Path: utzoo!attcan!uunet!lll-winken!ames!killer!vector!nobody From: map@gaak.LCS.MIT.EDU (Michael A. Patton) Newsgroups: comp.dcom.telecom Subject: Race conditions in a PBX Message-ID: Date: 11 Jan 89 18:44:31 GMT Sender: chip@vector.UUCP Lines: 32 Approved: telecom-request@vector.uucp X-Submissions-To: telecom@bu-cs.bu.edu X-Administrivia-To: telecom-request@vector.uucp X-TELECOM-Digest: volume 9, issue 13, message 3 From: hiraki@ecf.toronto.edu (Lester Hiraki) Date: Mon, 9 Jan 89 14:16:47 EST Does anyone know how to solve the following problem? Consider a simple PBX which works as follows: [... Describes race condition on bidirectional loop-start lines ...] Assuming loop-start lines, can this race condition be avoided? Note, not all business have ground-start lines. I understand ground-start lines eleminate this very problem? Can someone explain how ground- start lines work? I used to work for a company that made equipment for telephone connection. The answer to your question is that Loop-Start lines are not supposed to be used bi-directionally, except for cases where this does not matter (i.e. where the person answering the call and the person wanting to dial would often be the same person anyway). Full PBX equipment should not use Loop-Start bi-directionally. Frequently the people involved in an actual installation don't know this and so you get the problem you describe. We occasionally ran into the opposite problem where the phone company wanted to install Ground-Start lines because they assumed our equipment was a PBX when in fact it was terminate-only. In later models we provide small amounts of originate-like service and had to provide a Ground-Start interface to avoid exactly this problem. It is my impression that the conversion from Loop-Start to Ground-Start is fairly simple on most CO equipment (they managed to do about 10 lines in half an hour when we put in a PBX), but there are some low-end PBX vendors that can't provide Ground-Start trunking.