The DFS Testing function employs a 6 GHz radio module, amplifier and filters — it performs as an RF signal generator that simulates radar signal patterns. When an AP that is currently configured to use a DFS channel detects a radar signal it is then required to jump to another channel. Validation of an AP’s ability to detect and respond to radar events is the primary goal of WifiMETRIX’s DFS testing feature.

The AirHORN function is an RF signal / channel generator that transmits stable and accurate RF signals for each of the Wi-Fi channels, and aids in testing Wi-Fi antennas, RF shields and wireless networks.

The WifiPROBE function traverses each channel and measures the available transmit time. When the goal is to choose the best channel — the one with the highest available throughput — then WifiPROBE is the tool of choice.

WifiMETRIX operates in stand-alone mode and does not need to associate with an access point in order to perform its functions. It is a truly unique device that is tailored for troubleshooting Wi-Fi problems and computing the best channel.

When the device is powered-on it will display the main menu. Use the UP / DOWN arrow keys to highlight an option and then press the ENTER key to move to the next screen. You can always return to this top-level menu by pressing the MENU key. From this main menu you would select one of the following functionalities: AirHORN, WifiPROBE, or Radar Simulation (DFS Testing).

This is the primary menu for Radar Simulation (DFS Testing). Here you can select which DFS channel to test, the radar type (i.e. pulse pattern / waveform), and whether to transmit just the selected radar type or sequentially traverse all of them. We recommend starting-off with ‘Radar Type: FCC 2 (or FCC 3)’.  Also, though the traverse mode seems like a good idea but, in practice, doesn’t work as well. So, stick with ‘Single Radar Type’.

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This is the primary menu for AirHORN. Here you can select which Wi-Fi band to use, output power and which AirHORN mode to use.

This is the primary menu for WifiPROBE. Here you can select which Wi-Fi band to use, and which WifiPROBE mode to use.

Return to the main menu — i.e. OPERATIONAL MODE

Depending on the context, used to select highlighted menu option or toggle between ACTIVE and PAUSED.

Depending on the context, used to toggle between 2.4x and 5x GHz bands or return to previous menu.

Depending on the context, used to cycle through power output.

Depending on the context, used to cycle through channels.

When the device is powered-on it will display the main menu. Use the UP / DOWN arrow keys to highlight the ‘RADAR SIM (DFS)’ option and then press the ENTER key to move to the next screen.

This is the primary menu for DFS Testing. Here you can select which DFS channel to test, the radar type (i.e. pulse pattern / waveform), and whether to transmit just the selected radar type or sequentially traverse all of them. We recommend starting-off with ‘Radar Type: FCC 2 (or FCC 3)’. Also, though the traverse mode seems like a good idea but, in practice, doesn’t work as well. So, stick with ‘Single Radar Type’.

Single Radar Type

Here is an example of using ‘Single Radar Type’ mode. At first the transmitter is ‘PAUSED’ (see upper-right corner). When the ENTER button is pressed the device transmits a pulsing waveform and ‘ACTIVE’ is displayed in the upper-right corner. Pressing ENTER again will pause the transmission.

When the device is powered-on it will display the main menu. Use the UP / DOWN arrow keys to highlight the ‘AirHORN’ option and then press the ENTER key to move to the next screen.

This is the primary menu for AirHORN. Here you can select which Wi-Fi band to use, output power and which AirHORN mode to use. To change which Wi-Fi band to use, use the UP / DOWN arrow keys to select ‘WiFi Band’ and press the ENTER key to toggle between the 2.4x GHz and 5x GHz ISM bands. To change the output power, select ‘Power’ and press the ENTER key to cycle through available power outputs — 2 dBm through 18 dBm.

Also, in AirHORN mode there are 3 available ‘transmit’ modes — ‘Single’, ‘Traverse’ and ‘Pulse’ modes. Select one of the transmit modes and press the ENTER key.

Single Mode

Here is an example of using ‘Single’ mode — where you can select a channel in either the 2.4x or 5x GHz band and achieve close to 100% duty cycle on that channel (at the normal 22 MHz-wide channel width). When using the 2.4x GHz radio module you can choose from Wi-Fi channels 1 – 13. When using the 5x GHz radio module you can choose from Wi-Fi channels 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, 161, 165. There is also a TX mode option — the radio can transmit either 802.11 packets or raw RF energy. When transmitting packets, 802.11g is used by the 2.4x radio module and 802.11a is used by the 5x radio module. After specifying the channel and TX mode select NEXT and press ENTER.

The image below shows AirHORN in Single mode, using the channel 6 in the 2.4x GHz band and the power output set at 11 dBm. Pressing the ENTER key toggles transmission between PAUSED and ACTIVE. In the image below you can see that AirHORN is transmitting on channel 6.

