Nortel Networks 1000 - Manual (Page 28)

28 Overview
amplifies the result. On some of the line cards, the gain of these filters can
be programmed by the system controller. This allows the system to make
up for line losses according to the loss plan.
Balancing network
Depending on the card type, the balancing network provides a 600 3 /
4
,
900 3 /
4
, 3COM or 3CM2 impedance matching network. It also converts
the 2-wire transmission path (tip and ring) to a 4-wire transmission
path (Rx/ground and Tx/ground). The balancing network is usually
a transformer/analog (hybrid) circuit combination, but can also be a
monolithic Subscriber Line Interface Circuit (SLIC) on the newer line cards.
Line interface and foreign voltage protection
The line interface unit connects the balancing network to the telephone
tip and ring pairs. The off-premise line card (NT1R20) has circuitry that
protects the line card from foreign voltage surges caused by accidental
power line connections and lightning surges. This protection is necessary
if the telephone line leaves the building where the switch is installed.
The line interface unit has a relay that applies the ringing voltage onto
the phone line. See
Figure 4 "Typical analog line interface unit block
diagram" (page 27) . The RSYNC signal from the 20 Hz (nominal) ringing
voltage power supply is used to prevent switching of the relay during the
current peak. This eliminates switching glitches and extends the life of the
switching relay.
The off-hook detection circuit monitors the current draw on the phone line.
When the current draw exceeds a preset value, the circuit generates an
off-hook signal that is transmitted back to the system controller.
The message waiting circuit on message waiting line cards monitors the
status of the message waiting signal and applies –150 V dc power to the
tip lead when activated. This voltage is used to light the message waiting
lamps on phones that are equipped with that feature. The high voltage
supply is automatically disconnected when the phone goes off-hook.
Newer line cards can sense when the message waiting lamp is not
working and can report that information back to the system controller.
Digital line interface units
The NT8D02 Digital Line card provides voice and data communication
links between a switch and modular digital telephones. These lines
carry multiplexed PCM voice, data and signaling information as Time
Compression Multiplexed (TCM) loops. Each TCM loop can be connected
to a Nortel "Meridian Modular Digital" telephone.
Nortel Communication Server 1000
Circuit Card Reference
NN43001-311 02.06 Standard
27 August 2008
Copyright © 2003-2008 Nortel Networks
.