Specifications

General 13

802.11 a-radio Support in the Handset

802.11a Radar Protection, Dynamic Frequency Selection (DFS)

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 ra

dars.

To stop WLAN devices from interfering with radar installations, a radar detection feature must be

run on those channels. WiFi radios using this feature send out a specific probe to test for radar

existence before they can turn on the radio. When booting an AP it will look for channels that are

free from radar traffic and pick one of those. Many AP vendors therefore do not allow an adminis

trator to manually set the channel.

At regular intervals the AP continuously probes for radar detection and will move 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 asso

ciated 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 will be dropped. If

the handset is dropped by the AP due to such a switch, an ongoing call may experience a short

disruption. Because of this, it is recommended to avoid using DFS channels for voice. If DFS chan

nels must be used due to channel planning make sure that all non-DFS channels also are used.

NOTE: : Never use more than 8 channels for voice since this will introduce delayed roaming and

jitter.

The following table lists the DFS and non-DFS channels on the 5 GHz band:

Legacy data Voice 20 MHz

Data 20 MHz

40 MHz

Greenfield

mode

Note: Greenfield mode is not supported in the handset.

Band Channel ETSI (EU/EFTA etc) FCC (US etc)

UNII-1 36,40,44,48 Non-DFS Non-DFS

UNII-2 52,56,60,64 DFS DFS

UNII-2e 100,104,108,112,

116,120,124,128,132,

136,140

DFS DFS

Note: 120, 124, 128

excluded.

UNII-3 149,153,157,161 n/a Non-DFS

ISM 165

b/g/n a/n Comment