Wireless Local Area Networks (WLAN)

1
IT351 Mobile Wireless Computing
Wireless Local Area Networks (WLAN) Part-1
IEEE802.11

• Objectives
• To provide a detailed study of the WLAN
  architecture and system operation

2
Outline

• Wireless LAN main uses, advantages, disadvantages
• Classification of transmission technologies for
  WLAN
• Classification of WLAN IEEE802.11
• Infrastructure networks
• Ad Hoc networks
• WLAN IEEE802.11
• Architecture
• Protocols
• Physical layer
• MAC layer
• MAC management
• IEEE802.11-a/ b/... n

3
Overview of the main chapters
Chapter 10 Support for Mobility
Chapter 9 Mobile Transport Layer
Chapter 8 Mobile Network Layer
Chapter 4 Telecommunication Systems
Chapter 5 Satellite Systems
Chapter 6 Broadcast Systems
Chapter 7 Wireless LAN
Chapter 3 Medium Access Control
Chapter 2 Wireless Transmission
4
Mobile Communication Technology according to IEEE
WiFi
802.11a
802.11h
Local wireless networks WLAN 802.11
802.11i/e//n//z/aa
802.11b
802.11g
ZigBee
802.15.4
802.15.4a/b/c/d/e/f/g
Personal wireless nw WPAN 802.15
802.15.5, .6 (WBAN)
802.15.3
802.15.3b/c
802.15.2
802.15.1
Bluetooth
Wireless distribution networks WMAN 802.16
(Broadband Wireless Access)
WiMAX
Mobility
802.20 (Mobile Broadband Wireless
Access) 802.16e (addition to .16 for mobile
devices)
5
Wireless LAN (WLAN)

• Main uses
• Extension to existing LAN
• Cross building interconnect
• Nomadic access / wireless hotspots
• Ad Hoc networks
• Main Standard is IEEE 802.11
• Wireless extension for Ethernet
• Wi-Fi, Wireless-Fidelity, Alliance to certify
  products to the IEEE standard

6
Characteristics of wireless LANs

• Advantages
• very flexible within the reception area,
• allow for design of small independent devices
  (e.g. to be put in pockets)
• Ad-hoc networks without previous planning
  possible
• (almost) no wiring difficulties (e.g. historic
  buildings, firewalls)
• more robust against disasters like, e.g.,
  earthquakes, fire - or users pulling a plug...
• Cost is independent of the number of users

7
Characteristics of wireless LANs

• Disadvantages
• typically very low bandwidth compared to wired
  networks (1-10 Mbit/s) due to shared medium
• high error rates, low quality
• many proprietary solutions, especially for higher
  bit-rates, standards take their time (e.g. IEEE
  802.11n)
• products have to follow many national
  restrictions if working wireless, it takes a very
  long time to establish global solutions
• Safety security

8
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 (plug
  play)
• 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

9
Classifications of transmission technologies
infrared vs. radio transmission

• Infrared
• At 900 nm wavelength, 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 , can not
  penetrate objects
• low bandwidth (115kbps 4 Mbps
• 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
• Example
• Many different products

10
IEEE802.x standards

• 802 standards specify OSI layers 1 2
• Physical layer
• Encoding/decoding signals
• Preamble (for synchronization)
• Bit transmission/reception
• Link layer (Medium Access Control (MAC))
• Manage access to media
• Assemble/disassemble frames
• Addressing and error detection
• Interface with higher layers

11
ISM Unlicensed Frequency Bands
12
IEEE 802.11 WLAN

• 802.11 (Legacy, 1997) operates at 1-2 Mbps, with
  3 methods
• 1 infrared
• 2 radio access (FHSS, DSSS)
• 802.11a (1999) operates at 54Mbps (5GHz Freq.
  Band)
• 802.11b (Wi-Fi 1999) operates at 11Mbps (2.4GHz
  ISM Freq. Band - 2.400 2.4835 GHz)
• 802.11g (2003) operates at 54Mbps (2.4GHz Freq.
  Band)
• 802.11n (Oct 2009) operates at 540 M bps
  (typically 200Mbps) (2.4GHz or 5GHz Freq. Band)

Wireless Communications and Networks, W.
Stallings, Prentice Hall, N.J., 2001.

