TECHNISCHE UNIVERSITÄT ILMENAU IEEE 802.11 Integrated Hard- and Software Systems http://www.tu-ilmenau.de/ihs Characteristics System Architecture Protocol Architecture Physical Layer MAC Layer MAC Management Power Management Roaming Current Developments
Characteristics of Wireless LANs Advantages very flexible alternative to wired LANs (almost) no wiring difficulties (e.g. historic buildings, firewalls) ad-hoc networks without previous planning possible more robust against disasters, e.g. earthquakes, fire, or users pulling a plug... Disadvantages lower bandwidth compared to wired networks possible interference may reduce bandwidth no guaranteed service due to license-free spectrum need to consider security issues proprietary solutions, especially for higher bit-rates wireless products have to follow many national restrictions => long time to establish global standards like, e.g. IMT-2000 (UMTS) 2
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
Comparison: Infrared vs. Radio Transmission Infrared uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth Example IrDA (Infrared Data Association) interface available everywhere Radio typically using the license free ISM band at 2.4 GHz Advantages experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license-free frequency bands shielding more difficult, interference with other electrical devices Examples 802.11a/b/g, HIPERLAN1/2, Bluetooth, ZigBee 4
Comparison: Infrastructure vs. Ad-hoc Networks infrastructure network AP AP wired network AP: Access Point AP ad-hoc network 5
802.11: Architecture of an Infrastructure Network ESS STA 1 802.11 LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN STA 2 STA 802.11 LAN 3 Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) group of stations using the same radio frequency Access Point station integrated into the wireless LAN and the distribution system Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS 6
802.11: Architecture of an Ad-hoc Network 802.11 LAN STA 1 BSS 1 STA 3 Direct communication within a limited range Station (STA): terminal with access mechanisms to the wireless medium STA 2 Basic Service Set (BSS): group of stations using the same radio frequency BSS 2 STA 4 STA 5 802.11 LAN 7
IEEE Standard 802.11 mobile terminal fixed terminal server infrastructure network application TCP IP LLC access point LLC application TCP IP 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 8
802.11 Layers and Functions MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management PLCP (Physical Layer Convergence Protocol) clear channel assessment signal (carrier sense) PMD (Physical Medium Dependent) modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions PHY DLC LLC MAC PLCP PMD MAC Management PHY Management Station Management 9
802.11 Physical Layer 3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiated power 1 W (USA), 100 mw (EU), min. 1mW Infrared 850-950 nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchonization 10
FHSS PHY Packet Format 802.11 only, not 802.11a/b! Synchronization synch with 010101... pattern SFD (Start Frame Delimiter) 0000110010111101 start pattern PLW (PLCP_SDU Length Word) length of payload incl. 32 bit CRC of payload, PLW < 4096 (octets) PSF (PLCP Signaling Field) data rate of packet (1 or 2 Mbit/s) HEC (Header Error Check) CRC with x 16 +x 12 +x 5 +1 80 16 12 4 16 variable bits synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header 11
DSSS PHY Packet Format 802.11 (802.11b) Synchronization synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) 1111001110100000 Signal data rate of the packet (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: 802.11 compliant length of the payload HEC (Header Error Check) protection of signal, service and length, x 16 +x 12 +x 5 +1 128 16 8 8 16 16 variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header 12
