What is co-channel interference and how can we avoid it by using ARM?

Aruba Employee
Aruba Employee

Product and Software: This article applies to all Aruba controllers and ArubaOS 3.3.2.x and later.

Co-channel interference often occurs when access points (APs) are densely deployed. When the RF coverage begins to overlap, instead of increased coverage, transmission interference occurs. This interference is more common in the 2.4 GHz (802.11 b/g) band where there are typically only three channels available, and the radio transmissions propagate further. Dual-mode APs are typically deployed to support transmission in the 5 GHz band where 23 channels are available and transmission propagation is reduced.

In a dense deployment, the ARM algorithm notices this overlap in coverage and continues to reduce power and adjust channels on APs that are interfering with one another to balance the network. Even with this tuning in very dense deployments, the network the RF environment could still experience interference, which provides a suboptimal user experience. After tuning the APs, the system can use two methods to mitigate this interference.

The first method is to reduce the number of APs in the operating environment by enabling mode aware ARM. In this case, the system looks at the utilization of APs and interference, and when it is determined that too many APs are operating in the same environment, it converts underutilized APs into Air Monitors (AMs). This conversion can be done on a radio-by-radio basis. An overlapping 2.4 GHz radio will be converted into an AM and the 5 GHz radio is kept as an AP. This conversion process provides additional RF monitoring and Wireless Intrusion Protection monitoring and reduces interference and unnecessary client roaming.

The system takes into account the RF edge of the network, and it does not convert these edge APs to AMs. The system also does not convert APs that are currently serving clients. The mobility controller continues to monitor the RF environment and user load across the system. When the APs that are still serving clients begin to run out of capacity, the system converts the AMs back into APs to handle the load.

Mode-aware ARM is enabled as part of the ARM profile. You can edit this parameter in the AP group under Wireless LAN > RF Management > 802.11x Radio Profile > Adaptive Radio Management (ARM) Profile. Check the box for Mode Aware Arm. Often you use the same ARM profile for 802.11a and 802.11b/g radios, but you can configure separate profiles for finer grain control.

The second tool the Aruba system uses are automatically applied mitigation tools that are always running. These tools coordinate access to an individual channels in the system. The system performs actions such as not immediately forcing downward rate adaptation on collision detection, which keeps throughput high for all users. The system also directs an AP to reserve air time for a longer period of time when it has a large amount of data that needs to be delivered to one or more clients. These tools are work automatically and do not need to be configured.

The ARM band steering feature encourages dual-band capable clients to stay on the 5 GHz band on dual-band APs. This ability frees up resources on the 2.4 GHz band for single-band clients like VoIP phones. Band steering reduces co-channel interference and increases available bandwidth for dual-band clients, because the 5 GHz band has more channels than the 2.4 GHz band. Dual-band 802.11n capable clients may see even greater bandwidth improvements, because the band steering feature automatically selects between 40 MHz or 20 MHz channels in 802.11n networks. This feature is disabled by default, and must be enabled in a virtual AP profile.

Note: Band steering should work with and without local probe response enabled. AP and STM have the logic to "hide" from 5 GHz capable clients that are asking to connect on the 2.4 G band. However, if local probe responses are disabled, it may not work to allow persistent 2.4 GHz clients to associate on 2.4 GHz bands even after being identified as 5 GHz. The logic to let in persistent clients after 8 auth/probe req in 10 seconds is not in place on the controller.

So Aruba recommends that you enable local probe responses to let clients with connectivity issues associate properly.

Configure Band Steering Using the CLI

You must be in config mode to configure band steering in a virtual AP profile.

To enable band steering, issue the following command:
wlan virtual-ap <profile> band-steering

Specify an existing virtual AP with the <name> parameter to modify an existing profile, or enter a new name to create an entirely new virtual AP profile.

To disable band steering, include the noparameter:

wlan virtual-ap <profile> no band-steering

Configure Band Steering Using the WebGUI

1) Browse to Configuration > Wireless > AP Configuration and Edit AP group.

2) Within AP group, browse to Wireless LAN > Virtual AP and select virtual AP that is to be configured for band steering.

3) Check the band steering option, and click Apply to save the configuration.

Band steering, mode aware ARM, and the automatic mitigation tools help to balance the network and increase user throughput without any intervention on the client side. Because these are standards-based infrastructure control mechanisms, they can be deployed in any network with any client.

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Last update:
‎07-05-2014 06:59 AM
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