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）︰2009.12.05 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. Performance Anomaly of 802.11b IEEE 802.11b 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 way from their access point so that the quality of their radio transmissions is low. In this case current 802.11b produces degrade the bit rate from the nominal 11Mbps rate to 5.5, 2, or 1Mbps- when a host detects repeated unsuccessful frame transmission, 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 throughput of all host transmitting at the higher rate is degraded below the level of the lower rate. Such a behavior penalizes fast hosts and priviliges 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. 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 theretical throughput upper limit and a theoretical delay lower limit ecist for the IEEE 802.11 protocols. The existence of such limits indicates that by simple 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 hight. (A) Can you explain why? (B) Do you have solutions for the problem? (C) What 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. Cellular Technology (A) Explain what are sector (such as three-sector case) and what is cell splitting? Any corresponding benifit and disadvantage for sector and cell splitting (B) Usually different user might have different mobility behaviors, do you have any solutions to distinguish high speed and low speed customers and avoid unnecessary disconnections? (C) Direct Sequence Cellular system adopts universal frequency reuse policy. Explain what is universal freqeuncy reuse policy? What is the advantage and disadvantage of this policy? (D) If W=1.25 MHz, R=9600bps, and a minimum acceptable E/N is found to be 10db, determine the maximum number of users that can be supported in a single-cell CDMA system using omni- directional base station antennas and voice activity factor a=3/8, K=1/2, suppose the background noise is approximately neglected, where W/R is called the processing gain (E) CDMA also provides a natural way to exploit the burstly nature a source for added capacity. Can you explain how CDMA improves the capacity in the case of a two-way telephone conversation? 4. Power Saving The IEEE 802.11 standard recommends the following technique for power conservation. A mobile that wishes to conserve power may switch to sleep mode and inform the base station of this decision. The base station buffers packets received from the network that are destined for the sleeping mobile. The base station periodically transmits a beacon that contains information about such buffered packets. When the mobile wakes up, it listens for this beacon, and responds to the base station which then forwards the packets. This approach conserves power but results in additional delays at the mobile that may affect the quality of service (QoS) (A) Describe the difference between Active mode and Power Saving Mode. (B) How does Power Saving Mode station know when to wait up to get the TIM (Traffic Indication Map)? (C) Describes is the purpose of TIM (Traffic Indication Map) (D) How does Power Saving Mode station to inform AP that he is ready to accept the buffered data? (E) For BSS multicast services, how does AP transmit multicast or broadcast to all of the Stations if at least one of the stations are in power saving mode? (hint DTIM) 5. Mobile Communication (A) Discuss the similarities and differences between a conventional cellular radio system and a space-based (satellite) cellular radio system. What are the advantages and disadvantages of each system? Which system could support a larger number of users for a given frequency allocation? Why? (B) Today, airplanes seem to be the last remaining islands where mobile communications and Internet access are not available. Do you have any solution for deploying the mobile Communications and Internet access in the airplane? (C) Describe the difference between ad hoc (multi-hop) mode network and cellular infrasturucture (single-hop) mode network. Which one could be deployed dynamically? Which one might require distributed schemes? In your opinion, is that possible the next generation network will be infrastructure-less, self organized networks? -- 叶子的离开, 是因为风的追求, 还是树的不挽留 --

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