Nortel Circuit Card 311 Reference page 37

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Loop Start Mode
In Loop Start mode, the A and B bits meaning is:
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When a T1 channel is idle, the Lineside T1 card simulates a ground on the
tip lead and –48Vdc on the ring lead to the terminal equipment by setting its
transmit A bit to 0 and transmit B bit to 1. Accordingly, an on-hook channel
on the terminal equipment simulates an open loop toward the Lineside T1
card, causing the Lineside T1 card's receive bits to be set to A = 0 and
receive B = 1.
Incoming calls Incoming calls to terminal equipment attached to the
Lineside T1 card can originate either from stations that are local (served
by the PBX), or remote (served through the PSTN). To provide the ringing
signal to a telephone the Lineside T1 card simulates an additional 90V on
the ring lead to the terminal equipment by alternating the transmit B bit
between 0 and 1 (0 during ring on, 1 during ring off). When an incoming
call is answered by the terminal equipment going off-hook, the terminal
equipment simulates tripping the ringing and shutting off ringing, causing
the Lineside T1 card's receive A bit to be changed from 0 to 1.
Outgoing calls During outgoing calls from the terminal equipment,
a channel is seized when the station goes off-hook. This simulates a
low-resistance loop across the tip and ring leads toward the Lineside T1
card, causing the lineside T1's receive A bit to be changed from 0 to 1. This
bit change prepares the Lineside T1 to receive digits. Outward address
signaling is then applied from the terminal equipment in the form of DTMF
tones or loop (interrupting) dial pulses that are signaled by the receive A
bit pulsing between 1 and 0.
Call disconnect from far end PSTN, private network or local
Station When a call is in process, the central office may disconnect the
call from the CS 1000E, CS 1000M, and Meridian 1. If the Lineside T1
port has been configured with the supervised analog line (SAL) feature,
the Lineside T1 card responds to the distant end disconnect message by
momentarily changing its transmit A bit to 1 and then returning it to 0. The
duration of time that the transmit A bit remains at 1 before returning to 0
Copyright © 2003-2008, Nortel Networks
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Incoming calls
Outgoing calls
Calls disconnected by the CO
Calls disconnected by the telephone
Transmit from LTI:A bit = 0 (tip ground on); B bit = Ringing (0=on, 1=off)
Receive to LTI: A bit = Loop (0=open, 1=closed); B bit = 1 (no ring
ground)
Nortel Communication Server 1000
Circuit Card Reference
NN43001-311 01.04 Standard
Release 5.0 23 May 2008
Line cards 37