Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
6
• Protecting devices from access by unauthorized personnel.
• Give priority in the network for
some kind of traffic.
• Using role-based access rights and access to a VLAN
s depending on users group
membership.
• Create security rules and allow the use of internal firewalls.
It is important to understand that devices on separate VLANs will not be able to talk wit
h
each other if there are no devices in the network that will route the traffic between the
virtual networks.
Thus, if using separate VLANs for voice and data devices, for example having a voice
VLAN
with a Unite messaging server, there must be a route for the managing traffic coming from
the data network to the device and also for sending messages from a data device (PC) to
the Unite messaging server.
Note: D
o not implement VLAN without having a clear understanding of which devices that
need to t
alk with each other.
Note: Virtual LANs has nothing to do with today's
popular Virtual Machine Technology.
VLANs in the air
When using VLANs, a special tag is inserted into the wired dat
a frame, indicating which of
the VLANs a frame belongs to. This tag is not defined in a wireless frame and consequently
VLANs do not exist in the air. To logically separate traffic types in the air, it is possible to
create several SSIDs on the APs. Different SSIDs can be used for different staff categories
and guests. In the APs the SSIDs on the wireless side are mapped together with defined
VLANs on the wired side and thus give the impression of having VLANS defined in the
wireless media.
SSID information is sent out in the beacon packet from the AP normally ever
y 100ms as
broadcast
packets. Broadcast packets are sent out from the AP at the lowest configured
supported speed. Most vendors are using multiple beacons, one for each SSID. The total
airtime taken up by the beacons, probe requests and probe responses, will then rise
significantly especially if beacons have to be sent out at the lowest speed due to presence of
legacy 802.11b devices in the WLAN.
Some APs today allow configuration of up to 16 SSIDs per radio. This traf
fi
c can easily
consume more than 30% of the bandwidth. A WLAN client may also pick up SSID
information from neighbouring WLANs, which makes this effect even more pronounced.
It is recommended to limit the use of multiple SSIDs, and the lowest speeds should be turned
off.
2.2 Combination of Data and Voice Channel Assignments
The VoWiFi Handset supports both a and b/g, and it is recommended to have the data and
voice traffic on different bands, but not necessary have data on the -a band.
Depending on the existing data and/or voice network, and choice of new installation
prefer
ences, the WLAN can be set up as follows, see tables below: