Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
10
2.3 802.11 a-radio Support in the VoWiFi Handset
802.11a Radar Protection, Dynamic Frequency Selection (DFS)
Several of the radio channels (the DFS-channels) available in the 5 GHz band
are also used
by a multitude of radars both for civilian and military purposes; for an example in aviation,
weather radars.
To stop WLAN devices from interfering with radar in
stallations, a radar detection feature
must be run on those channels. WiFi radios using this feature send out a specific probe to
test for radar existence before they can turn on the radio. When booting an AP it will look
for channels that are free from radar traffic and pick one of those. Many AP vendors
therefore do not allow an administrator to manually set the channel.
At regular intervals the AP continuousl
y
probes for radar detection and will move away from
the channel if a radar is detected. Then the AP must dynamically select another channel to
use. The probing sequence is quite slow but happens without any disruption in the traffic
to/from the associated clients. When the AP moves to another channel, the client may be
disassociated for a short while.
The VoWiFi Handset supports 802.11h channel-switch announcements, but these are not
gua
r
anteed to make the switch seamless. For example, if the AP chooses another DFS
channel, the AP must probe for radar on that channel for 60 seconds; hence, the clients
associated will be dropped. If the VoWiFi Handset is dropped by the AP due to such a switch,
an ongoing call may experience a short disruption. Because of this, it is recommended to
avoid using DFS channels for voice. If DFS channels must be used due to channel planning
make sure that all non-DFS channels also are used.
Note: Never use m
o
re than 8 channels for voice since this will introduce delayed roaming
and jitter.
Legacy data Voice 20 MHz +
Data (HT)
40 MHz
non-DFS
Mixed mode
or
Voice 20 MHz +
Data (HT)
20 MHz
Mixed mode
Voice and data are both on the a/n radio.
Using 40 MHZ channels for data will red
u
ce the amount
of channels possible by half for the VoWiFi Handset.
Best combination is to move voice over to a/n.
Legacy data Voice 20 MHz
Data 20 MHz
or
40 MHz
Greenfield
mod
e
Note: Greenfiel
d mode is not supported in the VoWiFi
Handset.
b/
g/n a/n Comment