Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!decwrl!labrea!glacier!jbn
From: jbn@glacier.STANFORD.EDU (John B. Nagle)
Newsgroups: comp.dcom.modems
Subject: Re: call back
Keywords: glare
Message-ID: <17569@glacier.STANFORD.EDU>
Date: 19 Jul 88 16:36:18 GMT
References: <428@solaris.UUCP> <3208@edm.UUCP>
Reply-To: jbn@glacier.UUCP (John B. Nagle)
Organization: Stanford University
Lines: 34

In article <3208@edm.UUCP> steve@edm.UUCP (Stephen Samuel) writes:
> One serious problem with the shell script is that it leaves your
> system vulnerable to someone who manages to dial in between your
> hanging up and the system dialing out (you'd have a shell running
> at that point).

      Telephone people call this "glare".  There's a basic problem with
using the same phone line for originating and receiving calls.  There's
a risk that when the phone goes off-hook to originate a call, it is actually
answering a call for which ringing has not yet started.  Depending upon the
utilization of the line involved, this may happen very seldom or quite
frequently.  It happens constantly with PBX trunks, since one normally
orders enough outside lines for your PBX to meet the peak hour load.
So there's a solution.

      The trouble comes from the convention that the station equipment
(the phone) requests dial tone from the central office with the same
signal (placing a resistance of about 600 ohms between "tip" and "ring",
the two wires of the phone line) that it uses for answering after a ring.
These functions really should be performed with separate signals.  It
is possible to order trunks with "ground start", where they are.  On
a ground start trunk, it is necessary to ground one wire of the line
to get dial tone.  This also makes the line busy with regard to receiving
calls, eliminating the ambiguity.

      Are there commercial modems available that can utilize ground-start
trunks properly?  Sites that do heavy dial-out on dial-in lines, and 
anybody who uses "security call-back" systems, should have ground start
lines and suitable hardware.

      Some low-rent PBXs don't use ground-start lines.  The next time
you call a company with a PBX, and find yourself connected to someone
trying to place an outgoing call, you've found one.

      					John Nagle