AirHORN Single Mode — Two TX Modes:

When using the AirHORN feature in Single Mode there are two types of TX modes you can choose from: RAW and Packets. ‘RAW’ TX mode does not actually transmit 802.11 packets but, rather, a modulated, continuous, 22 MHz-wide channel shape. It should be unaffected by external RF activity, unless it is very strong and in close proximity to the unit. ‘Packets’ TX mode does transmit 802.11 packets and is affected by external RF activity.

WifiPROBE — Detecting Different Types of Interferers: RAW, ACI (Adjacent Channel Interference), CCC (Co-Channel Congestion)

When trouble-shooting a poorly performing Wi-Fi network it is often the case that RF interference from another wireless transmitter is the culprit. In order to better understand the problem and come up with a solution it helps if you can make the distinction between interference and congestion. There are 4 possible scenarios — 3 types of interference and one type of congestion:

[1] One type of interference occurs when a non-802.11 transmitter causes your Wi-Fi device to back-off. Included in the 802.11 standard is a sensing mechanism. Before attempting to transmit, a Wi-Fi device will check if the coast is clear – if not, then it will delay a random period before checking again. Interference from a non-802.11 transmitter is referred to as RAW (RF energy).

[2] Another type of interference occurs when an 802.11 device on an adjacent, overlapping channel, again causes your Wi-Fi device to back-off. This is referred to as Adjacent Channel Interference (or ACI) and is similar to [1]. By the way, ACI occurs in the 2.4 GHz ISM band since adjacent channels overlap, but not in the 5.x GHz ISM band (since channels do not overlap). ACI differs from RAW in that the Wi-Fi chip knows the interference comes from packets on an adjacent channel, but it can not decode those packets.

[3] The third type of interference occurs when a non-802.11 transmitter causes packets to be corrupted at the receiver – which may be referred to as the hidden node problem. For example, suppose you have an interferer that is far away from an AP – so, the AP thinks the coast is clear and continues to transmit. But the interferer may be close enough to the receiving client device and cause packets to be corrupted. This is another form of RAW interference since the culprit is a non-802.11 device.

[4] Congestion occurs when two or more Wi-Fi devices are using the same channel and have to take turns transmitting because the channel’s bandwidth is full. This is called Co-Channel Congestion (CCC). To be clear – there is nothing wrong with multiple Wi-Fi devices using the same channel. However, when the traffic exceeds that channel’s bandwidth, then the Wi-Fi devices have to arbitrate use of the channel – that is, take turns — and this causes performance to suffer.

Of these four scenarios, [3] is probably the worst. This is because it forces the 802.11 protocol and the transmitting device to re-transmit packets. Re-transmitting packets is a very inefficient use of the transmitter’s time, resources and CPU.

To demonstrate this feature we used two WifiMETRIX devices — one transmitting in AirHORN mode (packets on channel 6) and the other performing channel analysis in WifiPROBE mode. The image above comes from the WifiPROBE screen. It shows Co-Channel Congestion (CCC) on channel 6 and Adjacent Channel Interference (ACI) on adjacent channels. This helps a lot because now we know that channel 6 is affected by another 802.11 device (as opposed to a non-802.11 device).

WifiPROBE — Bar Decorations:

When WifiPROBE cycles through all the channels, the channel that is currently being measured is filled-in (e.g. channel 2, in this case). Also, the High and Low values are shown for each bar for the current session.