Nortel NT1R20 Description page 67

Call disconnect from line-side T1 terminal equipment
while a call is in process, the terminal equipment may disconnect by going
on-hook. The terminal equipment detects no loop current and sends signaling
to the line-side T1card that causes its receive A bit to change from 1 to 0. The
call is now released.
Table 4 outlines the line-side T1's A and B bit settings in each state of call
processing.
Ground Start Mode
In ground start mode, the A and B bits have the following meaning:
Transmit from LTI:A bit = Tip ground (0=grounded, 1=not grounded)
B bit = Ringing (0=on, 1=off)
Receive to LTI: A bit = Loop (0=open, 1=closed)
B bit = Ring ground (0=grounded, 1=not grounded)
When a T1 channel is idle, the line-side T1 card simulates a ground on the tip
lead and -48Vdc on the ring lead to the terminal equipment by setting the
transmit A bit to 1 and transmit B bit to 1. Accordingly, an on-hook telephone
simulates an open loop toward the line-side T1 card, causing the line-side T1
card's receive bits to be set to A = 0 and B = 1.
Incoming calls to terminal equipment that is connected to
Incoming Calls
the line-side T1 card can originate either from stations that are local (served
by the Meridian 1 PBX), or remote (served through the public switched
telephone network). To provide the ringing signal to the terminal equipment
the line-side T1 card simulates the 90V ring signal on the ring lead by
alternating the transmit B bit between 0 and 1 (0 during ring on, 1 during ring
off), and ground on the tip lead by setting the transmit A bit to 0. When an
incoming call is answered (by the terminal equipment going off-hook), the
terminal equipment simulates tripping the ringing and shutting off ringing by
causing the line-side T1's receive A bit to change from 0 to 1. The line-side
T1 card responds to this message by simulating loop closure by holding the
transmit B bit constant at 1.
Description Page 43 of 290
Alternatively,
Line cards Description