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Page 40 of 894 Overview
553-3001-211 Standard 2.00 September 2004
the system controller to operate the digital line interface circuits during calls.
The circuits receive outgoing call signaling messages from the controller and
return incoming call status information to the controller over the DS-30X
network loop.
Analog line call operation
The applications, features, and signalling arrangements for each line interface
unit are configured in software and implemented on the card through software
download messages. When an analog line interface unit is idle, it provides a
voltage near ground on the tip lead and a voltage near –48 V dc on the ring
lead to the near-end station. (The near-end station is the telephone or device
that is connected to the analog line card by the tip and ring leads.) An on-hook
telephone presents a high impedance toward the line interface unit on the
card.
Incoming calls
Incoming calls to a telephone that is connected to an analog line card can
originate either from stations that are local (served by the PBX), or remote
(served through the Public Switched Telephone Network (PSTN)). The
alerting signal to a telephone is 20 Hz (nominal) ringing. When an incoming
call is answered by the near-end station going off-hook, a low-resistance dc
loop is placed across the tip and ring leads (towards the analog line card) and
ringing is tripped. See Figure 6 on page 41.
Outgoing calls
For outgoing calls from the near-end station, a line interface unit is seized
when the station goes off-hook, placing a low-resistance loop across the tip
and ring leads towards the analog line card. See Figure 7 on page 42. When
the card detects the low-resistance loop, it prepares to receive digits. When
the system is ready to receive digits, it returns dial tone. Outward address
signaling is then applied from the near-end station in the form of loop
(interrupting) dial pulses or DTMF tones.