NT1R20 Off-Premise Station Analog Line card Page 167 of 894
Circuit Card Description and Installation

Incoming calls

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 67).

Table 67
Call connection sequence—near-end station receiving call (Part 1 of 2)
State
Signal / Direction
Far-end / Near-end Remarks
Line card unit idle Group on tip, battery on ring
High resistance loop
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.
Incoming call Ringing The system applies 20 Hz ringing to ring
lead.
Near-end station
off-hook
Low resistance loop
Two-way voice
connection
The system detects increase in loop
current, tips ringing, and put call through
to near-end station.
Near end station
hangs up first
High-resistance loop If near end station hangs-up first, the
line card detects the drop in loop
current.
Line card unit idle Group on tip, battery on ring
High resistance loop
Line card unit is ready for the next call.