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 Audio Video recordings of this lecture are available at http www cse wustl edu jain cse574 10 Washington University in St Louis CSE574s 6 1 2010 Raj Jain Overview IEEE 802 11 1 Features 2 MAC 3 Physical Layers Washington University in St Louis CSE574s 6 2 2010 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 2010 Raj Jain IEEE 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 off Washington University in St Louis CSE574s 6 4 2010 Raj Jain North American Channels 2 4 GHz Band 14 5 MHz Channels Only 12 in USA 20 MHz 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 56 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 5300 5825 5320 5350 2010 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 2010 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 2010 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 2010 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 wait 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 2010 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 2010 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 2010 Raj Jain IEEE 802 11 DCF Backoff MAC works with a single FIFO Queue Three variables Contention Window CW Backoff count BO 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 Washington University in St Louis CSE574s 6 12 2010 Raj Jain IEEE 802 11 DCF Backoff Initially and after each successful transmission CW CWmin After each unsuccessful attempt CW min 2CW 1 CWmax Washington University in St Louis CSE574s 6 13 2010 Raj Jain Typical 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 times Washington University in St Louis CSE574s 6 14 2010 Raj Jain DFS Example Slot Time 1 CWmin 5 DIFS 3 PIFS 2 SIFS 1 Ack DIFS SIFS Stn 1Data Stn 2 Stn 3 Stn 4 Time 0 Ack A DIFS CWmin R C Backoff Remaining Backoff D A 2 4 Washington University in St Louis CWmin R C 6 8 10 12 14 16 18 CSE574s 6 15 D 20 22 24 26 2010 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 Stations 2 3 4 set their NAV to 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 Announces a duration of 8 RTS SIFS CTS SIFS DATA SIFS ACK Station 2 and 4 pause backoff counter at 2 and 1 resp and wait till T 17 T 15 Station 3 finishes data transmission T 16 Station 3 receives Ack T 17 Medium becomes free Washington University in St Louis CSE574s 6 16 2010 Raj Jain DFS Example Cont T 20 DIFS expires Stations 2 and 4 start their backoff counter T 21 Station 4 …
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