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：电资学院 开课系所︰资工 考试日期（年月日）︰ 2010/11/05 考试时限（分钟）： 180 m 是否需发放奖励金： 是 （如未明确表示，则不予发放） 试题 : 1. Performance Anomoly of 802.11b IEEE 802.11 uses the CSMA/CA protocol to share the radio channel in a fair way. However, we have observed that in some common situations in a wireless environment, the method results in a considerable performance degradation. In a typical wireless local area network, some hosts may be far away from their access point so that the quality of their radio transmissions is low. In this case current 802.11b products degrade the bit rate from nominal 11Mb/s rate to 5.5, 2, or 1Mb/s- when a host detects repeated unsuccesful freame transmissions,it decreases its bit rate. If there is at least one host with a lower rate, a 802.11 cell presents a performance anomaly: the thoughtout of all host transmitting at the higher rate is degraded below the level ofthe lower rate. Such a behavior penalizes fast hosts and privileges the slow one. (A) Can you offer the reason for the 802.11b performance anomaly? (B) Can you offer a solution for the 802.11b performance anomaly? 2. Dynamic Rate Shifting (Auto rate) 0f 802.11b IEEE 802.11b uses the CSMA/CA protocol to share the radio channel. The following figure shows IEEE 802.11 DCF throughtput as a function of the number of stations from an indoor table top experiment for different date rates of 802.11b- 11Mbps, 5.5 Mbps, 2 Mbps and 1Mbps- and dynamic rate shifting ("auto rate") iplemented in Enterasys WLAN cards. At fixed data rates, the throughtput curves are realtively flat as the number of pocket PCs is increased from 2 to 18. When auto rate is enabled- the default mode in most WLAN cards- then aggregate throughput declines drastically as seen in Figure , reaching 1 Mbps at 16 wireless stations. (A) Can you explain the results? (B) Do you have solutions for auto rate performance degradation? (C) The way to enhance IEEE 802.11 protocol. 3. (Fairness in 802.11) In this question, we study the problem of maintaining fairness for upstream and downstream connections in wirelss local area networks (WLANs) based upon the IEEE 802.11 standard. Current implementations of 802.11 use the so-called Distributed Coordination Function (DCF), which provides similar medium access priority to all stations. Although this mode of operation ensures fair access to the miediom at the MAC level, it does not provide any provisions for ensuring fairness among the upstream and downstream connections. Connection unfairnesss may result in significant degradation of performance leading to users perceiving unsatisfactory quality of service. (A) Can you explain why current 802.11 might produre this so-called "critical unfairness" between upstream and downstream connect as the attached Figure. (B) Can you revise IEEE 802.11 to solve this problem? Can you ensure fairness among the upstream and downstream connections? 4. Large Area Interference Problem Sec I (A) Illustrate what is stronger-last collision? (B) Does Adaptive CSMA protocol (recently proposed by Intel Corporation) try to eliminate the exposed terminal problem and the hidden terminal problem? (C) Describe the advantage and disadvantage of Adaptive CSMA protocol. Sec II IEEE 802.11b uses the CSMA/CA protocol to share hte radio channel in a fair way. However we have observed that in some common situations in a wireless environment, the method results in a considerable performance degradation by the figure 1 and figure 2. (A) Can you offer the reason for the 802.11b large area interference problem? (B) Can you offer a solution for the 802.11b large area interference problem? 5. Wireless Random Loss & Burst Loss A burst loss event may be initiated by signal fading. Prolonged uncontrollable channel interference can lead to correlated packet losses. Yet, it generally occurs over a very short duration, leading to a loss of several consecutive segments at a time. On the other hand, a blackout due to mobility can lead to serial timer expirations for a connection so that multiple consecutive timer expirations and retransmissions of the same data segment take place within a single blackout period. The timeout period for the retransmission timer is doubled with each unsuccessful attempt until it reaches a value of at least 60s. Several consecutive retransmission failures can result in a terribly long period of inactivity of the connection even after thje network conditions have been restored to normal. (A) (Two state markov model for wireless channel): We define a two-state Markov process by the following transition probabilities. In the figure 1,p is the probability of successfully transmitting a frame given the previous frame was successfully transimitted, and 1-q is the probability of successfully transmitting a frame given the previous frame was in error. Devibe the frame error rate. (B) (Probing Strategy) Based on the above channel model, offer revised strategies for traditinal error control protocol (ARQ, automatic repeat request) and transmission Control Protocol (TCP). (A) -- 没有不可能的事, 只有不愿做的事 --

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