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WirelessRudy
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rts/cts hw-protection threshold needs revision

Wed Jan 04, 2017 11:35 pm

The station working in rts/cts (Request to send/Clear to send) basic 802.11abcg mode has some extra feature embedded to battle the 'hidden node' effect.
Years ago I already pointed MT about an small error in their preconfig state but more recently found the config is even changed and the manual is not representing this so now the config options are unclear.

Tge rts/cts is an extra control embedded in the 802.11abcg protocol that can be used to force stations working in a channel to listen and ask if the medium if free for sending.
More detailed info I suggest to start reading here; https://en.wikipedia.org/wiki/IEEE_802.11_RTS/CTS

Why this discussion now since we all work NV2 (=tdma) protocol these days? Well, since MT wireless developments are put on a low level network operators start looking for better alternatives.
One of the issues is the migration from one tdma platform towards another. The only way to do it without the need to changeout a whole P2MP network overnight is the intermediate legacy mode; 802.11 CSMA. To make sure another 802.11abcg AP can deal with all the CPE's usually the RTS/CTS help is needed to fight the 'hidden node' issue. (google!)

In the expert world it was already esthablished long time ago the RTS/CTS 'helper' should always be switched on for all stations in any circumstance. The slight extra overhead it creates in data is easily compensated by the collision avoidance and in nowadays high capacity networks with its higher connection rates not an issue at all.
So the config of plain 802.11 station should be made such the RTS/CTS always works....

The working of the rts/cts is actually controlled by the Hw-Protection-Threshold setting in the wireless advanced menu.
Setting '0' means "not set" or "no use". Any figure set makes the protocol to jump in when packages are bigger then the set threshold. So, the lower the figure, the earlier the protocol 'helper' jumps in, hence set to the lowest....

Now, in the old days the lowest setting a Mikrotik station would accept was 256 which then mend "always"...
My initial config script dating from 2005 is still using this to set the threshold to 256 which was the lowest possible in these days.
To my surprise I recently found that this setting is now allowed to go lower. I can set any value. Checking the manual there is nothing mentioned. It's still the same as in the old days.....

So my question; Does it makes sense to set lower then 256, and if so, "always on" would mean a setting of "1"?

Mikrotik should shine some light on this issue.... or anybody else...
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Rudy R. Puister

WISP operator based on MT routerboard & ROS.

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