802.11n LWAP 'draws low power' ??

[with_joerg]

Dear all,

we deployed the first 802.11n LWAPs. In some areas where the have anumber of clients associated we get the following alarm:

"Access point 'AP001d.70fa.6128' associated with controller 'Cisco_df:f8:e3' draws low power from Ethernet. Failure reason: 'The AP draws 15.4 watts from Ethernet'"

It seems as if the AP know two higher power levels (up to 20 watts?) whilst the switch only goes to 15.4 watts. Sometimes clients then get disassocieted from this AP. How could I fix this? Is it a firmware upgarde on the switch? Does the PoE switch HW need to be replaced? Or is it harmless and the disassociateions have probably an other reason?

--Joerg

[jeff.kish]

Hi Daniele,

That's correct. The 4507R-E switch cannot provide enough power to support the 1252s. This is a limit of the switch. To power the APs, you'll need to use power injectors or plug the device into the wall. Or consider purchasing a 3560E or 3750E, but that's the most expensive option for sure.

[Stephen Rodriguez]

Information on the 4500 supporting the 1252 is below:

4500 modules req switch to run 12.2(44)SG or later

WS-X4648-RJ45V-E

WS-X4648-RJ45V+E

power inline port maximum 20000

power inline auto max

HTH,

Steve

[Aaron]

Just to clarify again: the 1252 APs (and both radios) *can* be powered via 15.4W power, but with limited 802.11n performance. Details at http://www.cisco.com/en/US/docs/wireless/controller/release/notes/crn421120.html#wp366903 .

[with_joerg]

Aaron,

that is certainly true. But, what we observe is t hat if there is a higher WLAN client denisty (6 to 12 clients) on a 1252 AP powered with 15.4W only we get messages in the WCS saying "AP XY draws low power" and clients are disconnected arbitrary. So, it works, yes, but does not play nicely in all cases.

[Aaron]

You can turn off the "AP draws low power" noise in WCS thus: Go into go in "administration" -> "Settings" -> "severity configuration" and lower the severity for PoE alerts.

This noise will be addressed in a future WCS release via CSCsw79300.

As far as your client performance issues under load ... that would be require some in depth troubleshooting (incl. wireless packet capture) to determine the root cause.

[jeff.kish]

Aaron,

The doc you link states the following:

16.8 W-Both transmitters are used but at reduced power. Legacy data rates are not affected, but the M0 to M15 data rates are reduced. Throughput should be minimally impacted because all data rates are still enabled. The range is affected because of the lower transmit power. All receivers remain enabled.

15.4 W-Only a single transmitter is enabled. Legacy data rates and M0 to M7 rates are minimally affected. M8 to M15 rates are disabled because they require both transmitters. Throughput is better than the legacy rate but less than the 20 and 16.8 W power modes.

So even this document states that only a single radio can be enabled when on 15.4W. Am I missing anything?

[Aaron]

This wording is a bit confusing ... when it says "only a single transmitter is enabled" at 15.4W, it means a single transmitter *per radio*. I.e. you can have both the 2.4 and 5GHz radios turned on, but each radio will only use a single transmitter in 802.11n, and so you will only have the 11n MCS 0-7 rates available.

[Leo Laohoo]

HI Daniele,

Check my post.

If you don't want to upgrade the IOS of the switch, then upgrade the firmware of the WLC to version 6.X and this will allow the 1250 to operate both radios BUT AT A COST of lower MCS rates.

If you are running autonomous IOS, upgrade the IOS to 12.4(10b)JDA3 and this will 1250 to operate both radios (at 15.4 watts) BUT AT A COST of lower MCS rates.

Hope this helps.