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Access network design for branch, remote, outdoor and campus locations with Aruba access points, and mobility controllers.
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ARM Channel Churn: Empty Channel vs Error Threshhold

  • 1.  ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 04, 2015 10:25 AM

    Hi-

     

    I occasionally see a few 5GHz get caught in a 'churning' state whereby ARM flips them back and forth between a channel pair.  In this case, I've got 20MHz channels in place; and ARM flipped between channels 165/161 for a few hours.

     

    It always left 165 to go to 161 because "Empty Channel"

    Within 60 seconds, it would leave 161 due to "Error Threshold".  It would place it back on 165.

    Lather, rinse, repeat every 5-10 minutes for a couple of hours.

     

    Has anyone else seen such behavior?  I would think the Error Threshold would be reached due to high noise floor, interference or channel utilization - but it's not clear to me why ARM would think the 'bad' channel to be considered 'Empty'.  Does ARM strictly consider 'Empty' to mean no known RF neighbors on that channel (irrespectiive of interference sources) and/or simply evaluates noise floor?

     

    This is happening on AP-135s on 6.4.2.3.  I've been having this issue for a while though (read: including previous versions).  No clients connected at the time; but as a point of reference: client match enabled; client aware enabled; Min/Max TX 6dB apart; Channel Qual Aware ARM disabled. US Reg Domain.  DFS channels disabled [36-48/149-165].



  • 2.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 04, 2015 11:58 AM
    The interference index is used to determine what channels the AP can switch to. It contains wifi/non wifi sources of interference and channel activity. I don't know how often the index is refreshed but it may be possible that the index is trending downwards and then spiking all within a short time period resulting in this thrashing.

    There may be a way to tweak this in Arm but I think you should take a look at the spectrum either with an AP in spectrum monitor mode or Chanalyzer to locate the source of interference and respond appropriately.


  • 3.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 04, 2015 02:54 PM

    @thecompnerd wrote:
    The interference index is used to determine what channels the AP can switch to. It contains wifi/non wifi sources of interference and channel activity. I don't know how often the index is refreshed but it may be possible that the index is trending downwards and then spiking all within a short time period resulting in this thrashing.

    There may be a way to tweak this in Arm but I think you should take a look at the spectrum either with an AP in spectrum monitor mode or Chanalyzer to locate the source of interference and respond appropriately.

    I'm not sure I am convinced.  These events take place over a number of hours, so I am not convinced that the interference index is the metric used to determine "free channel".  This would only be plausible in my scenario if the interference was intermittent and recurring; and at a rate by which off-channel scanning would not have caught it to update the interference index (preventing the 'free channel' scenario).

     

     

     

     



  • 4.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 04, 2015 04:07 PM

    kjspgd,

     

    You could do the following :

     

    - In the ARM profile, increase the error rate threshold to 70%

     

    - In the ARM profile, increase the error wait time to 90 seconds.

     

    After you do that, see if it continues happening.



  • 5.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 04, 2015 04:10 PM

    Thanks Colin - will give this a shot.



  • 6.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 11, 2015 09:39 AM

    @cjoseph wrote:

    kjspgd,

     

    You could do the following :

     

    - In the ARM profile, increase the error rate threshold to 70%

     

    - In the ARM profile, increase the error wait time to 90 seconds.

     

    After you do that, see if it continues happening.


    Colin-

     

    So odd situation now since I adjusted Error rate threshold- where I had an AP channel thrash last night... except instead of churning between Interference/Empty Channel; I churned from Empty Channel/Empty Channel. Back and forth between the same two channels.

     

    Kevin

     

     



  • 7.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 11, 2015 10:08 AM

    Let's see your ARM profile...

