Re: IP OVER ATM

[Brian Candler <B.Candler at pobox.com>]

On Tue, Jun 03, 2003 at 09:56:38AM +0300, Mark Tinka wrote:
> Since ATM is more hardware than software driven, it can provide faster
> switching and processing speeds.

Than what?

> Bit rates can be either 155Mbps or 622Mbps.
> Speeds on ATM can reach as high as 10Gbps. ATM is truly a broadband service.

Modern hardware-based routers are just as fast.

You should be aware that if you buy into ATM, you are buying into legacy
technology. There are plenty of installed ATM networks still around, but
over time the cost of supporting your network is only going to increase.

The company I work for - Tiscali - used to have an ATM backbone. It has been
ripped out and replaced with MPLS.

The main reason ISPs built ATM networks in the first place was to be able to
do traffic engineering: that is, I want traffic from A to B not to take the
IP "lowest cost" path via C, but another path via D and E where I have spare
capacity. ATM lets you do that by building a mesh. MPLS does this in a more
scalable way, and without breaking your IP packets into 48-byte fragments.
It's also much easier to manage (you can do 'traceroute' through an MPLS
network, whereas an ATM link always appears as a single hop)

ATM probably still wins out for emulating old-style services (e.g. running a
simulated E1 circuit across your ATM network, to link two PABXes together).
In our case we are doing Voice-over-IP, in which case MPLS is a better
match.

One of the arguments for ATM is quality-of-service. This may be true for
voice circuits, but it's not good for IP packets. Trust me, I know from
bitter experience. If you perform rate policing on an ATM stream at the VP
(Virtual Path) level, the ATM switch may well throw away one cell which is
part of an IP datagram, but it doesn't know that. So an incomplete datagram
makes its way to the destination, doing nothing more than wasting bandwidth.

ATM is still used for one important service: DSL. DSL networks, DSLAMs and
CPE devices were all designed to use ATM over DSL, on the assumption that
people would connect them into an ATM core. That assumption is no longer
true, and there's no need for you to have an ATM core to support DSL (you
can convert ATM to IP at the ingress point). But you should be aware that if
you have DSL links, the traffic between the customer premises and your
ingress router will be running ATM.

Regards,

Brian.

-----
This is the afnog mailing list, managed by Majordomo 1.94.5

To send a message to this list, e-mail afnog at afnog.org
To send a request to majordomo, e-mail majordomo at afnog.org and put
your request in the body of the message (i.e use "help" for help)

This list is maintained by owner-afnog at afnog.org