Wi-Fi Configuration Recommendations - ASCOM Myco 3 Troubleshooting Manual

TROUBLESHOOTING GUIDE
Ascom Myco 3
The radio channels in the UNII-2 and UNII-2e bands are DFS-channels, which may be used by civilian and
military radar such as aviation and weather radar. Because radar always has a higher priority than a WLAN,
additional procedures must be employed to prevent LAN devices from interfering with radar when the radar
is using the DFS channels. This can increase latency and degrade the performance in a WLAN.
A client that does not support radar detection is not allowed to actively scan for APs in the DFS channels.
The client is only allowed to perform passive scanning, which means that it can only listen for beacons. For a
voice client, this will affect an ongoing call to some degree by introducing a slight increase in jitter in the
voice stream.
The handset can use the Dynamic Frequency Selection (DFS) channels in a voice enabled WLAN, but the
voice quality can be distorted as the DFS channels must be treated differently in the scanning process.
The probing process above is repeated each time the handset needs to find roaming candidates, that
means, when the signal quality on its current associated AP decreases.
Another reason that the use of DFS channels is not recommended in areas where radar may be operating, is
the requirement of automatic switching of channels and the non-service time gap that occurs. Note that
radar can be airborne, for example, used by aircraft for navigational purposes.
The regulations for using 5 GHz channels, generally known as DFS channels, in which radar operates, is that
the handset uses a different approach while scanning.
The DFS regulations require that Wi-Fi devices should check for radar that is currently operating before they
initiate a session. This requires special software features and is normally only included in APs, which are the
devices that are set to use a specific channel. If a radar signal is detected, the AP should invoke a special
procedure to automatically move to another channel. During this transfer to another channel, which takes a
while, the transmission of packets in that cell is stopped and as a result the call can be lost.
Portable devices, like a handset, are not able to detect radar and thus are not allowed to initiate a conversa-
tion. Active probing on the DFS channels is therefore prohibited until it has been determined that no radar is
present.
To detect an AP to roam to, the handset has to use passive scanning which means it has to listen to beacons
on every configured channel.
The default beacon period in most APs is set 102.4ms. Using a short scanning interval of let's say 20ms on a
channel the handset's chances of hearing a beacon is about one in five. To improve the chances of the
handset picking up a beacon the listening time on a channel can be extended or the beacon period made
shorter.
For the DFS channels the passive listening time for the handset is set up to 110ms, during which the handset
may or may not hear a beacon. If the handset hears a beacon during this period or if the time period expires,
it will immediately return to its currently working channel and exchange packets with the current AP that was
buffered during the scanning process.
Since scanning for many channels takes a considerable time, it is important that the channels used
in the scanning process are only those channels that have been setup for use in the WLAN system.
4.7

Wi-Fi Configuration Recommendations

This sections gives key configuration recommendations on how to create an optimized Wi-Fi environment
to enhance audio and voice performances as well as delivery of mission-critical data.
TD 93297EN / 19 December 2019 / Ver. B
The Handset as a WLAN Wireless Client
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