Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
27
8 Infrastructure Dependant Features
8.1 Automatic RF Adaptations in WLAN Systems
Many WLAN infrastructures make use of an internal tool that is changing the AP channels
and/or transmit power level in a dynamic way. The intention of the tool is to compensate for
changes in the RF environments due to layout changes of furnishings and/or AP failure.
However, these dynamic changes make the RF environment inconsistent and are not
recommended
when real-time applications like VoWiFi are deployed. The effects of dynamic
RF adaptations when APs switch channels are dropped speech frames and, at worst, the call
can be dropped.
If the power level is changed, the link budgets may b
e
asymmetrical with co-channel
interference as a result, which will make the WLAN system perform poorly. The VoWiFi
Handset m
onitors the output power of the APs and will automatically adapt itself to match
in best way possible.
8.2 Load Balancing
Some WLAN infrastructures have an “automatic load balancing” feature. The purpose is to
dynamically “move” stations between APs in order to avoid overload and to spread the load.
The “move” of stations is done by forcing them to connect to another AP than the current
one.
Unfortunately, IEEE 802.11 does not provide any proc
edure for a smooth transition of
stations between APs. Instead, the move is done by deauthenticating the station until it
associates to another AP.
This forced transition will cause a loss of speech frames, and in
worst case the call will be
disconnected.