MCI [ATM overhead]

Wed Mar 20 03:51:33 UTC 1996

> > If we are going to talk about ATM overhead when doing TCP/IP don't we need
> > to talk about the overhead of partially filled ATM cells? Won't that cost
> > you about 1/3 of your available bandwidth?
> > 
> >   -Jeff Ogden
> >    Merit
>
> Jeff, would you or anyone give an example or two of when you get 
> PARTIALLY filled ATM cells?  I've interviewed dave sincoskie, steve 
> tabaska, and Stephen von rump so far.  they all rather like ATM.  Don't 
> believe I heard about this problem from them.  You aren't talking about 
> the switch not being able to tell whether a cell is mangled before it 
> transmits by any chance are you?????

Gordon,

He's talking about the overhead due to carrying variable length IP packets
in fixed length ATM cells.  Consequently the last cell of an AAL5 frame
will contain 0 - 39(?) bytes of padding, which is wasted bandwidth.
Assuming random length distributions (which they're not), the average waste
is about 20 bytes per packet.

My rough estimate, based on an average packet size of 200 bytes (used to be
correct, not sure anymore), is that the waste due to cell padding is about
10%.  Due to the highly skewed packet size distribution the actual overhead
might vary substantially.  Note that this 10% is on top of the ~10%
overhead due to the 5 byte ATM cell headers (5/53 ~= 10%), and various
other overheads (some of which are also present in frame over Sonet
schemes).

There's beginning to be some expectation that there will be a transmission
capacity crunch in the carrier's Sonet nets, and this ~25% ATM cell tax may
be looked at carefully as packet over Sonet solutions emerge.

  -- Jim