IP over ATM overhead

The complete answer is that it depends upon packet size distributions.
Let's assume that typical Internet type traffic patterns, which tend to
have high percentages of packets at 40, 44, and 552 bytes.
If you have a DS3 backbone, and have PLCP turned on, the maximum cell rate
is 96,000 cells/sec. If you multiply this value by the number of bits in
the payload, you get:
        96000 cells/sec * 48 bytes/cell * 8 = 36.864 Mbps
But this is unrealistic, as it assumes perfect payload packing. There are
two other things that eat into the overhead value, namely the wasted
padding, and the 8 byte SNAP header.
It's the SNAP header which causes most of the problems, forcing most small
packets to consume two cells, with the second cell to be mostly empty
padding. This causes a loss of about another 16% from the 36.8 Mbps ideal
So the total throughput ends up being (for a DS3):
        DS3 Line Rate: 44.736
        PLCP Overhead - 4.032
        Per Cell Header: - 3.840
        SNAP Header & Padding: - 5.900