Nortel NT1R20, Description

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Page 34 of 290 Description
553-3001-105 Standard 5.00 June 1999
Analog line call operation
The applications, features, and signalling arrangements for each line interface
unit are configured in software and implemented on the card through software
download messages. When an analog line interface unit is idle, it provides a
voltage near ground on the tip lead and a voltage near –48 V dc on the ring
lead to the near-end station. (The near-end station is the telephone or device
that is connected to the analog line card by the tip and ring leads.) An on-hook
telephone presents a high impedance toward the line interface unit on the
card.
Incoming calls
Incoming calls to a telephone that is connected to an analog line card can
originate either from stations that are local (served by the Meridian 1 PBX),
or remote (served through the public switched telephone network). The
alerting signal to a telephone is 20 Hz (nominal) ringing. When an incoming
call is answered by the near-end station going off-hook, a low-resistance DC
loop is placed across the tip and ring leads (towards the analog line card) and
ringing is tripped (see Figure 12).
Outgoing calls
For outgoing calls from the near-end station, a line interface unit is seized
when the station goes off-hook, placing a low-resistance loop across the tip
and ring leads towards the analog line card (see Figure 13). When the card
detects the low-resistance loop, it prepares to receive digits. When the
Meridian 1 is ready to receive digits, it returns dial tone. Outward address
signaling is then applied from the near-end station in the form of loop
(interrupting) dial pulses or DTMF tones.
Message waiting
Line cards that are equipped with the message waiting feature receive
notification that a message is waiting across the Card LAN link (IPE cards)
or the SL-1 network loop (PE cards). On cards that drive a message waiting
light, the light is turned on by connecting the ring side of the telephone line
to the –150 V dc power supply. When the line card senses that the telephone
has gone off-hook, it removes the –150 V dc voltage until the telephone goes
back on-hook. Line cards that use an interrupted dial tone to indicate message
waiting do nothing until the receiver is picked up. The line card then
interrupts the dial tone at a regular interval to indicate that a message is
waiting.