FCC Restrictions on 802.11n channel use for TDWR

Jun 25, 2015 - Author: Sierra Wireless - 578 Views
This is an important 802.11n frequency interoperability advisory.

According to FCC regulations, use of certain 5GHz band 802.11n channels is restricted to prevent interference with Terminal Doppler Weather Radar (TDWR).

This is specified in the FCC document entitled Interim Plans to Approve UNII Devices Operating in the 5470 - 5725 MHz Band with Radar Detection and DFS Capabilities dated 10/14/2010 which states:

"Devices will not transmit on channels which overlap the 5600 – 5650 MHz band"

This means that 5600 MHz [120], 5620 MHz [124], and 5640 MHz [128] are disallowed. Therefore the oMG will not enable 802.11n connections to operate at these frequencies even though the channels may be selected in the oMG Local Craft Interface (LCI).

Furthermore FCC guidance is provided concerning adjacent channels as follows:
 
"In some instances it is possible that a device may be within 35 km of multiple TDWRs. In this case the device must ensure that it avoids operation within 30 MHz for each of the TDWRs."

This means that in addition to the outright ban on channels 120, 124, and 128, adjacent channel 116 (5580MHz) cannot be used within 35km of a TDWR operating between 5600MHz and 5610MHz and channel 132 (5660MHz) cannot be used within 35km of a TDWR operating between 5630MHz and 5650MHz.

Note that the oMG will send out probes on channels 116 and 132 if they are configured active, even if there is no access point in the vicinity. Therefore Sierra Wireless recommends, but does not enforce, that channels 116 and 132 be disabled.
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