• Two Modes
• Infrastructure Mode (LAN extension)
• Ad Hoc (wireless only)

13
Classifications of IEEE802.11 infrastructure
vs. ad-hoc networks
infrastructure network
AP Access Point
AP
AP
wired network
AP
ad-hoc network
14
802.11 - System architecture 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
• Each ESS has its own identifier ESSID

802.11 LAN
802.x LAN
STA1
BSS1
Access Point
Access Point
ESS
BSS2
STA2
STA3
802.11 LAN
15
IEEE802.11 System architecture infrastructure
network

• The distribution system is not specified in
  IEEE802.11.
• It could consist of IEEE LANs, wireless links or
  any other networks
• It handles data transfer between different APs
• To participate in a WLAN, you need to know the
  ESSID
• Stations can select an AP and associate with it
• The AP supports roaming (changing access points)
• APs provide synchronization within a BSS, support
  power management, and can control medium access

16
802.11 System architectureAd-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
• No specific node for data routing, or forwarding
  or exchange of topology information

802.11 LAN
STA1
STA3
IBSS1
STA2
IBSS2
STA5
STA4
802.11 LAN
17
IEEE802.11 Protocol architecture
fixed terminal
mobile terminal
infrastructure network
access point
application
application
TCP
TCP
IP
IP
LLC
LLC
LLC
802.11 MAC
802.3 MAC
802.3 MAC
802.11 MAC
802.11 PHY
802.3 PHY
802.3 PHY
802.11 PHY
18
802.11 Protocol Architecture

• PLCP Physical Layer Convergence Protocol
• clear channel assessment signal (carrier sense)
• Service access point (SAP)
• PMD Physical Medium Dependent
• modulation, coding
• PHY Management
• channel selection, MIB maintenance
• Station Management
• coordination of all management functions, higher
  layer functions (interaction with distribution
  system)

• MAC
• access mechanisms, fragmentation, encryption
• MAC Management
• Association/de-association, synchronization,
  roaming, MIB (management Information Base), power
  management to save battery power, authentication
  mechanism

Station Management
LLC
DLC
MAC
MAC Management
PLCP
PHY Management
PHY
PMD
19
802.11 - Physical layer (legacy)

• 3 versions 2 radio (typ. 2.4 GHz ISM), 1 IR
• data rates 1 or 2 Mbit/s
• All physical variants include the provision of
  the clear channel assessment (CCA). This is
  needed for MAC mechanisms.
• The Physical layer a service access point (SAP)
  with 1 or 2 Mbits/s transfer rate to the MAC
  layer.
• FHSS (Frequency Hopping Spread Spectrum)
• spreading, despreading using different hopping
  sequences (79 hopping channels for North America
  and Europe)
• Frequency Shift Keying (FSK) digital modulation

20
802.11 - Physical layer (legacy)

• DSSS (Direct Sequence Spread Spectrum)
• Spreading, despreading using 11-chip Barker code
• chipping sequence 1, -1, 1, 1, -1, 1, 1,
  1, -1, -1, -1
• Phase Shift Keying (PSK) digital modulation
• max. radiated power 1 W (USA), 100 mW (EU), min.
  1mW
• Robust against interference and multipath
  propagation
• More complex compared to FHSS
• Infrared
• 850-950 nm, diffuse light, typ. 10 m range
• Typically in buildings (classrooms, meeting
  rooms,..)
• Frequency reuse is simple, a wall is enough for
  shielding

21
802.11 - MAC layer - DFWMAC

• The MAC mechanisms are called Distributed
  Foundation Wireless Medium Access Control
  (DFWMAC)
• Functions medium access, support for roaming,
  authentication and power conservation
• Traffic services
• Asynchronous Data Service (mandatory)
• exchange of data packets based on best-effort
  no delay bounds
• support of broadcast and multicast
• Implemented using distributed coordination
  function (DCF) OR Point Coordination Function
  (PCF)
• For both infrastructure and ad Hoc
• Time-Bounded Service (optional)
• implemented using PCF (Point Coordination
  Function)
• Provides delay guarantees
• For infrastructure 802.11 only

