Rod (ruderod) wrote in wireless,

DFS on 802.11a


I found a wireless firmware update today that DISABLED some of the 802.11a channels because of the new FCC regulation.

Keep this in mind. . . might not be worth updating firmware if you lose 802.11a channels because of it. Basically the radio must have DFS on. . . or they are not playing the game right.

From what I understand DFS has to be there now. DFS scans constantly to see if any other 5 ghz noise is there. Noise, not just 802.11a. So if it sees weird radar, Motorola, any thing on that band it has to stop transmitting. . . until it is noise free. From what I see, that means the 802.11a wireless planning-mapping needs to be done to keep your 802.11a units from interfering. A good thing in a way as 802.11a units won’t interfere as much when improperly setup.

Keep in mind that it has to be -63dbm or stronger interference. So with proper surveying and planning, DFS shouldn’t disable your radios all that much. In theory!

I wish there was an option to say, if DFS is there, DO NOT change channels, just keep trying that channel. I haven’t seen that yet, but should be an option.

Sometimes we don’t want auto channel changing.

Some notes on it I found:

Customers in the U.S.: FCC Changes to Dynamic Frequency Selection (DFS) in the

5 GHz Band — Effective July 20, 2007, new FCC regulations on the use of the 5 GHz band,

the band used by radios supporting the IEEE 802.11a standard, prohibit the sale of radios

not meeting the new specifications.

The new ruling is based around Dynamic Frequency Selection (DFS2) and is being required to allow the coexistence of military and weather radar systems in the 5GHz band. The new ruling requires that Unlicensed National Information Infrastructure (UNII) devices operating in the 5.25 - 5.35 GHz and 5.47 - 5.725 GHz bands shall employ a DFS radar detection mechanism to detect the presence of radar systems and to avoid co-channel operation with radar systems. This regulation mandates that equipment operating in these spectrum’s must now use DFS to avoid in-use spectrum, and transmit power control (TPC) which throttles power to the minimum necessary for given communication.

About 18 months ago the FCC quietly added a new frequency band, UNII-3, which operates in the 5.47 - 5.725 GHz frequencies. This is an additional 255 MHz that will allow for approximately 8 more channels in the 5GHz spectrum for WLAN devices. This comes as a result of compromise between the Department of Defense and the “industry”. This will be advantageous as the 5GHz spectrum is likely to be used in the early profiles for WiMax.

What this means to us is that most of our WLAN equipment vendors are going to have to do some changes to accommodate this new regulation. Some older equipment will be exempt from this ruling but I would expect most vendors to offer software upgrades. My vendor, Extreme Networks, has already begun developing software to fix this issue and will be providing upgrades to that software in the near future. This ruling became effective July, 20 2007.References

The minimum DFS detection threshold for devices with a maximum e.i.r.p. of 200 mW to 1 W is -64

dBm. For devices that operate with less than 200 mW e.i.r.p. the minimum detection threshold is -62

dBm. The detection threshold is the received power averaged over 1 microsecond referenced to a 0 dBi

antenna. The DFS process shall be required to provide a uniform spreading of the loading over all the

available channels.

ii) Channel Availability Check Time. A U-NII device shall check if there is a

radar system already operating on the channel before it can initiate a transmission on a channel and when

it has to move to a new channel. The U-NII device may start using the channel if no radar signal with a

power level greater than the interference threshold values listed above is detected within 60 seconds.

(iii) Channel Move Time. After a radar’s presence is detected, all transmissions

shall cease on the operating channel within 10 seconds. Transmissions during this period shall consist of

normal traffic for a maximum of 200 ms after detection of the radar signal. In addition, intermittent

management and control signals can be sent during the remaining time to facilitate vacating the operating


(iv) Non-occupancy Period. A channel that has been flagged as containing a

radar system, either by a channel availability check or in-service monitoring, is subject to a nonoccupancy

period of at least

FCC Documentation
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