On Fri, Feb 23, 2001 at 12:02:05PM +1300, Stephen Donnelly wrote:
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.
The 5 bytes of header per cell is part of the cell tax; there's also the wastage caused by (a) AAL5 and possibly SNAP encapsulation, and (b) needing to populate an integer number of cells when the total frame-based payload is not a multiple of 48 bytes. Worst case, you will carry 49 bytes of payload per two cells. That's 57 bytes wastage in 106 bytes transmitted, or a cell tax of around 54%. The extent to which this effect is significant depends on the distribution of packet sizes in your traffic. Unfortunately, on the internet, a high proportion of packets are TCP ACKs with no data, which narrowly fail to fit inside a single cell. On the internet, a cell tax of 30% or so is quite normal.
In case you're interested, we have built a PCI card for passive monitoring OC48 POS. (no one wanted ATM, for some reason ;)
Cool :) Joe --------- To unsubscribe from nznog, send email to majordomo(a)list.waikato.ac.nz where the body of your message reads: unsubscribe nznog