802.11 MAC Layer DFWMAC Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on "best-effort" support of broadcast and multicast implemented using DCF (Distributed Coordination Function) Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods DFWMAC-DCF CSMA/CA (mandatory) Distributed Foundation Wireless MAC collision avoidance via randomized "back-off" mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) DFWMAC-DCF with RTS/CTS Extension (optional) avoids hidden terminal problem DFWMAC- PCF (optional) access point polls terminals according to a list 13
802.11 MAC Priorities defined through different Inter-Frame Spaces (IFSs) no guaranteed, hard priorities SIFS (Short Inter-Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF IFS) medium priority, for time-bounded service using PCF DIFS (DCF IFS) lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS t 14
802.11 Timing Details Transmission and Processing Delays Times for 802.11a SlotTime 9 μs SIFS 16 μs PIFS 25 μs DIFS 34 μs 15
802.11 CSMA/CA Access Method DIFS medium busy DIFS PIFS SIFS contention window (randomized back-off mechanism) next frame direct access if medium is free DIFS slot time t 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) 16
802.11 Competing Stations Simple Version DIFS DIFS bo e bo r DIFS bo e bo r DIFS bo e busy station 1 bo e busy station 2 station 3 busy bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t busy medium not idle (frame, ack etc.) bo e elapsed backoff time packet arrival at MAC bo r residual backoff time 17
802.11 CSMA/CA Access Method 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 sender DIFS data receiver SIFS ACK other stations waiting time DIFS contention data t 18
Hidden Terminal Problem A sends to B, C cannot receive A C wants to send to B, C senses a free medium -> CS fails collision at B, A cannot receive the collision -> CD fails A is hidden for C => Application of RTS/CTS A B C 19
802.11 DCF with RTS/CTS Extension Sending unicast packets station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS sender DIFS RTS data receiver SIFS CTS SIFS SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t NAV: network allocation vector (implicit in RTS and CTS) 20
Fragmentation sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t 21
802.11 PCF (Polling) SuperFrame defines time span for polling of (all) wireless stations by AP (including time to reply) t 0 t 1 SuperFrame medium busy point coordinator PIFS D 1 SIFS SIFS D 2 SIFS SIFS wireless stations U 1 U 2 stations NAV NAV D1: polling of wireless station 1 U1: station 1 responds to polling by sending its data D2: polling of wireless station 2 U2: station 2 responds to polling by sending its data 22
802.11 PCF (Polling) t 2 t 3 t 4 PIFS SIFS CFend point coordinator D 3 D 4 SIFS wireless stations U 4 stations NAV NAV contention free period contention period t D3: polling of wireless station 3 U3: no data -> no response by station 3 within SIFS D4 (after PIFS): polling of wireless station 4 U4: station 4 responds to polling by sending its data Discussion: unpredictable beacon delays unknown transmission duration of polled stations => no QoS guarantees 23
802.11 MAC Frame Format Types (frame control) control frames, management frames, data frames Sequence number important against duplicated frames due to lost ACKs Addresses receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous duration (NAV), checksum, data bytes 2 2 6 6 6 6 Duration/ ID Address 1 Address 2 Address 3 Sequence Control Frame Control Protocol version Type Subtype To DS More Frag Power Retry Mgmt 2 0-2312 4 Address Data 4 bits 2 2 4 1 1 1 1 1 1 1 1 From DS More Data WEP Order CRC 24
MAC Address Format scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System DA: Destination Address BSSID: Basic Service Set Identifier TA: Transmitter Address AP: Access Point SA: Source Address RA: Receiver Address Management frame format bytes 2 2 6 6 6 2 0-2312 4 Frame Duration DA SA BSSID Sequence Frame Body CRC Control Control 25
Control Frames: ACK, RTS, CTS Acknowledgement Request To Send Clear To Send ACK RTS CTS bytes 2 2 6 4 Frame Duration Receiver CRC Control Address bytes 2 2 6 6 4 Frame Duration Receiver Transmitter CRC Control Address Address bytes 2 2 6 4 Frame Duration Receiver CRC Control Address Duration is used to determine NAV value 26
