CCA in wifi

[Maccarony]

Hi,

Im trying to understand three terms in wifi and how it's related to each other and what it means:

CCA(clear channel assesment)

Energy threshold detector

Packet threshold detector

Noise floor

What purpose of each terms here anx how it affects throughput if energy threshold detector is very high/low?

[ammahend]

Wireless is a shared medium and generally speaking only one device can communicate (send or recieve) at a time (keeping some new enhancements aside) , so they need to make sure if the channel is free, so CCA is needed, its an important part of CSMA-CA algorithm to avoid collision and cause performance issues. 

ETD is also part of CCA, that measures the total RF energy on the channel, regardless of whether it’s a Wi-Fi signal or interference, WiFi is not the only thing on channel there can be bluetooth devices, microwave and others, this helps the device avoids transmitting when any significant RF energy (Wi-Fi or non-Wi-Fi) is present

PTD helps specifically detects valid Wi-Fi signals by identifying the preamble of Wi-Fi frames, this allows the device to distinguish Wi-Fi signals from other RF energy, enabling more precise channel access decisions

Noise floor is the baseline level of background RF energy typically -95dBm, it provides a baseline to calibrate CCA thresholds

[Maccarony]

I understand , thanks for clarity.

Two concerns,

Since enery detect level related to CCA, how it's affecting it? And what is standard/usual valye of energy detect threshold? .. therotically assume you can configure that threshold of detect energy .. is there a value could lead to high CCA report? Just to understand how it affects CCA report.

If Packet detect threshold found then why we care about ETD threshold and why needed? .. we only care about any signal that has higher energy than packet detect threshold then its wifi signal ..so here we care .. why ETD needed at all?

Second concern , generally lets assume noise floor is -100dbm , then how much in general the energy detect level and packet detect level?

[ammahend]

here is a good example with some calculations, let me know if this helps clarify your question 

[Maccarony]

Hi ,

Still not clear for me , why we need energy detect threshold?

See there is packet detect threshold which it tells , if signal energy > packet detect threshold then its wifi signal on the air.

So why we need energy threshold? And who has larger value energy detect threshold or packet detect threshold?

[ammahend]

ok, image an office space, there are a lot of interferes  which may not be WiFi for e.g. Bluetooth keyboards, mouse, microwaves etc. which will always be present no matter what, you can't get rid of them, the enerenergy detect threshold is usually set about 25 to 30dBm higher than noise floor, so if you assume noise floor to be -96 dBm, then EDT is about -66dBm (approx) this is strong signal, so unless one of these interferes are operating at high power you basically don't care and keep using the channel, but if you detect non-wifi singal high enough take this into account.

PDT specifically detects valid Wi-Fi signals, the value of PDT is usually 10-15dBm more than noise floor, so with -96dBm noise floor, it would be about -82dBm (approx), this is not a strong signal, now lets say you have a deployment with low data rate enabled, the client will keep shifting data rate and stay connected to the same AP even at lower signal strength as it moves further and further away. So even in this bad design the client has channel occupied and the system needs to take that into account to avoid collision on the channel. 

now what if we don't have EDT and only PDT, well first of all you are not taking into account non-WiFi interferes even if they occupy channel at high power and you will transmit causing performance issues. 

good thing is there are mechanism built into new WiFi standards like BSS coloring and OFDMA which helps avoid some of these legacy channel usage algorithm and allows multiple clients to use channel simultaneously and use tags on channel for better reuse of channels.