192620010 Mobile & Wireless Networking Lecture 7: Wireless LAN - PowerPoint PPT Presentation

192620010 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 2013 / 2014

Outline of Lecture 9 q Wireless LAN q General Characteristics / IEEE 802.11 standard q IEEE 802.11 Physical Layers q Medium Access Control l CSMA/CA l CSMA with RTS/CTS l MAC Quality of Service Enhancements q MAC Frame Format q MAC Management q IEEE 802.11: Important Developments 2 Mobile and Wireless Networking 2013 / 2014

Design goals for wireless LANs q global, seamless operation q low power for battery use q no special permissions or licenses needed to use the LAN q robust transmission technology q simplified spontaneous cooperation at meetings q easy to use for everyone, simple management q protection of investment in wired networks q security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) q transparency concerning applications and higher layer protocols, but also location awareness if necessary 3 Mobile and Wireless Networking 2013 / 2014

Comparison: infrastructure vs. ad-hoc networks infrastructure network AP: Access Point AP AP wired network AP ad-hoc network 4 Mobile and Wireless Networking 2013 / 2014

IEEE 802.11 - Architecture of an infrastructure network Station (STA) 802.11 LAN q terminal with access mechanisms 802.x LAN to the wireless medium and radio contact to the access point Basic Service Set (BSS) STA 1 BSS 1 q group of stations using the same Portal Access radio frequency Point Access Point Distribution System q station integrated into the wireless LAN and the distribution system Access ESS Portal Point q bridge to other (wired) networks BSS 2 Distribution System q interconnection network to form one logical network (ESS: Extended Service Set) based STA 2 STA 3 on several BSS 802.11 LAN 5 Mobile and Wireless Networking 2013 / 2014

IEEE 802.11 - Architecture of an ad-hoc network Direct communication within a 802.11 LAN limited range q Station (STA): terminal with access mechanisms to the wireless STA 1 STA 3 medium IBSS 1 q Independent Basic Service Set (IBSS): group of stations using the STA 2 same radio frequency Exception: q IEEE 802.11p for Vehicular IBSS 2 Networks: communication without setting STA 5 up a basic service set. STA 4 802.11 LAN 6 Mobile and Wireless Networking 2013 / 2014

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 2013 / 2014

IEEE 802.11 - Layers and functions PLCP Physical Layer Convergence Protocol MAC q clear channel assessment q access mechanisms, signal (carrier sense) fragmentation, encryption PMD Physical Medium Dependent MAC Management q modulation, coding q synchronization, roaming, PHY Management authentication, MIB, power q channel selection, MIB management Station Management q coordination of all management functions Station Management LLC DLC MAC MAC Management PLCP PHY PHY Management PMD 8 Mobile and Wireless Networking 2013 / 2014

The Physical Layer q IEEE 802.11 l Direct Sequence Spread Spectrum (DSSS) PHY – 2.4 GHz : RF : 1 – 2 Mbps l Frequency Hopping Spread Spectrum (FHSS) PHY – 2.4 GHz : RF : 1- 2 Mbps l Infrared (IR) PHY – Indoor : IR : 1 and 2 Mbps q IEEE 802.11b l High Rate DSSS PHY – 2.4 GHz : 5.5 Mbps – 11 Mbps q IEEE 802.11a l OFDM PHY – 5 GHz : 6-54 Mbps q IEEE 802.11g l OFDM PHY – 2.4 GHz : 6-54 Mbps q IEEE 802.11n l OFDM PHY + MIMO (up to 4x4) + 40 MHz channel (instead of 20 MHz) – Extension of a/g: 6.5 – 600 Mbps q IEEE 802.11ac l OFDM PHY + MIMO (up to 8x8) + Multi-User MIMO (up to 4 simultaneous users) + 80 MHz + 256 QAM – extension of n in 5GHz: up to 4 x 1.69 Gbit/s q IEEE 802.11ad l WiGig in 60 GHz, upto 7 Gbps Mobile and Wireless Networking 2013 / 2014

