(!) The cell tax is about 10% -- the SAR expands user data from 48 bytes to 53 bytes, plus an additional amount of overhead for internal fragmentation on the final cell (AAL-5), plus overhead for whatever encapsulation mode is being used.
The cell tax is actually about 20%, based on the traces of the backbone IP traffic. Reason for this is the additional overhead imposed of partially filled cells. Fixwest 960926: total packets seen = 15315121, total payload bytes seen = 4276560002 ATM framing bytes = 5155881287 ATM efficiency = 82.95% ATM w/snap framing bytes = 5387212772 ATM w/snap efficiency = 79.38% Funet 970608: total packets seen = 6699991, total payload bytes seen = 2336213700 ATM framing bytes = 2783454053 ATM efficiency = 83.93% ATM w/snap framing bytes = 2919019891 ATM w/snap efficiency = 80.03% Pasi