Nortel Circuit Card 311 Reference page 603

Ordinarily, an incoming call on a reverse-wired trunk without reverse-wiring
compensation presents ringing on the tip lead rather than on the ring lead.
Since the software expects to see a ground on the tip lead, it interprets
the end of the first ringing signal as a switchhook flash. But since the
interval between ringing signals exceeds the switchhook flash time of 512
milliseconds, the software assumes a far-end disconnect. This causes the
call to be presented to a console loop key and then immediately removed.
The reverse-wiring compensation feature operates as follows. If an
apparent disconnect takes place immediately after the first ringing signal,
the software time stamps the event and temporarily remove the call from
the console loop key.
If another such ringing/disconnect event occurs during the No Ringing
Detector (NRD) time, the trunk is considered "possibly reverse-wired"
and a threshold counter starts. Calls on trunks identified as possibly
reverse-wired are presented to the attendant during the initial ring, removed,
and then continuously presented after the second ring. If a call on a
possibly reverse-wired trunk is abandoned before the attendant answers, it
is disconnected after the NRD timer expires.
A trunk identified as possibly reverse-wired is switched by the software
to loop start processing after the second ring. This switching takes place
on a call-by-call basis. So if a previously correctly wired trunk becomes
reverse-wired, the next incoming call is marked as possibly reverse-wired
and the threshold count begins.
If the threshold count exceeds its limit, an error message is printed and the
trunk is registered as "positively reverse wired." Once identified as positively
reverse wired, the call is presented continuously from the first ring. When a
reverse-wired trunk becomes correctly wired, the first subsequent call clears
the threshold counter and normal ground start processing is implemented.
The Meridian 1 software includes a feature for detecting reverse wiring
(connection of near-end tip and ring leads to far-end ring and tip leads,
respectively) on ground start trunks with far-end answer supervision.
Ordinarily, an incoming call on a reverse-wired trunk without reverse-wiring
compensation presents ringing on the tip lead rather than on the ring
lead. Since software expects to see a ground on the tip lead, it interprets
Copyright © 2003-2008, Nortel Networks
.
Note 1: The far-end can reverse battery and ground upon receipt of
attendant answer.
Note 2: The near-end provides a high-impedance (>150k ohms)
disconnect signal of at least 50 ms before reconnecting the ground
detector.
Nortel Communication Server 1000
Circuit Card Reference
NN43001-311 01.04 Standard
Release 5.0 23 May 2008
Operation 603