ATM vs Point to point overhead

[SR]

Anyone have stats or a link?

 

[Fatt]

Just out of curiousity, what are you trying to do?

 

[ScottMac]

Unless it's all you can get, a single ATM @ T1 rates is probably not a good deal; the overhead will cost you several channels.

If you need end-to-end QOS (and your carrier allows it), ATM can be a wunnerful thing.

For straight data transfer, a point-to-point T1 will get you the best raw throughput.

What kind of bandwidth, and how is it being delivered to you? What are you going to use it with/for?

Gotta know...


Scott

 

[cmetz]

ATM puts 48 bytes of payload into a 53 byte cell. You normally end up sending IP packets that are 40 or 1500 bytes, neither of which are an even multiple of 48, so figure some bytes are plain wasted, and you've got AAL5 overhead too. So for rough numbers, say bye bye to about 10% of your capacity.

NOW, to make things a little more interesting, if you order an ATM circuit, sometimes what you are getting is a circuit with the quoted speed, and sometimes what you are getting is a circuit that can deliver IP packets at the quoted speed. That is, sometimes pre-overhead, and sometimes post-overhead. For example, many DSL ISPs quote post-overhead (a fine thing, since that overhead sucks so much!).

If you have a point-to-point T1 and run IP over HDLC over it, you're getting about as close to using all the capacity of that circuit as you can get. Very low overhead.