Mobile Systems

1
Mobile Systems

• The IEEE 802.11 WLAN
• Part I

Ver 1.1
2
IEEE Std. 802.11-1997, 1-2 Mbit/sec.

• Information Technology-
• Telecommunications and Information exchange
  between
• systems-
• Local and Metropolitian area networks-
• Specific requirements-
• Part 11 Wireless LAN Medium Access Control (MAC)
  and
• Physical Layer (PHY) specifications.
• Sponsor
• LAN MAN Standards Committee of the IEEE Computer
  Soc.
• Approved 26 June 1997.

3
ABSTRACT (I)

• The medium access control (MAC) and physical
  characteris-
• tics for wireless local area networks (LANs) are
  specified in
• this standard, part of a series of standards for
  local and me-
• tropolitian area networks. The medium access
  control unit in
• this standard is designed to support physical
  layer units as
• they may be adopted dependent on the availablity
  of the
• spectrum. This standard contains three physical
  layer units

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ABSTRACT (II)

• two radio units, both operating in the 2400-2500
  MHz
• band, and
• one baseband infrared unit.
• One radio unit employs the frequency-hopping
  spread spec-
• trum technique, and the other emplys the direct
  sequence
• spread spectrum technique.
• KEYWORDS ad hoc network, infrared, LAN, local
  area net-
• work, mobility, radio
  frequency, wireless

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The IEEE Standard 802.11 WLAN

• Similaries between WLAN (Wireless Local Area
  Networks)
• and Wired Local Area Networks.
• The IEEE 802.11 WLAN is designed to look like
  any
• IEEE 802 wired LAN.
• The 802.11 must support all protocols and LAN
• management tools, that operate on a wired
  network.
• The IEEE 802.11 is designed to the same
  interface as
• IEEE 802.3

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The IEEE Standard 802.11 WLAN

• Differences between WLAN (Wireless Local Area
  Networks)
• and Wired Local Area Networks.
• No wires (because air link), and mobility.
• The air link Radio or infrared.
• Data carried by a WLAN is not private or
  protected.
• Data is broadcast to all.
• IEEE 802.11 Wired Equivalent Privacy (WEP),
  protection
• at the same level as wired privacy.
• Electromagnetic Propagation.
• Reflection and/or attenuation (dæmpning) of the
  signal carrying
• LAN data.
• Small changes in physical position gt large
  changes in recieved
• signal strength.

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The IEEE Standard 802.11 WLAN

• Differences between WLAN (Wireless Local Area
  Networks)
• and Wired Local Area Networks.
• Problems introduced by mobility
• The location-based services lose the hook to a
  user
• location, when network addresses are not on a
  physical
• site.
• The notion of the nearest network printer
  must be re-
• defined.
• Increases the complexity of the service
  location provider.

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The IEEE 802.11 WLAN family

• First standard for WLAN IEEE Std. 802.11-1997,
  1-2 Mbit/sec
• Defines

• MAC Medium Access Control layer, management
  protocols and services.
• PHY PHysical Layer, consisting of
• PLCP Physical Layer Convergence Procedure
  sublayer.
• PMD Physical Medium Dependent sublayer.
• Three different physical layers
• Infrared (IR) baseband PHY
• frequency hopping spread spectrum (FHSS) radio
  in the
• 2.4 GHz band.
• direct sequence spread spectrum (DSSS) radio in
  the
• 2.4 GHz band.

MAC layer
PLCP sublayer
PMD sublayer
Wireless Media
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IEEE Std. 802.11-1997, 1-2 Mbit/sec.
Information technology- Telecommunications and
information exchange between sy- stems- Local and
metropolitian area networks- Specific
requirements- Part11 Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY)
specifications. Sponsor LAN MAN Standards
Committee of the IEEE Computer Society Approved
26 June 1997.

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The IEEE 802.11 WLAN family

• In 1999 two new physical layers are approved
• IEEE Std 802.11a is an Orthogonal Frequency
  Domaine Multiplexing
• (OFDM) radio in the UNII bands delivering up
  till 54 Mbit/sec.
• U-NII Unlicensed national information
  structure (US) at 5 GHz.
• IEEE Std 802.11b is an extension of the DSSS
  (Direct Sequence
• Spread Spectrum) PHY (Physical layer) in the
  2.4 GHz ISM band,
• delivering up till 11Mbit/sec. data rates.
• ISM Industrial, Scientific, and Medical band
  at 2.4 GHz.

