[Asterisk-Users] VoIP experiences with Cable and DSL

[Chris]

----- Original Message ----- 
From: "Andrew Kohlsmith" <akohlsmith-asterisk at benshaw.com>
To: <asterisk-users at lists.digium.com>
Sent: Tuesday, August 03, 2004 5:06 PM
Subject: Re: [Asterisk-Users] VoIP experiences with Cable and DSL


> 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.
>

Not really familiar with DOCSIS specs but I'm not sure cable IS actually
CSMA/CD, it may be ATM or FDMA or even TDMA... I guess it depends on the
provider?