Unit 11 IEEE 802 11 Wireless LANs Shyam Parekh IEEE 802 11 Wireless LANs References Standards Basics Physical Layer 802 11b 802 11a MAC Framing Details Management PCF QoS 802 11e Security Take Away Points TOC 802 11 References 802 11 Wireless Networks The Definitive Guide M Gast O Reilly 2002 ANSI IEEE Std 802 11 1999 Edition ANSI IEEE Std 802 11b 1999 ANSI IEEE Std 802 11a 1999 Most drawings used in the lectures are from this book TOC 802 11 References IEEE 802 Standards OSI Model Observe 802 11 MAC is common to all 802 11 Physical Layer PHY standards 802 11 PHY is split into Physical Layer Convergence Procedure PLCP and Physical Medium Dependent PMD sublayers TOC 802 11 Standards Related Standards Bluetooth HIPERLAN Originally intended for interconnecting computing and communication devices European standard for Wireless LANs IEEE 802 16 Broadband Wireless Addresses needs of fixed and mobile broadband wireless access replacing fibers cables etc TOC 802 11 Standards 802 11 Standards and Spectrum Key Standards Max Rate Spectrum U S Year 802 11 2 Mbps 2 4 GHz 1997 802 11a 54 Mbps 5 GHz 1999 802 11b 11 Mbps 2 4 GHz 1999 802 11g 54 Mbps 2 4 GHz 2003 2 4 2 5 GHz for all above except 802 11a referred to as C Band Industrial Scientific and Medical ISM Microwave ovens and some cordless phones operate in the same band 802 11a uses Unlicensed National Information Infrastructure bands 5 15 5 25 GHz 5 25 5 35 GHz 5 725 5 825 GHz TOC 802 11 Standards Basic Service Sets BSSs Independent BSSs are also referred to as Ad Hoc BSSs Observe that the AP in an Infrastructure BSS is the centralized coordinator and could be a bottleneck TOC 802 11 Basics Extended Service Set ESS BSSs in an ESS communicate via Distribution System A DS has to keep track of stations within an ESS Inter Access Point protocol IAPP is not yet fully standardized TOC 802 11 Basics Network Services Distribution Integration Association Reassociation Disassociation Authentication Deauthentication Privacy MAC Service Data Unit MSDU delivery TOC 802 11 Basics Seamless Transition Seamless transition between two BSSs within an ESS Between ESSs transitions are not supported TOC 802 11 Basics 802 11b HR DSSS PHY Use Complementary Code Keying CCK instead of Differential Quadrature Phase Shift Keying DQPSK used at lower rates Provides good performance in presence of interference and multipath fading 4 bit for 5 5 Mbps or 8 bit for 11 Mbps symbols form MAC layer arrive at 1 375 million symbols per second Each symbol is encoded using CCK code word ej 1 2 3 4 ej 1 3 4 ej 1 2 4 ej 1 4 ej 1 2 3 ej 1 3 ej 1 2 ej 1 1 2 3 and 4 are decided by symbol bits High Rate Direct Sequence Spread Spectrum TOC 802 11 Physical Layer 802 11b 802 11b HR DSSS PHY 2 Uses same channels as by the low rate DS In US channels 1 11 with center frequencies at 2 412 2 462 GHz and 5 MHz distance are available For 11 Mbps Channels 1 6 and 11 give maximum number of channels with minimum interference TOC 802 11 Physical Layer 802 11b 802 11b HR DSSS PHY 3 Long PLCP format Optional Short PLCP format is offered for better efficiency TOC 802 11 Physical Layer 802 11b 802 11a 5 GHz OFDM PHY Fundamental Orthogonal Frequency Division Multiplexing OFDM work was done in 1960s and a patent was issued in 1970 Basic idea is to use number of subchannels in parallel for higher throughput Issues with 802 11a Denser Access Point deployment needed due to higher path loss Higher power need TOC 802 11 Physical Layer 802 11a 802 11a 5 GHz OFDM PHY 2 OFDM is similar to Frequency Division Multiplexing except it does not need guard bands But need guard times to minimize inter symbol and inter carrier interference Relies on orthogonality in frequency domain TOC 802 11 Physical Layer 802 11a 802 11a 5 GHz OFDM PHY 3 In U S there are 12 channels each 20 MHz wide Spectrum layout TOC 802 11 Physical Layer 802 11a 802 11a 5 GHz OFDM PHY 4 Each channel is divided into 52 subcarriers 48 are used for data PLCP Protocol Data Unit PPDU format PHY uses rate of 250K symbols per second Each symbol uses all 48 subcarriers Convolution code is used by all subcarriers TOC 802 11 Physical Layer 802 11a 802 11a 5 GHz OFDM PHY 5 Modulation and Coding MAC Access Modes MAC Access Modes Distributed Coordination Function DCF Based on Carrier Sense Multiple Access Collision Avoidance CSMA CA Point Coordination Function PCF Restricted to Infrastructure BSSs Not widely implemented Access Point polls stations for medium access TOC 802 11 MAC Main Ideas of MAC CSMA CA Interframe Spacing IFS Short IFS For atomic exchanges PCF IFS For prioritized PCF access DCF IFS For Normal DCF access Extended IFS For access after error Medium Access TOC 802 11 MAC Main Ideas of MAC CSMA CA 2 If medium is idle for DIFS interval after a correctly received frame and backoff time has expired transmission can begin immediately If previous frame contained errors medium must be free for EIFS If medium is busy access is deferred until medium is idle for DIFS and exponential backoff Backoff counter is decremented by one if a time slot is determined to be idle Unicast data must be acknowledged as part of an atomic exchange TOC 802 11 MAC Interframe Spacing Interframe Spacing values are physical layer dependent SIFS and Slot Time are explicitly specified and the others are derived PIFS SIFS Slot Time DIFS SIFS 2 Slot Time EIFS SIFS DIFS Ack Time 1 Mbps For 802 11a and 802 11b SIFS is 16 s and 10 s respectively Slot Time is 9 s and 20 s respectively TOC 802 11 MAC Contention Window Backoff is performed for R slots R is randomly chosen integer in the interval 0 CW CWmin CW CWmax CWmin 31 slots and CWmax 1023 slots for 802 11b Up to CWmax CW CWmin 1 2n 1 where n 0 1 2 is re transmission number TOC 802 11 MAC Error Recovery Each frame is associated with a retry counter based on frame size as compared to RTS CTS threshold Short retry counter Long retry counter Fragments are given a maximum lifetime by MAC before discarding them TOC 802 11 MAC WLAN Problems Hidden Terminal and Exposed Terminal problems TOC 802 11 MAC RTS CTS Clearing RTS CTS Clearing Used for frames larger than RTS CTS threshold Tradeoff between overhead and retransmission costs TOC 802 11 MAC Virtual Carrier Sensing Virtual Carrier Sensing using Network Allocation Vector NAV TOC 802 11 MAC Fragmentation Burst Fragmentation and RTS CTS thresholds are typically set to the same value TOC 802 11 MAC Framing Details Format Generic 802 11 MAC Frame Frame Control Field Sequence Control Field
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