Apple connectivity issues revealed

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While troubleshooting connectivity issues with Apple we found that there was a near-far sitatuation: We could see the 802.11 open system authentication occur and the client stopped responding during the 802.1x authentication. Closer to the MacBook (2 meters) it appeared the client was heared at only -77 dBm. Now, how was this possible ? It appears that the local power constraint element as being advertised by access points has no strict relation to the actual TxPower and can differ per AP. In this case, the TPC power report was 13, the actual power 14 dBm and the local power constraint was 24 dB (18), this resulted in the MacBook lowering its power to a minumum which resulted in a near-far situation. Setting the TxPower static to 17 dBm resulted in a TPC power report of 14, a Local power Constraint of 0 dB and the connectivity was perfect !

As per the 802.11 8.4.2.16: The local maximum transmit power for a channel is thus defined as the maximum transmit power level specified for the channel in the Country element minus the local power constraint specified for the channel (from the MIB) in the Power Constraint element.

At channel 36 in the ETSI domain the max power is 23 dBm. An AP transmitting at 14 dBm should therefore not advertise 24 dB as a local power constraint. The temporary workaround is to tweak the TxPower of the AP.

The version being used is 6.5r3, the MacBooks 10.10.x.
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Frans Hendriksen

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Posted 2 years ago

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Gary Smith, Official Rep

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I wanted to add a little more detail to this thread. My intention is to make things a little more clear on what happens with this setting, which may have gone unnoticed by many.

In 6.6r1 Aerohive added a configurable feature called Client Transmit Power Control (TPC)

"Client TPC allows the AP to control the transmit power of its clients by sending a management frame containing a Power Constraint information element. The contents of information element provide supported clients with the maximum power level at which they can transmit. Client TPC is used to prevent co-channel interference by adjusting the transmit power of co-located stations.

You can use the following command to set a specified maximum transmit power for clients of the AP: "interface radio tx-power-control" 

Or, you can use the following command to allow the AP to specify the client transmit power based on the RF environment, which is the default setting:
"interface radio tx-power-control auto" "

http://docs.aerohive.com/330000/docs/help/english/documentation/HOS-HM_6.6r1_NewFeatures_330183-01.pdf


In HM6 this is a CLI configuration only. In HMNG this is configurable on the GUI and a good explanation is given in the HMNG help guide;

"Enable client transmission power control (802.1h): This allows the AP to control the transmit power of its clients by sending a management frame containing a power constraint information element. The contents of the information element provide supported clients with the maximum power level at which they can transmit. Client transmission power control is used to prevent co-channel interference by adjusting the transmit power of co-located stations. You can select from one of these modes:

Auto: When auto mode is selected for both Transmission Power and the Enable client transmission power control feature, the AP samples the RF environment, and then automatically adjusts its transmit power in conjunction with the client transmission power control feature. If you select Manual for Transmission Power, HiveManager NG does not allow you to select Auto for the Enable client transmission power control feature.

Manual: You can use the dBm slider to choose a power level from 1 dBm to 20 dBm."

Example;



The issue described in this thread is related to how the Apple client handles this local power constraint. 


Below is are some findings made when I tested this;

My test running HM6 and HiveOS 6.5r3a;

I set the  "acsp max-tx-power” to 10

The Radio power was set to 10 by ACSP

Because of the the "acsp max-tx-power” was adjusted and the acsp used the value of 10dBm, I also saw the “Tx power control” at 10dBm as per the note in HMNG help guide;

"Auto: When auto mode is selected for both Transmission Power and the Enable client transmission power control feature, the AP samples the RF environment, and then automatically adjusts its transmit power in conjunction with the client transmission power control feature."

AP230-0232c0#show interface wifi0 | include "tx power control"

Tx range=300m; Noise floor=-95dBm; Tx power control=10dBm;

We also see this in a packet capture;



Things to bear in mind;

  • This line;

"you can use the following command to allow the AP to specify the client transmit power based on the RF environment, which is the default setting: 

"interface radio tx-power-control auto"

In 6.6r1 onwards, including 6.5r3a but not 6.5r3b this is on by default in HM6. The current acsp power value (show acsp) will determine the local power constraint value. 

  • Is this setting suitable for your clients and deployment?
Carefully consider and test if this will work for you. This feature, like many others, has a use-case and will work very well in the right deployments. Many customers will not even have noticed the change and therefore should not be concerned. If it is deemed that this should be disabled, supplemental CLI with two simple commands will work just fine for HM6 deployments;

"no interface wifi0 radio tx-power-control"
"no interface wifi1 radio tx-power-control"

I hope that helps. Please let me know if there are any questions.

Kind Regards,
Gary Smith