802.11 MAC Management Synchronization try to find a LAN, try to stay within a LAN synchronization of internal clocks to coordinate access (e.g. SIFS, PIFS, etc.), send beacons, etc. Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network MIB - Management Information Base managing, read, write 27
Synchronization Using a Beacon (Infrastructure) beacon interval access point medium B B B B busy busy busy busy value of the timestamp B beacon frame t Beacon sent only by access point Beacon may be delayed due to busy medium; beacon interval is not influenced by this! 28
Synchronization Using a Beacon (Ad-hoc) beacon interval station 1 B 1 B 1 station 2 B 2 B 2 medium busy busy busy busy t value of the timestamp B beacon frame random delay Beacon may be sent by any station (sending of beacon employs a random delay to avoid collisions) 29
Power Management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) stations wake up at the same time Infrastructure Ad-hoc AP stores frames intended for sleeping stations AP transmits indication about stored frames in periodic beacons (Indication Maps) sent during awake interval Traffic Indication Map (TIM): List of unicast receivers Delivery Traffic Indication Map (DTIM): List of broadcast/multicast receivers Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?) 30
Power Saving with Wake-up Patterns (Infrastructure) TIM interval DTIM interval beacon indicates station that data are available station replies with PS (power save) poll and continues listening to the medium AP transmits data station acknowledges the data access point medium D B busy T T d D busy busy busy B station p d t T TIM D DTIM awake B broadcast/multicast p PS poll d data transmission to/from the station 31
Power Saving with Wake-up Patterns (Ad-hoc) ATIM window beacon interval B station 1 A D B 1 1 station 2 B 2 B 2 a d medium busy busy busy t B beacon frame random delay A transmit ATIM D transmit data awake a acknowledge ATIM d acknowledge data 32
802.11 Roaming No or bad connection? Then perform: Scanning scan the environment, i.e. listen into the medium for beacon signals or send probes into the medium and wait for an answer => time-consuming (scan all channels) Association Request station sends a request to an AP Association Response success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request signal the new station to the distribution system the distribution system updates its data base (i.e. location information) the distribution system may inform the old AP so it can release resources 802.11f standard (IAPP Inter AccessPoint Protocol) 33
WLAN: IEEE 802.11b Data rate 1, 2, 5.5, 11 Mbit/s, depending on SNR User data rate max. approx. 6 Mbit/s Transmission range 300m outdoor, 30m indoor Max. data rate ~10m indoor Frequency Free 2.4 GHz ISM-band Security Limited, WEP insecure, SSID Cost 25 adapter, 100 base station Availability Many products, many vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (unless polling is used, limited support in products) Manageability Limited (no automated key distribution, sym. encryption) Advantages/Disadvantages Advantage: many installed systems, lot of experience, available worldwide, free ISM band, many vendors, integrated in laptops, simple system Disadvantage: heavy interference on ISM band, no service guarantees, slow relative speed only 34
IEEE 802.11b PHY Frame Formats Long PLCP PPDU format 8 (corresponds to 802.11) 128 16 8 8 16 16 variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header 192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s Short PLCP PPDU format (optional) 56 16 8 8 16 16 variable bits short synch. SFD signal service length HEC payload PLCP preamble (1 Mbit/s, DBPSK) PLCP header (2 Mbit/s, DQPSK) 96 µs 2, 5.5 or 11 Mbit/s 35
Channel Selection (Non-overlapping) Europe (ETSI) channel 1 channel 7 channel 13 2400 2412 2442 2472 2483.5 22 MHz [MHz] US (FCC)/Canada (IC) channel 1 channel 6 channel 11 2400 2412 2437 2462 2483.5 22 MHz [MHz] 36
