Which is more efficient?

Bill Stewart nonobvious at gmail.com
Thu Jan 15 22:03:41 UTC 2009

On Wed, Jan 14, 2009 at 12:56 PM, Murphy, Jay, DOH
<Jay.Murphy at state.nm.us> wrote:
> In your humble opinion, which transmission method is more efficient, packet or cell?  ...
> Trying to make a decision on the transport mode for cost, delay, jitter, ROI, etcetera.

It really depends on what your applications are.
I've spent the last decade as the regional ATM specialist (among other
things) for an
international carrier, and since we can sell you koolaid in ATM,
Frame, MPLS, VPLS, IPv4, and IPSEC flavors,
I can be fairly neutral about technology recommendations for my customers.

The most efficient transmission method is the one for which you know
how to set up your router
to match the way the carrier's network works, so you'll need to train
your people.
If that's ATM, you may need to do some ATM-specific things, and
they're different for different carriers;
if it's Ethernet, you'll need to decide how to handle access line
failure detection.
And the work you need to do is much different if the
ATM/Frame/Ethernet is a Layer 2 end-to-end service
or if it's an access line for a routed service such as MPLS.
ATM can give you really good control over jitter, but only if you set
it up correctly.
Dedicated Ethernet access typically has lower jitter than shared
switched Ethernet access,
but it only comes in a couple of sizes and may cost more if that's
bigger than what you need.

As far as cost-effectiveness goes, ATM cells have about 10% overhead,
but some carriers price their services to charge you for it and some don't,
and they have different policies about bursting;
what you really care about is what price they're going to charge you for the
data circuits you need.
Ethernet also has a lot of overhead, if you're carrying lots of small packets;
it's very significant if you're carrying VOIP, and trivial on big file

These days circuit costs have decreased enough that router costs can be a
significant part of your total cost.  ATM cards are traditionally expensive,
but if you're buying a VLAN-based switched ethernet access service,
ask your router vendor what size router you'll need to handle traffic shaping -
even if the Ethernet is built-in, a large teal-colored box costs more
than a medium box.

My main concerns about ATM, other than whether it matches your applications,
are whether it'll scale to the size you need, and how long you'll be able to get
good router vendor support.   I don't see Frame/ATM interworking going away
as a method for handling lots of small endpoints like cash machines reliably,
at least until there are good ways to manage thousands of IPSEC sessions,
but it's not the technology you're going to want for OC48s.
DSL is usually ATM underneath, but that may or may not be how you
connect to your DSL carrier.

             Thanks;     Bill

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