Path: utzoo!attcan!utgpu!watmath!clyde!att!osu-cis!killer!vector!nobody From: hiraki@ecf.toronto.edu (Lester Hiraki) Newsgroups: comp.dcom.telecom Subject: Race conditions in a PBX Message-ID: Date: 9 Jan 89 19:16:47 GMT Sender: chip@vector.UUCP Lines: 23 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 10, message 3 Does anyone know how to solve the following problem? Consider a simple PBX which works as follows: All incoming calls from trunks are routed to the attendant console. Outgoing calls are processed as follows. From an internal extention, the user dials the trunk access code (usually 9) and then the some valid number according to the North American Numbering Plan. The PBX accumulates the valid digits first and after receiving the last digit, seizes the first free trunk and signals-out the dialing information in a burst to the local CO. Imagine now that an incoming call arrives at a trunk but the CO has not yet applied the ring voltage - ie connexion was made during the silent window (ringing is usually 2s on and 4s off, say). Just at this moment someone within the PBX is making an outside call & the PBX seizes this trunk before ringing starts, in effect answering the call. The incoming caller is connected to the person waiting for his outside call to be completed. 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?