ATM vs. DS3

Fri Jul 11 18:41:00 UTC 1997

Peter Kline wrote

>Another way of looking at this is that we know an ATM PVC over a DS3   
using
>PLCP  is configurable for a maximum of 96000 cells per second.  96000   
cells
>per second * 48 payload octets per cell * 8 bits per octet = 36864000   
bps,
>or 36.864 Mbps, not too far from what I observed above.  With PLCP   
turned
>off, the maximum PVC config was ~105000 cells per second, yielding ~40.3   
Mpbs.

The SAR function will add eight bytes to the payload.  The eight bytes
contain the CPCS-UU(1 bytes), CPI (1 byte), length of payload (two bytes)
and a CRC (4 bytes). This additional overhead needs to be taken into   
account
if you are segmenting small frames. The last cell after segmentation will   
have
these eight bytes at the end of the cell plus any padding needed to fill   
out the cell.

If you are segmenting a 96 byte payload for example you would think this
would actually fit into two cells (using the 48 byte payload), but it
really take three cells.  The last cell would only contain padding and   
this
overhead.

Steve Murray
Worldcom