Wireless LAN – WLAN
BAT54-Rail/F..
Release
7.54
06/08
3.10
WLAN according to 802.11h
121
Dynamic Frequency Selection (DFS) was stipulated to prioritize primary
applications. DFS initially assumes that no channel is available in the
corresponding frequency band. The WLAN device selects an arbitrary
channel at the start and performs what is known as a Channel Availability
Check (CAC). Before sending to a channel for 60 seconds (Channel
Observation Time, COT), a check is run to see if a different device is
already working on this channel and the channel is therefore occupied. If
this is the case, then a different channel is checked by the CAC. If not,
then the WLAN device can perform the transmission operation.
Even during operation, a check is run to see if a primary application such
as a radar device is using this channel. This exploits the fact that radars
frequently work according to the rotation method, whereby a tightly bun-
dled directional transmission signal is transmitted by a rotating antenna.
A remote receiver perceives the radar signal as a short pulse (radar
peak). If a device receives such a radar peak, then it initiates the trans-
mission operation and monitors the channel for further pulses. If addi-
tional radar peaks occur during the COT, then a new channel is selected
automatically.
Such as check has to take place every 24 hours. This is why interrupting
the data transmission for 60 seconds is unavoidable.
DFS is stipulated for the frequency ranges of 5250 - 5350 MHz and from
5470 - 5725 MHz. It is optional for the frequency range of 5150 - 5250
MHz.
D Transmission Power Control – TPC
Dynamically adjusting the transmission power is intended to reduce
interference from radio technology.
Dynamically adjusting the transmission power facilitates the shared use
of the 5250-5350 MHz and 5470 - 5725 MHz frequency bands with satel-
lite services. TPC should cause an average reduction in the transmission
power by at least 3 dB compared with the maximum permitted transmis-
sion power. TPC determines the minimum transmission power neces-
sary to maintain the connection with the partner (such as an access
point). If TPC is not used within these frequency bands, then the highest
permissible average EIRP and the corresponding maximum EIRP den-
sity are reduced by 3 dB. This restriction does not apply to the frequency
range of 5150 - 5350 MHz.
Without DFS and TPC, a maximum of only 30 mW EIRP is permitted.
When DFS and TPC are used, a maximum 1000 mW EIRP is permitted
as the transmission power (compared with 100 mW with 802.11 b/g, 2.4
GHz, DFS and TPC are not possible here). The higher maximum trans-
mission power not only compensates for the higher attenuation of 5 GHz
radio waves in air, it also makes noticeably longer ranges possible than
in the 2.4 GHz range.
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