Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
12
The 802.11n builds on the same frequency bands as the 802.11b/g and 802.11a radios
and is designed to coexist with older clients. Legacy clients will use lower speeds than the
802.11n clients.
To really benefit from 802.11n, a WLAN that utilizes the 802.11n enhanced standards
should be configured for Greenfield m
ode. This means that no non-802.11n devices should
be present in the coverage area. In most cases it is impossible to create such an
environment, so 802.11n will run in what is called a mixed/protected mode which will
reduce the maximum throughput.
The current 802.11n standards is really only benefici
al for data clients
like a laptop that are
set up for high definition video conferencing or for downloading large files from a server.
The implementation of 802.11n protocol features to be used in VoWiFi
Handsets have been
carefully examined, and features which will not benefit voice have not been implemented.
The MIMO features require more than one radio channe
l and antennas, which will consume
more power and hardware space in the VoWiFi Handset. Double sized channel (40MHz)
support reduces the amount of channels to half which makes channel planning much more
difficult. Using short guide interval (SGI) makes a client more sensitive to interference and
may not benefit a moveable client like a VoWiFi Handset.
Using 802.11n mixed mode frame when transmi
tting creates larger over
head (double
headers) than if using legacy mode.
The following table lists some 802.11n features in the VoWiFi Handset:
802.11n feature Supported Comment
Greenfield mode N Greenfield mode is unsupported.
40 MHz channel bonding N Channel bonding is used to increase bandwidth
and a VoWiFi
client will not gain much with these
higher rates. The VoWiFi Handset will not use
40 MHz channels but can operate in that
e
nvironment if allowed by the system. Bat
tery
lifetime is also negatively affected if using
40MHz channel width instead of 20MHz channel
width.
SGI N Using Short Guard Interval (SGI) increases the
probability fo
r transmission errors and is
therefore not applied by the VoWiFi Handset.
MIMO N The VoWiFi Handset uses SISO because it does
not need to communicat
e with higher bandwidth
and to extend battery lifetime.
Block ACK Y Block ACK is supported but not always beneficial
to
use for VoWiFi.
Note: The VoWi
Fi Handse
t supports, but does not make use of, 40 MHz channel bonding.
The VoWiFi Handset will prefer the use of legacy data rates in the uplink direction since the
MCS rates introduce more overhead.
The amount of channels that can be used for 2.4 and 5GHz bands is illustrated in the table
in the
section 5 Basic Cell Planning on page 18.