Traverse Mode

Here is an example of using ‘Traverse’ mode — where you can automatically scan through the set of 2.4x GHz or 5x GHz channels. When using the 2.4x GHz radio module this mode will automatically traverse channels 1 – 13. When using the 5x GHz radio module this mode with automatically traverse Wi-Fi channels 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, 161, 165. There is also a ‘Speed’ option — when ‘Slow’ is selected then AirHORN will transmit for 30 seconds before moving on to the next channel. When ‘Fast’ is selected then AirHORN will transmit for 5 seconds before moving on to the next channel. After specifying the ‘Speed’ NEXT and press ENTER. As with Single mode, the next screen displays the current band, channel and output power. Pressing the ENTER key toggles transmission between PAUSED and ACTIVE.

Pulse Mode

Here is an example of using ‘Pulse’ mode — where you “burst” on a specific channel, and then ‘off’ for a set interval. Here the options are channel, burst duration (in seconds) and ‘off’ interval (in seconds). As with the other AirHORN modes, the next screen displays the current band, channel and output power. Pressing the ENTER key toggles transmission between PAUSED and ACTIVE.

When the device is powered-on it will display the main menu. Use the UP / DOWN arrow keys to highlight the ‘WifiPROBE’ option and then press the ENTER key to move to the next screen.

This is the primary menu for WifiPROBE. Here you can select which Wi-Fi band to use and how the channel analysis results will be displayed. To change which Wi-Fi band to use, use the UP / DOWN arrow keys to select ‘Band’ and press the ENTER key to toggle between the 2.4x GHz and 5x GHz ISM bands.

In WifiPROBE mode there a 3 ways the channel analysis results can be displayed — ‘Traverse Channel Bars’, ‘Traverse Channel Table’ and ‘Selected Channels’. Select one of the analysis modes and press the ENTER key.

Traverse Channel Bars

Here is an example of using ‘Traverse Channel Bars’ — pressing the ENTER key toggles scanning between PAUSED and ACTIVE.. When using the 2.4x GHz radio module then WifiPROBE will traverse Wi-Fi channels 1 – 13. When using the 5x GHz radio module WifiPROBE will traverse Wi-Fi channels 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, 161, 165. As scanning occurs, WifiPROBE will compute the metric “% Available TX Time” for each channel. The higher the percentage the “healthier” the channel.

Pressing the ENTER key toggles transmission between PAUSED and ACTIVE. In the image below you can see that a narrow peak of RF interference at 2415 MHz blocks channels 1, 2, 3 — that is, causing their “% Available TX Time” to drop to 0%.

Traverse Channel Table

Here is an example of using ‘Traverse Channel Table’. This is similar to ‘Traverse Channel Bars’ above, except the results are presented in the form of a table (as compared to a bar graph).

Selected Channels

Here is an example of using ‘Selected Channels’. This mode differs from ‘Traverse Channel Bars’ and ‘Traverse Channel Table’ in that you specify which channels to traverse. It takes approximately 2 seconds to compute the “% Available TX Time” metric for each channel — i.e. close to 30 seconds to scan an entire band. But suppose you are really only interested in channels 1, 6, 11 — then specifying only these 3 channels can significantly shorten the refresh time between scans.

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.

The calibration feature can be accessed by selecting ‘Configuration’ from the main menu, followed by ‘Re-calibrate’ in the CONFIG MENU.

For the next step, the menu reminds you to ATTACH THE 50-OHM SMA TERMINATOR (“DUMMY LOAD”). Also, if you have a microwave oven available, we suggest you place the unit in the microwave oven during the duration of the calibration procedure (approximately 1 minute). The goal of the calibration step is to approximate 100% Available TX time, and the combination of dummy load plus microwave oven serves as a “poor man’s” Faraday cage.

Once you press the UP key there is a delay of 5 seconds before the calibration process begins. This allows a few seconds to place the device in a microwave oven before the calibration procedure begins traversing channels and recording their baseline reference values.

Once calibration has completed, remember to replace the SMA terminator with the dual-band antenna.

To enable the diagnostics screen, power ‘on’ the unit and while the splash screen displays then press and hold the RIGHT arrow key. When powered ‘on’ this way a new ‘Diagnostics’ menu option appears at the bottom of the screen.

While in Diagnostics mode, the following short-cut keys are in effect:
[RETURN] key: Change Wi-Fi band
[LEFT] and [RIGHT] keys: Change channel
[UP] and [DOWN] keys: Change output power level
[ENTER] key: Toggle Start / Stop
[MENU] key: Return to main menu