22
MAC Layer

• Asynchronous Data Service access method
• DFWMAC-DCF CSMA/CA (mandatory)
• collision avoidance via randomized back-off
  mechanism
• minimum distance between consecutive packets
• ACK packet for acknowledgements (not for
  broadcasts)
• DFWMAC-DCF w/ RTS/CTS (optional)
• avoids hidden terminal problem
• DFWMAC-PCF (optional)
• Time-bounded Service access method
• DFWMAC- PCF (optional)
• access point polls terminals according to a list

23
802.11 - MAC layer

• Priorities
• defined through different inter frame spaces
• 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, Distributed Coordination Function IFS)
• lowest priority, for asynchronous data service

DIFS
DIFS
PIFS
SIFS
medium busy
next frame
contention
t
direct access if medium is free ? DIFS
24
802.11 - CSMA/CA access method I

• 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 a DCF
  Inter-Frame Space (DIFS), the station can start
  sending
• 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)

contention window (randomized back-offmechanism)
DIFS
DIFS
medium busy
next frame
t
direct access if medium is free ? DIFS
slot time (20µs)
25
802.11 - competing stations - simple version
DIFS
DIFS
DIFS
DIFS
boe
bor
boe
bor
boe
busy
station1
boe
busy
station2
busy
station3
boe
busy
boe
bor
station4
boe
bor
boe
busy
boe
bor
station5
t
medium not idle (frame, ack etc.)
boe
elapsed backoff time
busy
packet arrival at MAC
bor
residual backoff time
26
802.11 CSMA/CA

• The contention window (CW) size affect the
  performance of the MAC scheme
• A small CW ensures shorter access delay but the
  probability of collision increases (more than one
  station can have the same backoff time)
• The contention window starts with a minimum value
  then doubles each time a collision occurs up to a
  maximum value (e.g. 7, 15, 31,63, 127, 255).
• This is called the exponential backoff algorithm
  (already used in CSMA/CD)

27
802.11 - CSMA/CA

• 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 (The sender has to compete
  again)

DIFS
data
sender
SIFS
ACK
receiver
DIFS
data
other stations
t
waiting time
contention
28
802.11 DFWMAC with RTS/CTS (method II)

• Sending unicast packets
• To solve the problem of hidden terminal
• 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 in the NAV (net
  allocation vector)

DIFS
data
RTS
sender
SIFS
SIFS
SIFS
ACK
CTS
receiver
DIFS
NAV (RTS)
data
other stations
NAV (CTS)
t
defer access
contention
29
802.11 DFWMAC with RTS/CTS (cont.)

• The scheme reserves the medium for one user
  (virtual reservation scheme)
• RTS/CTS can result in a non-negligible overhead
  causing a waste of bandwidth and higher delay
• A threshold based on frame size can be used to
  determine when to use the additional mechanism
  and when to disable it
• To reduce the bit error-rates in transmission,
  fragmentation can be used. However, for RTS/CTS
  scheme all fragments are sent by one RTS. Each
  fragment reserve the medium for the next
  fragment.

30
CTS/RTS with Fragmentation
DIFS
frag1
RTS
frag2
sender
SIFS
SIFS
SIFS
SIFS
SIFS
ACK1
CTS
ACK2
receiver
NAV (RTS)
NAV (CTS)
DIFS
NAV (frag1)
data
other stations
NAV (ACK1)
t
contention
31
DFWMAC-PCF with polling Method III (almost
never used)

• The two previous methods cannot guarantee a
  maximum delay or minimum bandwidth
• PCF provides time-bounded service
• It requires an access point that control medium
  access and polls the single nodes
• Ad Hoc network cant use this function so it
  provides only best-effort service
• The point coordinator in the access point splits
  the access time into super frame periods.
• A super frame comprises an contention-free period
  and a contention period
• If only PCF is used and polling is distributed
  evenly, the bandwidth is also distributed evenly
  static centrally controlled TDMA with TDD
  transmission
• Much overhead if nodes have nothing to send.