WLAN: IEEE 802.11g 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 150m outdoor, 20m indoor 54 Mbit/s up to 6 m Frequency 2.412~2.472GHz (Europe ETSI) 2.457~2.462GHz (Spain) 2.457~2.472GHz (France) Security Limited, WEP insecure, SSID Cost 50 adapter, 200 base station Availability Some products, some vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (same as all 802.11 products) Manageability Limited (no automated key distribution, sym. encryption) Advantages/Disadvantages Advantage: free ISM band, compatible with 802.11b standard Disadvantage: heavy interference on ISM band, no service guarantees 37
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 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band Security Limited, WEP insecure, SSID Cost 100 adapter, 200 base station Availability Some products, some vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (same as all 802.11 products) Manageability Limited (no automated key distribution, sym. encryption) 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, no service guarantees 38
WLAN: IEEE 802.11h Regulatory Details Spectrum Managed 802.11a power control dyn. channel/frequency selection 4 frequency bands: 5.150-5.250 GHz 4 usable channels (100 MHz) indoor only max. 30mW EIRP (.11a) TPC (Transmit Power Control) max. 60mW EIRP combined TPC and DCS/DFS (Dynamic Channel/Frequency Selection) max. 200mW EIRP Turbo Mode: combination of two carriers to reach 108 Mbps 5.250-5.350 GHz 4 usable channels TPC, DCS/DFS mandatory 5.470 5.725 GHz indoor and outdoor max. 1W EIRP disallowed in US not supported by all chipsets 5.725 bis 5.825 GHz disallowed in Germany EIRP (Equivalent Isotropic Radiated Power) Usage in Germany according to the German Federal Regulation (Vorschrift der Regulierungsbehörde für das Telekommunikations- und Postwesen, RegTP, 35/2002) 39
IEEE 802.11a 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 40
Operating Channels for 802.11a / US U-NII 36 40 44 48 52 56 60 64 channel 5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz] 16.6 MHz 149 153 157 161 channel center frequency = 5000 + 5*channel number [MHz] 5725 5745 5765 5785 5805 5825 [MHz] 16.6 MHz 41
OFDM in IEEE 802.11a (and HiperLAN2) OFDM with 52 used subcarriers (64 in total) 48 data + 4 pilot (plus 12 virtual subcarriers) 312.5 khz spacing pilot 312.5 khz -26-21 -7-1 channel center frequency 1 7 21 26 subcarrier number 42
IEEE 802.11a/b/g Data Rate Barker: 11-bits Barker coding sequence (DPSK/DQPSK modulation, fixed code) CCK: Complementary Code Keying (DPSK/DQPSK modulation, switching of the spreading sequence) PBCC: Packet Binary Convolutional Code (8PSK, complex version of CCK)) OFDM: Orthogonal Frequency Division Multiplexing (BPSK, QPSK, 16QAM, 64QAM) 43
WLAN: IEEE 802.11 Extensions and developments (05/2007) 802.11d: Regulatory Domain Update completed 802.11e: MAC Enhancements QoS completed Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol 802.11f: Inter-Access Point Protocol (IAPP) withdrawn 2006 Establish an Inter-Access Point Protocol for data exchange via the distribution system 802.11h: Spectrum Managed 802.11a (DCS, TPC) completed 802.11i: Enhanced Security Mechanisms completed Enhance the current 802.11 MAC to provide improvements in security 802.11j: MAC and PHY Specifications for Operation in 4.9-5 GHz Band in Japan completed 802.11n: Throughput enhancement to 108-320 Mbps draft Study Groups 5 GHz (harmonization ETSI/IEEE) closed Radio Resource Measurements started High Throughput started See http://standards.ieee.org/getieee802/802.11.html for an update 44
