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Integrated Communication Systems Group Ilmenau University of Technology
IEEE 802.11
Integrated Communication Systems Group
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. UMTS
Advanced Mobile Communication Networks, Master Program 2
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program 3
Integrated Communication Systems Group
Comparison: Infrastructure vs. Ad-hoc Networks
Advanced Mobile Communication Networks, Master Program
infrastructure network
ad-hoc network
APAP
AP
wired network
AP: Access Point
4
Integrated Communication Systems Group
Distribution System
Portal
802.x LAN
Access Point
802.11 LAN
BSS2
802.11 LAN
BSS1
Access Point
802.11: Architecture of an Infrastructure Network• 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
STA1
STA2
STA3
ESS
Advanced Mobile Communication Networks, Master Program 5
Integrated Communication Systems Group
802.11: Architecture of an Ad-hoc Network
• Direct communication within a limited range
– Station (STA):terminal with access mechanisms to the wireless medium
– Independent Basic Service Set (IBSS):group of stations using the same radio frequency
802.11 LAN
IBSS2
802.11 LAN
IBSS1
STA1
STA4
STA5
STA2
STA3
Advanced Mobile Communication Networks, Master Program 6
Integrated Communication Systems Group
IEEE Standard 802.11
mobile terminal
access point
server
fixed terminal
application
TCP
802.11 PHY
802.11 MAC
IP
802.3 MAC
802.3 PHY
application
TCP
802.3 PHY
802.3 MAC
IP
802.11 MAC
802.11 PHY
LLC
infrastructure network
LLC LLC
Advanced Mobile Communication Networks, Master Program 7
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program
PMD
PLCP
MAC
LLC
MAC Management
PHY Management
PH
YD
LC
Sta
tio
n M
anag
emen
t
8
Integrated Communication Systems Group
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, synchronization
Advanced Mobile Communication Networks, Master Program 9
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program 10
Integrated Communication Systems Group
802.11 MAC
• Priorities– defined through different Inter-Frame Spaces (IFSs)– no guaranteed or 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
Advanced Mobile Communication Networks, Master Program
t
medium busy
DIFSDIFS
next framecontention
direct access if medium is free DIFS
SIFS
PIFS
11
Integrated Communication Systems Group
t
medium busy
DIFSDIFS
next frame
contention window(randomized back-offmechanism)
802.11 – CSMA/CA Access Method
• 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
Random() = Pseudorandom integer drawn from a uniform distribution over the interval
[0, CW]
CW = an integer between CWmin and CWmax
• If another station occupies the medium during the back-off time of the station, the
back-off timer stops (fairness)
• If another collision happens, the stations double CWAdvanced Mobile Communication Networks, Master Program
slot time
direct access if medium is free DIFS
SIFS
PIFS
12
Integrated Communication Systems Group
802.11 – Competing Stations – Simple Version
Advanced Mobile Communication Networks, Master Program
t
busy
boe
station1
station2
station3
station4
station5
packet arrival at MAC
DIFS
boe
boe
boe
busy
elapsed backoff time
borresidual backoff time
busy medium not idle (frame, ack etc.)
bor
bor
DIFS
boe
boe
boe bor
DIFS
busy
busy
DIFS
boe busy
boe
boe
bor
bor
13
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program
t
SIFS
DIFS
data
ACK
waiting time
otherstations
receiver
sender data
DIFS
contention
14
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program
t
SIFS
DIFS
data
ACK
defer access
otherstations
receiver
sender data
DIFS
contention
RTS
CTSSIFS
SIFS
NAV (RTS)NAV (CTS)
NAV: network allocation vector (implicit in RTS and CTS)
15
Integrated Communication Systems Group
Fragmentation
t
SIFS
DIFS
data
ACK1
otherstations
receiver
senderfrag1
DIFS
contention
RTS
CTSSIFS
SIFS
NAV (RTS)NAV (CTS)
NAV (frag1)NAV (ACK1)
SIFS
ACK2
frag2
SIFS
Advanced Mobile Communication Networks, Master Program 16
Integrated Communication Systems Group
802.11 – PCF (Polling)
PIFS
stations‘NAV
wirelessstations
point coordinator
D1
U1
SIFS
NAV
SIFS
D2
U2
SIFS
SIFS
SuperFramet0
medium busy
t1
D1: polling of wireless station 1U1: station 1 responds to polling by sending its dataD2: polling of wireless station 2U2: station 2 responds to polling by sending its data
SuperFrame defines time span for polling of (all) wireless stations by AP (including time to reply)
Advanced Mobile Communication Networks, Master Program 17
Integrated Communication Systems Group
802.11 – PCF (Polling)
t
stations‘NAV
wirelessstations
point coordinator
D3
NAV
PIFS
D4
U4
SIFS
SIFS
CFend
contentionperiod
contention free period
t2 t3 t4
D3: polling of wireless station 3U3: no data -> no response by station 3 within SIFSD4 (after PIFS): polling of wireless station 4U4: station 4 responds to polling by sending its data
Discussion: unpredictable beacon delays unknown transmission duration of polled
stations=> no QoS guarantees
Advanced Mobile Communication Networks, Master Program 18
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program 19
Integrated Communication Systems Group
Synchronization Using a Beacon (Infrastructure)
Advanced Mobile Communication Networks, Master Program
beacon interval
t
medium
accesspoint
busy
B
busy busy busy
B B B
value of the timestamp B beacon frame
• Beacon sent only by access point• Beacon contains a timestamp and other management information used for
power management and roaming• Beacon may be delayed due to busy medium; beacon interval is not influenced
by this!
