Nortel Circuit Card Installation Manual page 578

Page 578 of 906
Ground start operation
553-3001-211 Standard 3.00
NT8D14 Universal Trunk card
Ground start operation is configured in software and implemented through
software download messages. In the idle state, the tip conductor from the CO
is open and a high-resistance negative battery is present on the ring lead.
Incoming calls
In an incoming call, after ground is detected on the tip, the universal trunk
card scans for a ringing detection signal before presenting the call to an
attendant and tripping the ringing. When the attendant answers, a low
resistance is placed across the tip and ring conductors, which trips CO ringing
and establishes a speech path. See Figure 119 on
page 580.
Reverse-wiring compensation
The CS 1000 Release 4.5 software includes a feature for detecting reverse
wiring (connection of the near-end tip and ring leads to the far-end ring and
tip leads) 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 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.
August 2005
page 579 and Figure 120 on