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Wi-Fi (IEEE 802.11)
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Background
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  • The Standards:
    • IEEE 802.11 (a.k.a. ISO/IEC 8802-11)
  • The Certification:
    • Wireless Fidelity (Wi-Fi)
    • Given by the Wi-Fi Alliance [formerly Wireless Ethernet Compatibility Alliance (WECA)]
The Physical Layer
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  • Purpose:
    • Defines the radio wave modulation and signalling characteristics for data transmission
  • Alternative Modes for 802.11b/g:
    • Direct Sequence Spread Spectrum (DSSS) - encodes each bit into a bit pattern (called a chipping code or Barker sequence; spreads the energy in a signal over a wider frequency range using 11 channels)
    • Frequency Hopping Spread Spectrum (FHSS) - jumps between one of 79 channels (using a fixed hop pattern)
    • Infrared
  • Mode for 802.11a:
    • Orthogonal Frequency-Division Multiplexing (OFDM) - multiple narrowband data streams at different frequencies
The Physical Layer (cont.)
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  • Frequency Bands:
    • 802.11b and 802.11g DSSS uses 11 channels in the 2.4GHz band, each of which is 22MHz wide
    • 802.11a uses 9 channels in the 5.8GHz range
  • Distance Limitations:
    • In theory, 802.11b has a range of 100 meters indoors and 200 meters outdoors
    • In practice, many things can interfere with the signal, including microwave ovens that operate at 2.45GHz (recall that this part of the spectrum is unlicensed)
The Physical Layer (cont.)
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  • DSSS Channels in 802.11b/g:
    • 1: 2.401-2.423GHz
    • 2: 2.405-2.428GHz
    • 3: 2.411-2.433GHz
    • 4: 2.416-2.438GHz
    • 5: 2.421-2.443GHz
    • 6: 2.426-2.448GHz
    • 7: 2.431-2.453GHz
    • 8: 2.436-2.458GHz
    • 9: 2.441-2.463GHz
    • 10: 2.446-2.468GHz
    • 11: 2.451-2.473GHz
  • An Important Observation:
    • Channels 1, 6 and 11 do not overlap so can be used at the same time
Topologies
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  • Completely Connected:
    • Ad hoc Wi-Fi networks communicate peer-to-peer
  • Star:
    • Infastructure networks use a Wireless Access Point (AP/WAP) to transmit between devices
    • [A basic service set (BSS) consists of one or more stations and an access point.]
    • [Multiple APs can be connected to form a distribution system (DS).]
Media Access Control (MAC) Layer
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  • Recall:
    • Ethernet uses Carrier Sense Multiple Access/Collision Detection (CSMA/CD)
  • A Difficulty:
    • This is impractical for wireless when an adapter can't transmit and receive at the same time (because you can't listen to determine if a collision has occurred while transmitting)
MAC Layer (cont.)
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  • Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA):
    • Listen [for an amount of time called the distributed inter frame space (DIFS)] to see if idle
    • If idle, transmit
    • After transmitting, wait for an acknowledgment (ACK)
    • The recipient transmits an ACK [after waiting for an amount of time called the short inter frame space (SIFS)]
    • If no ACK is received, backoff (as in Ethernet) and repeat
  • A Potential Problem:
    • Carrier sense does not work when two devices are too far from each other
MAC Layer (cont.)
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  • Fixing this Problem:
    • Replace the "direct carrier sensing" process with a "virtual carrier sensing" process
  • Virtual Carrier Sensing:
    • A device sends a Request To Send (RTS) to the wireless access point
    • If appropriate, the WAP sends a Clear To Send (CTS) that identifies the device that can send and the amount of time its been allocated
    • All devices receive the CTS and, hence, know whether they can send (i.e., the requesting device) or not (i.e., all other devices)
MAC Layer (cont.)
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  • Terminology:
    • CSMA/CA is referred to as the Distributed Coordination Function (DCF)
    • Virtual carrier sensing is referred to as the Point Coordination Function (PCF)
  • Of Interest:
    • DCF is required
    • PCF is optional
MAC Layer Frames in IEEE 802.11
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  • Frame Control (FC):
    • Two octets that indicate whether it is a control, management or data frame
  • Duration/Connection ID (D/I):
    • Two octets indicating the number of milliseconds allocated for transmission
  • Address 1:
    • Six octets that depend on frame type but often containing the MAC address of the transmitter
  • Address 2:
    • Six octets that depend on frame type but often contain the MAC address of the receiver
  • Address 3:
    • Six octets that depend on frame type but often contain the MAC address of the destination
  • Sequence Control (SC):
    • Two octets; a 4-bit fragment number and a 12-bit sequence number
  • Address 4:
    • Six octets that depend on frame type but often contain the MAC address of the source
  • Data:
    • 0-2312 octets
  • Frame Check Sequence (FCS):
    • Four octets used for a cyclic redundancy check (CRC)
MAC Layer Frames in IEEE 802.11 (cont.)
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  • Control Frames:
    • RTS, CTS, ACK, Contention-free end (CF-End), CF-ACK, a few others
  • Data Frames:
    • Data, Data+CF-ACK, a few others
  • Management Frames:
    • Used to manage associations between stations and APs
Physical Layer Frames in IEEE 802.11
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  • There are Several:
    • Different for 802.11b and 802.11a
  • Contain Fields For:
    • Data rate, length, synchronization, and other things
Security
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  • The Issue:
    • Since Wi-Fi uses an unguided medium, it its very easy for "outsiders" to receive the signal
  • Services:
    • Authentication - Used by stations to establish their identity (several schemes are supported)
    • Confidentiality/Privacy [e.g., wired equivalent privacy (WEP) which is fairly weak; Wi-Fi protected access (WPA)]
    • comics/Hackles-WiFiSecurity.png
      (Courtesy of Hackles)
      Expand
    • comics/SOTI_DiscussingTechnologyWithRelatives.jpg
      (Courtesy of SomethingOfThatIlk.com)
      Expand
Some Other 802.11 Standards of Interest
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  • 802.11e, 802.11i: Security mechanisms (including WPA)
  • 802.11n: Increased throughput
  • 802.11p: Vehicular environments
  • 802.11r: Fast roaming (i.e., fast BSS transition)
There's Always More to Learn
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