Let's estimate the ATM bandwidth available on an OC-3. ...
You can find a paper that addresses this question at
http://www.msci.magic.net/docs/magic/overhead.ps or
ftp:ftp.magic.net/pub/magic/overhead.ps
It covers OC-3c, OC-12c, 100 and 140 Mbps TAXI, and DS3 framing, and
gives the effective bandwidth at each layer.