Re: Low latency communication over wifi

[Johannes Berg <johannes@sipsolutions.net>]

On Wed, 2015-09-23 at 13:03 +0200, Pavel Machek wrote:
> Hi!
> 
> I'm trying to transfer some audio data over wifi. Latency constrains
> are such that if it would took more than 20msec to wait for
> transmission, the packet should be dropped instead, and new one 
> should be transmitted.

This is typically not supported, at least not by the drivers right now.
Some hardware might support it, not sure.

>   	  qi->tqi_cw_min = ath5k_cw_validate(0);
>           qi->tqi_cw_max = ath5k_cw_validate(20);
> 
> ...but I don't think it did what I wanted. What am I missing?

That just changes the content window settings, which changes the
probability of getting medium access.

> (I could use Intel 3945ABG-based card instead, but I figured out 
> ath5k might be easier to hack?)
> 
> Is there way to manipulate type of service from userland to get
> similar behaviour without patching kernel?

If your AP has WMM/QoS then you can use setsockopt(SO_PRIORITY) to
change the TID and thus the AC and get your packets classified as voice
(VO) on wifi, which makes them much more likely to get access to the
medium.

johannes