Wireless Access

I was digging through a recent packet capture trying to educate myself on the HT capabilities that are advertised in beacon frames.  I noticed that under  HT capabilities > RX Supported Modulation and Coding Scheme Set,  “TX Maximum Number of Spatial Streams Supported” is equal to 1 spatial stream.  Also, HT Capabilities Info reports Rx support of one spatial stream.  We have AP105’s so I was expecting to see 2 spatial stream support being advertised, rather than 1.

 

I have an 802.11n client with a very good signal on 5GHz and it's reporting 130Mbps Tx/Rx rates, which matches an MCS index of 15 w/2 spatial streams on 20MHz.  Also, the PHY mode for the client reported in the controller is a-HT-20-2ss which is correct.  I'm pretty sure that the clients that support 2 spatial streams are sending 2 as they should, but I'm just thrown off by what I'm seeing in the beacons.  Any thoughts?  Also, is it possible to change the amount of spatial streams allowed?

A screen capture of the packet would be nice.

 

Here you go.

That parameter is for Spatial Stream support for STBC  http://en.wikipedia.org/wiki/Space–time_block_code, a diversity technique as opposed to Spatial Multiplexing http://en.wikipedia.org/wiki/Spatial_multiplexing which are parallel streams which improve throughput.

In the packet capture If you look under Mac flags drop down (Mac/phy) you should able to see this information there along with MCS mechanism

Thanks for the info!

 

There are several more places where a spatial stream is referenced.  vfabian, I can't find where you're referring to.  I've attached another screenshot showing where it says only 1 stream is supported.

Compnerd,

 

REAL TALK:

 

- You need to capture with a wireless adapter that either supports the same or more capabilities as the client you are capturing, or you will have missing or incomplete data

- Many off-the-shelf adapters and packet capture software cannot keep up with the flow of traffic and will miss frames, giving you an incomplete picture of what you are capturing.  Sometimes the frame is captured, but might be corrupted and the parameters *could* be incorrect or missing.

- Commercial software like Omnipeek has custom drivers for specific cards which enables the software to have very good control over what the WLAN adapter does.  Omnipeek is also expensive.

- If you do not use specific packet capture software that can keep the adapter on the right channel, that WLAN adapter will scan other channels and miss frames, and give you an incomplete picture of what you are capturing.

- You can use free software like Wireshark and AirPcap, as long as you understand their limitations above

 

Probably the best guide about WLAN packets and their true meaning is the CWAP book from the CWNP website here:  http://www.cwnp.com/certifications/cwap/  If you are really interested in packet capturing it is an excellent read.

 

Thanks for the useful info, Colin. I was using an AirPcap adapter and Eye Pa software.