Wi-Fi CSMA/CA – Going Deep

Wi-Fi CSMA/CA – Going Deep

In my career as a Wireless Engineer I have read a whole bunch of articles on Wireless Arbitration and how it works and to be perfectly honest, I have not ever really looked deep enough to fully understand how the carrier medium is actually marked as busy by the PHY and MAC layers.

I learnt pretty early on that at the beginning of the transmission of a frame, there is a field somewhere that tells all the other stations to stop transmitting for a given period of time.  But I glossed over where exactly it was placed because, well, it just did not seem that important.  The medium gets marked as busy,  I send my frame at my chosen rate and then everyone waits politely until i get my ACK, and we all start again after a nice communal DIFS. Right? Simple!

Hah!  As they say: the devil is in the details.

I must admit that I never fully understood Andrew Von Nagy’s post Understanding Wi-Fi Carrier Sense until now.  It all finally came together for me whilst studying for my CWAP exam and reading the IEEE 802.11-2012 standard along wth it and his post suddenly brought it all together for me. So kudos to Andrew!

HERE IS THE THING.

The implementation of Wi-Fi carrier sense and how it works is actually very important. This is in my view, one of the MOST critical things to understand when designing a WLAN. It will affect your network in many ways including the size of the contention domain, the presence and effect of hidden nodes in your network, and the effects of minimum rate selection when you commence with your attempts to optimise your design.

I don’t want to re-hash what other people have written so much about (and I feel many have done a great job before me) but I do want to focus on the parts that I failed to fully grasp early on, in the hope that it will help someone else get the penny to drop.

So if you have done any reading on this topic (and you can read the articles above for free) you will know that a wireless station is not allowed to transmit until it has determined the Physical Medium to be Idle (Clear Channel Assessment is determined to be True) AND the Network Allocation Vector must be zero.  But how does all this actually work?!

Ok we are going to get into this, but first!

CCA – Energy Detect

802.11 is a polite protocol.  Stations in an 802.11 based WLAN are required to wait for other stations to finish their transmissions before sending their own. They are supposed to share.  But what about scenarios where someone else, who is not using Wi-Fi, is using the same medium?  What about a bluetooth radio? Or something using Zigbee? Or one of those dreaded 5GHz dect phones?  Brace yourselves friends, the IoT is coming…

(*cough* Please note my subtle joke about LTE-U and LAA/LWA *cough*) 

In the event that there is some signal that cannot be decoded by the station, the 802.11 standard provides for polite interoperation with the unknown entity in the form of Clear Channel Assessment  using Energy Detect.

If there is any signal above the specified Energy Detect (ED) threshold, 802.11 based or otherwise, the  affected station detecting the energy in the channel must mark the channel as busy and wait for it to stop.

The Energy Detect threshold is typically calculated as a number ABOVE the minimum required receive sensitivity of the radio.  In 802.11, the original requirement for receive sensitivity was to be able to receive 2Mbps (using DQPSK) at an RSSI of -80dBm with a given error rate (something tiny).  In 802.11a and beyond, the ED threshold was set to 20dB ABOVE the minimum receive sensitivity laid out in the standard.

In the original 802.11 standard the ED threshold was defined as:

  • -80dBm for stations using a transmit power of 100mW or more.
  • -76dBm for stations using a transmit power of more than 50 mW
  • -70dBm for stations using a transmit power of less than or equal to 50mW

In later amendments the threshold was changed as follows:

  • 802.11b (HR-DSSS): -76dBm, -73dBm and -70dBm respectively following the same pattern as defined for DSSS above
  • 802.11a /g/n/ac: -62 dBm  (using a 20MHz Channel)

Vendors will typically implement an ED threshold of just less than -62dBm to be compliant with the standard.  Using a metric of -65dBm in your designs for an ED threshold is quite reasonable.

So, in this scenario, any received signals above the ED threshold will cause the channel to be marked as busy and stations will defer access.   This is called CCA – Energy Detect.

CCA – Carrier Sense (CS)

Now that we know when to defer to very loud and/or non-802.11 radio activity, let’s see how 802.11 stations interact with each other.

If you were a receiving station, with nothing to say, you would be in IDLE mode and you would be listening to the communication medium on your chosen channel.

Let’s assume another station on your channel starts transmitting.

NOTE: This can be ANY station, not just another in your Basic Service Set or an STA sending something addressed to you.  It can literally be any station using the same channel as you and whose transmissions you can successfully decode.

The transmitting station starts by sending the appropriate PLCP preamble for the chosen PHY type (these are just wave forms and training sequences that help receivers sync up with the transmission and lets them know of the imminent arrival of an 802.11 transmission).

