Re: Purchasing more routers

[T Gillett]

Hi Anish

The wifi algorithm dynamically sets the Modulation Coding Scheme (MCS) mode which is often displayed with (or as) a theoretical bit rate for the type of modulation/coding that is being used.
I assume that this is what you are referring to.

There is a good explanation here:

    http://www.wirelessedge.com.au/understanding-modulation-and-coding-scheme-mcs-index-values/

On the SECN Status page you can see MCS and Bit Rate as in the example below:

Number of connected clients: 3
Station MAC Addr     Signal    2.4Ghz
18:3D:A2:6C:12:34  -69 dBm / -95 dBm (SNR 26)  750 ms ago
	RX: 65.0 MBit/s, MCS 7, 20MHz                  29473 Pkts.
	TX: 72.2 MBit/s, MCS 7, 20MHz, short GI        52180 Pkts.

You can (attempt to) set the Modulation/Coding scheme (aka bitrates) using the "iw" command as in the reference below. Note that YMMV with different devices and drivers.

On a AR9331 1T1R device, you can use MCS values up to 7.

I am not sure whether this will address the issue that you mentioned, but it is worth a test.

Regards

Terry

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Ref:    https://wireless.wiki.kernel.org/en/users/documentation/iw

Modifying transmit bitrates

iw supports modifying TX bitrates, both legacy and HT MCS rates. It does this by masking in the allowed bitrates, and also lets you clear the mask.

Modifying tx legacy bitrates

You can set preference for transmitting using only certain legacy bitrates. For example:

iw wlan0 set bitrates legacy-2.4 12 18 24

Here's how to enable what some folks call “Purge G” which disables 802.11b associations:

iw wlan0 set bitrates legacy-2.4 6 12 24

Modifying tx HT MCS bitrates

Setting preference for transmitting using MCS rates is supported by letting you specify the band and MCS rate. Note that whether or not the device actually listens to your petition will vary depending on the device driver and cooperation from the firmware. For example:

iw dev wlan0 set bitrates mcs-5 4

iw dev wlan0 set bitrates mcs-2.4 10

To clear all tx bitrates and set things back to normal:

iw dev wlan0 set bitrates mcs-2.4
iw dev wlan0 set bitrates mcs-5

On Mon, Dec 5, 2016 at 9:04 PM, Anish Mangal <ani...@umich.edu> wrote:

Terry, I have another question. WiFi dynamically adjusts the bandwidth (speed in mbps) based on signal quality between router and client. As a result, what I saw happen a few times (I could be wrong though) was that the bandwidth was set quite high (far more than what is needed for good user experience), and only when the signal degraded quite a bit, after a while was the bandwidth changed. This resulted in temporary loss of quality which is unacceptable in the case of VoIP. If the starting bandwidth was not that high, the temporary loss of quality perhaps would not have happened since the reception sensitivity at low bandwidths is quite high (-94dbm for AR150 at 1mbps). I guess what I am saying is that if there were a way to cap the maximum wifi bandwidth to a user (lets say to 10mbps) then they signal they would receive might be a lot more stable. Does this make sense?

[Anish Mangal]

Thank you. This is work experimenting when I get more routers, and setup an experimental mesh.

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Anish

[Anish Mangal]

From the datasheet of AR9331 (https://www.openhacks.com/uploadsproductos/ar9331_datasheet.pdf), page 320

HT20, MCS0, Rx sensitivity is -88dbm. Compared with -71 dbm for MCS7

Also, R(adj - adjacent channel rejection) is 34db in MCS0 compared with 18db at MCS7. So would work far better in noisy WiFi environment.

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Anish