Wireless Local Area Networks (WLANs) Part IWiFiIEEE 802.11 FeaturesNorth American ChannelsIEEE 802.11 Physical LayersHidden Node Problem4-Way HandshakeIEEE 802.11 MACIEEE 802.11 PrioritiesTime Critical ServicesIEEE 802.11 DCF BackoffIEEE 802.11 DCF BackoffTypical Parameter ValuesDFSDFS: Example (Cont)DFS: Example (Cont)SummaryHomework 6Reading List6-1©2010 Raj JainCSE574sWashington University in St. LouisWireless Local Area Wireless Local Area Networks (WLANs)Networks (WLANs)Part IPart IRaj JainProfessor of CSE Washington University in Saint LouisSaint Louis, MO [email protected]/Video recordings of this lecture are available at:http://www.cse.wustl.edu/~jain/cse574-10/6-2©2010 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 1. Features2. MAC3. Physical LayersOverviewOverview6-3©2010 Raj JainCSE574sWashington University in St. LouisWiFiWiFi Almost all wireless LANs now are IEEE 802.11 based Competing technologies, e.g., HiperLAN can’t compete on volume and cost 802.11 is also known as WiFi = “Wireless Fidelity” Fidelity = Compatibility between wireless equipment from different manufacturers WiFi Alliance is a non-profit organization that does the compatibility testing (WiFi.org)6-4©2010 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 FeaturesIEEE 802.11 Features Original 802.11 was at 1 and 2 Mbps. Newer versions at 11 Mbps, 54 Mbps, 108 Mbps, 200 Mbps Supports both Ad-hoc and base-stations Spread Spectrum ⇒ No licensing required.Three Phys: Direct Sequence, Frequency Hopping, 915-MHz, 2.4 GHz (Worldwide ISM), 5 GHz, and Diffused Infrared (850-900 nm) bands. Supports multiple priorities Supports time-critical and data traffic Power management allows a node to doze off6-5©2010 Raj JainCSE574sWashington University in St. LouisNorth American ChannelsNorth American Channels5 GHz Band: 12 non-overlapping channelsChannel 1Channel 3Channel 5Channel 6Channel 7Channel 9Channel 112402 2422 2432 2442 2452 2462 2472 248224122483.524002.4 GHz Band: 14 5-MHz Channels. Only 12 in USA.20 MHz => Only 3 non-overlapping channels5180 5200 5220 5240 5260 5280 5300 5320 5350515036 40 44 48 52 56 60 645725 5745 5765 5785 5805 5825149 153 157 1616-6©2010 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 Physical LayersIEEE 802.11 Physical Layers Issued in four stages First part in 1997: IEEE 802.11 ¾ Includes MAC layer and three physical layer specifications¾ Two in 2.4-GHz band and one infrared¾ All operating at 1 and 2 Mbps Two additional parts in 1999:¾ IEEE 802.11a-1999: 5-GHz band, 54 Mbps/20 MHz, OFDM¾ IEEE 802.11b-1999: 2.4 GHz band, 11 Mbps/20 MHz Fourth part:¾ IEEE 802.11g-2003 : 2.4 GHz band, 54 Mbps/20 MHz, OFDM6-7©2010 Raj JainCSE574sWashington University in St. LouisHidden Node ProblemHidden Node Problem A can hear B, B can hear C, but C cannot hear A. C may start transmitting while A is also transmitting ⇒ A and C can't detect collision. Only the receiver can help avoid collisionsA B CABC6-8©2010 Raj JainCSE574sWashington University in St. Louis44--Way HandshakeWay HandshakeAccessPointAccessPointMobileNodeMobileNodeReady to sendDataClear to sendAck6-9©2010 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 MACIEEE 802.11 MAC Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) Listen before you talk. If the medium is busy, the transmitter backs off for a random period. Avoids collision by sending a short message: Ready to send (RTS)RTS contains dest. address and duration of message.Tells everyone to backoff for the duration. Destination sends: Clear to send (CTS)Other stations set their network allocation vector (NAV) and wait for that duration Can not detect collision ⇒ Each packet is acked. MAC level retransmission if not acked.6-10©2010 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 PrioritiesIEEE 802.11 Priorities Initial interframe space (IFS) Highest priority frames, e.g., Acks, use short IFS (SIFS) Medium priority time-critical frames use “Point Coordination Function IFS” (PIFS) Asynchronous data frames use “Distributed coordination function IFS” (DIFS)BusySIFSPIFSDIFSContention WindowRandom BackoffCarrier SensedTimeFrame6-11©2010 Raj JainCSE574sWashington University in St. LouisContention-Free PeriodTime Critical ServicesTime Critical Services Timer critical services use Point Coordination Function The point coordinator allows only one station to access Coordinator sends a beacon frame to all stations.Then uses a polling frame to allow a particular station to have contention-free access Contention Free Period (CFP) varies with the load.TimeBeaconDCF AccessPCF AccessSuper FrameContention Period6-12©2010 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 DCFIEEE 802.11 DCFBackoffBackoff MAC works with a single FIFO Queue Three variables: ¾ Contention Window (CW)¾ Backoff count (BO)¾ Network Allocation Vector (NAV) If a frame (RTS, CTS, Data, Ack) is heard, NAV is set to the duration in that frame. Stations sense the media after NAV expires. If the medium is idle for DIFS, and backoff is not already active, the station draws a random BO in [0, CW] and sets the backoff timer. If the medium becomes busy during backoff, the timer is stopped and a new NAV is set. After NAV, back off continues.6-13©2010 Raj JainCSE574sWashington University in St. LouisIEEE 802.11 DCFIEEE 802.11 DCFBackoffBackoff Initially and after each successful transmission:CW = CWmin After each unsuccessful attemptCW = min{2CW+1, CWmax}6-14©2010 Raj JainCSE574sWashington University in St. LouisTypical Parameter ValuesTypical Parameter Values For DS PHY: Slot time = 20 us, SIFS = 10 us, CWmin = 31, CWmax = 1023 For FH PHY: Slot time = 50 us, SIFS = 28 us, CWmin = 15, CWmax = 1023 11a: Slot time = 9 us, SIFS= 16 us, CWmin= 15, CWmax=1023 11b: Slot time = 20 us, SIFS = 10 us, Cwmin= 31, Cwmax=1023 11g: Slot time = 20 us or 9 us, SIFS = 10 us, Cwmin= 15 or 31,Cwmax=1023 PIFS = SIFS + 1 slot time DIFS = SIFS + 2 slot times6-15©2010 Raj JainCSE574sWashington University in St. LouisDFSDFS Example: Slot Time = 1, CWmin = 5, DIFS=3, PIFS=2, SIFS=1, Stn 4Stn 3Stn 2Stn 1DataDIFSCWminRA02468101214TimeSIFSAckCWminR1816 20BackoffRemaining BackoffADIFSAckC DC D22 24 266-16©2010 Raj JainCSE574sWashington University in St. LouisDFS: Example (Cont)DFS:
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