On Fri, 23 Feb 2001, Stephen Donnelly wrote:
dean(a)flatnet.gen.nz wrote:
Yeah Joe's right.
Given the fact that there is no way of sar-ing over oc48, most major players are now moving to POS. Even if you don't need oc48 or oc192 now, it makes little sense to embrace ATM for a new network and then have to throw it in the bin next year anyway. Also the higher the speed the more the celltax costs you. Eg if you pay for an oc12 on southern cross then you loose the equiv of an oc3 in overhead. Thats a heckload of cash. Loosing an oc12 on an oc48 link is even worse.
The 'cell tax' is 5 bytes of overhead for every 48 bytes of data, or about 1/10. So for OC12 (around 622Mbps), it's about 60Mbps, not 155, and for oc48 it's around 250Mbps. That is over an OC3, but much less than an OC12.
I beleive the extra overhead is in carrying IP over ATM - the smallest packet in IP is 64 bytes, which is split into 2 cells, 1 with 32 bytes of padding in it due to fixed cell length. Of course not all packets are 64 bytes long, but this is the by far the largest excuse for overhead with IP over ATM I have heard cheers -- Brendan Black - Evil Engineer Extraordinaire - ratfink(a)xtra.co.nz UK mobile: +44 7790 525596 Linux User# 44680 "You know, it's at times like this when I'm trapped in a Vogon airlock with a man from Betelgeuse and about to die of asphyxiation in deep space that I really wish I'd listened to what my mother told me when I was young!" "Why, what did she tell you?" "I don't know, I didn't listen!" -- Douglas Adams, "Hitchhiker's Guide to the Galaxy" --------- To unsubscribe from nznog, send email to majordomo(a)list.waikato.ac.nz where the body of your message reads: unsubscribe nznog