When operating in AirHORN mode, WifiMETRIX uses its 802.11 chip to promiscuously transmit without having to first associate with an access point. Furthermore, when transmitting in Single mode, the transmission can be in the form of 802.11 packets or RAW energy (a modulated, continuous, 22 MHz-wide channel shape). When transmitting in Traverse and Pulse modes, the transmission is always in the form of 802.11 packets. Below we show examples of what the Single, Traverse and Pulse mode transmissions look like when monitored by an RF spectrum analyzer.
The above image shows AirHORN transmitting in Single Mode where ‘TX Mode’ is PACKETS.
The above image shows AirHORN transmitting in Single Mode where ‘TX Mode’ is RAW.
The above image shows AirHORN transmitting in Traverse Mode.
The above image shows AirHORN transmitting in Pulse Mode — where Duration is 5 seconds, and Interval is 10 seconds.
The WifiPROBE function traverses each channel, measuring its available transmit (TX) time. This is reported as ‘% Available TX’, compared to a baseline reference. This metric — % Available TX — is a measure of a channel’s “health”. The higher this value then the more bandwidth is available for transmitting data packets. Using this metric, WifiPROBE can be used to rank channels from best-to-worst. However, a few more points should be clarified:
- Should you always use the channel with the highest ‘% Available TX’? For example, suppose your 802.11N channel has a theoretical bandwidth of 100 Mbps, but because of interference and congestion that has decreased to 40 Mbps — i.e. the ‘%
Available TX’ is 40%. Should you avoid using that channel at all costs? The answer is ‘not necessarily’. If your current bandwidth needs are only 5 Mbps — which is much less than the available 40 Mbps — then it is perfectly fine to continue using that channel because it still has plenty of capacity to handle your current traffic load. Only when your traffic load begins to approach the bandwidth ceiling is it then necessary to consider a different channel. WifiPROBE can tell you what that ceiling is, but it does not tell you the current traffic load.
- When monitoring the 2.4x GHz band you should always choose either channel 1, 6, or 11 — even if other channels have a higher ‘% Available TX’ value. There are few exceptions to this rule — and here’s why…
The image above shows the results of WifiPROBE scanning all the 2.4x GHz channels in a typical office environment. As WifiPROBE cycles through all the channels, the channel currently being measured is filled-in (e.g. channel 2 in this example). We see that some channels are higher than others — but all are greater than 50%. Since this is the 2.4x GHz band, we really should focus just on channels 1, 6, and 11. We would interpret this result to mean it is okay to use channels 1, 6, 11 for your AP.
In the image above we’ve used another WifiMETRIX in AirHORN mode transmitting on channel 6. As you can see, the AirHORN transmission significantly affects channel 6, along with adjacent channels. In this scenario or similar ones, it would then be best to use either channel 1 or 11.
In the image above we’ve again used another WifiMETRIX in AirHORN mode transmitting on channel 6. In this table view you can again see that channel 6 and adjacent channels are severely affected. Furthermore, the ‘CCC’ and ‘ACI’ tags give you a clue that the problem is caused by an 802.11 device (i.e. AirHORN). If we didn’t see those tags, then the problem probably is caused by a non-802.11 device.
In the image above we’ve again used another WifiMETRIX in AirHORN mode transmitting on channel 6. In this Selected Channels view you can again see that channel 6 is severely affected by the congestion caused by AirHORN transmitting on channel 6.
In the image above we’ve used an RF signal generator, transmitting at frequency 2440 MHz, which is near the center of channel 6 (2437 MHz). As you can see, even though this is a very narrow band interferer, it affects not only channel 6 but also neighboring channels.