larry@uunet.uu.net (Larry Lippman) (04/08/90)
In article <6111@accuvax.nwu.edu> john@bovine.ati.com (John Higdon) writes: > In the real world of modern CO switches (1ESS or newer) glare is a > negligible problem. Unlike SXS and crossbar, electronic/digital > switches apply ring current simultaneously with the connection to the > called party. Once any PBX sees that ring, the trunk is instantly > taken out of the pool for outside calls. Therefore, even systems with > loop start trunks need not segregate the available lines for incoming > vs outgoing. A well-designed ringup detection circuit in a PABX (or any other device, for that matter) will provide a time constant of several hundred milliseconds before furnishing a logical signal to the trunk circuit that ringing is in fact detected. The purpose of such delay is to prevent line transients from being falsely detected as ringing. While it is true that ESS and some modified XBAR will supply immediate ringing to the called party, the ringup detection circuit time constant still offers a "window of opportunity" for glare on a loop start trunk. The above condition is not a problem on a ground start trunk, however, since the CO *immediately* supplies ground on the tip side of the line to mark it busy. Under these circumstances, ringup detection delay is immaterial. Ground start trunks have always worked this way, even in SxS offices. Glare may still occur in ground start trunks due to delay in the PABX between the time it selects an idle trunk (i.e., ascertains that tip is open), and actually seizes the trunk (placing ground on ring). <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {boulder|decvax|rutgers|watmath}!acsu.buffalo.edu!kitty!larry <> TEL 716/688-1231 || 716/773-1700 {utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 || 716/773-2488