32
DFWMAC-PCF (cont.)
33
802.11 - Frame format

• Types control frames, management frames, data
  frames
• Sequence numbers
• important against duplicated frames due to lost
  ACKs
• Addresses
• receiver, transmitter (physical), BSS identifier,
  sender/receiver (logical)
• Miscellaneous
• Duration (to set the NAV), checksum, frame
  control, data

bytes
2
2
6
6
6
6
2
4
0-2312
Frame Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence Control
Address 4
Data
CRC
bits
1
1
1
1
1
1
2
2
4
1
1
Protocol version
Type
Subtype
To DS
More Frag
Retry
Power Mgmt
More Data
WEP
From DS
Order
34
802.11 - Frame format

• Frame Control
• Protocol version 2 bits
• Type (management 00, control 01, data 10)
• Subtype (e.g. Management- association 0000,
  beacon 100 Control RTS 1011, CTS
  1100)
• More fragments 1 if another fragment to follow
• Retry 1 if retransmission of an earlier frame
• Power Management 1 if the station will go to
  power save mode
• More Data A sender has more data to send
• Wired Equivalent Privacy (WEP) Standard security
  mechanism applied
• Order frame must be processed in strict order

bytes
2
2
6
6
6
6
2
4
0-2312
Frame Control
Duration/ ID
Address 1
Address 2
Address 3
Sequence Control
Address 4
Data
CRC
bits
1
1
1
1
1
1
2
2
4
1
1
Protocol version
Type
Subtype
To DS
More Frag
Retry
Power Mgmt
More Data
WEP
From DS
Order
35
MAC address format
DS Distribution System AP Access Point DA
Destination Address SA Source Address BSSID
Basic Service Set Identifier RA Receiver
Address TA Transmitter Address
36
802.11 - MAC management

• MAC management plays a central role in an
  IEEE802.11 as it controls all the functions
  related to system integration, i.e., integration
  of a wireless station into a BSS, formation of an
  ESS, synchronization of stations,..etc
• The major functions are
• Synchronization
• try to find a WLAN and stay within it
• synchronization of internal clock (timing
  synchronization function (TSF)
• For power management
• For coordination of PCF (super frame)
• For synchronization of hopping sequence in FHSS
  systems
• Generation of beacon signals

37
802.11 - MAC management (cont.)

• Power management
• To control transmitter activity for power
  conservation
• sleep-mode without missing a frame
• periodic sleep, frame buffering, traffic
  measurements
• Association/Re-association
• integration into a WLAN
• roaming, i.e. change networks by changing access
  points
• scanning, i.e. active search for a network
• MIB - Management Information Base
• managing, read, write, update

38
Synchronization using a Beacon (infrastructure)

• Within a BSS, timing is conveyed by the periodic
  transmission of a beacon frame
• A beacon contains a timestamp and other
  management information (identification of BSS,
  power management, roaming)
• In infrastructure-based networks, the beacon is
  sent by the access point periodically. However,
  it may be delayed if medium is busy, but beacon
  interval is not shifted if one beacon is delayed.
• The time stamp is used by a node to adjust its
  local clock

beacon interval (20ms 1s)
B
B
B
B
access point
busy
busy
busy
busy
medium
t
B
value of the timestamp
beacon frame
39
Synchronization using a Beacon (ad-hoc)

• Each node maintains its own timer and starts
  transmission of a beacon frame after the beacon
  interval
• Using random backoff algorithm, one beacon only
  wins
• All other stations adjust their internal clock
  according to the received beacon

beacon interval
B1
B1
station1
B2
B2
station2
busy
busy
busy
busy
medium
t
B
value of the timestamp
beacon frame
random delay
40
Power management

• Power-saving mechanisms are crucial for wireless
  devices
• Standard WLAN protocols assume that stations are
  always ready to receive data. This permanent
  readiness consumes much power
• Idea switch the transceiver off if not needed
• States of a station sleep and awake
• Timing Synchronization Function (TSF)
• stations wake up periodically at the same time
• Buffering of data at senders
• Senders announce destination during wake periods
• Longer off periods save battery life but reduce
  average throughput and increase delay

