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From: vjs@rhyolite.wpd.sgi.com (Vernon Schryver)
Newsgroups: comp.dcom.modems
Subject: Re: PEP turnaround time
Message-ID: <106126@sgi.sgi.com>
Date: 24 May 91 21:24:15 GMT
References: <9105210700.AA14518@ucbvax.Berkeley.EDU> <1991May24.060836.7247@netcom.COM>
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Organization: Silicon Graphics, Inc., Mountain View, CA
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In article <1991May24.060836.7247@netcom.COM>, gandrews@netcom.COM (Greg Andrews) writes:
>                                                                       If you
> >turn on the speaker, you can hear the modem laboriously switching between
> >7 and 18 baud (or whatever the exact numbers).
> 
> Yes, an uncompressed implementation of SLIP will make PEP thrash between
> different sized packets.  It's not a "baud" rate switch at all.  It's a
> simple matter of packet size.

I think it really is a "baud change."  I understand from Telebit that while
running micropackets they use something like 18.3 baud, but doing big
packets they use about 7 baud.  That is, the signal transmitted by the
modem changes either 7+ or 18+ times/second.  Of course, the instantaneous
bit rate at the DTE-DCE interface never changes.
(Yes, the old lecture that "buad" != "bps")

> ...
> Thus the packet thrashing you hear over the speaker.  My understanding of 
> compressed SLIP is that the SLIP overhead is reduced from ~40 bytes down to
> ~5 bytes.

The "VJ compression" takes 3 or 4 bytes for TCP/IP plus one of SLIP
framing.  Thus, at best VJ-cslip replaces the 20 byte IP and 20 byte TCP
headers with 4 bytes.  Another commercially available scheme with slightly
different checksum and compression trade-offs gets it down to 3 bytes
including SLIP framing.  The checksum difference is that Van Jacobson did
not add a link-level checksum but did retain the TCP checksum.  That's good
if all of your traffic has TCP and UDP checksums (think of Sun's NFS) and
if you don't trust router memories and buses.  (Yes, I know people who run
NFS over SLIP, tho with machines that do use the UDP checksum.)  There's
room for only 1 SLIP compression standard, so don't worry about the
alternate.  It was developed when there were restrictions on shipping the
VJ code.



Vernon Schryver,   vjs@sgi.com