IEEE 802.11 Wireless Standards

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IEEE 802.11 standards are standards for wireless networking based on spread spectrum radio transmission in the 2.4 GHz and 5 GHz bands.

• aka Wi-Fi
• define the Physical layer media by which data is encoded into a radio carrier signal using a modulation scheme
  • properties of radio waves include:
    • amplitude
      • the height of peaks and troughs
    • frequency
      • the number of peaks per unit of time
    • phase
      • the angle of a wave at a point in time
  • Modulation changes one or more of those properties to encode a signal
• use different carrier methods to provide sufficient resistance to interference from noise and other radio sources
• Wi-Fi Alliance certifies products conforming to Wi-Fi standards

CSMA/CA

• a wireless radio transmitting and receiving within a particular range of frequencies with the same modulation scheme is a half-duplex shared access medium (a physical bus)
• 802.11 uses carrier sense multiple access with collision avoidance (CSMA/CA)
  • to cope with contention
• under CSMA/CA,
  • when a station receives a frame, it performs error checking
  • If the frame is intact, the station responds with an acknowledgment (ACK)
  • If the ACK is not received, the transmitting station resends the frame until timing out
• 802.11 also defines a virtual carrier sense flow control mechanism
  • to further reduce the incidence of collisions
  • A station broadcasts a request to send (RTS) with
    • the source and destination
    • the time required to transmit
  • receiving station responds with a clear to send (CTS)
    • all other stations in range do not attempt to transmit within that period

Implementation

• Wi-Fi typically uses logical star topology to establish a wireless local area network (WLAN)
  • access point (AP) mediates connections between client device radios (stations)

Frequency Bands

Two main frequency bands used by the IEEE 802.11 standards:

• 2.4 GHz standard

  • better at propagating through solid surfaces, giving it the longest signal range
  • does not support a high number of individual channels and is often congested, with both other Wi-Fi networks and other types of wireless technology, such as Bluetooth
  • microwave ovens work at frequencies in the 2.4 GHz band
  • increased risk of interference
  • the maximum achievable data rates are typically lower than with 5 GHz

• 5 GHz standard

  • less effective at penetrating solid surfaces
    • does not support the maximum ranges achieved with 2.4 GHz standards
  • supports more individual channels
    • suffers less from congestion and interference
    • supports higher data rates at shorter ranges

• nominal indoor range for Wi-Fi is 45 m (150 feet) over 2.4 GHz

  • 30 m (100 feet) over 5 GHz

• clients are only likely to connect at full speed from a third to a half of those distances

  • Depending on the wireless standard used, building features that may block the signal, and interference from other radio sources