DFS Information

jna · Thu Apr 28, 2005 11:18 am

I am interested in the DFS (Dynamic Frequency Selection) feature now available. However there is little documentation on it.

What is the difference between radar detect or no radar detect?

What triggers a change in channel? If one end starts to receive interference and it make the link unstable will it search a better channel? If so what is the threshold on the trigger? Packet loss, noise, signals?

Answers to the above and real world implementation would be great. I know the OS Gemini units have DFS and they monitor all channels and will use the clearest channel. Are we speaking the some concept here?

Thanks,
John

Thu Apr 28, 2005 11:30 am

some more info in the manual (radar detect for example): http://www.mikrotik.com/docs/ros/2.9/interface/wireless

jna · Thu Apr 28, 2005 11:40 am

Thanks, I already read that and is in fact where I found out about the DFS. However the ONLY mention of DFS and Radar detect is the following:

dfs-mode (none | radar-detect | no-radar-detect; default: none) - used for APs to dynamically select frequency at which this AP will operate
none - do not use DFS
no-radar-detect - AP scans channel list from "scan-list" and chooses the frequency which is with the lowest amount of other networks detected
radar-detect - AP scans channel list from "scan-list" and chooses the frequency which is with the lowest amount of other networks detected, if no radar is detected in this channel for 60 seconds, the AP starts to operate at this channel, if radar is detected, the AP continues searching for the next available channel which is with the lowest amount of other networks detected

however this does not tell you what radar detect is? How is it used? What good is it? Also it mentions it scans the channel list and chooses the channel with the lowest amount of networks detected. What if this changes for the worse 30 minutes later, a day later, etc??? Will it detect that the current channel is no longer good and use a better one? Or does it stay fixed regardless once it determines the channel was initially good even though it may not stay that way?

This type of stuff is not addresses anywhere and is some of the basics I am trying to find out about.

Thanks,
John

GJS · Thu Apr 28, 2005 6:50 pm

To add to jna's post, what interruption to traffic flow is there while channel scanning and changing takes place?

Thanks.

Thu May 05, 2005 10:44 am

no radar detect means that RouterOS scans once for the most unoccupied frequency and uses that. radar detect mode means that the RouterOS will scan again after a certain period of time.

jna · Thu May 05, 2005 11:03 am

When you say radar mode will scan initally and then again after a certain amount of time does the connection get dropped during the scan or is it transparent while the connection remains active? Also when the change in channel is made if determined the need be how much delay is there in the switch?

Thanks,
John

Thu May 05, 2005 11:07 am

no, clients will not disconnect. only traffic will stop for a moment, the ap will tell the clients to wait until it looks for a better frequency

jna · Thu May 05, 2005 11:14 am

That is what I am worried about, what is that delay, there is a 60 delay initially. Will this mean it will stop passing traffic for 60 seconds? When used in a backhaul situation that would be very noticable to tower feeds.

Thanks.

Thu May 05, 2005 11:23 am

yes, 60 seconds is the DFS standard, you can look it up in the internet. if you think the delay is too big, you can just not use dfs radar detect

jna · Thu May 05, 2005 11:29 am

Yes 60 seconds is a very noticed by clients in the middle of doing something and get timed out or page can not be displayed for a whole minute. However how many times does it scan? Every hour, six hours, 24 hours? Can it be set to scan nightly after hours?

On a side note if we were running DFS with radar and the channel became unstable because of interferance would DFS know this and automatically start looking for a better channel or would it sit there untill the next scheduled scan?

Thank you for the info so far.

John

stephenpatrick · Thu May 05, 2005 12:34 pm

... and, is use of "radar detect" **required** in the EU by regulation at 5.8GHz ?
Would be great to have this clarified.

Regards

Thu May 05, 2005 12:45 pm

acutally is is just like the manual says and nothing else:

radar-detect - when enabled AP scans channel list from "scan-list" and
chooses the one with lowest amount of other networks detected. Then
tries to detect radar on this channel for 60sec. If radar detected,
chooses next "best" channel and so on. If no radar detected, starts AP.
During operation of AP also detects radars and when radar detected,
restarts with scanning.

clients _will_ disconnect at the time when it changes the channel.

GJS · Thu May 05, 2005 4:20 pm

Stephen,

There are no EU regulations that apply to use of 5.8GHz band in the UK. There is an ISM band lower down (at about 5.3 I think) which is what you get when you buy an 802.11a card at PC world. These are limited to 200mW power and are for nomadic use only, no fixed installations.

5.8 (5725 - 5850) is what Ofcom call Band C and is allocated to fixed wireless broadband access. Equipment operated in this band is required to conform with IR2007 which includes DFS because it is still used by some radar equipment. A "light license" is required which is £50/year.

Let me know if you want more details, there are quite a few quirks to this bit of spectrum in the UK.

stephenpatrick · Thu May 05, 2005 4:45 pm

Thanks Guy,

I was aware OFCOM was different than rest-of-EU, and yes DFS is needed as well as TPC.
We put kit in all over the world so it the regional variations which are very much of interest.
Yes it would be great to chat sometime, do drop me a line -

Regards

jna · Fri May 06, 2005 5:33 am

Still would like the following answered please:

"On a side note if we were running DFS with radar and the channel became unstable because of interferance would DFS know this and automatically start looking for a better channel or would it sit there untill the next scheduled scan?"

Thank You.

surfnet · Tue May 10, 2005 1:28 am

When I turn DFS on with radar detect, I get no usable connection, it just keeps resetting the signal, I guess my area is so crowded that it thinks all channels are full, and it just keeps looking

if I turn DFS on with no radar detect, it scans the frequencys and always chooses the lowest channel. I am in a very noisey area and on channel 5745(the lowest in my scan list) I can't download at all, I cant even login to the other side via telnet. I can hower upload at full speed.. very strange. By manually choosing 5785 I can download at 17 megs and upload at 7 megs on a 15 mile shot with nstreme.

so in short, DFS kills my connection with or without radar

Trisc · Mon May 23, 2005 3:04 pm

so in short, DFS kills my connection with or without radar

I see this too but only on one client - I have an Atheros AR5211 AP using channel 5475. If I turn on DFS mode on this AP I have one client that cannot connect - also an AR5211 - and another that does work - an AR5213.

Why cannot these two identical cards connect using DFS?

Trisc

jea · Mon May 23, 2005 7:17 pm

Hello All;

The radar detector is required to protect the RADAR operation, NOT the AP or client.
This is because in many countries, the radar is the primary operation, and
wireless conectivity is secondary in the 5 GHz bands. All unlicensed operations
are secondary to licensed operations.
Exact requirements vary from country to country. In the US, the requirements can
be found at http://www.fcc.gov.

-John