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 throughput. 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 ========= 30.960 Mbps So, from a DS3 line rate of 44.736 Mbps, the total overhead is about 31%. Hope this helps. Prabhu Kavi Manager, Network Architecture Group Cascade Communications. ______________________________ Reply Separator _________________________________ Subject: IP over ATM overhead Author: Stephen Balbach <stephen@clark.net> at SMTPLINK Date: 3/13/97 10:47 AM We are installing an ATM backbone connection and wondering what level of overhead can be expected. Ive read from %10 to %50 - this will be a LAN connection so we can assume almost no cell loss. Our provider has said on average %12 bandwidth is overhead. It will be a Cisco->Cisco LAN configuration. Thanks! Stephen Balbach VP ClarkNet