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Can My Sniffer Smell 802.11n? PDF Print
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Written by Devin Akin   
Friday, 31 August 2007

With 802.11n certified devices popping up all over the place (most due to the Wi-Fi Alliance's new certification testing), how long will it be before 802.11n APs become rogues?  Well, that's already happened.  How do we detect them?  Fortunately, backwards compatibility is mandatory in 802.11n devices.  DSSS/CCK (when using 2.4 GHz) or clause 17 OFDM rates (when using 5 GHz) are used for Beacons when either 20 MHz mode or 20/40 MHz mode is used.  While Space-Time Block Coded (STBC) Beacons are supported (called Secondary Beacons), legacy Beacons still must be transmitted as the primary Beacon. 

Data frames transmitted by 802.11n devices may or may not use STBC (and therefore be detected by 802.11a/g WIPS), but rest assured, at some point, even an STBC capable AP (and client device) will transmit non-STBC frames and be detected by the WIPS.  Additionally, Transmit Beamforming (TxBF) functionality will make it a little more difficult for WIPS sensors to capture all of the data stream between an STBC client and STBC AP, but nevertheless, the point isn't to analyze rogue traffic but rather to eliminate it.
Comments (3)Add Comment
...
written by Kalyan, September 09, 2007
So I can use my Netgear /a/b/g card with Atheros chipset to capture 802.11n traffic. How to identify it as the .11n traffic though?
...
written by Devin Akin, September 09, 2007
No. Perhaps you misunderstood. Only the traffic transmitted using non-HT PHYs can be understood by an a/b/g analyzer. HT (high throughput) APs must send some frames using non-HT PHYs, and therefore can be found by non-HT capable analyzers. Non-HT capable analyzers cannot understand HT frames.
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written by Larry, September 10, 2007
It's important when selecting your sniffer application to understand the optional supported PHY features of the 11n capture device.

An example would be if device [a] was attempting to capture traffic between devices and [c].

If was transmitting Greenfield packets to [c], but [a] was not able to receive Greenfield packets, then the sniffer capture would have a lot of missing data - or a lot of corrupt data.

Consider the previous scenario for the following PHY features.

STBC (as described above)
Greenfield
Short Guard Interval in 20MHz
Short Guard Interval in 40MHz
40MHz
Spatial Streams

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