Re: CUSM, ISDN & Mac, questions

Christopher Davis (
Wed, 15 Feb 1995 22:02:56 -0500

DSB> == David S Bennahum <>

DSB> Too bad, an old trick is to set your port to maximum, regardless of
DSB> your modem speed/ISDN speed, because no matter what, Internet
DSB> traffic will *never* get data to you at top speed (it is always
DSB> 7-15% of actual rated).

Actually, you set the port speed higher because, on compressible data, a
modem using MNP5 or V.42bis will *effectively* transmit faster than the
modem<->modem speed. (In the extreme case, a file consisting solely of
the letter "A" will reach the maximum speed of the computer<->modem links.)

DSB> So, for example, I always set my port to 56K/sec when dialing in at
DSB> 14.4K, this actually will boost transmission from 1K/sec to a
DSB> whopping 2K/sec (yay) while doing ftp or whatever (try it).

This is because you may be transmitting noncompressible data, as well as
the fact that the IP headers get in the way of compressing the data
somewhat (they tend to throw off the compression algorithm a bit, which is
why Van Jacobson's header compression scheme is so useful.

DSB> So, I wish I could do the same for my ISDN box which, at best, ftps
DSB> at 6K/sec even though it can handle 56K/sec.

You're mixing kilobits and kilobytes, David. 6kbytes/sec *is* about
56kbits/sec (do the math).

DSB> Boosting the port setting to 215K would probably send ftp speeds to
DSB> a "blistering" 12k/sec!

Only if you were using both B channels, or had both a compressing TA and
suitably compressible data.

This is one reason Ethernet interfaces on ISDN hardware are so popular.
No port speed bottlenecks, no "fast serial port" hardware needed...

 Christopher Davis * <> | "It's 106 ms to Chicago, we've got a full |  disk of GIFs, half a meg of hypertext,
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