Re: QoS in wireless networks

[Francisco Javier Simo Reigadas <jsimo@xxxxxxxxxxxxxx>]

> There are many problems with QoS on a wireless (802.11) networks.
> QoS mechanisms impelented by HTB, CBQ etc. are designed for a dedicated
> point-to-point link with full controll all the queues at the sender side.
>
> Today IEEE 802.11 does not support QoS at the link layer...
I know, I have written myself that sencence several times in other 
documents :-)
In my previous message I should have written "partial QoS support".

> It is impossible to provide hard QoS, only some statistical QoS.
>  
I aggree.

> IEEE 802.11 provides a fair channel access for all the nodes including APs
> and clients. (I do not know about PCF mode implemented)
>  
It seems that it was abandoned because it didn't work as expected, only 
private extensions like Karlnet's Turbocell can do that (polling from a 
central point) and I am not interested in private extensions.

> After that you must keep best effort traffic
> below the maximal capacity. It is not so easy, because uplink and downlink
> shares the same medium, so the must be summed.
> Like this:
> BE_BW = BE_BW_DL+BE_BW_UL <= WLAN_BW - RT_BW_UL - RT_BW_DL - GUARD_BW
> where
>
>
>  
I have implemented something already and it works in the laboratory, the 
problem is that we cannot know how much is WLAN_BW. At the lab we can 
have 6Mbps and then doing a good outdoor installation with long links, 
the same configuration drops to 1Mbps or less, and that can still change 
as the weather evolves. That is why I made the question in the list: I 
can say that I have 6Mbps in the laboratory and then I create a HTB with 
subclasses, I reserve 1Mbps for VoIP and 5Mbps for BE; fine, it works. 
But what is going to happen when the real troughput drops? That doesn't 
happen in wired LANs (it happened in old phone connections though) but 
is very common in wireless longhaul installations. Let's put those well 
configured routers in their outdoor possitions, separated 15miles one 
from the other. The same configuration will then give a throughput of 
1Mbps or less. How HTB is going to do? I VoIP going to get the whole 
link? is BE going to collapse the link? are they going to get 
proportional shares (170kbps/830kbps) ? And, as opposed to HTB, what 
would be the behaviour of CBQ in the same situation? That is my doubt.

> You mentioned there are streaming and real time video traffic in your
> network. Are there any uplink UDP video? There should not be any, or you
> have to build a complex bandwidth controller in each nodes.
>  
Yes, we will do videoconference, there will be UDP audio and video in 
both uplink and downlink.

Thanks Ferenc for your message

Javier

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