80Mhz Channels in 802.11ac and the “Backward Compatibility”

With the introduction of 802.11ac, we met 80Mhz channels. It meant more data because more spectrum is utilized when a packet is being transmitted.

Of course, there are other consequences when using 80Mhz channels, such as being more prone to interference, less channels to use etc. However, what about the backward compatibility?

We know that still some devices out there using 20Mhz channels in 5Ghz band. Yes, these are legacy 802.11a devices. And also there are 40Mhz devices. And now we have 80Mhz client devices.

What will happen when there are three types of devices (20Mhz, 40Mhz and 80Mhz) connected to an AP which supports 80Mhz? How is backward compatibility working here?

Another question is about Co-Channel Interference. What will happen if there are two 80Mhz APs sharing the same 80Mhz portion of the spectrum? How are they handle the CCI issue?

So, the backward compatibility is solved nicely with 80Mhz channels. Every 80Mhz channel consists of sub 40 and 20Mhz channels.

Screen Shot 2017-12-07 at 12.12.25

For the example, the above image represents an 80Mhz channel. It consists of 4 blocks and each block is 20Mhz.

For example, let’s look at the lower portion of the spectrum in the 5Ghz band, which is UNII-1 band.

UNII-1 is clean from DFS and sometimes we are hit badly by weather RADARs, so sometimes it is better to stay on UNII-1. What is DFS and how is it being affected by RADARs? These could be the subjects of another blog post.

So, when we put 2 80Mhz APs in the same band, say, UNII-1, which has 4 x 20Mhz channels which are 36, 40, 44 and 48; two APs will cover the same spectrum in the entire UNII-1 band. Two APs will be broadcasting on 36+40+44+48.

However, automatically (based on the gear you are using), APs will select different 20Mhz and 40Mhz sub-portions despite their same 80Mhz channel.

Let’s look at the below image, again:

Screen Shot 2017-12-07 at 12.12.25

Assume that A is channel 36. B is channel 40. C is channel 44 and D is channel 48.

Your first 80Mhz AP is on A+B+C+D

And, your second 80Mhz is AP is also on the same band: A+B+C+D

However, according to your radio management mechanism that you use on your APs, they will be selecting different “sub”channels for the 40Mhz and 20Mhz operation.

For example, if you are a 20Mhz client, which sub-channel you’ll use on the first AP? Which sub-channel be used by the second AP? Of course, they will be selecting different channels for 20Mhz operation even though they are on the same 80Mhz channel. At the same time, they will be selecting different 40Mhz sub-channels.. However, the selected 40Mhz sub-portion, should include the 20Mhz subportion.

For example, the first AP is on A+B+C+D as 80Mhz channels.

“A” can be the 20Mhz channel. This will be used for beaconing, NAV, mgmt frames, 802.11a frames, all 20Mhz supported clients’ frames etc.

A+B can be the 40Mhz channel. This will be used for the 40Mhz clients. The 40Mhz portion should include the “A” here. So it must be A+B. For example, B+C portion cannot be used as the 40Mhz sub-portion. C+D cannot be the 40Mhz portion for this AP, as well.

On the other hand, the second AP will be selecting different 20Mhz (hence, 40Mhz) subportion.

For example, AP1 is on

  • A+B+C+D as 80Mhz.
  • A+B as 40Mhz.
  • A as 20Mhz.

AP2 will select like:

  • A+B+C+D as 80Mhz.
  • C+D as 40Mhz.
  • C as 20Mhz.

So, this will decrease the CCI within the ABCD (80Mhz) block and this will also provide simultaneous transmissions to different 20Mhz or 40Mhz channels by different APs. For example, given the channels selected above, AP1 will be communicating with a 20Mhz client on channel A, and at the same time, AP2 will be able to communicate another 20Mhz client on channel C.

So, 80Mhz channels also provides backward compatibility for older 20 or 40Mhz clients and, at the same time, the radio resource allocation mechanism are assigning different sub-channels to different APs, to utilize the air better.

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