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IEEE 802.11 Architecture

• Support networks with decisions in mobile
  stations,
• thus eliminating bottlenecks of a centralized
  structure.
• Error tolerant in WLAN equipment.
• Flexible Supporting
• small transient networks, and
• large semipermanent or permanent networks.
• Deep power saving modes to prolong battery life
  without
• losing network connectivity.

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IEEE 802.11 Architecture

• Architectural Elements
• The Station,
• the Access Point (AP, which is a Station), the
  wireless medium.
• the Basic Service Set (BSS). Stations that
  communicate.
• The Distribution System (DS), and
• the Extended Service Set (ESS).

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The Station and The Basic Service Set

• Station Mobile, portable or stationary.
• Other names might be Network adapter or network
  interface card.
• It always consists of a
• MAC Medium Access Control
• PHY Physical Layer (Antenna, Radio, etc.)
• Station services
• Authentication Prove the identity of one
  station to another.
• Deauthentication Eliminate a previously
  authorized user from acces.
• Privacy Equivalent level of protection,
  compared to a wired network.
• Data delivery Reliable delivery of data frames
  from the MAC in
• one station to the MAC in one or more stations.

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The Station and The Basic Service Set

• Station services
• Authentication Prove the identity of one
  station to another.
• Deauthentication Eliminate a previously
  authorized user from acces.
• Privacy Equivalent level of protection,
  compared to a wired network.
• Data delivery Reliable delivery of data frames
  from the MAC in
• one station to the MAC in one or more stations.

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The Station and The Basic Service Set

• Basic Service Set (BSS) Set of stations
  communcating with one another.
• Independent Basic Service Set (IBSS) All
  stations communicating direct-
• ly with one another. Also often denoted an ad
  hoc network.

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The Extended Service Set (ESS)

• The access points (AP) communicate among
  themselves to forward
• traffic from one BSS to another.
• The APs perform this communication via an
  abstract medium called
• the Distribution System (DS).

An Access Point is a station, with access to a
distribution sys- tem.
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Distribution System and Services
Distribution System

• One AP Communicating with another to exchange
  frames for
• stations in their BSSs,
• forward frames to follow mobile stations from
  one BSS to another,
• exchange frames with wired networks.

Services

• Station Services Authentication,
  deauthentication, privacy, delivery
• of data.
• Distribution Services Association,
  disassociaton, reassociation,
• distribution, integration.

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Station Services (I)

• Authentication
• Prove the identity of one station to another.
• Without this, the station is not allowed to use
  the WLAN for
• data delivery.
• Deauthentication
• Eliminate a previously authorized user from any
  further use
• of the network.

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Station Services (II)

• Privacy
• Equivalent level of protection as that provided
  by a wired network
• with restricted physical access to the network
  plant.
• Delivery
• Data delivery similar to that provided by other
  IEEE 802 LANs.
• Reliable delivery of dataframes from one MAC in
  one station to
• the MAC in one or more other stations, with
  minimal dublication
• and minimal reordering.

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Distribution Services (I)

• Association
• Establish a logical connection between a mobile
  station and an AP.
• Necessary for the DS to deliver data to the
  mobile station.
• Invoked once, when the station enters the WLAN
  for the first time,
• after power on, or when rediscovering the WLAN
  after some time.
• Reassociation
• As association, but include information about
  the AP, with which
• the mobil station was previously associated.

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Distribution Services (II)

• Disassociation
• Mobile Station (MS) inform AP that it does not
  need service.
• AP inform one or more MS that logical connection
  can no longer
• be provided.
• Distribution
• Frame sent to its own basic service set (BSS) or
• to another mobile station associated with
  another AP or
• to a network outside the IEEE 802.11 WLAN.

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Distribution Services (III)

• Integration Service
• Connect the IEEE 802.11 WLAN to other LANs,
  including
• one or more wired LANs, or other IEEE 802.11
  WLANs.
• The integration is performed by a portal, which
  is an abstract
• architectural concept.
• The integration service translates IEEE 802.11
  frames to
• frames that may traverse another network, and
• translate frames from other networks to frames
  that may be
• delivered by an IEEE 802.11 WLAN.

