[Asterisk-Users] VoIP experiences with Cable and DSL

[Andrew Kohlsmith]

On Tuesday 03 August 2004 19:44, Chris Shaw wrote:
> > QoS isn't going to help you get to talk in a crowded CSMA/CD network.
>
> I might be misunderstanding you about QoS, but I know for a fact that it
> does help greatly because whether you use DSL or Cable, your bridge device
> (it's not a modem no matter how much people want to call it that, it's a
> bridge!) uses large buffered queues to achieve sustained transfer rates...
> this is awesome for bulk downloads but makes your VoIP conversation sound
> like you're on a cellphone under a bridge in a windstorm... Also if the ISP
> is using QoS and they classify users by the MAC address of your bridge
> device, they can create something similar to ATM PVCs, allowing traffic to
> flow more orderly and evenly across THEIR network...

What I am saying is that you are shaping your ethernet to your cable modem 
(and yes I call it a cable MODEM -- you're still modulating and demodulating 
-- it's just DMT or some superhypermega modulation method) -- once it hits 
your cable modem you're playing the CSMA/CD game and if you collide you're 
SOL, there goes your timely packet.

And yes I know all about huge queues...  The cure for that (at least with DSL) 
is to get a Sangoma S518 -- it's a PCI ADSL modem with drivers for 
everything...  I just prioritise packets now (no rate limiting) and get my 
full 4M/800kbit without any nonsense.  I can flood the link in both 
directions and my VOIP sounds perfect.

You just can't do that with an external modem -- tested 3 different ones 
(Speedstream one that comes with Bell HSE, an "industrial" grade one that 
comes with commercial DSL and also an old FP2100 -- the Bell one was by far 
the worst -- I had to rate limit to 400kbps or it would start queueing up the 
packets like crazy.

> Bear in mind that when you're using QoS you're shaping YOUR traffic as it
> goes out YOUR link... you can do nothing about what happens to it once it
> crosses your ISP's router into the rest of the InterNet.

Exactly -- you're shaping your upstream and with a busy CSMA/CD or CA network 
you won't have much luck since your prioritised packets are getting delayed 
on their way to the head unit.

-A.