Tài liệu Mobile & wireless networking – lecture 7 wireless lan - geert heijenk

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262001 Mobile & Wireless Networking Lecture 7: Wireless LAN [Schiller, Section 7.3] [Reader, Part 6] [Optional: "IEEE 802.11n Development: History, Process, and Technology", Perahia, IEEE Communications Magazine, July 2008] Geert Heijenk Mobile and Wireless Networking 2009 / 2010 Outline of Lecture 7   Wireless LAN       General Characteristics / IEEE 802.11 standard IEEE 802.11 Physical Layers Medium Access Control             CSMA/CA CSMA with RTS/CTS MAC Quality of Service Enhancements MAC Frame Format MAC Management IEEE 802.11: Important Developments 2 Mobile and Wireless Networking 2009 / 2010 Design goals for wireless LANs                   global, seamless operation low power for battery use no special permissions or licenses needed to use the LAN robust transmission technology simplified spontaneous cooperation at meetings easy to use for everyone, simple management protection of investment in wired networks security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) transparency concerning applications and higher layer protocols, but also location awareness if necessary 3 Mobile and Wireless Networking 2009 / 2010 Comparison: infrastructure vs. ad-hoc networks infrastructure network AP: Access Point AP AP wired network AP ad-hoc network 4 Mobile and Wireless Networking 2009 / 2010 IEEE 802.11 - Architecture of an infrastructure network Station (STA) 802.11 LAN STA1 802.x LAN   Basic Service Set (BSS) BSS1 Portal Access Point Access Point ESS   group of stations using the same radio frequency Access Point Distribution System   station integrated into the wireless LAN and the distribution system Portal   BSS2 bridge to other (wired) networks Distribution System   STA2 terminal with access mechanisms to the wireless medium and radio contact to the access point 802.11 LAN STA3 interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS 5 Mobile and Wireless Networking 2009 / 2010 IEEE 802.11 - Architecture of an ad-hoc network Direct communication within a limited range 802.11 LAN   STA1 STA3 IBSS1   STA2 Station (STA): terminal with access mechanisms to the wireless medium Independent Basic Service Set (IBSS): group of stations using the same radio frequency IBSS2 STA5 STA4 802.11 LAN 6 Mobile and Wireless Networking 2009 / 2010 IEEE standard 802.11 fixed terminal mobile terminal infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY 7 Mobile and Wireless Networking 2009 / 2010 IEEE 802.11 - Layers and functions PLCP Physical Layer Convergence Protocol MAC   access mechanisms, fragmentation, encryption MAC Management   synchronization, roaming, authentication, MIB, power management   clear channel assessment signal (carrier sense) PMD Physical Medium Dependent   modulation, coding PHY Management   channel selection, MIB Station Management LLC MAC MAC Management PLCP PHY Management PMD coordination of all management functions Station Management PHY DLC   8 Mobile and Wireless Networking 2009 / 2010 The Physical Layer   IEEE 802.11   Direct Sequence Spread Spectrum (DSSS) PHY –  2.4 GHz : RF : 1 – 2 Mbps   Frequency Hopping Spread Spectrum (FHSS) PHY –  2.4 GHz : RF : 1- 2 Mbps   Infrared (IR) PHY –  Indoor : IR : 1 and 2 Mbps   IEEE 802.11b   High Rate DSSS PHY –  2.4 GHz : 5.5 Mbps – 11 Mbps   IEEE 802.11a   OFDM PHY –  5 GHz : 6-54 Mbps   IEEE 802.11g   OFDM PHY –  2.4 GHz : 6-54 Mbps   IEEE 802.11n   OFDM PHY + MIMO (up to 4x4) + 40 MHz channel (instead of 20 MHz) –  Extension of a/g: 6.5 – 600 Mbps Mobile and Wireless Networking 2009 / 2010 WLAN: IEEE 802.11a Data rate       6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) 6, 12, 24 Mbit/s mandatory Transmission range   100m outdoor, 10m indoor   E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m Frequency   Free 5 GHz ISM-band Special Advantages/Disadvantages     Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band