20
Integrated Communication Systems Group
Synchronization Using a Beacon (Ad-hoc)
Advanced Mobile Communication Networks, Master Program
t
medium
station1
busy
B1
beacon interval
busy busy busy
B1
value of the timestamp B beacon frame
station2
B2 B2
random delay
Beacon may be sent by any station (sending of beacon employs a random delay to avoid collisions)
21
Integrated Communication Systems Group
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– 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– Ad-hoc Traffic Indication Map (ATIM)
• announcement of receivers by stations buffering frames
• more complicated - no central AP
• collision of ATIMs possible (scalability?)Advanced Mobile Communication Networks, Master Program 22
Integrated Communication Systems Group
Power Saving with Wake-up Patterns (Infrastructure)
TIM interval
t
medium
accesspoint
busy
D
busy busy busy
T T D
T TIM D DTIM
DTIM interval
BB
B broadcast/multicast
station
awake
p PS poll
p
d
d
ddata transmissionto/from the station
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
Advanced Mobile Communication Networks, Master Program 23
Integrated Communication Systems Group
Power Saving with Wake-up Patterns (Ad-hoc)
awake
A transmit ATIM D transmit data
t
station1
B1 B1
B beacon frame
station2
B2 B2
random delay
A
a
D
d
ATIMwindow beacon interval
a acknowledge ATIM d acknowledge data
medium busy busy busy
Advanced Mobile Communication Networks, Master Program 24
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program 25
Integrated Communication Systems Group
Channel Selection (Non-overlapping)
2400
[MHz]
2412 2483.52442 2472
channel 1 channel 7 channel 13
Europe (ETSI)
US (FCC)/Canada (IC)
2400
[MHz]
2412 2483.52437 2462
channel 1 channel 6 channel 11
22 MHz
22 MHz
Advanced Mobile Communication Networks, Master Program 26
Integrated Communication Systems Group
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 indoor54 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
Advanced Mobile Communication Networks, Master Program 27
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program 28
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program
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)
29
Integrated Communication Systems Group
Operating Channels for 802.11a / US U-NII
5150 [MHz]5180 53505200
36 44
16.6 MHz
center frequency = 5000 + 5*channel number [MHz]
channel40 48 52 56 60 64
149 153 157 161
5220 5240 5260 5280 5300 5320
5725 [MHz]5745 58255765
16.6 MHz
channel
5785 5805
Advanced Mobile Communication Networks, Master Program 30
Integrated Communication Systems Group
WLAN: IEEE 802.11 – Extensions and developments (11/2011)
• 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 – completed
• 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
Advanced Mobile Communication Networks, Master Program 31
Integrated Communication Systems Group
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.
Advanced Mobile Communication Networks, Master Program 32
Integrated Communication Systems Group
WLAN: IEEE 802.11 – More DetailsWithin the IEEE 802.11 Working Group,[6] the following IEEE Standards Association Standard and Amendments exist:• IEEE 802.11-1997: The WLAN standard was originally 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and
infrared (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 Mbit/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 Mbit/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 802.11-2007: A new release of the standard that includes amendments a, b, d, e, g, h, i and j. (July 2007)• IEEE 802.11k: Radio resource measurement enhancements (2008)• IEEE 802.11n: Higher throughput improvements using MIMO (multiple input, multiple output antennas) (September 2009)• IEEE 802.11p: WAVE—Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (July 2010)• IEEE 802.11r: Fast BSS transition (FT) (2008)• IEEE 802.11s: Mesh Networking, Extended Service Set (ESS) (July 2011)• IEEE 802.11T: Wireless Performance Prediction (WPP)—test methods and metrics Recommendation cancelled• IEEE 802.11u: Improvements related to HotSpots and 3rd party authorization of clients, e.g. cellular network offload (February 2011)• IEEE 802.11v: Wireless network management (February 2011)• IEEE 802.11w: Protected Management Frames (September 2009)• IEEE 802.11y: 3650–3700 MHz Operation in the U.S. (2008)• IEEE 802.11z: Extensions to Direct Link Setup (DLS) (September 2010)• IEEE 802.11-2012: A new release of the standard that includes amendments k, n, p, r, s, u, v, w, y and z (March 2012)• IEEE 802.11aa: Robust streaming of Audio Video Transport Streams (June 2012)• IEEE 802.11ad: Very High Throughput 60 GHz (December 2012) - see WiGig• IEEE 802.11ae: Prioritization of Management Frames (March 2012)In process• IEEE 802.11ac: Very High Throughput <6 GHz;
[28] potential improvements over 802.11n: better modulation scheme (expected ~10% throughput increase), wider channels (estimate in future time 80 to 160 MHz), multi user MIMO;[29] (~ February 2014)
• IEEE 802.11af: TV Whitespace (~ June 2014)• IEEE 802.11ah: Sub 1 GHz sensor network, smart metering. (~ January 2016)• IEEE 802.11ai: Fast Initial Link Setup (~ February 2015)
Advanced Mobile Communication Networks, Master Program 33
Integrated Communication Systems Group
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
Advanced Mobile Communication Networks, Master Program 34