Nortel Networks Circuit Card - Manual (Page 53)

Overview Page 53 of 906
Circuit Card Description and Installation
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.