The CSMA/CA protocol utilizes a line of defense to ensure that any Wi-Fi radio does not transmit while another is already transmitting on the same channel. The 802.11-2016 standard defines a physical carrier sense mechanism to determine if the radio frequency (RF) medium is busy.
Physical carrier sense is performed constantly by all Wi-Fi radios that are not transmitting or receiving. When a station performs a physical carrier sense, it is actually listening to the channel to see whether any other RF transmissions are occupying the channel. Physical carrier sense has two purposes:
To achieve these two physical carrier sense goals, 802.11 radios use a clear channel assessment (CCA) to appraise the RF medium. The CCA involves listening for RF transmissions at the Physical layer. 802.11 radios use two separate CCA thresholds when listening to the RF medium. As shown in Figure 1, the signal detect (SD) threshold is used to identify any 802.11 preamble transmissions from another transmitting 802.11 radio. The preamble is a component of the physical layer header of 802.11 frame transmissions. The preamble is used for synchronization between transmitting and receiving 802.11 radios.
The SD threshold is sometimes referred to as the preamble carrier sense threshold. The signal detect (SD) threshold is statistically around 4 dB signal-to-noise ratio (SNR) for most 802.11 radios to detect and decode an 802.11 preamble. In other words, an 802.11 radio can usually decode any incoming 802.11 preamble transmissions at a received signal at about 4 dB above the noise floor.
The energy detect (ED) threshold is used to detect any other type of RF transmissions during the clear channel assessment (CCA). Remember that the 2.4 GHz and 5 GHz bands are license-free bands and other non-802.11 RF transmissions may occupy a channel. As shown in Figure 1, the ED threshold is 20 dB higher than the signal detect threshold. For example, if the noise floor of channel 36 were at –95 dBm, the SD threshold for detecting 802.11 transmissions would be around –91 dBm, and the ED threshold for detecting other RF transmissions would be –71 dBm. If the noise floor of channel 40 were at –100 dBm, the SD threshold for detecting 802.11 transmissions would be around –96 dBm, and the ED threshold for detecting other RF transmissions would be –76 dBm.
The definition of both of these CCA thresholds is somewhat vague in the 802.11-2016 standard, which has often resulted in a misunderstanding of the actual threshold values. The interpretation of these thresholds by WLAN manufacturers of 802.11 clients and AP radios will often differ. To complicate matters, please remember that the receive sensitivity capabilities between radios can vary widely. Because of the difference in receive sensitivity, the perception of the noise floor can be quite different between 802.11 radios. Therefore, these two CCA thresholds may also vary due to differences in radio receive sensitivity. The CCA thresholds discussed in this blog are based on transmissions on a 20 MHz channel. A discussion of CCA thresholds becomes even more complex when discussing primary and secondary channels for bonded 40 MHz channels.