41
Power Management

• Infrastructure
• Access point buffers all frames destined for
  stations operating in power-save mode
• With every beacon sent, a Traffic Indication Map
  (TIM) is transmitted
• TIM contains a list of unicast receivers
  transmitted by AP
• Beacon interval TIM interval
• Additionally, the AP maintains a Delivery
  Traffic Indication Map (DTIM)
• list of broadcast/multicast receivers transmitted
  by AP
• DTIM interval multiple of TIM interval
• The TSF assures that sleeping stations will
  wake-up periodically and listen to the beacon and
  TIM
• If TIM indicates a unicast frame buffered for a
  station, the station stay awake to receive it
• Stations always stay awake for muti-cast/
  broadcast transmission
• Stations also wake-up when they have frames to be
  transmitted

42
Power saving with wake-up patterns
(infrastructure)
TIM interval
DTIM interval
D
T
T
D
B
B
d
access point
busy
busy
busy
busy
medium
p
d
station
t
43
Power saving with wake-up pattern (Ad hoc)

• 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?)
• APSD (Automatic Power Save Delivery)
• new method in 802.11e replacing above schemes

44
802.11 - Roaming

• No or bad connection? Then perform
• Scanning
• scan the environment,
• Passive scanning listen into the medium for
  beacon signals
• Active scanning send probes into the medium and
  wait for an answer
• Reassociation Request
• Choose best AP (e.g. based on signal strength)
• station sends a request to one or several AP(s)
• Reassociation 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)
• typically, the distribution system now informs
  the old AP so it can release resources
• Fast roaming 802.11r
• e.g. for vehicle-to-roadside networks

45
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
• DSSS, 2.4 GHz ISM-band
• Security
• Limited, WEP insecure, SSID (service set
  identifier)
• 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)
• Special 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

46
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
• 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)
• 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, no QoS

47
WLAN IEEE 802.11 current developments

• 802.11j Extensions for operations in Japan
• Changes of 802.11a for operation at 5GHz in Japan
  using only half the channel width at larger range
• 802.11-2007 Current complete standard
• Comprises amendments a, b, d, e, g, h, i, j
• 802.11k Methods for channel measurements
• Devices and access points should be able to
  estimate channel quality in order to be able to
  choose a better access point of channel
• 802.11m Updates of the 802.11-2007 standard
• 802.11n Higher data rates above 100Mbit/s
• Changes of PHY and MAC with the goal of 100Mbit/s
  at MAC SAP
• MIMO antennas (Multiple Input Multiple Output),
  up to 600Mbit/s are currently feasible
• However, still a large overhead due to protocol
  headers and inefficient mechanisms
• 802.11p Inter car communications
• Communication between cars/road side and
  cars/cars
• Planned for relative speeds of min. 200km/h and
  ranges over 1000m
• Usage of 5.850-5.925GHz band in North America
• 802.11r Faster Handover between BSS
• Secure, fast handover of a station from one AP to
  another within an ESS
• Current mechanisms (even newer standards like
  802.11i) plus incompatible devices from different
  vendors are massive problems for the use of,
  e.g., VoIP in WLANs
• Handover should be feasible within 50ms in order
  to support multimedia applications efficiently

48
WLAN IEEE 802.11 current developments

• 802.11s Mesh Networking
• Design of a self-configuring Wireless
  Distribution System (WDS) based on 802.11
• Support of point-to-point and broadcast
  communication across several hops
• 802.11T Performance evaluation of 802.11
  networks
• Standardization of performance measurement
  schemes
• 802.11u Interworking with additional external
  networks
• 802.11v Network management
• Extensions of current management functions,
  channel measurements
• Definition of a unified interface
• 802.11w Securing of network control
• Classical standards like 802.11, but also 802.11i
  protect only data frames, not the control frames.
  Thus, this standard should extend 802.11i in a
  way that, e.g., no control frames can be forged.
• 802.11y Extensions for the 3650-3700 MHz band in
  the USA
• 802.11z Extension to direct link setup
• 802.11aa Robust audio/video stream transport
• 802.11ac Very High Throughput lt6Ghz
• 802.11ad Very High Throughput in 60 GHz
• Note Not all standards will end in products,
  many ideas get stuck at working group level
• Info www.ieee802.org/11/, 802wirelessworld.com,
  standards.ieee.org/getieee802/