Wireless NetworkingIEEE 802.11

IEEE 802.11IEEE 802.11 is a set of standards implementing wireless local area network (WLAN) computer communication in the 2.4, 3.6 and 5 GHz spectrum bands.

You should know . . .802 is the IEEE LAN project11 is the working groupg is the task groupi (lower case) is a task group working on 802.11-dependent security standardsF (upper case) is a task group working on independent roaming protocols (e.g., that could be used in non-WLAN applications as well)

Important relationshipsc = *fC = speed of light (meters/second)~ 3 * 108 meters/second = wavelength in meters (meters/cycle)f = frequency in Hertz (Hz) (cycles/second)

Q: What Wavelength corresponds to 2.4 GHz?

Wireless AdvantagesMobility and increased reachEase and speed of deploymentFlexibility and ease of configurationCost savings (sometimes)New business opportunities (m-commerce)

Wireless ChallengesScarce total bandwidth availableCompared to wired, wireless is generally,Slower (partially due to limited analog bandwidth)Less reliable (dynamic, noisy environments)Less secure (lack of physical boundaries)Less able to ensure QoS

The IEEE 802 family and its relation to the OSI Model

ComponentsSTA StationAP Access PointBSS Basic Service SetIBSS : Infrastructure BSS : QBSSESS Extended Service SetA set of infrastructure BSSsConnection of APsTracking of mobilityDS Distribution SystemAP communicates with another AP

ServicesStation services - authentication, de-authentication, privacy, delivery of dataDistribution Services -associationdisassociationre-associationdistributionintegration

Components of 802.11 LANs

Overview - 802.11 ArchitectureSTASTASTASTASTASTASTASTAAPAPESSBSSBSSBSSBSSExisting Wired LANInfrastructure NetworkAd Hoc NetworkAd Hoc Network

Independent and Infrastructure BSSs Also called Ad-Hoc NetworksAlso called Infrastructure based NetworksIf one station wants to talk to another station in an Infrastructure BSS, the AP acts as a relayWhat are the advantages of an Infrastructure BSS?

Extended Service Set

Distribution system in common 802.11 access point implementations

Overlapping BSSs in an ESS

Overlapping network types

BSS transition

ESS transition

Station ProblemsTypically nodes are all trying to talk with a central (wired) hubSometimes (but not always) the nodes can hear one anotherHidden station problem When one node cant hear another and tromps on transmission to a third nodeExposed station problemWhen one node hears another node and so wont transmit to a third, even through it could

IEEE 802.11 MAC Protocol: CSMA/CA802.11 sender1 if sense channel idle, then transmit entire frame (no CD)2 if sense channel busy then start random back-off timetimer counts down while channel idletransmit when timer expiresif no ACK, increase random back-off interval, repeat 2802.11 receiver- if frame received OK return ACK (ACK needed due to hidden terminal problem) receiver

802.11: Channels, association802.11b - 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequenciesAP admin chooses frequency for APinterference possible - channel can be same as that chosen by neighboring AP!Host - must associate with an APscans channels, listening for beacon frames containing APs name and MAC addressselects AP to associate withwill typically run DHCP to get IP address in APs subnet

802.11 frame - addressingAddress 2: MAC addressof wireless host or AP transmitting this frameAddress 1: MAC addressof wireless host or AP to receive this frameAddress 3: MAC addressof router interface to which AP is attachedAddress 4: used only in ad hoc mode

Frequency Hopping and Direct Sequence Spread Spectrum TechniquesSpread Spectrum used to avoid interference from licensed and other non-licensed users, and from noise, e.g., microwave ovensFrequency Hopping (FHSS)Using one of 78 hop sequences, hop to a new 1MHz channel (out of the total of 79 channels) at least every 400millisecondsRequires hop acquisition and synchronizationHops away from interferenceDirect Sequence (DSSS)Using one of 11 overlapping channels, multiply the data by an 11-bit number to spread the 1M-symbol/sec data over 11MHzRequires RF linearity over 11MHzSpreading yields processing gain at receiverLess immune to interference

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Its not that hard! One division and keeping your units straight will do it. Hint: In inches, the answer is about 5.