Path: utzoo!utgpu!attcan!uunet!lll-winken!netsys!vector!nobody From: hou2d!peter@clyde.att.com Newsgroups: comp.dcom.telecom Subject: Re: Race conditions in a PBX Message-ID: Date: 13 Jan 89 20:25:10 GMT Sender: chip@vector.UUCP Lines: 33 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 18, message 4 The condition with PBX's where two simultaneous seizures (call attempts) may be connected together is called glare. In PBX trunking, there are two common signaling arrangements: loop start and ground start. Loop start is a two state signaling (on-hook and off-hook), which may allow simultaneous seizures. That is, an outgoing call may, in the absense of ringing, seize a trunk at the same time as an incoming call is waiting for the ringing generator to place ringing on the line. Remember that there is a pause in the ringing signal which is typically four seconds. To eliminate this condition with loop start signaling, PBX trunks are usually engineered as one-way in or one-way out. This directionality refers to the call set up direction, not the transmission path. From the PBX's perspective, a one-way in trunk can receive outside calls, but the PBX cannot place calls on that trunk. This eliminates the possibility of a trunk being seizied from both ends simultaneously. The other PBX signaling, ground start, uses more than two states to contain signaling information. A seizure from central office to PBX is initiated by placing a ground on the tip. A PBX to office seizure is initiated with a ground on the ring. After detecting a ground, the trunk is considered busy, and is not seized at the detecting end. >From here ground start gets complicated. In the office, a ring ground is responded to with a tip ground and a dial tone, the tip ground causes a loop closure at the PBX, which is detected in the CO, and a DP or DTMF detector (where applicable) recieves the digits (and causes dial tone to vanish). The call is now set up from the station set to the office. On an incoming call, an office tip ground is also accompanied with office ringing, which initiates a loop closure in the PBX, and sets the call up. (Disclaimer: I haven't studied this in a while, and address signaling isn't addressed here.) Hope this answered your questions...