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VigorAP Interference Monitor Usage

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DrayTek's Interference Monitor feature tests signal quality through the VigorAP's web interface, with a visual representation and statistics to determine whether wireless channels are free for potential use, or would be able to transmit wireless signals effectively. This aids with the setup and troubleshooting of wireless networks, both locally and working remotely from the access point.

The Interference Monitor can scan all channels at once to visualise the channel usage and possible interference on all channels in a wireless band, then recommend which channel to use for the best signal quality at the time of scanning.

Interference Monitor visualises channel usage in two ways:

Channel Utilisation - IEEE 802.11 wireless ethernet packets detected on the channel
Channel Energy - Energy levels detected on that frequency band, which can include both wireless transmissions and other, non 802.11 sources of energy on that frequency band

Applicable Products:

VigorAP 710
VigorAP 810
VigorAP 900
VigorAP 902
VigorAP 903
VigorAP 910C

The Interference Monitor can be found in the [Diagnostics] > [Interference Monitor] menu:

There are two sections available in the Interference Monitor:

Current Channel

Displays details of the currently active wireless channel on the selected band, with a graph displaying the history of the Channel Utilisation & Channel Energy results.

This can be useful for troubleshooting wireless issues

All Channels

Displays Energy and Utilisation for all channels on the selected 2.4GHz or 5Ghz Band.

This can be useful for determining the clearest wireless channel in the VigorAP's current location

Current Channel
All Channels

Current Channel

Interference Monitor - Current Channel

The Current Channel tab of Interference Monitor shows details of the currently active wireless channel for the selected Band, with a usage graph and statistics of that wireless channel displayed below. This can be set to contuously monitor by selecting the "Auto-Refresh" option.

Channel Information Table

Band	Wireless interface that Interference Monitor will display information for. This option is available on Dual-Band (2.4GHz & 5GHz) VigorAP access points.
Channel	Currently active wireless channel on the selected Band.
TX Power	Power output specified for the wireless interface as selected in [Wireless LAN] > [Advanced Setting].
Country Code	Wireless region that the wireless interface operates in.
Mode	802.11 wireless modes supported / allowed by the wireless interface.
Bandwidth	Specified Channel Bandwidth of the wireless interface.

Channel Status Table

Channel Utilisation	Detected 802.11 wireless signals (WiFi pre-amble) either from the VigorAP or other 802.11 WiFi devices. 0% - Channel idle, 100% - Channel fully utilised. Indicates how frequently the wireless channel is in use either by the VigorAP, its wireless clients, or other nearby wireless access points.
Channel Energy	Wireless energy detected on that frequency band, both from 802.11 WiFi signals and other sources. 0% - no energy detected on this Channel, 100% - high energy detected on this Channel. The energy indicated is that of its own wireless radio, wireless clients and nearby wireless access points. Wireless clients that are further from the VigorAP may require more transmission power which can cause the Channel Energy value to be higher than the Channel Utilisation value. It can be used to determine possible interference by comparing its value plotted on the graph alongside the Channel Utilisation. If the Channel Energy increases and decreases with the Channel Utilisation, as it does in the graph above, only normal 802.11 traffic is detected. If the graph's Channel Energy line does not move in-line with the Channel Utilisation, it could be indicative of possible interference from something transmitting on the frequency that isn't an 802.11 wireless network.
FalseCCA	A counter value displaying the number of times during one second that the Clear Channel Assessment has determined the channel is in use. 0 - Channel has been assessed to be free for use. If the channel is in use at the time of sampling, it increases the FalseCCA count. This can be indicative of either wireless interference or a busy wireless channel, which can reduce wireless throughput and increase latency.
TX Fail	Number of failed WiFi packet transmissions. 0 - no packets have failed to transmit. Failed transmissions will cause packet loss and significantly reduce wireless throughput.
TX Retry OK	Number of WiFi packets that failed an error check, then succeeded after retransmission. 0 - no packets have required retransmission to be sent successfully. When packets need to be retransmitted, this increases the time spent sending that packet. High Retry counts will significantly increase latency / delays for wireless clients and may reduce wireless throughput. This value will typically show some retries, which is normal given the nature of wireless network. If this value is frequently high, the wireless channel or network may be too busy.
Primary Channel Busy	Usage of the primary wireless channel (specified in Wireless LAN > General Setup). 0% - Channel is idle, 100% - Channel is fully utilised.
Secondary Channel Busy	Usage of the secondary wireless channel (specified in Wireless LAN > General Setup). 0% - Channel is idle, 100% - Channel is fully utilised.

Channel Status Graph

The graph displays the Channel Utilisation and Channel Energy values over the selected time period (X axis), with the graph scaling to fit the percentage values defined on the left (Y axis).

Select a different 5 minute time period to view wireless usage on that channel over the last 30 minutes:

All Channels

Interference Monitor - All Channels

The All Channels tab of Interference Monitor shows a list of the available channels on that band with indicators of how channels are currently utilised.

