Path: utzoo!attcan!uunet!husc6!bloom-beacon!apple!rutgers!bellcore!faline!thumper!ulysses!smb
From: smb@ulysses.homer.nj.att.com (Steven Bellovin[jsw])
Newsgroups: comp.dcom.modems
Subject: Re: Break definition (Some Answers from the Standards, LONG)
Message-ID: <10607@ulysses.homer.nj.att.com>
Date: 16 Sep 88 14:59:22 GMT
References: <402@ucrmath.UUCP> <6095@galbp.LBP.HARRIS.COM> <39157@pyramid.pyramid.com>
Organization: AT&T Bell Laboratories, Murray Hill
Lines: 14

My thanks to csg@pyramid.pyramid.com (Carl S. Gutekunst) for an
excellent article.  One point I should add:  many (most? all?) modems
use a ``long break'' -- on the order of 3 seconds -- to indicate
disconnect.  That is, modems can be strapped so that when they wish to
hang up (for example, if the host drops DTR), they will first transmit
a 3-second break to the remote modem.  Similarly, when they receive
such a break signal, they will hang up, too.  I suspect (but do not
know for certain) that this usage dates back to half-duplex modems,
which of necessity do not transmit a continuous carrier.  Rather, they
only generate carrier when they wish to transmit.  Thus, the remote end
cannot tell when the local end has hung up -- no carrier may simply
mean that the modem doesn't have anything to say.

		--Steve Bellovin