As we know 802.11 is half-duplex, so devices must check the wireless medium to determine if it is free or busy before it can send traffic, No issues there.
Where it gets interesting is when we look at the thresholds (at a high level) that determine if the medium is busy. Part of the mechanism used to determine if it is available is called Clear Channel Assessment (CCA). Within CCA there are two thresholds – Signal Detect (SD)and Energy Detect (ED).
– Signal detect (SD): listen for any transmitting 802.11 frames
– Energy detect (ED): list for any other none RF transmissions that is 20dB stronger than SD (won’t go into this in this post)
SD is used to detect a transmitting 802.11 preamble, which is contained within the physical layer header of an 802.11 frame, if it can decode the preamble, it will attempt to sync with the incoming transmission, which is sent at the lowest basic rate for the Band, 1 Mbps for 2.4GHz and 6Mbps for 5GHz. The SD threshold is usual set at 4dB SNR, so if the noise floor is at -96dBm, an 802.11 device is able to demodulate the preamble at an RSSI of -92dBm, which would then trigger CCA, causing devices to deferrer.
So how far is -92dBm or 4 dB SNR? The below simulation shows you how far, represented by a simulated Cisco 3802 AP with the following configuration: 1mW, 2.4GHz and no obstruction,
The colour coding represents the following signal areas,
: The wireless coverage we have designed for, where we want our clients to be associated
: The area that can cause CCA
and every where else is where it becomes noise below the required CCA threshold
- Note the picture above is only the Radius.
Using 1mW (equates to 0dBm) and with a cell edged measure at -92dBm the total distance is 500m.
In some of the environments that I work in our industrial wireless AP can vary anywhere from 2Watts (33dBm) – 4Watts (36dBm) in the 2 .4GHz and 5GHz band and our enterprise AP around 25mW (14dBm) +/-, so the potential coverage area for SD to cause devices to deferrer can be quite large
Understanding this is important when it comes to our wireless designs. We design for a required cell edge i.e. -67dBm/25dB SNR for devices, but we also need to account for the signal we don’t care about as it continues on, so we can minimise Co- channel interference (CCI).
Minimising CCI is possible in 5GHz (depending on AP capacity), but not possible in 2.4GHz. It becomes an even greater pain in the butt when we add client STA’s into the mix, as the client STA are one of the biggest contributors to CCI.
- Image sourced from https://blog.wifidownunder.com/
While we may have done our best to minimise AP CCI, as soon as a mobile client roams to the outer edges of a cell, it has the potential to extend the contention domain and interfere with another AP on the same channel, causing devices to defer. This is due to SD as mention previously
This in turn increases medium contention, due to devices having to defer, which eats up your airtime – equaling less throughput.
What can we do to help reduce this?
- Understand your AP and client STA receive sensitivities levels
- Consider your CCI boundaries in your designs
- Use directional antennas where possible
Tom Carpenter (2016). Certified Wireless Analysis Professional (CWAP-402). US: CertiTrek Publishing
Devin Akin (2018) Certified Wireless Design Professional Training course