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What does this tag mean? Tag: Power Capability Min: 249, Max :19

asked 2018-04-07 00:36:13 +0000

HeyEddie gravatar image

updated 2018-04-07 00:36:33 +0000

Doing an 802.11 pcap, looking at an association request I see:

Tag: Power Capability Min: 249, Max :19

Is this dBm? mW? Some other metric?

Trying to understand what those numbers actually mean.


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answered 2018-04-07 05:58:54 +0000

Guy Harris gravatar image

If that's the Power Capability element from IEEE 802.11-2016 "Power Capability element", then the minimum and maximum transmit power are signed 8-bit integers "in units of decibels relative to 1 mW", i.e. they're signed dBm. 249 is 0xF9 which, if it were correctly interpreted as a signed 8-bit value, would be -7 dBm.

If Wireshark is interpreting those fields in a Power Capability element as unsigned values, that's a bug in Wireshark; please report it on the Wireshark Bugzilla.

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Came to the same conclusion after some more research and asking some folks I know on twitter.

Also, Omnipeek shows the value calculated at -7dBm.

I'll file the bug report now as well.

Glad there are so many smart people out there.


HeyEddie gravatar imageHeyEddie ( 2018-04-08 17:53:33 +0000 )edit

That's Bug 14593, and it's now fixed.

Guy Harris gravatar imageGuy Harris ( 2018-04-08 20:10:39 +0000 )edit

answered 2018-04-07 02:12:43 +0000

Bob Jones gravatar image

Does this help:

802.11 Power Capability information element More...

Data Fields
u8  id
    Power Capability ID: 33. 
u8  len
    Power Capability length: 2. 
u8  min_txpower
    Minimum possible TX power, dBm. 
u8  max_txpower
    Maximum possible TX power, dBm.
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Thank you!

So, if "dBm" Is what this should be showing the min 249 makes no sense.

Now, I'm thinking either something is being mis-interpreted by Wireshark, or is there some conversion I am supposed to do?

HeyEddie gravatar imageHeyEddie ( 2018-04-07 02:27:12 +0000 )edit

Either 1) "u" in "u8" doesn't mean "unsigned", 2) those aren't the minimum/maximum possible TX power in dBm but the minimum/maximum possible TX power in some other representation that prevents it from becoming negative, or 3) the iPXE developers didn't correctly read the 802.11 spec, because that spec clearly says those values are signed.

Guy Harris gravatar imageGuy Harris ( 2018-04-08 20:13:27 +0000 )edit

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Asked: 2018-04-07 00:36:13 +0000

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Last updated: Apr 07 '18