IEEE 802.11a Data rate q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) q 6, 12, 24 Mbit/s mandatory Transmission range q 100m outdoor, 10m indoor l 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 q Free 5 GHz ISM-band Special Advantages/Disadvantages q Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band q Disadvantage: stronger shading due to higher frequency 10 Mobile and Wireless Networking 2013 / 2014

IEEE 802.11g Frequency q Free 2.4 GHz ISM-band q But shared with many legacy WLANs and other systems, e.g. Bluetooth Backward compatible with IEEE 802.11b Data rate (as 802.11a) q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput significantly lower in presence of 802.11b stations Transmission range q Somewhat higher than 802.11a Special Advantages/Disadvantages q Advantage: backward compatibility, better propagation conditions q Disadvantage: crowded band, lower speed due to backward compatibility 11 Mobile and Wireless Networking 2013 / 2014

802.11 b/g Channel selection (non-overlapping) Europe (ETSI) channel 1 channel 7 channel 13 2400 2412 2442 2472 2483.5 [MHz] 22 MHz US (FCC)/Canada (IC) channel 1 channel 6 channel 11 2400 2412 2437 2462 2483.5 [MHz] 22 MHz 12 Mobile and Wireless Networking 2013 / 2014

IEEE 802.11a/g – PHY frame format 4 1 12 1 6 16 variable 6 variable bits rate reserved length parity tail service payload tail pad PLCP header PLCP preamble signal data 12 1 variable symbols 6 Mbit/s 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s 13 Mobile and Wireless Networking 2013 / 2014

802.11 - MAC layer I Access methods q DCF CSMA/CA (mandatory) l collision avoidance via randomized „back-off“ mechanism l minimum distance between consecutive packets l ACK packet for acknowledgements (not for broadcast) q DCF w/ RTS/CTS (optional) l reduces hidden terminal problem q PCF (optional) l access point polls terminals according to a list q HCF EDCA (optional) l CSMA/CA with priority levels q HCF CCA (optional) l Improved polling 14 Mobile and Wireless Networking 2013 / 2014

802.11 - MAC layer II Priorities q defined through different inter frame spaces q no guaranteed, hard priorities q SIFS (Short Inter Frame Spacing) l highest priority, for ACK, CTS, polling response q PIFS (PCF IFS) l medium priority, for time-bounded service using PCF q DIFS (DCF, Distributed Coordination Function IFS) l lowest priority, for asynchronous data service DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free ≥ DIFS 15 Mobile and Wireless Networking 2013 / 2014

802.11 - CSMA/CA access method (DCF) I contention window (randomized back-off DIFS DIFS mechanism) medium busy next frame t direct access if medium is free ≥ DIFS slot time q station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) q if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) q 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) q if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) 16 Mobile and Wireless Networking 2013 / 2014

802.11 - competing stations - simple version DIFS DIFS DIFS DIFS bo e bo r bo e bo r bo e busy station 1 bo e busy station 2 busy station 3 bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t medium not idle (frame, ack etc.) elapsed backoff time busy bo e packet arrival at MAC bo r residual backoff time 17 Mobile and Wireless Networking 2013 / 2014

802.11 - Binary Exponential Backoff q Stations choose their backoff time randomly from contention window q Ideal contention window size is trade-of between acceptable load and experienced delay q Initial contention window size (CW min ) is 7 slots (backoff time between 0 and 7) q After collision (no ack), contention window is “doubled” until CW max = 255 is reached: 7 -> 15 -> 31 -> 63 -> 127 -> 255 18 Mobile and Wireless Networking 2013 / 2014

802.11 - CSMA/CA access method (DCF) II q Sending unicast packets q station has to wait for DIFS before sending data q receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) q automatic retransmission of data packets in case of transmission errors DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time contention 19 Mobile and Wireless Networking 2013 / 2014