Avaya 1000 Series, Справочник

Страница: 41 / 700

Balancing network
Depending on the card type, the balancing network provides a 600 ¾, 900 ¾, 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 on page 40. 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.
The digital line interface card contains one or more digital line interface units. See Figure 5:
Digital line interface unit block diagram on page 42. Each digital line interface unit contains
a Digital Line Interface Circuit (DLIC). The purpose of each DLIC is to demultiplex data from
the DS-30X Tx channel into integrated voice and data bitstreams and transmit those bitstreams
as Bi-Polar Return to Zero, Alternate Mark Inversion (BPRZ-AMI) data to the TCM loop. It also
Line cards
Circuit Card Reference July 2011 41