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Limit MCS on radio

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Limit MCS on radio

I am referencing a past post:
Manual MCS control? | Amateur Radio Emergency Data Network (

   Does anyone know if the suggestion to add a addition as follows will still work?  I have been spending considerable time trying to understand why some of our nodes(ALL RF)  are having some throughput issues.  RF nodes shooting through some trees  are subjected to varying signals and it would be interesting to see if limiting the speed to create a more reliable, but slower connection. 

Glenn WA3LAB
K2VIZ,   We may be able to lock the rate, by limiting the list of rates the radio will choose from.  There is a setting for this.   This can be tested by editing /etc/config/wireless on one of the nodes.   Find and make this section look like this, by adding the "supported_rates" list.  

config 'wifi-device' 'radio0'
    option network 'wifi'
    option mode 'adhoc'
    option ssid 'AREDN-5-v3'
    option encryption 'none'
    list supported_rates '6000 9000 12000'

Note, MCS 5, 6, and 7 all use the same modulation of 64QAM, thus it would take a jump from 7 down to 4 to change modulation technique.   Also, do consider that when lowering the rate, it takes longer to transmit the same data.  This additional air time means it increases the probability of a collision or interference. 

I suspect there is more to explain in the environment, and other options to improve performance. 



nc8q's picture
some of our nodes are having some throughput issues.
"see if limiting the speed to create a more reliable, but slower connection. "

Hi, Glenn:

 If your issue is too much throughput, then limiting the speed is likely too alleviate the issue.

If your issue is too little throughput and since the software is self adjusting for maximum throughput,
likely an increase in antenna gain, an increase in power, a clearer link path, or a shorter distance between nodes
would alleviate the issue.

 If you have a situation wherein more than 2 nodes are sharing the same channel, throughput seems to
degrade in geometric proportion to the number of nodes. Exposed nodes and hidden nodes also
degrade throughput.

I hope this helps,

Piggy-backing NC8Q's suggestions

Without specifics, it's difficult to tell what's going on for your poor-performing nodes.  Limiting the MCS rate is not going to help because, as Chuck NC8Q said, the radios are self-adjusting.  Just guessing...   if you have nodes at a particular location which are sharing the same band, SSID, channel, and channel width then the performance will degrade, as Chuck NC8Q mentioned.  The solution is to put collocated nodes on different channels and DTD link them at the site.  As Chuck NC8Q also suggested, if you have nodes without clear line of sight to each other -- trying to link through tree cover -- then the solution is to raise the nodes above any obstructions -- more power and higher gain antennas are not going to help if your link path is obstructed. How high above obstructions is high enough?  You need not only clear line of sight but also the majority of the Fresnel Zone must be clear.  If less than 20% of the Fresnel Zone is obstructed there is little signal loss, but any blockage beyond 40% will cause significant signal loss and could make the path unusable.

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