Co-Channel Interference, RRM & MCS rates

[Rob Simkins]

Question for the gurus!

Does RRM use MCS rates for AP neighbour RSSI? 

If yes, then in a dense .11n only deployment would it be a good idea to remove low rates in the same way as we do for 802.11ag? 

I'm looking to enable 16-QAM and better only to see if I can reduce channel utilisation on 2.4GHz.  But if RRM doesn't use it I won't bother.

interested to hear if there any stories out there of negative side effects when disabling lower MCS rates?

Cheers!

[Saurav Lodh]

RRM refers to an algorithm which is used to minimise the interference, air quality, maximise SNR and efficient radio power.RRM reduces the need to perform exhaustive site surveys, increases system capacity, and provides automated self-healing functionality to compensate for RF dead zones and AP failures. It has nothing to do with MCS ( modulation coding scheme) index. MCS index is just a number mapping to different Modulation types from BPSK (0) to 64-QAM (31). Different modulation techniques have come up to increase the throughput , data rates. They have nothing to do with RRM algorithm.

[Leo Laohoo]

If yes, then in a dense .11n only deployment would it be a good idea to remove low rates in the same way as we do for 802.11ag?

In a dense environment, it's a good idea to disable the low-speed data rates of 802.11 b/g.  In a dense deployment, I would always disable the data rates from 1 Mbps to 12 Mbps.  I would make 18 Mbps as "Mandatory" and the rest are "Supported".

Because of the amount of the channels available for 802.11a/n, I tend to leave all the Data Rates enabled.

[George Stefanick]

Unrelated but a fun nugget I learned and made me have a "oh!" moment.

When I looked at the rrm neighbor info I seen rssi values. I adjusted the power of the aps and the next read of rrm neighbor information didn't change .. Well I changed the tx power surely my rssi values should change .

I learned when the rrm packet is sent. It uses the lowest data rate 1 phy and the highest power of the radio. Regardless of static config ..





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[Nolan Herring]

Say whaaat !

Dang I didn't know that.

So for example, if you have a perfect square, and 4 access points in each corner (simple fake setup). Bottom Left can see Top Right at -55 RSSI with power level set to 1 on both.

You then change power level to 8 on both , and you are telling me that we are still going to see RSSI neighbor information at -55 ?

How are we suppose to use the RSSI information to our advantage then if it never changes when we adjust settings manually?  =(

Also, back to original posters question, which I don't think really got an answer.

If you disable a/n data rates all the way up to 54mbps for example, the MCS rates are still enabled from 0 - 23.  Should we or should we not go in and manually uncheck the lower MCS rates (0, 1, 2...etc.) Otherwise those are sending at 7mbps and up etc., which in my mind leaves your cell size basically larger than you think it is.

Thoughts?

[Stephen Rodriguez · Accepted solution]

RRM is sent at 1mbps, and max power.  The WLC does some math around max and actual power for that AP, to decided if it needs to raise or lower power.

Now, for the 802.11n only deployment, yes you could go in and disable MCS0-7 and only keep the higher rates like we do with normal a/b/g deployment.

Tune, test, tune repeate till you have the results you are looking for

HTH,
Steve

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[migilles]

You would only want to do that if all clients are 11n MIMO capable.
Otherwise 11n SISO clients would only be able to use 802.11ag rates with such a config.
So if you have 11n SISO clients then should enable some lower MCS rates (e.g. MCS 1 - MCS 7).

FYI, Cisco 11n Endpoints such as the DX650 have a single antenna so would not be able to utilize rates MCS 8 - MCS 15; can only utilize MCS 0 - MCS 7.


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[ericgarnel]

George,

Why not just disable RRM?

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008072c759.shtml

I know it goes against the whole point of controller based APs, but in a stable environment, how often does RRM change power and/or channel?  

[Jacob Snyder]

I think he is referring to trimming the lowest MCS data rates, not the numerically lowest MCS numbers. So turning off 0,1,2,8,9,10,16,17,18 would turn off the BPSK/QPSK for all spatial streams.

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[ericgarnel]

You may not want to disable 0,  some mobile devices are impacted by this

[Rob Simkins]

Thanks all - particularly George & Steve

Its good to know RRM is relative to a fixed modulation rather than whatever is enabled administratively.

I guess we make the assumption that BPSK range is equal for 802.11ag and 802.11n....

[ericgarnel]

Did some more research on this....

Even with static settings,   RRM will be functional and

it will still send neighbor messages to establish the RRM groups, and this cannot be shut off.

You can however increase the neighbor message interval under WIRELESS| 802.11B>RRM>General

Monitor Intervals (60 to 3600 secs)

Channel Scan Interval      

Neighbor Packet Frequency 

Eric