WIRELESS LANS

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WIRELESS LANS

• By Ian Ridsdill
• November 23rd 2007

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WIRELESS LANS

• So what IS all this jibber-jabber nonsense,
  anyway?

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Wireless LANS

• Becoming increasingly popular.
• Office buildings, airports, universities, cafés,
  sidewalks, bathrooms, bowling alleys
• Can operate in one of two modes
• Ad-hoc (w/o base station) and
• Access Point (w/ base station)
• Both use 802.11 standard

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802.11 Protocol Stack

• Members of the IEEE 802 standard are
• Ethernet (802.3)
• Wi-Fi (802.11)
• Bluetooth (802.15.1)

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802.11 Protocol Stack

• Fun happens here.

your data here

• Corresponds to OSI physical layer fairly well.

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802.11 Protocol StackData Link Layer
your data here

• Logical Link Layer
• hides the differences between the different 802
  variants.
• makes them indistinguishable as far as the
  network layer is concerned.

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802.11 Protocol StackPhysical Layer

• FHSS (Frequency Spread Spectrum)
• DSSS (Direct Sequence Spread Spectrum)
• OFDM (Orthogonal Frequency Division
  Multiplexing)
• HR-DSSS (High Rate Direct Sequence Spread
  Spectrum)

your data here
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802.11 Protocol StackMAC Sublayer Protocol

• MAC (Medium Access Control) Sublayer
• Quite different from that of Ethernet
• Due to inherent complexity of a wireless
  environment versus wired.
• Ethernet waits until the ether is clear and
  starts transmitting.
• If it does not receive a noise burst within the
  first 64 bytes, the frame is almost assuredly
  been delivered correctly.

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802.11 Protocol Stack Distributed Coordination
Function

• Hidden station
• transmissions occurring in one part of a cell
  may not be received elsewhere in the cell.
• Ad-hoc network problem
• DCF (Distributed Coordination Function) is used
  in ad-hoc networks to correct problem.

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802.11 Protocol Stack Point Coordination Function

• PCF (Point Coordination Function) used for
  access point based networks.

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802.11 Protocol Stack Point Coordination Function

• Base station broadcasts beacon frame
  periodically (10 to 100 times per second).
• Beacon contains hopping sequences and dwell
  times (for FHSS), clock synchronization, new
  station/client invites (to sign up for polling
  service).
• polling service guarantees a client a certain
  fraction of the bandwidth making it possible to
  give quality of service guarantees.

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802.11 Protocol Stack Point Coordination Function

• Base station polls the other stations.
• Asks them if they have any frames to send.
• No collisions. Transmission order controlled by
  base station.

SIFS (Short InterFrame Spacing), PIFS (PCF
InterFrame Spacing), DIFS (DCF InterFrame
Spacing), EIFS (Extended InterFrame Spacing)
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802.11 Protocol Stack Point Coordination Function

• SIFS (Short InterFrame Spacing),
• Let receiver send a CTS in response to an RTS,
  let receiver send ACK for fragment or full frame,
  let sender transmit next fragment without sending
  an RTS first.
• PIFS (PCF InterFrame Spacing),
• Receiver sends beacon or poll frame. Allows a
  client sending a data frame or fragment sequence
  to finish without anyone getting in the way.
• DIFS (DCF InterFrame Spacing),
• Any client may attempt to acquire the channel to
  send a new frame.
• EIFS (Extended InterFrame Spacing)
• Used only by client that has just received a bad
  or unknown frame. This interval allows it to
  report a bad frame.

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802.11 Protocol Stack Frame Structure
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802.11 Protocol Stack Frame Structure
Protocol Version allows two versions of protocol
to operate at the same time. (eg, 802.11b and
802.11g)
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802.11 Protocol Stack Frame Structure
Type data, control, or management
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802.11 Protocol Stack Frame Structure
Subtype RTS or CTS
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802.11 Protocol Stack Frame Structure
Coming FROM or going TO an intercell distribution
system (eg, Ethernet)
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802.11 Protocol Stack Frame Structure
More Fragments will follow.
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802.11 Protocol Stack Frame Structure
Marks a retransmitted frame.
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802.11 Protocol Stack Frame Structure
Frame is encrypted using a WEP (Wired Equivalent
Privacy) algorithm.
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802.11 Protocol Stack Frame Structure
A sequence of frames must be processed strictly
in order.
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802.11 Protocol Stack Frame Structure
Duration how long the frame and its
acknowledgements will occupy the channel.
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802.11 Protocol Stack Frame Structure
Address 12 are used for Sender/Receiver.
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802.11 Protocol Stack Frame Structure
Address 3 4 are used for Sender/Receiver access
points for intercell traffic..
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802.11 Protocol Stack Frame Structure

• 16 bits available to number fragments
• 12 identify the frame
• 4 identify the fragment.

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802.11 Protocol Stack Frame Structure
Payload
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802.11 Protocol Stack Frame Structure
Checksum for payload.