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nicolas
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5GHz, DFS and radar-detect

Tue Apr 17, 2007 9:17 am

Hi,

I'm using 5.480 to 5.700 GHz frequencies and I have to use DFS and radar-detect to comply to my local regulation (Europe).
It seems to me that radar-detect has a weird behaviour because my AP keeps saying through the logs that it detects radars on every frequency it tries...and then my APs never use any channel.

When I specify "no-radar-detect", everything works fine.

Does anyone use succesfully dfs AND radar-detect ?

I've running the tests with RouterOS 2.9.40 on AP side.

Thanks,

Nicolas
 
karyal
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Re: 5GHz, DFS and radar-detect

Sun Apr 22, 2007 3:24 pm

Does anyone use succesfully dfs AND radar-detect ?
Depends what you mean for successfully :D
Actually DFS + Radar detect is supposed to switch frequency when interference is detected on the channel is using and listen on each channel to decide where to switch.
If rf environment is very busy (or if you have some device that does frequency hopping too, i guess) DFS + radar detect will switch channel, and it's actually doing what's supposed to do..
 
wwalcher
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Re: 5GHz, DFS and radar-detect

Sun Apr 22, 2007 4:15 pm

Hi,

I'm using 5.480 to 5.700 GHz frequencies and I have to use DFS and radar-detect to comply to my local regulation (Europe).
It seems to me that radar-detect has a weird behaviour because my AP keeps saying through the logs that it detects radars on every frequency it tries...and then my APs never use any channel.

Does anyone use succesfully dfs AND radar-detect ?
I had the same problem when I tried to use DFS. It would keep bouncing from channel to channel. I don't think it works correctly.
 
karyal
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Re: 5GHz, DFS and radar-detect

Sun Apr 22, 2007 4:25 pm

I don't think it works correctly.
On what basis?
3.1.6 Dynamic Frequency Selection (DFS)
3.1.6.1 Theory of the DFS declared by the manufacturer
“The Equipment shall employ a Dynamic Frequency Selection (DFS) mechanism to detect interference
from other Systems and therefore is able to avoid co-channel operation with other systems, notably
radar systems. Dynamic Frequency Selection associated with the channel selection mechanism shall be
required to provide a uniform spread of the loading of the equipment across a minimum of 14 channels
( or 330MHz), or 255MHz in the case of equipment used only in the band 5470MHz to 5725MHz to
further facilitate sharing with satellite services.”
The RLAN products will provide the specified DFS feature on the Access point device via the following
method:
During initial startup (before communication between an Access point and any Stations in the RLAN
service) and during normal operation when the Access Point is associated and communicating with one
or more Stations, the radio screens all packets received by the Access Point. The radio and controller
hardware records any event that caused the receiver to sense a signal above the specified threshold. This
information is reported to the software driver including an estimate of the size of the event and an
indication of when the event occurred. In addition, the driver logs if a valid RLAN packet was received
in order to determine if the recorded event in not caused by an RLAN that signals.
Once communicated to the driver, the driver sorts through all the events and searches for repetitive
signals (such as radars). The driver builds a histogram which captures the % of time of the channel is
occupied with valid 11a traffic at each power level as well as all remaining non-RLAN signals sorted by
magnitude. A periodogram is then calculated for each of the magnitude bins. The periodicities that are
present in the events. Radar signals show two basic periodic natures. First, a given pulse bursts
themselves have a quiet interval between them. Therefore the pulse bursts themselves have a periodic
nature.
The output of the periodogram is processed by the algorithm for a number of characteristics:
1). Periods present that correspond to a known radar system/signal.
2). Any type of periodicity (i.e. an unfamiliar radar systems).
3). Periodicity corresponding to the natural periods of RLAN packets.

