Modem-to-Modem Flow Control (was Re: The Final Word on HFC)

Marc Weinstein mhw at fithp
Mon May 20 14:45:14 AEST 1991

>From article <2825 at public.BTR.COM>, by thad at public.BTR.COM (Thaddeus P. Floryan):
> In article <1991May19.030549.3983 at> floyd at (Floyd Davidson) writes:
> {comments re: HFC on the 3B1 per the extractions:}
>>	HFC doesn't have anything to do with the rate of data transfer
>>	between the modems.  The sending modem is NOT going to detect
>>	anything as a result of the receiving modem asserting HFC.
>>	[...]
>>	I would suggest that anyone else who wants further
>>	information on this should bring it up in the comp.dcom.modems
>>	group.
> Outgoing HFC works just fine on the 3B1, both over hardlines and modems. Thus
> I disagree with the comment above re: "...sending modem is NOT going to detect
> anything..." because calling from my office at 2400 baud into a T2500 connected
> to my 3B1 (fixed at 9600 baud) works just fine ... the 3B1 does "somehow"
> detect my office computer's HFC signal to STOP to prevent its (the office
> computer's) buffer from overflowing when the 3B1 is sending at 9600 baud but
> the office computer is receiving at 2400 baud.  Magic?  Swinging dead chickens
> over one's head while dancing under a full moon in one's Jockey shorts?  Dunno.

No magic involved.  Unless you're using plain, vanilla V.22bis for your
2400 baud connection.  Then, I dunno.

Both MNP and V.42 error-correction protocols support Flow Control.  I'm
not sure how it's done, but the protocols handle it.  Modems which 
implement these protocols typically REQUIRE HFC on the computer, since
it then becomes likely that the modems will detect and exert flow control.
If your system doesn't recognize it, you're screwed.

If you support HFC and are using these protocols, then you have
computer-to-computer flow control.  So, unless the T2500 doesn't support
the protocols, you can get rid of the dead chickens.  Or, have them for

Marc Weinstein
{simon,royko,tellab5}!linac!fithp!mhw		Elmhurst, IL
-or- {internet host}!!fithp!mhw

More information about the Comp.sys.3b1 mailing list