Nortel Enterprise 1000 Reference Manual page 57

Analog Trunk Cards - NT8D14 Universal Trunk Card
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
will time stamp 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. Thus, if a previously correctly wired trunk becomes
reverse-wired, the next incoming call is marked as possibly reverse-wired and
the threshold count will begin.
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.
Note 1: The far-end may 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.
Outgoing calls
For outgoing calls, the trunk provides a ground to the ring lead. The CO
responds by grounding the tip and returning dial tone. After the tip ground is
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