The periodogram will detect the above characteristics even if there are additional events due to thermal
noise fluctuations or missed RLAN packets.
If the Access Point detects the presence of a radar signal or other unknown periodic signal, it will switch
to another channel and then repeat the detection/listen sequence to determine if there are radars present
on that channel. If the channel is clear, it begins sending beacons on that new channel so that Stations can
re-associate with the Access Point. Otherwise it switches to another channel and repeats the process
until it finds a channel with no radar signals.
The above functionality is limited to the Access Point device in an RLAN installation. One feature will be
configured for Station devices (e.g. WLAN PC Cards, etc.): All Station devices will be constrained to
passive scanning mode. This ensures that Station do not transmit probe requests before detecting an
available Access Point in the area. This prevents a Station from interfering with radar systems while trying
to locate an Access Point. Passive scanning in fully supported by the 802.11a protocol.
The procedure for channel selection by the Access Point is as follows: The Access Point profiles all
channel before starting communication. First, any channels in which radar signals are suspected would
not be used in any circumstances. Second, the Access Point would avoid any channels that are already
heavily loaded with RLAN traffic. Finally, among the remaining channels that neither radar nor RLAN
traffic, the Access Point chooses a channel randomly to begin communication. This priotization and
method for channel selection ensures the most uniform spreading of the traffic while minimizing potential
interference to other users of the bands. This completes the description of the radar avoidance
mechanisms.
 
wwalcher
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Re: 5GHz, DFS and radar-detect

Sun Apr 22, 2007 4:41 pm

I don't think it works correctly.
On what basis?

[
On the basis that, even testing in my office, when I turn on DFS, it supposedly "sees a radar" on every channel, continously moving from channel to channel in the list, never staying on one. I'm not saying that DFS itself does not work correctly. I am saying that I think it is broken in RouterOS. Either that or I am missing an essentail setting or settings necessary to make it work.
 
karyal
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Re: 5GHz, DFS and radar-detect

Sun Apr 22, 2007 5:09 pm

On the basis that, even testing in my office, when I turn on DFS, it supposedly "sees a radar" on every channel, continously moving from channel to channel in the list, never staying on one. I'm not saying that DFS itself does not work correctly. I am saying that I think it is broken in RouterOS. Either that or I am missing an essentail setting or settings necessary to make it work.
We use DFS as well,as we're in europe too, and i can tell you that it does work as supposed to (to be true, sometimes we do need a larger separation between channels and need to set up the appropriate scan list, DFS tends to use the closest possible channels). There are no settings to add, just select if you want dfs or dfs + radar detect.

I guess you're using 5Ghz band, are you sure there's nothing around trasmitting at that frequency? what's the noise level?
 
wwalcher
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Re: 5GHz, DFS and radar-detect

Sun Apr 22, 2007 5:24 pm

[quote="karyal]

We use DFS as well,as we're in europe too, and i can tell you that it does work as supposed to (to be true, sometimes we do need a larger separation between channels and need to set up the appropriate scan list, DFS tends to use the closest possible channels). There are no settings to add, just select if you want dfs or dfs + radar detect.

I guess you're using 5Ghz band, are you sure there's nothing around trasmitting at that frequency? what's the noise level?[/quote]

Do you use DFS plus Radar Detect, or just DFS?
 
karyal
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Re: 5GHz, DFS and radar-detect

Sun Apr 22, 2007 5:34 pm

Do you use DFS plus Radar Detect, or just DFS?
dfs + radar
 
wwalcher
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Sun Apr 22, 2007 6:37 pm

hmm... Everytime I tried to test it, even when the Mini PCI card was not hooked up to an antenna, it would think it was finding a radar on every channel. I wonder why it does that for me and the earlier poster, but not for you?
 
karyal
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Sun Apr 22, 2007 6:42 pm

hmm... Everytime I tried to test it, even when the Mini PCI card was not hooked up to an antenna, it would think it was finding a radar on every channel. I wonder why it does that for me and the earlier poster, but not for you?
Maybe.. note that usually is not a good idea to run without antenna, and i don't know if DFS works properly indoor anyway.
If you turn wireless debugging on it will report more info on the activity detected the radar detection number for each channel.
May be useful.
Bye,
Ricky
 
nicolas
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Mon Apr 23, 2007 9:39 am

Hi,

So what would be the best configuration, in terms of frequency "distance" and scan-list settings, knowing that I have several APs on the same mast and that they could interfere themselves ?

Thanks
 
karyal
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Mon Apr 23, 2007 10:24 am

Hi,

So what would be the best configuration, in terms of frequency "distance" and scan-list settings, knowing that I have several APs on the same mast and that they could interfere themselves ?

Thanks
You should set the scan list in such way that you leave as many channels free to be switched with at least 20mhz between the APs
It really depends on how may AP you have, antennas polarization ad type and AP distance.

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