6
Wireless LANs I
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Instructions
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Introduction
1. a) At what layers do wireless LANs operate?
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b) Do wireless LAN standards governed by OSI or TCP/IP standards? Justify your answer.
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2. a) Distinguish between 802.3 standards and 802.11 standards.
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b) What is the actual difference between 802.11 and Wi-Fi?
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c) Do we use the two terms interchangeably?
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3. a) In a Wi-Fi LAN, do two wireless hosts usually send frames directly to one another? Explain.
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b) Why does the access point connect to the corporate Ethernet LAN?
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Radio Signal Propagation
b) In 802.11 Wi-Fi networks, can simple installation rules usually reduce propagation effects to nonissues?
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5. a) What is a transceiver?
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b) Is wireless radio transmission usually described in terms of wavelength or frequency?
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c) What is a hertz?
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d) At what range of frequencies do most wireless systems operate?
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6. a) Distinguish between omnidirectional and dish antennas in terms of operation.
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b) Under what circumstances would you use an omnidirectional antenna?
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c) Under what circumstances would you use a dish antenna?
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d) What type of antenna normally is used in WLANs? Why?
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7. a) If you quadruple propagation distance, how much will signal intensity change at the receiver?
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c) If the signal strength from an omnidirectional radio source is 8 mW at 30 meters, how strong will it be at 150 meters, ignoring absorptive attenuation? Show your work.
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d) What will it be at 200 meters?
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8. a) Contrast inverse square law attenuation and absorptive attenuation.
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b) What causes dead zones?
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c) What is the most serious propagation problem in WLANs?
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d) How is it controlled?
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e) What two propagation problems become worse as frequency increases?
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Service Bands and Bandwidth
9. a) Distinguish among the frequency spectrum, service bands, and channels.
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b) In radio, how can you send multiple signals without the signals interfering with one another?
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d) Are the set of frequencies used for police communication in a city channels or a service band? Explain.
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10. a) Does a signal travel at a single frequency, or does it spread over a range of frequencies?
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b) If the lowest frequency in a channel is 1.22 MHz and the highest frequency is 1.25 MHz, what is the channel bandwidth? Give your answer in kHz.
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c) If you want to transmit seven times as fast, how much wider must the channel be?
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d) Why is large channel bandwidth desirable?
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e) What do we call a system whose channels are wide?
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11. a) Do WLANs today use licensed or unlicensed service bands?
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b) What is the advantage of using unlicensed service bands?
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c) What is the downside?
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12. In Figure 6-12, there are question marks between several pairs of access points. For each of these pairs, list their channels of operation and whether they will interfere.
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13. a) In what two unlicensed service bands does 802.11 usually operate?
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b) How many 20 MHz nonoverlapping channels does the 2.4 GHz band support?
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c) Why is this a problem?
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d) Why are companies moving rapidly into the 5 GHz band?
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e) If you triple channel bandwidth, what happens to the number of channels in a service band? (The answer is not directly in the text.)
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Spread Spectrum Transmission
14. a) In the 2.4 GHz and 5 GHz service bands, what type of transmission method is required by regulators?
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b) What is the benefit of spread spectrum transmission for business communication?
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c) Is spread spectrum transmission done for security reasons in commercial WLANs?
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d) Does spread spectrum transmission increase transmission speed thanks to its wider channels?
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15. a) What spread spectrum transmission method dominates today?
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b) Why does it divide the channel into subcarriers?
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802.11 WLAN Operation
16. a) Why must an access point remove an arriving packet from the frame and place the packet in a different frame when it sends the packet back out?
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17. a) What is a roaming in 802.11?
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b) What characteristics do all access points in a corporate network share?
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c) Over what transmission system do access points communicate with each other to accomplish roaming?
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d) Distinguish between a BSS, an ESS, and an SSID.
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18. a) All wireless hosts and the access point that serves them transmit on the same channel. What problem does this cause?
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b) How does media access control (MAC) address this problem?
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c) Does media access control apply to wireless hosts, access points, or both?
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d) Can a wireless access point and one of the wireless clients in its BSS transmit simultaneously?
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More Information: Media Access Control (MAC)
19. a) What does CS mean? (Do not just spell out the abbreviation.)
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b) How is carrier sensing used in multiple access?
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c) Why is CA desirable?
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d) Does a frame’s receiver transmit an ACK immediately or after a random delay?
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e) Is CSMA/CA+ACK reliable or unreliable?
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f) Why was 802.11 made reliable?
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g) Is CSMA/CA+ACK efficient?
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20. a) Describe RTS/CTS.
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b) Is CSMA/CA+ACK required or optional?
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c) Is RTS/CTS usually required or optional?
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d) Which is more efficient, RTS/CTS or CSMA/CA+ACK?
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e) When does it make sense to use RTS/CTS?
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802.11 Transmission Standards
22. a) Compare the rated speeds of 802.11n and 802.11ac.
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b) Compare the market status of 802.11n and 802.11ac.
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c) If you needed an access point providing 3 Gbps service, what choice do you have when considering 802.11n and 802.11ac?
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23. a) You are using an access point with a rated speed of 4 Gbps. Why will you experience much less speed?
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b) What will happen to your speed as you move away from the access point?
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24. a) How does MIMO use spatial streams to increase transmission speed?
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b) What is the main benefit of MIMO?
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c) What is its other benefit?
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25. a) What is beamforming?
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b) What benefits can it bring?
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c) Distinguish between MIMO and multiuser MIMO (MU-MIMO).
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Box: 802.11/Wi-Fi Notes
26. a) You are considering a laptop computer that uses 802.11ay. (802.11ay is discussed in the next subsection.) Will your existing 802.11ac access point be able to communicate with the new device?
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b) What standard will they use in the communication if communication is possible?
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c) What principle does this communication exemplify?
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27. a) Why does the Wi-Fi Alliance release compatibility testing profiles in waves instead of combining the entire standard’s features initially?
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b) When someone says that an access point is a Wave 1 802.11ac device, what improvements do you expect to receive with a Wave 2 802.11ac device?
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28. a) What is the main promise of 802.11ax over 802.11ac?
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b) Why is the 60 GHz unlicensed band attractive?
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c) What problems does it pose for Wi-Fi?
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d) How is 802.11ay likely to be better than 802.11ad?
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End-of-Chapter Questions
Thought Questions
6-1. Why might a company decided to use 80 MHz channels in 802.11ac instead of 160 MHz channels?
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6-2. a) What do physical layer standards govern
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b) What do data link layer standards govern?
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For the following lettered question parts, say whether the concept is a Layer 1 concern or a Layer 2 concern. Explain your reasoning.
c) Multipath interference.
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d) Media Access Control.
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e) MIMO.
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f) Converting between 802.11 frames and 802.3 frames.
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g) Wireless propagation problems.
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h) Roaming.
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i) 802.11ac.
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