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課程名稱︰無線多媒體系統研究
課程性質︰多媒體
課程教師︰吳曉光
開課學院：電資
開課系所︰資工
考試日期（年月日）︰2009.12.05
考試時限（分鐘）：180
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
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?
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