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Interaction Between Some Services (I)

• The IEEE 802.11 std. requires that each station
  must
• maintain two variables that are dependent on
• the authentication/deauthentication service, and
• association/reassociation/disassociation
  services.
• These two Boolean variables are
• authentication state
• association state.

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Interaction Between Some Services (II)

• A station may be authenticated with many
  different stations simultaneously.
• A station may be associated with only one other
  station
• at a time.

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Interaction Between Some Services (III)The
General Frame Format
MAC Header
2
2
6
6
6
2
6
0-2312
4
FCS
Address 2
Frame Body
Address 4
Address 1
Sequence Control
Duration/ID
Frame Control (2 bytes)
Address 3
Frame body max. 18496 bits
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Interaction Between Some Services (IV)
Class 1 Frames.
State 1 Unauthenticated,
Unassociated
DeAuthentication Notfication
Successful authentication
Class 1 2 Frames
State 2 Authenticated, Unassociated
Disassociation, Notification
Successful Association or reassociation
State 3 Authenticated, and
Associated.
Class 1, 2 3 Frames
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Interaction Between Some Services (V) A Station
moving between Access Points.
a Find AP1 and authenticate and associate. e
Disassociate stations. b Preauthenticate with
AP2, when moving. f Find another AP3 for
authen- c Reassociate with AP2.
tication and association. d
Terminate the association with AP1.
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Medium Access Control (MAC)

• The IEEE 802.11 medium access control supplies
  the
• required function for
• reliable delivery mechanism for
• user data over
• noisy, unreliable wireless media,
• while providing advanced LAN services, beyond
  those
• of existing wired LANs.

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Medium Access Control (MAC)

• MAC Functionality
• Reliable data delivery service, through a frame
  exchange protocol.
• Fairly control access to the shared wireless
  medium, through two
• different access mechanisms
• The basic access mechanism, called the
  Distributed Coordination
• Function (DCF), and a
• centrally controlled access mechanism, called
  the Point Coordination
• Function (PCF).
• Privacy service called Wired Equivalent Privacy
  (WEP) for encryption.

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MAC Frame Exchange Protocol (I)

• The media used by IEEE 802.11 WLAN are very noisy
  and unreliable.
• Thus the MAC implements a frame exchange
  protocol, which allows the
• source of a frame to determine,
• when the frame has sucessfully been received at
  the
• destination.
• The frame exchange protocol requires the
  participation of all stations in the WLAN. Every
  station decodes and reacts to information in the
  MAC header of every frame it receives.

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MAC Frame Exchange Protocol (II)

• The minimal MAC frame exchange protocol consists
  of two frames
• a frame sent from the source to the destination,
  and
• an acknowledgement from the destination of
  correctly received frame.
• If the source did not receive an acknowledgement
  it will retransmit.
• A frame and its acknowledgement, constitute an
  atomic
• unit of the MAC protocol, and cannot be
  interripted by
• transmission from any other station.

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MAC Frame Exchange Protocol (III)

• If the source does not receive the
  acknowledgement because
• the destination did not send one, due to error
  in the ori-
• ginal frame, or because
• the acknwledgement was corrupted,
• the source will attempt to transmit the frame
  again.
• Reduces the error rate of the medium, at the cost
  of
• reduced bandwidth.

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The Hidden Node Problem (I)
Fact Not every WLAN station can be expected to
commu- nicate directly with every other WLAN
station.
Station A communicates directly with B. Station
B communicates directly with C.
Station A cannot communicate with station C.
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The Hidden Node Problem (II)
If station A was sending a frame to station B,
the frame could be corrupted by a transmission
initia- ted by station C !
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Hidden The Node Problem (III)

• Announce to all station in the neighborhood of
  both the
• source and destination, the impending
  transmision.
• Source sends Request to Send (RTS).
• Destination answers Clear to Send (CTS).

• Then the Source sends
• the dataframe(s).
• The Destination sends
• acknowledgement.

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The Hidden Node Problem (IV)
Four frame exchange protocol, which is atomic in
the MAC (Medium Access Control) protocol. They
cannot be interrupted by transmissions of
other stations.
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MAC Frame Exchange Protocol (IV)

• Now extend into a four frame protocol for
  communicating
• between different MACs
• The source sends a Request To Send (RTS) to the
  destination.
• This is also received by other stations too.
• The destination returns a Clear To Send (CTS) to
  the source.
• This is also received by other stations too.
• The source sends the Data Frame to the
  destination.
• The destination sends an Acknowledgement to the
  source, thus
• completing the data transfer.

