more on the HFC saga

DoN Nichols dnichols at
Thu May 23 23:37:42 AEST 1991

In article <1991May22.042143.13250 at fithp> mhw at fithp (Marc Weinstein) writes:
>From article <103431 at becker.UUCP>, by bdb at becker.UUCP (Bruce D. Becker):

	[ ... ]

>>       For most
>> 	things compression is irrelevant, or they
>> 	are done in the host as in news batches
>> 	or uucp files.  Doing compression in the
>> 	modem seems wasteful of resources due
>> 	to the fact that uncompressed data gets
>> 	pumped thru the serial interface with
>> 	an interrupt service routine invocation
>> 	for each character!
>Hmmm - don't understand the logic here.  If I want to send a file to
>someone, and I know my modem will compress the file anyway, then I don't
>have to bother with compressing the file before the fact.  Less wory,
>less bother.

	If you pre-compress the file, that's fewer characters through your
serial interface, and therefore fewer interrupts.  And, since the
transmission speed between modems is fixed (after compression), the fewer 
interrupts per unit time (on the average).  It's the high rate of interrupts
that really brings the system to its knees.

	Since the ethernet interface is also a source of interrupts, (I
think at the same or higher priority), having it running at the same time as
a high-speed uucp transfer can cause lost characters.  (I can get away with
running the interface to my TeleBit at 19200 IF the ethernet is stopped, and
the system at the other end is running at 9600, so the average data rate is
slow enough to let the 3B1 empty the buffers.)  If ethernet is running, I
get lots of retrys, and eventual timeouts and failures to the same system at
19200, but it survives with the 3B1's interface speed at 9600.  If I try to
go to the uucp 'e' protocol, I get failures at 9600 with the ethernet
running.  I haven't tried it with the ethernet stopped.  The system at the
other end is a PC/RT running AIX, and the serial interface is a dumb
multi-port board, so IT can't keep up with 19200, therefore it doesn't hurt
much to tie my system to 9600 :-)

	[ ... ]

	Good Luck
Donald Nichols (DoN.)		| Voice (Days):	(703) 664-1585
D&D Data			| Voice (Eves):	(703) 938-4564
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