Nortel Circuit Card Installation Manual page 169

Incoming calls
Table 68
Call connection sequence—near-end station receiving call (Part 1 of 2)
State
Line card unit idle
Incoming call
Near-end station
off-hook
Two-way voice
connection
Near end station
hangs up first
Line card unit idle
NT1R20 Off-Premise Station Analog Line card
Incoming calls to a telephone connected to the NT1R20 OPS analog line card
originate from stations that can be local (served by the PBX) or remote
(served through the public switched telephone network). The alerting signal
to telephones is 20 Hz (nominal) ringing. When an incoming call is answered,
ringing is tripped as the telephone goes off-hook, placing a low-resistance dc
loop across the tip and ring leads toward the OPS analog line card. (see
Table 68).
Signal / Direction
Far-end / Near-end
Group on tip, battery on ring
High resistance loop
Ringing
Low resistance loop
High-resistance loop
Group on tip, battery on ring
High resistance loop
Remarks
No battery current drawn.
Far-end station goes off-hook and
addresses (dials-up) the near-end
station. The system receives the
incoming call on a trunk and determine
the TN.
The system applies 20 Hz ringing to ring
lead.
The system detects increase in loop
current, tips ringing, and put call through
to near-end station.
If near end station hangs-up first, the
line card detects the drop in loop
current.
Line card unit is ready for the next call.
Circuit Card Description and Installation
Page 169 of 906