     

    show rf arm-profile <name of arm profile>

     



  • 8.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 11, 2015 10:26 AM

    Parameter                                                                    Value
    ---------                                                                    -----
    Assignment                                                                   single-band
    Allowed bands for 40MHz channels                                             None
    80MHz support                                                                Disabled
    Client Aware                                                                 Enabled
    Max Tx EIRP                                                                  18 dBm
    Min Tx EIRP                                                                  12 dBm
    Rogue AP Aware                                                               Disabled
    Scan Interval                                                                10 sec
    Aggressive scanning                                                          true
    Active Scan                                                                  Disabled
    ARM Over the Air Updates                                                     Enabled
    Scanning                                                                     Enabled
    Multi Band Scan                                                              Enabled
    VoIP Aware Scan                                                              Enabled
    Power Save Aware Scan                                                        Disabled
    Video Aware Scan                                                             Enabled
    Ideal Coverage Index                                                         10
    Acceptable Coverage Index                                                    4
    Free Channel Index                                                           25
    Backoff Time                                                                 240 sec
    Error Rate Threshold                                                         70 %
    Error Rate Wait Time                                                         90 sec
    Channel Quality Aware Arm                                                    Disabled
    Channel Quality Threshold                                                    70 %
    Channel Quality Wait Time                                                    120 sec
    Minimum Scan Time                                                            8
    Load aware Scan Threshold                                                    1250000 Bps
    Mode Aware Arm                                                               Disabled
    Scan Mode                                                                    all-reg-domain
    Cellular handoff assist                                                      Disabled
    Client Match                                                                 Enabled
    Client Match report interval (sec)                                           30
    Allows Client Match to automatically clear unsteerable clients after ageout  Enabled
    Client Match Unsteerable Client Ageout Interval                              2 Days 0 Hours
    Client Match Band Steer G Band Max Signal (-dBm)                             45
    Client Match Band Steer A Band Min Signal (-dBm)                             75
    Client Match Sticky Client Check Interval (sec)                              3
    Client Match Sticky client check SNR (dB)                                    18
    Client Match SNR threshold(dB)                                               10
    Client Match Sticky Min Signal                                               65
    Client Match Restriction timeout (sec)                                       10
    Client Match Load Balancing threshold (%)                                    20
    Client Match IOS Steer Backoff interval (sec)                                300
    Client Match VBR Stale Entry Age (sec)                                       120
    Client Match Max steer failures                                              2
    Client Match Load Balancing client threshold                                 30
    Client Match Load Balancing SNR threshold (dB)                               30
    Client Match Load Balancing signal delta bound (dB)                          5
    Client Match 11v BSS Transition Management                                   Enabled



  • 9.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 11, 2015 10:33 AM

    kjspgd,

     

    Which channel is it going back and forth between?  What type of access point is this and what version of ArubaOS is this?

     



  • 10.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 11, 2015 10:35 AM

    In the case of the Empty/Empty movement it was flopping between 149/153. AP-135; AOS 6.4.2.3.



  • 11.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 11, 2015 12:52 PM

    kjspgd,

     

    You have client-aware on.  Does the access point change channels when clients are associated?

     

    You might want to open a support case, so that they can look at all of your parameters, as well as the AP tech support to see if there is a bug.  It would be painful to ask you for all of that information here.  From the limited information given, it appears you are doing everything correct.

     

    EDIT:

    #1 "EMPTY" channel means a channel with no interference, NOT that there are no access points on it. 

    #2 "EMPTY" channel changes are "active" client aware, NOT "associated" client aware, so the access point will change to an "empty" channel when there are no active clients associated.

     

    Regardless, TAC should take a look at your configuration to make sure that the channel changes are occurring for real reasons.



  • 12.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 04, 2015 04:27 PM

    @kjspgd wrote:
    Im not sure I am convinced.  These events take place over a number of hours, so I am not convinced that the interference index is the metric used to determine "free channel".  This would only be plausible in my scenario if the interference was intermittent and recurring; and at a rate by which off-channel scanning would not have caught it to update the interference index (preventing the 'free channel' scenario).

    I'm going off an Aruba document which references how the interference and coverage indexes are calculated and used for channel/power selection.  You may find it useful as I have: http://www.arubanetworks.com/pdf/technology/whitepapers/wp_ARM_EnterpriseWLAN.pdf

     

    Although you may solve your issue of channel thrashing via threshold tuning, you may still be ignoring an interference or PHY issue that could be impacting client connectivity.  This is where an OTA packet capture and spectrum monitoring tool would come in handy.



  • 13.  RE: ARM Channel Churn: Empty Channel vs Error Threshhold

    Posted Feb 04, 2015 04:56 PM

    thecompnerd,

     

    You are correct.  There is always a chance of that.  We have a chance to measure the errors independently if it is not thrashing..  It is more difficult if it is changing all of the time.