802.11k

One of the problem with today’s Wi-Fi operations is a fact the mobile client is a device, which makes a decision when and why to roam to a different Access Point. The most important factor to make such a decision is the Received Signal Strength Indicator (RSSI). If the mobile client’s sginal will drop to the minimum acceptable value or exceed it, it will start looking for an AP with a better signal. This process involves active scanning, simply the mobile client will send a probe request with specific SSID on specific channels.

To improve and speed the process up, the 802.11k standard was introduced. It has to be  supported by the mobile client and enabled on APs/WLC.

I will present how to it works with usage of Extreme Wing APs and WLC. The setup is pretty simple, 1 x WLC with 2 x APs broadcasting an SSID ‘study’.

The configuration is pretty simple, 802.11k is to be configured in WLAN section:

WLC(config-wlan-study)#radio-resource-measurement ?
   channel-report Configure support for including the channel-report element
             in Beacons and Probe Response
   neighbor-report Configure support to respond for neighbor-report requests

There are 2 available options:

  1. Channel-report
  2. Neighbor-report

1. The channel report will inform the mobile clients about channels used for specific sites. In Wing environment, the Smart RF policy is responsible for setting, which channels are used for specific site, group of sites etc. In this example channels 36-64 were configured.

As we can see below, the AP sends an information in the beacon about channel list on which the APs may operate. Same information is sent in a probe response.

1

2. The neighbor report will work with usage of action frames.

The mobile client will send an action frame category code 5, which is radio measurement with action code 4, which is neighbor report request.

2

The AP will response with an action frame category code 5, which is radio measurement with action code 5, which is neighbor report response. In this example, there is only one extra AP with the same SSID operating on channel 60.

3

With 802.11k the mobile client has a possibility to have a better picture of used channels on which APs broadcast. Similarly to scanning methods, the mobile client may have it passively by looking into beacon frames or actively: either with probe requests/response when not authenticated to the AP or using action frames with neighbor request/response while authenticated to the AP.

Example from the Apple page related to 802.11k:

“The roam scan runs more quickly if you turn on 802.11k on your control plane. This helps because iOS uses the first six entries in the neighbor report and reviews them to prioritize its scans. If you don’t turn on 802.11k, iOS has to scan more methodically. This can add several seconds to the discovery process.”

More information about support of 802.11k by the most popular mobile clients:
Apple IOS About wireless roaming for enterprise
Apple Wi-Fi network roaming with 802.11k, 802.11r, and 802.11v on iOS
Samsung’s Enhanced Roaming Algorithm
Microsoft’s Fast Roaming with 802.11k, 802.11v, and 802.11r

 

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