Jim Forster <forster@cisco.com> wrote:

He's talking about the overhead due to carrying variable length IP packets in fixed length ATM cells. ... There's beginning to be some expectation that there will be a transmission capacity crunch in the carrier's Sonet nets, and this ~25% ATM cell tax may be looked at carefully as packet over Sonet solutions emerge.

Given the bimodailty of IP traffic size distribution (about 40% of packets are small, like TCP ACKs or telnet/rlogin keystrokes) the ATM "cell tax" is closer to 32%. I.e. a dual clearline DS-3 actually carries as much user data as OC-3c ATM. Which, incidentally, was why SprintLink backbone design is easily expandable to dual links (that includes carefully considering implications for routing). Sean presented that design on NANOG a year ago, BTW. Funny thing, the design is expandable beyond that, too, so OC-3 ATM is already obsolete. --vadim