Decoding An 802.11ac Beacon Frame

vht-beaconWifi 802.11ac with 80 MHz channels (Wave 1) have been on the market for some time now and high end notebooks and smartphones have started supporting the new standards. I’m still stuck with 802.11n on 5 GHz with 40 MHz channels but I do have an 802.11ac capable access point. So while I can’t use the functionality for the moment with my notebook I decided I could at least have a look at the Access Point’s 802.11ac capabilities by doing a Layer 2 Trace and capturing a Beacon Frame.

The Access Point I traced was an AVM Fritzbox 7490. To make it short, like probably most other consumer Access Points, the 7490 supports Wave 1 802.11ac channels, i.e. it aggregates 80 MHz of spectrum in the 5 GHz band. In terms of spacial multiplexing it supports 3×3 MIMO. Like in previous incarnations of the 802.11 specifications many many optional features have been specified. As far as I can tell the 7490 supports none of them, i.e. zero! That’s probably not out of the ordinary for an early 802.11ac device.

As even an excerpt of the 11ac beacon frame would make this entry very long I’ve instead linked to Wireshark’s text export of the beacon frame here. The VHT (Very High Throughput) section that contains 802.11ac information can be found in the second half of the frame.

If you happen to have Wireshark traces of other 802.11ac beacon frames please drop me a note, it would be quite interesting to make a comparison!