Overview
5GHz Radar Protection in DFS Channels
• Reducing the broadcast domain.
• Separating traffic types.
• Protecting devices from access by unauthorized personnel.
• Giving priority in the network for some kind of traffic.
• Using role-based access rights and access to a VLAN depending on user group membership.
• Creating security rules and allowing the use of internal firewalls.
It is important to understand that devices on separate VLANs are not able to talk with each other
if there are no devices in the network that route the traffic between the virtual networks.
Thus, if using separate VLANs for voice and data devices, there must be a route for managing
traffic coming from the data network to the device.
NOTICE:
Do not implement VLAN without having a clear understanding of which
devices need to talk with each other.
NOTICE:
Virtual LANs have 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
Service Set IDentifiers (SSIDs) on the APs. Different SSIDs can be used for different staff cate-
gories 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 broad-
cast 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 then rises significantly.
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-
boring WLANs, which makes this effect even more pronounced.
It is recommended to limit the use of multiple SSIDs, and turn off the lowest speeds.
5GHz Radar Protection in DFS Channels
This section briefly explains how radar detection in Dynamic Frequency Selection (DFS) chan-
nels works, and how to mitigate its impacts on wireless networks.
Several of the radio channels (the DFS-channels) available in the 5 GHz band are also used by
a multitude of radars both for civilian and military purposes, for an example in aviation, weather
radars.
At regular intervals the AP continuously probes for radar detection and moves away from the
channel if a radar is detected. Then the AP must dynamically select another channel to use. The
probing sequence is quite slow but happens without any disruption in the traffic to/from the as-
sociated clients. When the AP moves to another channel, the client may be disassociated for a
short while.
The handset supports 802.11h channel-switch announcements, but these are not guaranteed to
make the switch seamless. For example, if the AP chooses another DFS channel, the AP must
probe for radar on that channel for 60 seconds; hence, the clients associated are dropped. If the
A31003-M2000-P103-01-76A9, 29/04/2020
System Planning, Planning Guide
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