Nortel Networks 553-3001-211 manual Overview Page 51

Overview Page 51 of 894

Incoming Calls

Incoming calls to terminal equipment that is connected to the lineside T1 card can originate either from stations that are local (served by the PBX), or remote (served through the public switched telephone network). To provide the ringing signal to the terminal equipment the lineside 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 lineside T1’s receive A bit to change from 0 to 1. The lineside T1 card responds to this message by simulating loop closure by holding the transmit B bit constant at 1.

Outgoing Calls

During outgoing calls from the terminal equipment, a channel is seized when the terminal equipment goes off-hook, simulating a ground to the ring lead toward the lineside T1 card by causing the lineside T1’s receive B bit to change from 1 to 0. In turn, the lineside T1 card simulates grounding its tip lead by changing the transmit A bit to 0. The terminal equipment responds to this message by removing the ring ground (lineside T1’s receive B bit is changed to 1) and simulating open loop at the terminal equipment (lineside T1’s receive A bit is changed to 0).

Call disconnect from far end (PSTN, private network or local station

While a call is in process, the far end might disconnect the call. If the lineside T1 port has been configured with the Supervised Analog Line (SAL) feature, the lineside T1 will respond to the distant end disconnect message by opening tip ground. This causes the lineside T1 card to change the transmit A bit to 1. When the terminal equipment sees the transmit A bit go to 1, it responds by simulating open loop causing the lineside T1’s receive A bit to change to 0. The call is terminated and the interface is once again in the idle condition.

For the lineside T1 card to support distant end disconnect in ground start mode, the following configuration parameters must exist:

•The Supervised Analog Line (SAL) feature must be configured for each lineside T1 port.

Circuit Card Description and Installation