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：電資學院
開課系所︰資工
考試日期（年月日）︰ 2010/11/05
考試時限（分鐘）： 180 m
是否需發放獎勵金： 是
（如未明確表示，則不予發放）
試題 :
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)
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