MCI [ATM overhead]

Wed Mar 20 04:10:13 UTC 1996

Jim Forster <forster at 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