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MAC Frame Exchange Protocol (V)

• Reduce the four-way frame exchange protocol to a
  two-way
• protocol if risk of contention is low.
• The management information base (MIB)
• If length of frame gt dotRTSThreshold attribute
• Then use the four way protocol,
• Else use the two way protocol.
• Allow for tuning of the network, by reducing the
  communi-cation overhead.

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Retry Counters

• Two Retry counters, when transmission fails.
• Short retry counter for frames lt
  dotShort11RTSThreshold,
• Long retry counter for frames lt
  dotLong11RTSThreshold.
• There is also a lifetime timer associated with
  each frame
• transmitted from a MAC.

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Basic Access Mechanism (I)

• The basic access mechanism from
• an Access Point (AP) to the
• Physical medium (Radio or Infrared carriers) is
• Carrier Sense Multiple Access (CSMA) with
• Collision Avoidance (CA) with
• binary exponential backoff.
• This is also denoted CSMA/CA.

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Basic Access Mechanism (II)

• A station will listen (Carrier Sense, CS) before
  beginning a transmission.
• If the medium is already carrying a
  transmission,
• Then the station that is listening will not
  begin its own
• transmission, and
• the station enters a deferal period.
• The duration is determined by a random number
  which
• represents the amount of time, that must elapse
  while
• there are not any transmissions.

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Basic Access Mechanism (III)

• The waiting time is called the Contention Window
  the size
• of which doubles with every attempt to access
  the medium
• which is deferred.
• The random number for the exponential backoff
  algorithm
• is uniformly distributed in the Contention Window
  interval.
• The window is re-initialized when a transmission
  is sucess-
• fully completed.

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Basic Access Mechanism (IV)

• The Network Allocation Vector (NAV)
• The amount of time, that remains, before the
  medium will
• become available.
• The NAV is updated through duration values,
  transmtted
• in all frames.
• The NAV is virtual carrier sense mechanism,
  which is com-
• bined with the physical carrier sense, into the
  MAC colli-
• sion avoidance part of the CSMA/CD access
  mechanism.

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Timing Intervals (I)
The 5 timing intervals needed for control of
transmission.
Time
Short Interframe Space (SIFS), determined by PHY.
The Slot Time (ST), determind by PHY.
The Priority Interframe Space (PIFS) SIFS
ST. The Distributed Interframe Space (DIFS)
SIFS 2 x ST. The Extended Interframe Space
(EIFS) gtgt DIFS. Used for error correc.
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Timing Intervals (II)

• The 5 timing intervals is used for implementing
  two diffe-
• rent control mechanisms for transmission
• The Distributed Coordinating Function (DCF),
  used in
• Independent Basic Service Sets (IBSS) also
  denoted ad
• hoc networks.
• The Centrally Controlled Access Mechanism, used
  in a
• Poll and Response protocol to eliminate
  contention for
• the medium, thus obtaining higher bandwidth
  than the
• DCF.

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The Distributed Coordination Function (DCF)
The DCF is used in an Independent Basic Service
Set (IBSS), also denoted an ad-hoc network
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The Distributed Coordination Function (DCF)

• When the MAC is requested to transmit a frame,
  the follow-
• ing is carried out
• Check the physical and virtual carrier sense
  mechanism
• (NAV) if the medium is not in use for an
  interval of
• DIFS Distributed Interframe Space.
• If the medium is in use, apply the back-off
  mechanism,
• and increment the retry counter.
• Decrement the back-off value if the medium is
  idle, by
• on slot time interval. When expire then
  transmit.

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The Distributed Coordination Function (DCF)
Contention Window
End of previous transmission
Next Transmission
DIFS
time
Slots
DIFS Distributed Interframe Space
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Centrally Controlled Access Mechanism (I)

• The Centrally Controlled Access is used when
• there is an Access Point (AP)

Method Use a poll and response protocol, to
eliminate contention for the medium. Point
Coordination Function (PCF).
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Centrally Controlled Access Mechanism (II)

• A point coordinator (PC) controls the PCF.
• A PC is always located in an Access Point (AP).
• The PCF operation
• A mobile requests the PC to register it on a
  polling list,
• residing in the Access Point (AP).
• The PC regularly polls the stations for traffic,
  and
• the PC delivers traffic to mobile stations.
• Near-isochronous service to the stations on the
  polling list.
• www.webster.com Isochronous Uniform in time.

