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[WLC/WLA] How to improve roaming by reducing the cell size

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Article ID: KB28153 KB Last Updated: 29 Oct 2013Version: 1.0
Summary:

Many times when site surveys are not performed or when the physical characteristics change over time (new walls, new APs, etc.) you will end up in a situation where roaming between WLAs produces unwanted results: delays, packet drops or even total connection loss. This article explains how to improve roaming by instructing the WLAN system to discard sessions coming from stations that are far away.

Symptoms:

Cause:

Solution:

Roaming is the Achilles' heel of a large WLAN infrastructure. Many times when site surveys are not performed or when the physical characteristics change over time (new walls, new APs, etc.) you will end up in a situation where roaming between WLAs produces unwanted results: delays, packet drops or even total connection loss.

It is very important to realize that the roaming decision is purely a client decision based on the information it collects from the environment. So the first step towards good performance is to have a properly configured client, with up-to-date drivers or firmware, with a good power supply and NOT in power saving mode. Any mobile device that is operating in PS mode will have a significantly lower chance of performing well when roaming.

The only aspect we can control from the WLAN infrastructure perspective is to have a good plan, so a site survey is mandatory for modern WLAN networks.

However, if you observe that your clients are still connected to a far away AP while roaming and sitting near a new AP (which has a better RSSI), you could try to instruct the WLAN system to discard sessions coming from stations that are far away. This is done by inspecting the data rates that the client can support as a distant client will only support low data rates. By configuring rate-enforcement, you can define a minimum data rate that any client has to support in order to be connected to an AP.

Let's say I want that all my clients to be able to support at least 36Mbit/s as data while operating in the 5Ghz band. I will configure the following:

# set service-profile <name> transmit-rates 11a mandatory 36.0,48.0,54.0 disabled 6.0,9.0,12.0,18.0 beacon-rate 36.0
# set service-profile <name> transmit-rates 11na mandatory 36.0,48.0,54.0 disabled 6.0,9.0,12.0,18.0 beacon-rate 36.0

# set radio-profile <name> rate-enforcement enable

The configuration is similar for the 2.4Ghz band; just keep in mind that the beacon-rate has to be configured to have the same value as the lowest value in the mandatory transmit rates list.

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