Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
5
Physical separation
A WLAN network can either operate on the IEEE 802.11 2.4 GHz (b/g) or a 5 GHz (a) band.
Dependin
g on the WLAN APs used, a network may support either one of those bands or
both if the AP is equipped with dual radios. In such a case, the WLAN network can be
thought of as two independent WLANs which are physically separated by the usage of
different frequencies.
An AP that has only one radio must be using protocol features that m
i
tigate the effects of
having different traffic types and patterns in the WLAN.
Physical separation of traffic types in
a wireline network is achieved by pulling two cables
side by side. It is quite common that IT departments build a second totally independent
network used only for management of infrastructure devices that have additional
management ports, for example a WLAN controller. The management network will still be
functional if the normal network breaks down. Physical separation of WiFi traffic is,
however, not possible in any another way than using different radio channels for different
traffic types.
If voice has to share the channels with any other type of data, WMM p
riority pr
otocol must
be used.
Logical separation
All clients in a wireless cell have equal access rights to the air if priority schemes are not
us
ed. Laptops that uses s
treaming audio and video applications, like a video conferencing
tool, require not just high bandwidth but they will also require steady regular access to the
network. The large video packets will take up a lot of the bandwidth and thus the available
airtime for a voice call will be less.
Using the IEEE 802.11e standard or WMM will give voic
e p
ackets, if configured correctly, a
higher probability to use the air than other types of packets. This standard will stop data
clients from monopolising the WLAN.
In a network it is possible to use informatio
n found in the headers of the packets to identify
traffic types and to treat the traffic differently on its route to the destination based on that
information.
The information that is written to or read from the hea
ders can be used to prioritize a
certain traffic type above another type.
Logical separation of Voice and Data
traffic on the same channel
In a wired converged data network, traffic type
s are often logically separated using Virtual
LANS (VLANS). This allows the administrator of the network to set up rules in the switches
and routers that treat the traffic types differently depending on the VLAN association of a
device. Having devices on separate VLANs (but still on the same physical LAN) will hide the
visibility of a device from any other device that is not on the same VLAN. It will also reduce
the impact of broadcasts sent in the LAN since only devices in the same VLAN will receive
broadcasts. The LAN will actually be divided in smaller broadcast domains, each with its own
range of IP-addresses.
Some of the benefits of using VLANs are:
• The possibility to create a sepa
ra
te subnet for management of devices and thus blocking
any normal users from tampering with configuration.
• The separation of guest traffic from corporate data t
raffic which only give guests access
to the Internet.
• Reducing the broadcast domain.
• Separating traffic types.