The Interference Monitor displays a list of the channels available on the specified wireless band once a scan has completed:

Channel	The 802.11 wireless channel number on the selected band. 2.4GHz - Channel numbers 1 to 13 5GHz - Channels 36 to 48, 52 to 64 and 100 to 140
Channel Utilisation	Detected 802.11 wireless utilisation of that channel
Channel Energy	Detected energy of signals on that channel, which includes both 802.11 wireless and unidentified signal energy / noise
APs	Quantity of wireless access points detected on that channel

Usage & Troubleshooting

Select the Band to display information for, then click Refresh to begin the scan.

Please note that this scan will stop communication with wireless clients until it completes, which takes around 10 seconds:

The VigorAP will recommend which channel to use from the findings of its scanning at the time. This is based upon which channel has the lowest Channel Utilisation, Channel Energy and fewest wireless APs on that channel.

The VigorAP's Auto Channel algorithm determines which channel to use based on this information, on VigorAP's with this feature.

The tabs below are scenarios in which Interference Monitor can be used to aid network setup and optimisation:

Detecting non-802.11 Interference - How to use Interference Monitor to observe possible sources of wireless interference
Channel Planning - How to use Interference Monitor to determine the best wireless channel to use in a location

Detecting non-802.11 Interference

A high amount of Channel Energy on one or more 802.11 channels without corresponding Channel Utilisation, may be indicative of possible noise on the scanned frequencies.

It may be possible to find sources of interference by clicking Refresh to re-scan while operating electrical devices such as cordless phones and microwave ovens.

Possible non-802.11 Interference

In this example, the Channel Utilisation shows low values for each channel but comparatively high Channel Energy.

The Channel Energy levels do not change in line with the Channel Utilisation in the way that the example to the right shows normal 802.11 channel usage.

This indicates that there was possibly a source of non-802.11 compliant wireless energy / interference when the scan was performed.

Normal 802.11 Signal

In this example, there is significant Channel Energy on some channels, which increases where the Channel Utilisation also increases on that same channel.

This indicates that it's less likely that there non-802.11 sources of interference when the scan was performed.

Channel Planning with 2.4GHz Wireless Networks

The 2.4GHz wireless spectrum spaces each channel 5MHz apart, while each channel can actually use 20MHz of bandwidth. This creates a significant overlap between the channels which can cause interference between access points if the channels are not spaced far enough apart. More information on the 2.4GHz band's overlapping channels can be found here.

It's recommended to space channel usage apart by at least 4 channels, because each channel occupies the space of the frequencies around it. For instance Channel 8 overlaps significantly with Channels 5, 6, 7, 9, 10, 11 and overlaps slightly Channels 4 and 12 because the 20MHz channel bandwidth each of those wireless channels overlaps with the 20/22MHz channel width of Channel 8.

The example to the right shows an active wireless transmission on Channel 11. There is significant Channel Utilisation & Energy detected on channels 10, 11 and 12, with some Channel Energy detected on channels 9 and 13.

That overlap could limit the throughput possible if using a wireless channel that fits within that overlap, such as Channel 9.

In the example to the right, it may be best to use a channel that does not overlap and has low utilisation, even when there are other access points detected on that channel, such as Channel 6.

Channel Planning with 5GHz Wireless Networks

The 5GHz wireless spectrum does not have overlaps between channels, but this frequency band can make significant use of Channel Bonding, where the wireless radio bonds multiple 20MHz channels to increase throughput. This is one of the main speed improvements with 802.11ac wireless networks, which use 80MHz (4x20MHz) channels to achieve higher throughput. This is explained in more detail here: The 5GHz Wireless Band

Selecting channels in the 5GHz band when using 802.11ac 80MHz channels is more limited because the 80MHz channel requires 4 continuous 20MHz bands. This allows these channels in the UK:

Channels 36 to 48
Channels 52 to 64 (requires Dynamic Frequency Selection with 60 second scan time)
Channels 100 to 112 (requires Dynamic Frequency Selection with 60 second scan time)
Channels 116 to 128 (requires Dynamic Frequency Selection with 600 second scan time due to Terminal Doppler Weather Radar on this frequency range)

Smaller channel bands can be used by setting the wireless radio to operate in 20MHz/40MHz Channel Bandwidth mode from [Wireless LAN] > [Advanced Setting]. This reduces the potential throughput of each individual 802.11ac AP but allows for more access points to utilise the 5GHz spectrum in that space, for an overall increase in throughput.

In the example below, there are many APs detected on channel 36, which could potentially cover channels 36, 40, 44 and 48 if the access points are 802.11ac capable.

Channel 100 has an 802.11ac access point which is actively transmitting, using channels 100, 104, 108 and 112. This could limit throughput if using those channels; it may be best to use Channels 52 to 64 in this scenario, which has a small number of access points on it, with low Channel Utilisation and Channel Energy.

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: First Published: 09/03/2017; Last Updated: 22/04/2021

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VigorAP Interference Monitor Usage

Applicable Products:

Current Channel

All Channels

Current Channel

Interference Monitor - Current Channel

Band

Channel

TX Power

Country Code

Mode

Bandwidth

Channel Utilisation

Channel Energy

FalseCCA

TX Fail

TX Retry OK

Primary Channel Busy

Secondary Channel Busy

Channel Status Graph