LM CCN Section 5.2: 802.11 Wireless Networks
Computer and Communication Networks
802.11 Wireless Networks John Easton
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Learning Objectives
o Understand wireless networking (IEEE 802.11) – Use of channels
– Transfer of data to wired LANs
– Alternative wireless standards
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Wireless LANs
o Architecture(2models)
– Hostscommunicatingwithbasestation
o Infrastructure network – Hostsonly
o Ad-hoc network e.g. VANET o 802.11BasicServiceSet
– Infrastructurenetwork o One or more clients o Access Point (AP)
– Identifiedby48-bitMAC o ExtendedServiceSet
Router
Access Point
Edge Devices
– LogicalunitformedfromoneormoreBSS
– IdentifiedbySSID(usuallynaturallanguage)
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LM CCN Section 5.2: 802.11 Wireless Networks
Wireless LANs (Cont.)
o 802.11buses2.4GHz–2.485GHz – Dividedinto11channels
– Partiallyoverlapping
o NetworkadminchooseschannelforeachAP
o InterferencepossibleifneighbouringAPsusesamechannel o Infrastructurenetwork
– HostsassociatewithAP
o Scan channels for beacon frames with Service Set Identifier (SSID) and MAC
o Selects AP to associate with o Authenticates (if enabled)
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Wireless LANs (Cont.)
o 14 possible channels in 2.4GHz band
o For 802.11b 4 channels are guaranteed not to
overlap
– Other combinations overlap partially
– C6 strongly interferes with 5 & 7 etc.
o Precise selection of channels authorised for use is determined by local authorities
– UK doesn’t allow channel 14
– 12 and 13 not used in US and Canada
o Some US Toshiba laptops won’t see these even if available
2.4GHz channels by Michael Gauthier, Wireless Networking in the Developing World, via Wikimedia Commons https://tinyurl.com/p77ehxg – Non-overlapping channels by Liebeskind (Own work), via Wikimedia Commons https://tinyurl.com/h9bgc33 – both [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
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Wireless LANs (Cont.)
o TwoneighbouringAPsonsamefrequency o Howdoesinterferenceeffectperformance?
– Wirelessnetworkmaycontinuetowork
– Sharedchannelmeanssignificantly higher risk of collision
o Poor performance
– Theeffectivemaxtransmissionrateonbothnetworksishalved
o Remedy?
– Don’tuseoverlappingchannelsifatallpossible – ONSreport90%havehomebroadband
Office for National Statistics (2017) Statistical bulletin: Internet access – households and individuals: 2017 available online via https://www.ons.gov.uk/peoplepopulationandcommunity/householdcharacteristics/homeinternetandsocialmediausage/bulletins/internetaccesshouseholdsandindividuals/2017
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LM CCN Section 5.2: 802.11 Wireless Networks
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Wireless LANs (Cont.)
o 802.11 data frame (+ size in bytes)
o 4 addresses
– Ad.1isMACofhost/APtoreceiveframe
– Ad. 2 is MAC of host / AP transmitting frame – Ad. 3 is MAC of router AP is attached to
– Ad. 4 used in ad-hoc mode only
o AP is a link layer device
– Doesn’t speak IP, hence 3rd address
o “Default” length of 32 bytes (non-data)
F.C. 2
Dur. 2
Ad. 1 6
Ad. 2 6
Ad. 3 6
S.C. 0-2
Ad. 4 6
Payload 0-2312
CRC 4
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Wireless LANs (Cont.)
o Duration is reserved transmission time
o Sequence control contains sequence number for reassembly o Type is RTS, CTS, ACK, DATA
F.C. 2
Dur. 2
Ad. 1 6
Ad. 2 6
Ad. 3 6
S.C. 0-2
Ad. 4 6
Payload 0-2312
CRC 4
Protocol Version 2
Type 2
Subtype 4
To AP 1
From AP 1
More Frag. 1
Retry 1
Power Mgt. 1
More Data 1
WEP 1
Rsvd 1
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LM CCN Section 5.2: 802.11 Wireless Networks
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Linking Wireless to Wired LANs
Mobility Within Subnet
o MigrationbetweenAPs – Associationchanges – IPremainsconstant
o ExternalroutetoH1remainsthe same (no change for router)
o Local changes?
– H1detectsweakeningsignal
from AP1, rescans, detects AP2
beacon frame
– H1disassociatesfromAP1,
associates with AP2
– AP2issuesabroadcastEthernet frame to switch with H1’s MAC
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Power Management
o Powerisatapremiuminmobiledevices o 802.11allowssleepstates
– Setpowermanagementbitto1tonotifyAP
– Wakejustbeforenextbeaconframe(around
100ms)
– APbuffersframesforsleepingnodes
– Nextbeaconframeincludeslistofnodeswith
buffered frames
o If no frames waiting return to sleep state o If frames waiting, node polls AP
o Canresultinsleepupto99%oftime
– Active250microsecondsto1msper100ms
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LM CCN Section 5.2: 802.11 Wireless Networks
Alternative Wireless Standards
o 802.11 is aimed at high-power, medium range (~100m) communications
o Other standards within the 802 family cover different use-cases
o Two important examples are: – Bluetooth
– Zigbee
o Non-802.11 standards also exist, including:
– 4GLTE
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Bluetooth
o IEEE802.15.1
o Low-power,short-rangereplacementforcables o Operatein2.4GHzband
– Interferencewithwireless,microwaveovens,cordlessphonesetc.
o Dividesspectruminto79channels1MHzwide+upper&lowerguardbands
– Hopsbetweenchannelsevery625microseconds o Ad-hocarchitecture
– Formpiconetsofupto7slavedevicesplusonemaster(8total) o Master communicates in odd-numbered slots
o Salves only communicate when spoken to
– Upto255“parked”devicesmaywaittojoin/leaveasneeded
Image by Anagoria (Own work) CC BY 3.0 (http://creativecommons.org/licenses/by/3.0), via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/c/ce/1200_Harald_Bl%C3%A5tand_anagoria.jpg
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Zigbee
o IEEE 802.14.5
o Low-power, low data rate, low duty
cycle communications
– Room temperature sensors, wall switches, etc.
o Simple and cheap
o Full-function and reduced function
devices
– Full function devices can form a mesh network
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LM CCN Section 5.2: 802.11 Wireless Networks
Cellular Internet Access (Up to 3G)
o Networkusedbymobiles o Geographicareadivided into hexagonal cells
– Eachcellservedbya Base Station Transceiver (BST)
– BaseStation Controllers (BSC) allocate channels to subscribers in the cell
– MobileSwitchingCentres(MSC)handleuserauthorisation,call establishment, and handoff
– GatewayMSClinkstopublictelephonenetwork
– DataisroutedseparatelyviaGPRSCoreNetworktothepublic
Internet
Image by Pan Camel (Own work) [CC BY 3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons https://upload.wikimedia.org/wikipedia/commons/4/44/GSM_ArchitecturePL.svg
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4G LTE
o 4GLong-TermEvolutionStandard
o All-IP network, unifying voice and data F5
o Uses orthogonal frequency domain multiplexing F4
(OFDM)
– Verylowinterference
o Mobilesallocatedoneormore0.5msslotson one or more channel frequencies
– Additionalslots(ifavailable)maybe allocated to give higher data rates
o Similar to audio compression
o Max100Mbpsdown/50Mbpsupon20MHz spectrum
F3 F2 F1
0.5 1.0 1.5 2.0 2.5 3.0 3.5
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Summary
o Understand wireless networking (IEEE 802.11) – Use of channels
– Transfer of data to wired LANs
– Alternative wireless standards
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