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Centrally Controlled Access Mechanism (III)

• PCF is built on the DCF (Distributed Coordination
  Function).
• Both operate simultaneously.
• The PCF controls and operates the
• Contention Free period (CFP), where all acces is
  controlled
• by the Point Coordinator, and the DCF is not
  allowed access
• to the medium.

Alternate
DCF
PCF
PCF
time
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Centrally Controlled Access Mechanism (IV)

• The Contention Free Period (CFP) starts when PC
  gains ac-
• cess to the medium, from the DCF.
• The PC transmits a Beacon frame containing
• Time stamp
• Beacon interval
• Capability information.
• The PC now delivers traffic to its Basic Service
  Set (BSS), through
• Polling mobile stations on its polling list,
• Stations returning data and acknowledgements, one
  frame for each
• Contention Free Poll (CF-Poll).
• Beacon Signal for guidance.

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Centrally Controlled Access Mechanism (V)

• Improve the efficiency of media utilization by
• Piggyback both the
• Acknowledgement and
• CF-Poll (Contention Free Poll) onto data frames.
• The dataframe from a station to the PC, may
  include
• acknowledgement of the frame just received
  from the PC.

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Centrally Controlled Access Mechanism (VI)

• The Point Coordinator (PC) residing in the Access
  Point (AP),
• may do the following
• sending a frame to one station along with a
  CF-Poll, and
• acknowledge a frame received from a different
  mobile.

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Centrally Controlled Access Mechanism (VII)
PCF Timing
SIFS Short Interframe Space.
Data CF-Ack from Station 1

Data CF-Ack CF-Poll to Station 2
Data CF-Poll
ACK from Station 2
Priority Interframe Space
PIFS
CF-Poll to Station n
Data CF-Poll To Station n1
CF-End
time
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Centrally Controlled Access Mechanism (VIII)

• During the Contention Free Period (CFP), the PC
• Ensures the interval between frames on the
  medium
• is lt PIFS. PIFS Priority Interframe Space.
• Thus a station operating under DCF cannot gain
  access
• to the medium.
• Sends a frame to a station, and expect the
  responding
• frame (ack. or data) within the SIFS interval.
• SIFS Short Interframe Space.
• If not received, the PC transmits the next
  frame before
• a PIFS expires.

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Centrally Controlled Access Mechanism (IX)

• As the CFP (Contention Free Period) is not a true
  isochronous
• Service, where the timing is known in advance,
  the Point
• Coordinator (PC)
• announces the end of the CFP by transmitting a
  CF-End
• frame, which concludes the CFP.
• Then the mobile stations, which had set their NAV
  (Network Allocation vector), from the initial
  Beacon frame, resets
• The NAVs to zero.
• Thus the stations are free to operate in the DCF
  again.

58
References (I)

• Bob OHara, Al Petrick, The IEEE 802.11
  Handbook
• IEEE Press, 1999.
• Reading material
• p. 1 - 69,
• p. 88 (from Power Management) - 98 (not
  including Combining
• Management Tools).
• 2 Jennifer Bray, Charles F. Sturman,
  Bluetooth, Prentice-Hall 2001.

59
References (II)

• IEEE Std. 802.11/1997
• Part 11 Wireless LAN Medium Access Control
  (MAC) and Physical
• Layer (PHY) specifications.
• IEEE Standards Board, June 26, 1997.
• Download www.dtv.dk -gt Search literature -gt
  Search of other
• literature at DTV -gt IEEE/IEE Electronic
  Library -gt Standards -gt
• 802.11 GO

60
References (III)

• 4 Brian P. Crow, Indra Widjaja, Jeong Geun
  Kim, Prescott T. Sakai
• IEEE 802.11 Wireless Local Area Networks
• IEEE Communications Magazine, September 1997,
  pp. 116-126.
• Reading material p. 116, p. 117, p. 118
  excluding the Physical
• Layer.
• P. 119, p. 120, p.
  121.