1. Whatisthe purposeof abeaconframe inWiFi(802.11) networks?

• Abeaconframe allowsamobile deviceto signalthatitisreadyto receiveaframe.
• A beacon frame allows a mobile node to determine the direction in which it shouldmovein order toobtainanincreasingsignal strength.
• A beacon frame allows a node with a directional antenna to aim the antenna towardsthe beacon point to maximize the quality of the send and receive signal.A beacon frameallows an access point to advertise its existence, and the frequency channel it isoperatingon, todevicesthat wanttoconnecttoan access point.

2. Whyare link-layerACKs usedinWiFi(802.11)networks?[Hint:checktwooftheboxesbelow].

• Wireless links are noisier than wired links, and so bit level errors are more likely tooccur,makinglink-layererrorrecoverymorevaluablethat inless-noisywiredlinks.
• HearingareceiverACK,allotherstationswillstoptransmitting.Thisreduces collisions.
• Becauseofthehidden terminalproblem,anodethatis transmittingandhearsn collisionsstill doesn'tknowiftherewasa collisionatthereceiver.
• The sender can used the differences in the signal strength in an ACK to infer whetherthereceiver ismovingtowards, orawayfrom, thesender

3. Why does the WiFi (802.11) link-layer frame have three addresses? [Note: WiFi has four MACaddressesinthe frame, butwe'reonlyfocusinghereonthethreewidelyusedones].

• Because both the access point that will relay this frame to the intended link-layerreceiving host or router interface, as well as that intended destination host or routerinterfaceneedtobespecified.
• Because the sender of this frame can be either the access point or a link-layer host orrouterinterface,andwe need to identify whichofthesetwoisthesender.
• Because there may be two hosts or routers that are possible destinations for this link-layerframeandwe need toidentifywhich ofthese is the intendedreceiver.

4. What is the purpose of RTS (request to send) and CTS (clear to send) frames in WiFi (802.11)networks?Selectoneormoreofthe answers below.[Hint:checktwoanswersbelow].

• A CTS allows a receiver to let the sender (who sent that RTS) know that it (the receiver)hasenoughbufferstoholda frametransmitted bythat sender
• RTC/CTS frames helps nodes in a wireless network mitigate the effects of the hiddenterminalproblem.
• A CTS that is sent allows a receiver to force other nodes (other than the intended senderwho sent the RTS) to refrain from transmitting, thus allowing the sender who sent theRTStothentransmit aframewith lesslikelihoodof acollision.
• RTC/CTSframesallow asendertogatherCTSframesfromallothernetwork nodes, sothat itknowsitcanthensendwithoutcollisions.

5. Whichofthe followingstatementare trueaboutthe802.11(WiFi)MACprotocol?

• The 802.11 MAC protocol performs collision avoidance.That is, an 802.11 sender andreceiver can use approaches such as RTS/CTS, inter-frame spacing, and explicitacknowledgments to try avoid, rather than detect, colliding transmissions from another node.
• The 802.11 MAC protocol performs carrier sensing.That is, it listens before transmittingandwill only transmitif thechannel is sensed idle.
• The802.11MACprotocolperforms collisiondetection.Thatis,an802.11sender willlisten to the channel while it is transmitting and stop transmitting when it detects acolliding transmissionfromanother node.