WLAN: IEEE 802.11 Details 802.11d aims to produce versions of 802.11b that work at other frequencies, making it suitable for parts of the world where the 2.4GHz band isn't available. Most countries have now released this band, thanks to an ITU recommendation and extensive lobbying by equipment manufacturers. The only holdout is Spain, which may follow soon. 802.11e add QoS capabilities to 802.11 networks. It replaces the Ethernet-like MAC layer with a coordinated Time Division Multiple Access (TDMA) scheme, and adds extra error-correction to important traffic. The technology is similar to Whitecap, a proprietary protocol developed by Sharewave and used in Cisco's 802.11a prototypes. A standard was supposed to be finalized by the end of 2001, but has run into delays thanks to arguments over how many classes of service should be provided and exactly how they should be implemented. 802.11f tries to improve the handover mechanism in 802.11 so that users can maintain a connection while roaming between two different switched segments (radio channels), or between access points attached to two different networks. This is vital if wireless LANs are to offer the same mobility that cell phone users take for granted. 802.11h attempts to add better control over transmission power and radio channel selection to 802.11a. Along with 802.11e, this could make the standard acceptable to European regulators. 802.11i deals with 802.11's most obvious weakness: security. Rather than WEP, this is an entirely new standard based on the Advanced Encryption Standard (AES), the U.S. government's "official" encryption algorithm. 45
WLAN: IEEE 802.11 More Details IEEE 802.11 - THE WLAN STANDARD was original 1 Mbit/s and 2 Mb/s, 2.4 GHz RF and IR standard (1997), all the others listed below are Amendments to this standard, except for Recommended Practices 802.11F and 802.11T. IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001) IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mb/s (1999) IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard (2001) IEEE 802.11d - International (country-to-country) roaming extensions (2001) IEEE 802.11e - Enhancements: QoS, including packet bursting (2005) IEEE 802.11F - Inter-Access Point Protocol (2003) Withdrawn February 2006 IEEE 802.11g - 54 Mb/s, 2.4 GHz standard (backwards compatible with b) (2003) IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility (2004) IEEE 802.11i - Enhanced security (2004) IEEE 802.11j - Extensions for Japan (2004) IEEE 802.11k - Radio resource measurement enhancements (proposed - 2007?) IEEE 802.11m - Maintenance of the standard; odds and ends. (ongoing) IEEE 802.11n - Higher throughput improvements using MIMO (multiple input, multiple output antennas) (pre-draft - 2009?) IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (working - 2009?) IEEE 802.11r -Fast roaming Working "Task Group r" - 2007? IEEE 802.11s -ESS Extended Service Set Mesh Networking (working - 2008?) IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics Recommendation (working - 2008?) IEEE 802.11u - Interworking with non-802 networks (for example, cellular) (proposal evaluation -?) IEEE 802.11v - Wireless network management (early proposal stages -?) IEEE 802.11w - Protected Management Frames (early proposal stages - 2008?) IEEE 802.11y - 3650-3700 Operation in the U.S. (early proposal stages -?) 46
Reference Books on 802.11: Franz-Joachim Kauffels: Wireless LANs: Drahtlose Netze planen und verwirklichen, der Standard IEEE 802.11 im Detail, WLAN-Design und Sicherheitsrichtlinien. 1. Aufl., mitp-verl., Bonn, 2002 Frank Ohrtman: WiFi-Handbook Building 802.11b wireless networks. McGraw-Hill, 2003 Jochen Schiller: Mobile Communications (German and English), Kap 7.3, Addison-Wesley, 2002 Details on 802.11e: Anders Lindgren, Andreas Almquist, Olov Schelén. Quality of service schemes for IEEE 802.11 wireless LANs: an evaluation. Mobile Networks and Applications, Volume 8 Issue 3, June 2003 Daqing Gu; Jinyun Zhang. QoS enhancement in IEEE 802.11 wireless local area networks. Communications Magazine, IEEE, Volume: 41 Issue: 6, June 2003 Qiu Qiang; Jacob, L., Radhakrishna Pillai, R., Prabhakaran, B.. MAC protocol enhancements for QoS guarantee and fairness over the IEEE 802.11 wireless LANs. 11th Intl. Conf. on Computer Communications and Networks, Oct. 2002 Mangold S, Choi S, May P, Klein O, Hiertz G, and Stibor L. IEEE 802.11e wireless LAN for quality of service. Proc. Of European Wireless (EW2002), Feb. 2002 Web Links: The IEEE 802.11 Wireless LAN Standards http://standards.ieee.org/getieee802/802.11.html Introduction to the IEEE 802.11 Wireless LAN Standard http://www.wlana.org/learn/80211.htm 47