Well Stephen, Here at ACSI, our entire national backbone is ATM, the overhead so far seems to be about 12-14%. This is taking into account the 48/53 byte percentage and the time to reassemble the cells into packets at the remote end. I have run tests in our lab and we can totally saturate a DS3 and an OC-3 link via ATM. This is in contrast to a clear channel DS-3 which itself loses some bandwidth to conversions and overhead. I would guess that the difference of DS-3 ATM and clear channel is around 9% of your bandwidth but I need to run more tests in the lab to make a more educated guess. But you don't run an ATM backbone if your just offering IP service, we use it to offer Frame/ATM/IP services all over the same wire. Now, packet of sonet seems the way to go for high speed IP with little overhead, but it is only available at 0C-3 and higher. I have not tested it yet to see the overhead or how good it works. Anyone out there really tested the POS cards from Cisco yet? Eric _______________________________________________________ Eric D. Madison - Senior Network Engineer - ACSI - Advanced Data Services - ATM/IP Backbone Group 24 Hour NMC/NOC (800)291-7889 Email: noc@acsi.net On Thu, 13 Mar 1997, Stephen Balbach wrote:

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

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

It probably depends on what you define as overhead. You might define it as that 16 bytes of AAL5/SNAP header & trailer per IP datagram, plus 5 bytes per cell, plus whatever trailing bytes are wasted in the last cell. Compare this with packet over sonet (1 byte of IFG, 4 bytes of ppp encapsulation, and 4 bytes of CRC). Peter Lothberg gathered some packet size distributions at various internet routers in January, and found that using the above definitions, atm overhead consumed 22% of bandwidth, vs. 3.1% for POS overhead. Looking at it another way, POS can move 24% more payload than ATM using the same packet size distribution. I've taken other snapshots at other routers since then, and the results come very close. /Darren