In 802.11, I have been studying different types of CSMA/CA. And I come across the following

DCF - Distributed Coordination Function

  • Relies on the ability of computers to physically listen before they transmit
    • When a node wants to send a message:
      • First listens to make sure that the transmitting node has finished, then
      • Waits a period of time longer
  • Each frame is sent using stop
    • By waiting, the listening node can detect that the sending node has finished and
    • Can then begin sending its transmission
    • ACK/NAK sent a short time after a frame is received,
    • Message frames are sent a somewhat longer time after (ensuring that no collision will occur)

California State University, Long Beach, IS 355 - Wi-Fi LAN

I have few questions regarding DCF and its relationship with Clear Channel Assessment (CCA) (the threshold that indicates that the channel is busy)

  1. Is DCF implemented using Energy detection or is it using preamble detection?

  2. does the node that wish to transmit, detect energy at its antenna and then defer transmission? Is there a certain gain threshold (CCA threshold) in dbm that nodes should compare to and how do they calculate it?

Thanks, looking forward for your discussion or references.

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2 Answers 2


IEEE 802.11-2012 controls for OFDM encoded wireless transmissions:

CCA shall detect a medium busy condition when the carrier sense/clear channel assessment (CS/CCA) mechanism detects a channel busy condition.

The definition of "busy" depends on whether or not a valid OFDM transmission is demodulated. If one is and the receive level is equal to or greater that the minimum MCS sensitivity (e.g. -82 dBm for 20 MHz channels), then CCA will report the channel busy within 4 microseconds with a probability greater than 90%. If a valid OFDM transmission is not received (i.e. a preamble is not successfully decoded), then the receiver defines a channel as busy if energy is greater than 20 dB above the MCS minimum sensitivity (e.g -62 dBm for 20 MHz channels).

A node is required to determine the state of the channel before it determines if it can transmit and must defer if the channel is not clear.


Additionally to Cliff Alligood answer: the coordination function is independent of the way the state of the channel is detected.

IEEE 802.11-2012 Annex E.1 defines that Energy Detection is to be used only in certain operating classes, presently 13, 14 & 15, which include channels 131 to 138 (in the 3GHz range).

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