Low Latency MAC Bug with 802.11n APs

During a recent VoWLAN deployment, I ran across sporadic voice performance during lab testing prior to production rollout. After double-checking all the recommended setting for a voice deployment, including physical layer RF, everything seemed to be correct.

A search for open caveats in the Cisco controller release notes for version 6.0.188.0 revealed an interesting finding...

Looking up the bug on the Cisco bug toolkit reveals an issue with Low Latency MAC operation with 802.11n access points. This specific bug requests that Low Latency MAC should have no effect on 11n APs until the original bug is fixed. This was implemented as of currently in-development code version 7.0.94.121, and is really only a workaround.

CSCtc73527 Bug Details

Make Low Latency MAC a no op for 11n APs, till CSCsy66246 is addressed

Symptom: An AP1252 or AP1142 may be found to be stubbornly transmitting unicasts at the maximum supported rate. Despite the fact that the transmissions are failing, the AP does not downshift. Conditions: Low Latency MAC ("Voice MAC Optimizations") configured. Workaround: Turn off Low Latency MAC. Further Problem Description: Low Latency MAC is not supported on the 802.11n APs. (See CSCsy66246.) When this bug CSCtc73527 is Resolved, configuring LLC will have no effect on the APs. Once CSCsy66246 is addressed, LLM will be supported.

The original bug details the issue with 11n APs failing to properly downshift to lower voice data rates (also referred to as 'nominal' data rates in Autonomous software) when attempting frame re-transmission. Note that this bug is still open at this time.

CSCsy66246 Bug Details

1250 doesn't downshift rates for retries when low latency mac is enabled

Symptom: The AP1250 doesn't downshift rates for retries when low latency mac is enabled. It will send 3 retransmissions, but the data rate for the retransmissions is the same data rate that the initial packet was sent at. Condition: Using the AP1250 with low latency mac enabled. Workaround: Do not enable low latency mac

To briefly explain the issue, Low Latency MAC is a feature which limits the attempted number of packet retransmissions to 3 attempts for packets in the voice queue to age them out quickly. Voice is a real-time protocol and frames become useless very quickly if delayed too long. They can disrupt a voice conversation and actually be counter-productive. It also controls the nominal data rates used to transmit packets in the voice queue, thereby reducing data rate shifting to a minimal set of data rates to ensure successful delivery of packets and avoid retransmissions.


So rather than continue to repeatedly attempt frame re-transmission, the network will only try 3 times at which point it will drop the frame. This will help clear up network and access point resources to deliver frames which still have validity for a voice call.


If you're deploying voice on 11n APs, be sure to disable Low Latency MAC... for now (and keep a close eye on the status of this bug ID). You can still safely enable EDCA, which controls the Adaptive Inter-Frame Spacing and Contention Window values in a QoS Basic Service Set, to any of the available settings without issue (WMM, Spectralink, Voice Optimized, Voice and Video).

Andrew