Path: utzoo!attcan!uunet!lll-winken!ames!killer!vector!nobody From: westmark!dave@rutgers.edu (Dave Levenson) Newsgroups: comp.dcom.telecom Subject: Re: Race conditions in a PBX Message-ID: Date: 13 Jan 89 02:09:11 GMT Sender: chip@vector.UUCP Lines: 52 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 14, message 3 In article , hiraki@ecf.toronto.edu (Lester Hiraki) writes: > 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... ... > 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? Lester's question refers to what telephony people call glare. When bi-directional trunks are used between two switches, either end may seize a trunk at any time. How do we handle the case where both ends of the same circuit are seized at the same time? Ground-start lines are typically used between PBX and Central Office switches. A separate mark-busy channel in each direction is provided, as follows: The CO marks the circuit busy by applying a high-impedence ground to the TIP side of the loop. The customer equipment marks the circuit busy by applying a high-impedence ground to the RING side of the loop. Either end may send its busy-mark to the other end while testing the other end's busy-mark. The standard protocol is that the PBX tests the CO's busy-mark (by checking for the ground on TIP) before it bids for the line (by grounding RING). The CO does the same thing in reverse. If the PBX and the CO both bid at the same time, the standard protocol requires that the PBX release the circuit and seize another outgoing trunk, and the CO will complete the incoming call. The probability of glare is reduced, if possible, by having the PBX hunt for a trunk from the top down, while the CO hunts from the bottom up. If ground-start service is not available, the usual practice is to use two trunk groups, one for incoming service, and the other for outgoing service. If this is not done, there is no reliable way of avoiding glare. -- Dave Levenson Westmark, Inc. The Man in the Mooney Warren, NJ USA {rutgers | att}!westmark!dave