Auto-Channel Timing and the Issues it can Cause

All wireless network vendors have some Auto-RF Management of some manner, RRM for Cisco, AirMatch (Formerly ARM) for Aruba, etc. Most of the industry uses these features for about 95% of their installs to handle power level changes, channel changes based on interference or utilization. But something I have noticed time and again is the number of installs that use the default values for these Auto-RF algorithms to run.

So the question is, why do we care about this?

When using this for control of power I typically do not see a big issue in using default values for timing of the algorithm. However, for channel assignment I have seen lots of problems over the years using defaults values and the issues it can cause clients.

What is auto-channel management?

Simply, auto-channel management is exactly what it says, centralized automatic management of the channels being used in the network by an RF or mobility master. Each manufacturer has their own way of managing and handling these changes but the concept behind it is universal. We will look into each manufacturers way of doing it in another blog. This one is simply how it generally works.

During normal operation of the wireless network access points collect data about the RF environment, either from dedicated sensors, off-channel scanning, RSSI values that clients are being seen at, as well as neighbor messages from surrounding APs in the same RF group or neighborhood. This data contains client load, interference seen from radar, microwaves, Bluetooth or other networks in the surrounding area.

All of this data gets sent back to the RF Master, typically the wireless controller on the network or a master controller that is handling these duties. This master then takes all of this data to make the calculations for the APs in the network for an optimized channel plan to help mitigate interference as much as possible.

Once this data is compiled on the master the changes are sent back to the network based on anchor times and interval settings. Cisco does this default every 10 minutes starting at midnight. Aruba sends this at 5 am local time to the Mobility Master by default. A common misconception I have run into over the years is just because RRM runs every 10 minutes, does not mean that the channels are necessarily changing every 10 minutes.

 

Why is this an issue for clients?

With the addition of 802.11h the Management Frames Information Elements now include Element ID 37 for Channel Switch Announcements as shown below from the IEEE.

             
  Element ID Length Channel Switch Mode New Regulatory Class New Channel Number Channel Switch Count
Octets: 1 1 1 1 1 1

The Channel Switch Announcement is sent from an AP that has been marked as needing to change channel by the AutoRF calculations. The important parts of the element are the Channel Switch Mode, New Channel Number and Channel Switch Count.

The Channel Switch mode informs the clients on the AP that is scheduled to change channels that a change is going to occur. If this value is set to 1 the clients should cease transmitting data to the AP until the change has occurred, which will cause a disruption in communication for a short period until the change is complete. If the value is set to 0, there are no restrictions on the clients transmitting during the channel change.

The New Channel Number is pretty basic, this the new channel that AP will be on after then channel change is complete.

The Channel Switch Count is basically the countdown timer for the channel switch.  If the count is set to 0 the channel change could occur at anytime. If it is some other number, that is the remaining time before the change occurs.

So with this very basic overview, why does it matter to a client?

In wireless networking a client’s channel is based on the AP it is connected to. If the client is connected to an AP on channel 11, the client will communicate on channel 11. But again, why does this matter?

When an AP changes channels based on RRM calculations, every client associated to that AP must change as well. So our AP that was on channel 11 changes to channel 6 now every client associated to that AP need to change to channel 6 based on the Channel Switch Announcement and the values within that element. Based on the Channel Switch Count, if a client is downloading a file, making a video call, or just doing basic online tasks from their computer there would be a disruption to that client. It could be very brief, but it depends on how long it takes the client to reassociate or roam to the new channel for the AP. With time sensitive applications this can seem like jitter or lag or even just slowness on the network. This can equate to the dreaded, “The network sucks right now”.

Back to the opening, if the defaults time is set to use say 10 minutes, there is a possibility that a network that is seeing interference from surrounding wireless networks, high channel overlap, lower RSSI values, etc. could change channels on AP that frequently. So clients that are connected to these APs are changing channels as well every 10 minutes which could be confused for a small service disruption or just poor network quality. This topic will be looked at in-depth in a coming post.

In the next post we look at some other issues this constant changing of channels can produce as well as how a couple of different manufacturers handle AutoRF within their products.

 

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