[asterisk-users] VoIP over Alvarion Wireless

[Gordon Henderson]

On Wed, 7 Mar 2007, Matt wrote:

> Hi,
> This question isn't specifically asterisk related, but perhaps someone here
> can shed some light or offer some insight.
> Is anyone else here running VoIP over Alvarion wireless?    If yes, do you
> have any suggestions for what you've done to make it "work"?   It seems that
> no amount of traffic shaping, checking installs for error rates, lowering
> error rates, or setting contention windows makes VoIP work.     It just
> plain blows over Alvarion wireless gear.   Cable and DSL?  Yup.. it works
> great.    Wireless?  Forget it, pickup your cell phone.

Many many moons ago I was involved with community WiFi broadband systems 
and we used a variety of wireless systems for "backhaul" I think we 
trialled Alvarion but never used it - we did use Orthogon, WiLAN and 
Apperto kit though.

The biggest issues we had (with both the consumer facing WiFi and the more 
industrial "backhaul" kit) at the time were to do with the packet size and 
the units ability to cope with small packets. At the nuts & bolts level, 
all the low-end stuff is really half-duplex, so there is a link 
turn-around-time, and if that exceeds the packet send time, then you're in 
trouble - VoIP typically requires a full duplex link with small packets 
going in both directions - you can emulate full duplex with a half duplex 
link if you can do the turn-around quick enough and/or accept a lower 
overall data rate, higher latency, etc., but I'm guessing here that the 
Alvarion kit can't cope - they are typically optimised to stream large 
quantities of data in full sized MTU packets back to back. (so streaming 
video and audio seems great, but interactive audio is less-so).

Increasing the packet size at the end points may help, if possible - but 
I'm not sure where to start - Grandstream phones have a "Voice Frames per 
TX:" parameter, but I've no idea whether that (or it's equivalent 
elsewhere) would help, or just make things like latency worse...


Things may have changed in the 3 years since I was actively involved in 
this though, but we spent a lot of time playing with iperf and various 
makes of links and so on. (and daisy-chaining links back to back, which we 
found was a really bad thing to do with the 802.11b kit we were using - 
each hop would halve the throughput )-: So one kiddy running a video phone 
application killed everything else, or another kiddy doing huge uploads 
would kill it for people trying to do downloads. Traffic shaping helped, 
but it really needs to be done at the access point level, and not 2-3 hops 
further up the network where our routers were typically positioned.

If you want good PtP backhaul kit, then get Motorola Canopy units, but I 
used to work for them (before the company was bought by motorola) so I may 
be biased ;-)

Gordon