There are a few outstanding Cisco bugs regarding the Low Latency MAC feature on wireless LAN controllers when 802.11n access points are used. Today, I happened to check in on these bugs and found out that one of them is closed (resolved).
For quick recap, low latency mac enables the wireless AP to quickly age out voice packets ins QoS user priority 6, reducing the number of re-transmission attempts down to 3. This helps clear out AP buffers and prevent sending voice packets that are of no more use in the voice conversation.
The two bug IDs are CSCsy66246 and CSCtc73527.
CSCsy66246 - 1250 doesn't downshift rates for retries when low latency mac is enabled.
Essentially, low latency mac does not work for any 802.11n APs since they don't downshift data rates appropriately when re-transmitting frames. This affects all of Cisco's 802.11n APs, not just the 1250 AP referenced. This bug is still open (unresolved).
CSCtc73527 - Make Low Latency MAC a no op for 11n APs, till CSCsy66246 is addressed.
This bug is a follow-up to the first bug ID, and states that the low latency mac feature should be re-coded to prevent 802.11n APs from making use of the feature until the first bug is fixed. Essentially, this is a workaround for the first bug so that customers can still enable low latency mac on their network for older 802.11a/b/g radios, and the 802.11n radios will simply ignore the feature.
This second workaround bug has been fixed as of code version 220.127.116.11. This is somewhat good news. Customers can now safely enable low latency mac in a mixed AP environment for benefit on 802.11a/b/g APs without worrying about negative side-effects of the first bug on the 802.11n APs.
Verification can be seen when enabling low latency mac on the controller. The following warning is displayed:
(Cisco Controller) >config advanced 802.11a voice-mac-optimization enable
This feature is not supported for 1140/1250/3500 platforms if more than 3 data rates are enabled.
Now... just to get that first bug fixed!