Wireless Local Area Networks WLANs Part I Raj Jain Professor of CSE Washington University in Saint Louis Saint Louis MO 63130 Jain cse wustl edu These slides are available on line at http www cse wustl edu jain cse574 08 Washington University in St Louis CSE574s 6 1 2008 Raj Jain Overview IEEE 802 11 1 Features 2 MAC 3 Physical Layers Washington University in St Louis CSE574s 6 2 2008 Raj Jain WiFi 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 Washington University in St Louis CSE574s 6 3 2008 Raj Jain IEEE 802 11 Features Original 802 11 was at 1 and 2 Mbps Newer versions at 11 Mbps and 54 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 2 GHz and Diffused Infrared 850 900 nm bands Supports multiple priorities Supports time critical and data traffic Power management allows a node to doze off Washington University in St Louis CSE574s 6 4 2008 Raj Jain North American Channels 2 4 GHz Band Only 3 non overlapping channels Channel 9 Channel 5 Channel 7 Channel 3 2400 Channel 1 2402 2412 Channel 6 2422 2432 2483 5 Channel 11 2442 2452 2462 2472 2482 60 64 5 GHz Band 12 non overlapping channels 5150 36 40 44 5180 5200 5220 5240 5260 5280 149 153 157 161 5745 5765 5785 5805 5725 Washington University in St Louis 48 52 CSE574s 6 5 56 5300 5825 5320 5350 2008 Raj Jain IEEE 802 11 Physical Layers Issued in four stages First part in 1997 IEEE 802 11 Includes MAC layer and three physical layer specifications Two in 2 4 GHz band and one infrared All operating at 1 and 2 Mbps Two additional parts in 1999 IEEE 802 11a 1999 5 GHz band 54 Mbps 20 MHz OFDM IEEE 802 11b 1999 2 4 GHz band 11 Mbps 20 MHz Fourth part IEEE 802 11g 2003 2 4 GHz band 54 Mbps 20 MHz OFDM Washington University in St Louis CSE574s 6 6 2008 Raj Jain Hidden Node Problem C B A A B C 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 collisions Washington University in St Louis CSE574s 6 7 2008 Raj Jain 4 Way Handshake Access Access Point Point Ready to send Mobile Mobile Node Node Clear to send Data Ack Washington University in St Louis CSE574s 6 8 2008 Raj Jain IEEE 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 do not transmit for that duration Can not detect collision Each packet is acked MAC level retransmission if not acked Washington University in St Louis CSE574s 6 9 2008 Raj Jain IEEE 802 11 Priorities DIFS PIFS Busy SIFS Contention Window Random Backoff Frame Time Carrier Sensed 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 Washington University in St Louis CSE574s 6 10 2008 Raj Jain Time Critical Services Super Frame Contention Free Contention Period Period PCF Access DCF Access Time Beacon 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 Washington University in St Louis CSE574s 6 11 2008 Raj Jain IEEE 802 11 DCF Backoff MAC works with a single FIFO Queue Two variables Contention Window CW Backoff count BO BO is a pseudorandom integer in 0 CW Initially and after each successful transmission CW CWmin After each unsuccessful attempt CW min 2CW 1 CWmax The stations wait for BO If another station starts transmitting the waiting stations pause their backoff counter and restart it DIFS after the end of frame again Washington University in St Louis CSE574s 6 12 2008 Raj Jain DFS Example Slot Time 1 CW 5 DIFS 3 PIFS 2 SIFS 1 Backoff Remaining Backoff Ack Ack SIFS DIFS DIFS Station 1 Frame CWmin Station 2 Station 3 Station 4 Time 0 Frame CWmin Frame 2 Washington University in St Louis 4 6 8 10 12 CSE574s 6 13 14 16 18 20 2008 Raj Jain DFS Example Cont T 1 Station 2 wants to transmit but the media is busy T 2 Stations 3 and 4 want to transmit but the media is busy T 3 Station 1 finishes transmission T 4 Station 1 receives ack for its transmission SIFS 1 T 5 Medium becomes free T 8 DIFS expires Stations 2 3 4 draw backoff count between 0 and 5 The counts are 3 1 2 T 9 Station 3 starts transmitting Station 2 and 4 pause backoff counter at 2 and 1 resp T 13 Station 3 finishes transmission T 14 Station 3 receives Ack T 15 Medium becomes free T 18 DIFS expires Stations 2 and 4 start their backoff counter T 19 Station 4 starts transmitting Washington University in St Louis CSE574s 6 14 2008 Raj Jain Summary 1 802 11 uses Frequency hopping Direct Sequence CDMA OFDM 2 802 11 PHYs 802 11 802 11a 802 11b 802 11g 3 Allows both Ad Hoc vs Infrastructure based 4 BSS ESS AP 5 802 11 supports single FIFO Q Uses SIFS PIFS DIFS Washington University in St Louis CSE574s 6 15 2008 Raj Jain Homework 6 Two 802 11 stations get frames to transmit at time t 0 The 3rd station has just finished transmitting a long packet at t 0 The transmission parameters are Slot time 1 SIFS 1 DIFS 3 Cwmin 5 Cwmax 7 Assume that the pseudo random number generated are 1 3 The frame size is 3 slots Draw a transmission diagram How many slots before the two packets will get acknowledged assuming no new arrivals Washington University in St Louis CSE574s 6 16 2008 Raj Jain
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