What is the purpose of "Off Channel Scanning Defer" priority?

[mstrz]

On the Cisco 4400 controller there is an option under the WLAN > Advanced tab, "Off Channel Scanning Defer", and the "Priority" option has check boxes 0-7. I've searched but there isn't a lot of information explaining what those numbers mean, and the help page just says to use number 6.

Can someone explain what the 0-7 means? And which one(s) should be checked?

(I'm using outdoor mesh 1522's, and indoor 1131's)

[Scott Fella]

Here you go. 4,5,6 are default for voice and video QoS level.

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_tech_note09186a008072c759.shtml

Note: Off-chanel scanning is essential to the operation of RRM, which gathers information about alternate channel choices, such as noise and interference. Additionally, off-channel scanning is responsible for rogue detection. Devices that need to defer off-channel scanning must use the same WLAN as often as possible. If there are many of these devices, and the possibility exists that off-channel scanning could be completely disabled by the use of this feature, you must implement an alternative to local AP off-channel scanning, such as monitor access points or other access points in the same location that do not have this WLAN assigned.

Assignment of a QoS policy (bronze, silver, gold, and platinum) to a WLAN affects how packets are marked on the downlink connection from the access point, regardless of how they were received on the uplink from the client. UP=1,2 is the lowest priority, and UP=0,3 is the next higher priority. These are the marking results of each QoS policy:

Bronze marks all downlink traffic to UP= 1

Silver marks all downlink traffic to UP= 0

Gold marks all downlink traffic to UP=4

Platinum marks all downlink traffic to UP=6


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[mstrz]

Thank you, I have already read that document, and it seems incomplete.

First of all, the top paragraph you pasted explains off-channel as an RRM feature, and from what I understand, the access points periodically switch to different channels to scan them for rogues or interference. But I don't know what it means to "defer" it, or what the numbers 0-7 mean.

Also, it does not even mention the second setting of the "off channel scanning defer", which is the "scan defer time (ms)". The default on that is 100ms, but that's not even mentioned here.

In fact, the second paragraph is about the assignment of QoS policy. It doesn't seem to relate to the off-channel scanning setting.

I want to know what the off channel scanning should be set to if I am not using voice or video on this WLAN.

[Scott Fella]

It means that the AP will not go off channel to scan. You don't want that when you have voice or video. So that is why 4,5,6 is checked by default. There is really no need to change these values.

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[mstrz]

One of things that document mentions is that it's for "power-save" clients, such as medical devices:

"This feature improves the way that QoS interacts with the RRM scan defer feature. In deployments with certain power-save clients, you sometimes need to defer RRM normal off-channel scanning in order to avoid missing critical information from low-volume clients, such as medical devices that use power-save mode and periodically send telemetry information."

So does that also include mobile devices that shut off WiFi when going into a "sleep" or low-power mode? We've had issues with mobile phones that keep dropping off. Would increasing the defer time to a higher value have any affect?

[Scott Fella]

I don't think the off channel is the issue. Might be something else. What exactly is happening.

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[Stephen Rodriguez]

so 0-7 are the 802.11 UP values.

If the AP has traffic in those UP values, say a voice call, it will defer ( put off ) going off channel to scan for 100ms.  This should allow the traffic to either roam or cease.

So long as there is traffic in the UP value detected, that AP will not go off channel so that it doesn't impact the client performance/experience.

HTH,
Steve

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[mstrz]

Ah, I think I understand.. basically it doesn't switch channels for 100ms if there are certain QoS-level packets coming through.

It's just delaying the off channel scan until there are no priority 4,5, or 6 packets coming through.

But why is silver UP=0? If bronze is 1, shouldn't silver be higher than bronze?

[Stephen Rodriguez]

Hey now that's logic, and that has no place here!! Kidding. I know I asked that question a wHile back, but I don't remember the answer.

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[Rasika Nayanajith]

Hi Stephen,

If you could explain this little bit more that would be helpful.

"If the AP has traffic in those UP values, say a voice call, it will  defer ( put off ) going off channel to scan for 100ms.  This should  allow the traffic to either roam or cease."

Which direction traffic (upstream - towards AP from client, downstream - AP to clinet) could trigger this feature ? Is this applicable to both directions ? My understanding was it only applicable to upstream direction traffic(I may be wrong)

Let's say if I want to differ this for Best Effort traffic (Silver Profile). Should I tick 0 & 3 as UP values  or only 0 ? What is the importance of mentioning downlink marking with this feature  in the config guides ?

Regards

Rasika

[Ravi Singh]

In deployments with certain power-save clients, you sometimes need to  defer RRM's normal off-channel scanning to avoid missing critical  information from low-volume clients (for example, medical devices that  use power-save mode and periodically send telemetry information). This  feature improves the way that QoS interacts with the RRM scan defer  feature.

You can use a client's WMM UP marking to configure the access point to  defer off-channel scanning for a configurable period of time if it  receives a packet marked UP.

Off-Channel Scanning Defer is essential to the operation of RRM, which  gathers information about alternate channel choices such as noise and  interference. Additionally, Off-Channel Scanning Defer is responsible  for rogue detection. Devices that need to defer Off-Channel Scanning  Defer should use the same WLAN as often as possible. If there are many  of these devices (and the possibility exists that Off-Channel Defer  scanning could be completely disabled by the use of this feature), you  should implement an alternative to local AP Off-Channel Scanning Defer,  such as monitor access points, or other access points in the same  location that do not have this WLAN assigned.

Assignment of a QoS policy (bronze, silver, gold, and platinum) to a  WLAN affects how packets are marked on the downlink connection from the  access point regardless of how they were received on the uplink from the  client. UP=1,2 is the lowest priority, and UP=0,3 is the next higher  priority. The marking results of each QoS policy are as follows:

•Bronze marks all downlink traffic to UP= 1.

•Silver marks all downlink traffic to UP= 0.

•Gold marks all downlink traffic to UP=4.

•Platinum marks all downlink traffic to UP=6.