Nortel Networks 553-3001-211 manual Loop Start Mode, Incoming calls

Overview Page 47 of 894

Call operation will be described by categorizing the operation into the following main states:

•Idle (on-hook)

•Incoming calls

•Outgoing calls

•Calls disconnected by the CO

•Calls disconnected by the telephone

Loop Start Mode

In Loop Start mode , the A and B bits have the following meaning:

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)

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.

Circuit Card Description and Installation