EEE521 Computer and Data Communication Networks
Semester 2 2011/2012
Preparatory questions for CSMA/CA and WLANs
These questions facilitate the study of random access methods in wireless computer networks.
Students should read the text book, Chapter 12.1 CSMA/CA part and Chapter14 Wireless LAN (14.1
only). It is recommended for students to complement their study with other supplementary reading
resources. Students are advised to attempt as many questions as they can before the lecture and
complete the answers during and after the lecture. It is not compulsory to hand in the answers to these
questions but the option is always available.
1. Why is the design of random contention access for the wireless network different from that for the
wired network?
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2. Figure 1 below shows 3 labtops A, B and C in a wireless local area network. The transmission
ranges for A and B are given by the lined ovals.
Figure 1
Laptop A is sending data to Laptop B. During the transmission, laptop C also has a data to be sent
to laptop B. Anticipate the problem that can arise in this situation and identify the reason.
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3. According to CSMA/CA, what actions must be taken by station A, B and C to avoid this
problem?
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4. Figure 2 below shows 4 laptops in a wireless local area network. The transmission ranges of A, B,
C and D are given by the lined ovals.
Figure 2
B is transmitting to A. Meanwhile, C also has to send data to D. Anticipate the problem that can
occur in this situation.
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5. What must C do to avoid the potential problem?
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6. What are the three strategies used by CSMA/CA to regulate medium access in wireless network?
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7. What actions must be taken by the wireless stations prior to sending their request to send (RTS) to
its intended destination in an adhoc wireless LAN? Explain the importance of every action taken.
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8. What actions must be taken by the wireless station when they are alerted by a RTS from a source?
Explain the importance of every action taken.
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9. What action must be taken by the sending wireless station after it has successfully received the
CTS from the destination? Explain the importance of the action.
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10. How do wireless stations know that they have gained the right to use the channel for sending
data? What action must be taken by the wireless station if they are told that another station has
gained the right to access the medium?
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11. Based on the operation of the CSMA/CA, completes the timing diagram in Figure 3 below with
the appropriate frames and waiting times.
Figure 3
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12. What layers are covered by the IEEE specifications for 802.11?
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13. Describe the two kinds of network architectures defined in IEEE 802.11.
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14. What is the difference between an ad hoc architecture and infrastructure architecture in the Basic
Service Set?
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15. What are the two MAC sublayers defined by IEEE 802.11?
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16. Briefly explain how the medium access is coordinated using the PCF in the IEEE 802.11 based
network?
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17. Briefly explain how the PCF access method ensures that polled station which has time-critical
information gained higher priority than stations without critical data.
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18. Briefly explain how the Access Point, Polled Stations and Contention Stations coordinated their
access to ensure that both contention and contention-free traffic are delivered in the wireless
LAN?
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19. What is the importance of the MAC layer data fragmentation in wireless network?
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