The next field to follow straight after the preamble is the PLCP Header.  In the PLCP Header there is a SIGNAL field that contains two pieces of information:

  1. The length (in octets) of the coming 802.11 frame
  2. The data rate or modulation and coding scheme of the data to follow.

NOTE: For legacy DSSS (802.11) and HR-DSSS (802.11b) radios, the PLCP Header is preceded by a Start of Frame Delimiter of 16 bits.  The DSSS / HR-DSSS PLCP Header contains a Length Field defining the period of time in microseconds that the channel will be busy for.  The newer PHY types (802.11a/g/n/ac) simply provide the frame length in bytes and the PHY Rate and let the receiving stations figure out how long to be quiet for.

Any receiving station that can decode the PLCP Header and the SIGNAL field must maintain Physical Medium Busy Status for the duration specified by the two parameters above.  This is called Physical Carrier Sense.  The station heard a PLCP header it could decode, it establishes that the channel is busy, so it shuts up.

It is important to know:  The PLCP Preamble and the PLCP Header are sent AT THE LOWEST RATE using THE MOST ROBUST MODULATION SCHEME.  This basically ensures that ANY other station within range of the transmitting station will be silenced by the PLCP Preamble and Header.

A Common Misconception:

The IEEE 802.11 standard and its amendments (a/g/n/ac) define the minimum required signal level at which PLCP preambles and PLCP Headers must be decoded for 802.11a/g/n networks as -82dBm.

Many people misinterpret this to mean that below -82dBm, the CCA stops having any effect and I can ignore incoming transmissions even if I can decode them…  THIS IS A MISTAKE!

6 Mbps at a signal level of -82dBm is the minimum required receive sensitivity to meet the minimum required 802.11 standards.  Most if not ALL enterprise WLAN equipment has receive sensitivities that are vastly better than this required value.  Some even typically go lower than -95dBm!

In practical terms, this could mean that the signals from your APs are travelling four times further than you originally anticipated, and your contention domain is a whole LOT larger than you originally thought…

The point here is that if a station can detect a preamble and decode it at ANY signal level, the station is obligated to indicate that CCA is false and the medium is busy.

Duration / ID and the Network Allocation Vector

After the PLCP Header is received, the 802.11 frame itself actually starts arriving at the receiving stations.  The 802.11 frame is transmitted at the PHY rate specified in the PLCP Header.

The 802.11 frame contains the MAC Header in which we have a Duration/ID field.  This Duration/ID field is used to tell the wireless stations on the same channel about *future frames* that may follow as part of the MAC protocol in use with this frame transmission.   For example, we know that if we are using the good old DCF that any frame transmission will be followed by a SIFS and and ACK.  The Duration Field in this case provides a time value in microseconds equal to the time taken to wait for a SIFS and receive the ACK, keeping the medium open for the required frame exchange to be successfully completed.

ALL stations who heard the MAC Header in the transmission will now update something called the Network Allocation Vector to add in the extra time needed to remain silent for future frame exchanges. This is called Virtual Carrier Sense.  

 

In Summary:

Accessing the medium is made up of three parts:

  1. CCA – Energy Detect: Is anything currently occupying the channel above the specified Energy Detect threshold?
  2. CCA – Carrier Sense: Have I detected / decoded any 802.11 frames on my channel that are currently being transmitted?
  3. NAV:  Am I waiting for other frames to be sent as part of an exchange between other STAs on my channel?

Other STAs can only enter into contention to access the channel once all three conditions are met.

Some important notes to remember here:

  1. The PLCP Header is always sent at the lowest PHY rate defined in the IEEE 802.11 standard for that PHY Type regardless of what you set your minimum rate to (more on this later).
  2. The MAC Header containing the Duration Field is sent at the PHY Rate / MCS Index defined for the 802.11 frame in the PLCP header.
  3. The information contained in the PLCP Header and the MAC Header are processed by ALL stations on the same channel that are nearby enough to successfully decode the information.
  4. Channel overlap and contention due to CCA – Carrier Sense may be occurring over a MUCH bigger domain than you originally anticipated.  6Mbps at -82dBm receive sensitivity is THE MINIMUM requirement for 802.11a/g/n/ac.  Check your APs’ receive sensitivity tables to see just how far away a PLCP Header might be heard!
  5. Acknowledgement to Andrew Von Nagy over at Revolution Wi-Fi for the remark in his blog about the NAV being used for Future Frames. That was when the penny dropped!

Finally, I would like to acknowledge Keith R Parsons and Jared Griffith and the larger twitter Wi-Fi community for providing me with the discussions necessary to wrap my head around these concepts. Thanks all!

Hope this helped,

Rob

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