mlm@ftel.net (Mark Milhollan) writes:
"Patrick W. Gilmore" writes:
PPP overhead is not even close to 200 Kbps on a T1.
Without information on the traffic being handled this statement cannot be supported. I agree that its doubtful.
Actually, we can probably do better than that. For HDLC, the worst case is all ones user data, which expands 6/5, plus 7 bits per packet of shared flags. The PPP overhead, assuming no header compression, is four bytes of header plus two bytes of CRC per packet. Now, if we assume 256 byte packets (not a bad assumption of average given IP traffic measurements I've seen) with worst case data, the user portion will expand from 2048 to 2458 bits, and the HDLC/PPP overhead adds 55 bits. Thus, our efficiency is .815, or 284Kbps of overhead. With random data, rather than worst-case data, things are much better. The expansion is 161/160 on random data, which means that our 2048 bits of data go as 2061 encoded. The efficiency is .968, or 49Kbps.
there are no "cells" in PPP.
Its certainly possible that cells are being used, but their use should not impact the delivered capacity.
(!) The cell tax is about 10% -- the SAR expands user data from 48 bytes to 53 bytes, plus an additional amount of overhead for internal fragmentation on the final cell (AAL-5), plus overhead for whatever encapsulation mode is being used. On a T1, you'd be lucky to get away with only 154Kbps wasted in cell overhead, let alone any of the L2 stuff. -- James Carlson, Consulting S/W Engineer <carlson@ironbridgenetworks.com> IronBridge Networks / 55 Hayden Avenue 71.246W Vox: +1 781 402 8032 Lexington MA 02421-7996 / USA 42.423N Fax: +1 781 402 8092 "PPP Design and Debugging" --- http://people.ne.mediaone.net/carlson/ppp