Specifications

8 General
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 net
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work to the device and also for sending messages from a data device (PC) to the Unite messaging
server.
NOTE: : Do not implement VLAN without having a clear understanding of which devices that need
to talk 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 data 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 every 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 traffic can easily consume
more than 30% of the bandwidth. A WLAN client may also pick up SSID information from neigh
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boring 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.