CNET304 - Wireless Technology: Notes on Multiple Access & IEEE802.11 MAC

Distributed Coordination Function (DCF)

  • DCF is the fundamental access method of 802.11 communications.
  • Based on the CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) process.
  • Key Components of CSMA/CA:
    • Physical Carrier Sense: Determines if the medium is busy or if a frame is being transmitted.
    • Virtual Carrier Sense: Uses the Duration/ID field in the MAC header to prevent transmission during active exchanges.
    • Pseudo-random Backoff Timer: Elected timer that waits before transmitting when the medium is busy.
    • Interframe Spaces: Various waiting periods to prevent data collisions.

Physical and Virtual Carrier Sense

  • Physical Carrier Sense:

    • Detects if the medium is clear before transmission.
    • Synchronizes with ongoing transmissions if the medium is busy.

  • Virtual Carrier Sense:

    • Occurs at Layer 2 of the OSI model.
    • Utilizes the Duration/ID field that indicates the time needed for ongoing exchanges.

MAC Service Data Unit (MSDU)

  • Divided into two sublayers:
    • IEEE 802.2 LLC (Logical Link Control)
    • MAC Sublayer (Media Access Control)
  • MSDU is the data passed from the LLC to the MAC corresponding to upper-layer data.

MAC Protocol Data Unit (MPDU)

  • Components of MPDU:
    • MAC Header
    • Frame Body
    • Frame Check Sequence (FCS)
  • The MAC header is crucial for identifying frame properties and ensuring proper delivery.

802.11 MAC Header

  • Contains layer 2 information, visible without encryption.
  • Comprises nine fields, significant for addressing and frame control.
  • Frame Control Field includes subfields like Protocol Version, Type, Subtype.

Frame Types

  • Different frame types identified by fields:
    • Management Frames: Discover access points (APs).
    • Control Frames: Acknowledge and reserve the medium.
    • Data Frames: Carry upper-layer payloads.

Addressing in 802.11

  • 802.11 frames can have up to four addresses:
    • Source Address (SA): The original sender.
    • Destination Address (DA): Final recipient of the frame.
    • Transmitter Address (TA): Address of the transmitting radio.
    • Receiver Address (RA): Intended recipient's radio address.
  • BSSID: Identifies the basic service set.

Sequence Control Field

  • 16-bit field used to manage fragmented frames.
  • Essential for maintaining integrity in transmitting segmented data.

Frame Body

  • Not all frames contain a body:
    • Management Frames: Include control without carrying upper-layer data.
    • Control Frames: Manage channel usage without a body.
    • Data Frames: Typically carry the MSDU payload, possibly encrypted.

802.11 Trailer

  • Contains the FCS (Frame Check Sequence) for data integrity validation.

Management Frames

  • Essential for BSS traffic:
    • Help join and leave BSS.
    • Include types like Association Request, Reassociation Response, etc.

Scanning Processes

  • Passive Scanning: Receives beacons for necessary BSS info; performed every 102.4 ms.
  • Active Scanning: Sends probe requests to identify available APs across multiple channels.

Authentication and Association

  • Open System Authentication: No verification occurs.
  • Association: Establishes layer 2 connectivity with an AP to join a BSS.

Basic and Supported Rates

  • APs configure basic and supported rates;
    • Basic rates - necessary for association.
    • Supported rates - optional, advertised in beacon frames.

Reassociation

  • Involves sending a reassociation request to transition to a new AP.

Control Frames

  • Assist in the delivery of data frames, operate at basic rates.
  • Include types like RTS, CTS, ACK, and Block ACKs to optimize communication.

Data Frames

  • Carry payloads and facilitate communication across the layers.
  • Two primary types: simple data and QoS data frames; 15 subtypes exist overall for various functionalities.