Inspect Cisco software versions to determine if they are marked DF (deferred release) by Cisco. If so, avoid these version.
Cisco ecosystems typically use features which attempt to dynamically learn and improve the RF environment. While beneficial, these features, such as Radio Resource Management (RRM), Dynamic Channel Assignment (DCA), Auto Transmit Power, Coverage Hole Detection (CHD), and Off-Channel-Scan-Defer, engage in constant processing that can negatively impact the RF stability necessary for voice applications.
Zebra strongly recommends carefully analyzing the usage of these features throughout the deployment, during the enabling phases and after reconfiguration, as well as using wireless survey, RF tools, and frequent monitoring of the health and impact of these features. If such precautions are not possible, Zebra recommends disabling them completely in voice deployments.
Following are best practices for RRM, DCA, CHD, and related features. Consider the particular deployment to determine if they are applicable.
Recommendations for DCA when set to Automatic:
The DCA Channel List is used to assign a channel to each radio/band of APs.
Set Sensitivity Threshold to Low.
Set Interval of DCA to 24 hours.
For several other DCA parameters that use the Avoid … terminology, follow Cisco guidelines.
Recommendations for RRM, CHD, and Auto Transmit Power:
Set the monitoring interval and frequency values to maximum (lowest frequency) where possible, respective to tasks, such as AP channel scans and neighbor-packets-scans.
Set the Transmit Power minimum and maximum within a range of 6 dB. For example, min = 12, max = 18. Note: This is a command-line interface (CLI) only parameter.
Recommendations for Off Channel Scan Defer:
For Scan Defer Priority selection for voice (platinum, UP = 6), set the Scan Defer Time to the maximum value supported (lowest frequency of the scan).
Take care when setting the mandatory and supported rates:
Set Beacons to the lowest mandatory rate (Cisco default).
Disable rates below lowest-mandatory, unless there is specific reason to make the cell sizes appear smaller than the range (distance) that data/voice packets can travel. This is typically not the case.
Recommendations for Aironet IEs:
In typical voice deployments, enabling Aironet IEs in the controller is required when CCKM is used for fast roaming. Otherwise, Aironet IEs sub-features are ineffective for voice enterprise and have been replaced by other standards.
If CCKM is not used for fast roaming, disable Aironet IE.
Recommendations for DTPC (Dynamic Transmit Power Control):
DTPC is a mechanism in which the AP requests CCX-enabled clients to set their transmit power to a specific value determined by dynamic algorithms of the RRM. In a voice deployment with the careful configuration of RRM parameters with respect to the environment, the DTPC may help resolve corner cases of localized imbalance issues (for example, the AP not hearing the device), and otherwise cause no harm.
Disable the DTPC in the following situation: In a complex RF environment, DTPC changes may be excessive system-wide, reflecting the AP-side RRM changes. As a result, because a device moves faster than the RRM relearns and rebalances in surrounding areas, the device may remain the DTPC value of the prior RF area, rather than adjusting to the value of the new RF area. In this way, the DTPC may possibly create the imbalance that it is designed to resolve. The AP RRM indexes from the new RF area would continually loopback and make more power changes to resolve the issues created by DTPC. This recursive loop could negatively impact voice quality.
Set the EAP-Request-Identity Timeout to 30 seconds, if connected devices on the relevant SSID are not only mobile devices. For example, laptops in which the EAP identity exchange (user/password) with the EAP server may involve human interaction via typed-in credentials.