Disadvantage: stronger shading due to higher frequency 10 Mobile and Wireless Networking 2009 / 2010 WLAN: IEEE 802.11g Frequency     Free 2.4 GHz ISM-band But shared with many legacy WLANs and other systems, e.g. Bluetooth Backward compatible with IEEE 802.11b Data rate (as 802.11a)     6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput significantly lower in presence of 802.11b stations Transmission range   Somewhat higher than 802.11a Special Advantages/Disadvantages     Advantage: backward compatibility, better propagation conditions Disadvantage: crowded band, lower speed due to backward compatibility 11 Mobile and Wireless Networking 2009 / 2010 802.11 b/g Channel selection (non-overlapping) Europe (ETSI) channel 1 2400 2412 channel 7 channel 13 2442 2472 22 MHz 2483.5 [MHz] US (FCC)/Canada (IC) channel 1 2400 2412 channel 6 channel 11 2437 2462 22 MHz 2483.5 [MHz] 12 Mobile and Wireless Networking 2009 / 2010 IEEE 802.11a/g – PHY frame format 4 1 12 1 rate reserved length parity 6 16 tail service variable 6 variable payload tail pad bits PLCP header PLCP preamble 12 signal data 1 6 Mbit/s variable symbols 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s 13 Mobile and Wireless Networking 2009 / 2010 802.11 - MAC services MAC services Asynchronous Data Service Time-bounded Service Contention Contention-Free Distributed Coordination Function (DCF) Point Coordination Function (PCF) CSMA/CA Polling Ad-hoc and Infra-structure mode Infra-structure mode only optional mandatory 14 Mobile and Wireless Networking 2009 / 2010 802.11 - MAC layer I Access methods   DCF CSMA/CA (mandatory)         DCF w/ RTS/CTS (optional)     access point polls terminals according to a list HCF EDCA (optional)     avoids hidden terminal problem PCF (optional)     collision avoidance via randomized „back-off“ mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcast) CSMA/CA with priority levels HCF CCA (optional)   Improved polling 15 Mobile and Wireless Networking 2009 / 2010 802.11 - MAC layer II Priorities       defined through different inter frame spaces no guaranteed, hard priorities SIFS (Short Inter Frame Spacing)     PIFS (PCF IFS)     highest priority, for ACK, CTS, polling response medium priority, for time-bounded service using PCF DIFS (DCF, Distributed Coordination Function IFS)   lowest priority, for asynchronous data service DIFS medium busy DIFS PIFS SIFS contention direct access if medium is free ≥ DIFS next frame t 16 Mobile and Wireless Networking 2009 / 2010 802.11 - CSMA/CA access method I DIFS DIFS medium busy direct access if medium is free ≥ DIFS         contention window (randomized back-off mechanism) next frame t slot time station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) 17 Mobile and Wireless Networking 2009 / 2010 802.11 - competing stations - simple version DIFS DIFS station1 station2 DIFS boe bor boe busy DIFS boe bor boe busy boe busy boe bor boe boe busy station3 station4 boe bor station5 busy bor t busy medium not idle (frame, ack etc.) boe elapsed backoff time packet arrival at MAC bor residual backoff time 18 Mobile and Wireless Networking 2009 / 2010 802.11 - Binary Exponential Backoff         Stations choose their backoff time randomly from contention window Ideal contention window size is trade-of between acceptable load and experienced delay Initial contention window size (CWmin) is 7 slots (backoff time between 0 and 7) After collision (no ack), contention window is “doubled” until CWmax = 255 is reached: 7 -> 15 -> 31 -> 63 -> 127 -> 255 19 Mobile and Wireless Networking 2009 / 2010 802.11 - CSMA/CA access method II   Sending unicast packets       station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) automatic retransmission of data packets in case of transmission errors DIFS sender data SIFS receiver ACK DIFS other stations waiting time data t contention 20 Mobile and Wireless Networking 2009 / 2010
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