Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
7
2.2.1 Legacy Network Not Using Any 802.11n APs
b/g a Comment
Customer is running single radio APs. Most vendor
s single
radio APs are using the b/g band.
Data/Voice - Due to the limited amount of channels av
ailable, any WiFi
device must share the airtime. Since voice requires a steady
access to the media, it is important to minimize the impact
of the other devices in the WLAN by changing the
randomness of getting access to the channel.
Standard 802.11 implementation does not sup
port any
type of
admission or congestion control; data is served to
clients on a “best effort“ basis. The adoption of WiFi
alliance's WMM specification will help, but not solve all of
the problems with admission/congestion control.
If data clients must operate in t
he same band as VoWi
Fi
Handsets, they must be Wi-Fi Multimedia (WMM)
compliant and support 802.11g. Any 802.11b only clients
will reduce the overall performance and is not
recommended to use.
If any legacy b/g-client causes heavy traffic or does not
sup
por
t WMM (QoS), this device should possibly be phased
out, be replaced with more modern equipment and moved
over to the a-radio band.
2.2.2 Customer Is Running Dual Radios a/b/g APs
b/g a Comment
Data (b-radio) Data + Voice This scenario is common with older APs that use only 
b-radio i
n the 2.4 GHz band.
Sharing the bandwidth for data and voice on the
a-band
is essentially the same
as for the b/g radio. Since the
bandwidth is shared by the two traffic types, WMM shall
be used.
This leaves the b-radio free for any legacy clients like
bar-code scanners. M
ost bar-code scanners send little
amo
unts of data.
There are more non-overlapping channels to choose from
in the 5 GHz b
and, but special considerations m
ust be
taken to plan for the limitations of the available channels
due to the use of radars in the same band. (See 802.11a
Radar Protection, Dynamic Frequency Selection (DFS) on
page 1
0.)
Data (b/g) Voice If the WLAN contains of a lot of b/g data clients, it can be
preferable to keep them in th
e 2.4 GHz band and have all
voice clients use the 5 GHz band.
The same planning considerations apply if DFS-channels
no
t are used.