Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
23
Even if APs that use the same channel are placed far away, there can be STAs present in the
cells that are closer and thereby causing transmission interruptions, see figure 4 on page 23.
Figure 6.
Figure 4. CCA might cause problems even for far away STAs
VoWiFi Handset a/b/g
If the VoWiFi Handset detects an energy level that is stronger then
-70 dBm or confirmed
802.11 traffic it will consider the air as occupied and not transmit. For example, if it hears an
AP with -80 dBm and can identify it as 802.11 traffic, it will not transmit. A non 802.11
disturbance at -72 dBm will, however, not stop the VoWiFi Handset from transmitting.
6.2 Hidden Node Problem
The “Listen before Talk” mechanism, mentioned in 6.1 Clear Channel Assessment, CCA on
page 22, works as long as all STAs in a cell can hear each other. However, when STAs are
positioned
at the cell boundaries on opposite sides of the
AP, they can not hear each others