While this is certainly true, HDLC is a point-to-point protocol
and not a network protocol, like frame relay, SMDS or ATM. And
HDLC itself isn't quite enough, IMHO, you really need PPP.
And the efficiency lost to ATM is not 40% as often claimed on this
list, but rather it is 12% less efficient than PPP for TCP.
10% is the cell header overhead and 2% is due to modulo 48 padding,
given actual traffic at FIX West as measured by kc at NLANR.
We return to our regularly scheduled ATM tweaking program now in