[Asterisk-Users] VoIP experiences with Cable and DSL

[Chris Shaw]

----- Original Message -----
From: "Andrew Kohlsmith" <akohlsmith-asterisk at benshaw.com>
To: <asterisk-users at lists.digium.com>
Sent: Tuesday, August 03, 2004 4:05 PM
Subject: Re: [Asterisk-Users] VoIP experiences with Cable and DSL


> On Tuesday 03 August 2004 12:07, Steve Szmidt wrote:
> > But with VoIP it has to go both ways and things like latency can easily
> > become a big issue. (I have cable and it seems that I get sound
> > degradations much easier than I'm comfortable with, yes it's a shared
> > connection with occational POP traffic. Also, I'm only talking about
> > dedicated network connections for final implementation.)
>
> As the old Rogers Cable and Bell HSE commercials used to slog it out with
> "With cable you're all sharing a link, with HSE it's individual links" --
> there is some truth in that.
>
> You have a dedicated TX/RX interface with DSL; once you hit the DSLAM you
are,
> of course, just part of some gigantic ATM flood but at least the bandwidth
on
> that ATM network is likely far beyond what is normally available.  With
cable
> you're fighting to talk; something that QoS isn't going to help with in a
> CSMA/CD network.
>
> > So, what I realized was that I have no real data to operate with is, and
> > has anyone done an evaluation of typical needs which shows DSL better
> > suited for VoIP? F.ex. cable shares the pipe and unless QoS is
implemented
> > can reasonably have more traffic issues than DSL.
>
> QoS isn't going to help you get to talk in a crowded CSMA/CD network.
>
> -A.


Being a cable user, the other thing I notice is that cable (or at the very
least my ISP) also seems to suffer from ARP flooding...

Billions and Billions of Are you there? Yes I am! Who Is at blah? I am at
Blah! Crap every second, probably wasting like 512kbit of bandwidth just for
DHCP and BOOTP crap... But for the most part I gotta say that the sustained
transfer rates are WAY better than they ever were with DSL... And I don't
notice too much difference in latency between the two...

> As the old Rogers Cable and Bell HSE commercials used to slog it out with
> "With cable you're all sharing a link, with HSE it's individual links" --
> there is some truth in that.

You guys probably remember the old ethernets where the ether was this long
thick yellow cable (ThickNet) HFC is something like that, everyone is
sharing the same link like with the old ThickNet and BNC networks, it is not
switched at all until you get to the headend and as more people use the
link, the more congested it becomes until it becomes unusable because even
ARP messages can't go through...

> 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...

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.

-Chris

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