deferment due to CTS. (D) Can you offer a solution for this needless transmission deferment due to CTS. 2. (Maximum Throughput Limit) The IEEE 802.11 protocol family provides up to 54-Mbps data rate, whereas the industry is seeking higher data rates. Recent papers (IEEE communication letter 2002) show that a theoretical throughput upper limit and a theoretical delay lower limit exist for the IEEE 802.11 protocols. The existence of such limits indicates that by simply increasing the data rate without reducing overhead, the enhanced performance, in terms of throughput and delay, is bounded even when the data rate goes into infinitely high. (A) Can you explain why? (B) Do you have solutions for the problem (C) Ehat are the relative advantages and disadvantage of basic CSMA/CA(without RTS/CTS) and CSMA/CA with RTS/CTS protocols? (D) Explain "adaptive CSMA" solution proposed by Intel Corporation. 3. Network Mobility Consider vehicles such as ships, aircrafts and trains that contain mobile networks with a potentially large number of devices requiring global connectivity. There devices may or may not be sophisticated nodes. By employing a special entity namely a Mobile Router(MR) to act as a gateway, all devices within the network can achieve global connectivity irrespective of their capabilities. Nemo working group identifies 3 types of nodes that would be supported by a Mobile Router. Local Fixed Nodes, i.e, nodes which belong to the mobile netowrk and cannot move with respect to the mobile Router, would tyically not be able to achieve global connectivity without the support of the MR. There could also be nodes which can move with respect to the MR namely Local Mobile Nodes (home link belongs to Mobile network) and visiting mobile nodes (home link does not belong to mobile network) Describe your solution to extend mobile IP to support local fixed nodes, visiting mobile nodes and visiting network. 4. TCP over Satellite Networks Future broadband satellite networks relying on GEO, MEO,LEO constellations intend to provide a wide variety of high and medium bit-rate data services to the end user. Given that IP applications in general and TCP/IP (WWW,email, filetransfer) more specifically are by far the most popular in the current data netwokrs, it becomes quite evident that TCP will be the transport protocol of a significant part of the traffic of all networks, including satellite networks. (A) For typical satellite Networks with long propagation delays and high link error rates, explain the possible adverse effects(Performance degration) of TCP REno. (B) Propose an end to end revision solution of TCP Reno to address the adverse effects. 5. Consider an indoors 802.11a LAN that operate in a small office environment. Suppose the transmitter and receive are located 6m apart and there is a reflection point on the ceiling that contributes to multipath effects. (A) if the ceiling height is 4m relative to the transceive levels, ehat is the time delay between signals going direcylt from the transmitter to the receiver and those bouncing off the reflection point? (B) if the system operates at 54Mbps , what is the effect of the delay on the signal spread? (C) Does 802.11a offer solution to avoid exposed terminal problem? (D) How does 802.11a deal with hidden terminal problem? (E) Elthough 802.11 offer higher data rates (up to 54Mbps), it may suffer from RTS/CTS (basic rate) and collision overhead to degrade the correponding total throughout. Do you have performance improving solutoins for 802.11a? -- 叶子的离开, 是因为风的追求, 还是树的不挽留 --

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