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Appendix B

Line Interface Signaling

Introduction

This appendix describes how to interpret signaling to and from the AG 2000 line interfaces. Note: The telephony protocol, embodied by a TCP (Trunk Control Program) running on the AG 2000 board, automatically controls and monitors the line signaling bits. This information is provided for reference only. Controlling the signaling bits manually may violate local telecommunications regulations.
The following table describes the two signaling directions: Signaling Type Description Transmit The signaling that the board sends out onto the phone line via the line interface. The transmit signal is used to control the line or phone. Receive This signaling comes from the phone line through the line interface to the board. An application can monitor this signal to detect loop current or ringing.
The line interfaces on the board convert the signaling into the line condition appropriate for the line type (for example, loop start). They also convert incoming information into digital signals recognizable by AG 2000-based applications.

Signaling Type	Description
Transmit	The signaling that the board sends out onto the phone line via the line interface. The transmit signal is used to control the line or phone.
Receive	This signaling comes from the phone line through the line interface to the board. An application can monitor this signal to detect loop current or ringing.

Loop Start Line Interfaces

Loop Start Transmit Signaling

With loop start interfaces, the transmitted signaling A bit in the signaling timeslot causes the interface to seize the line (go off-hook) or release the line (go on-hook).
If the A bit is set to 1, the line goes off-hook. If the A bit is set to 0, the line goes on-hook.
Bits B, C, and D are reserved, and should be set to 0.
Figure 21 shows transmit signaling for loop start line interfaces: Figure 21. Loop Start Transmit Signaling
This table summarizes the transmit signaling for loop start line interfaces: Bit Hex Bitmask To take line off-hook To put line on-hook A bit 0x08 0x08 0 Note: If you reset the switch, all bits are set to 0.

Bit	Hex Bitmask	To take line off-hook	To put line on-hook
A bit	`0x08`	`0x08`	`0`

Loop Start Receive Signaling

Depending on how the transmitted signaling A bit is set, the line has been placed on-hook or off-hook. Depending on the hook state, the received signaling A bit acts either as a ring signal detector or a loop current indicator. When the line is on-hook, monitoring the A bit tells you if the line is ringing. When the line is off-hook, monitoring the A bit indicates whether there is loop current flowing. The B bit indicates the polarity of tip and ring. If the B bit is set to 1, the loop current direction is reverse. Bits C and D are reserved, and should be ignored.
Regulations require that loop start equipment must function regardless of idle state polarity. The B bit normal state is undefined. The information in the B bit is in the change of state.
Figure 22 shows receive signaling for loop start line interfaces: Figure 22. Loop Start Receive Signaling
The following table summarizes the receive signaling for loop start line interfaces: Bit Hex Bitmask If line is off-hook If line is on-hook A bit 0x08 Detects loop current: Value Description 0 No loop current. 0x08 Current is flowing. A bit toggles with ring frequency. Idle state = 0. B bit 0x04 Loop current direction: Value Description 0 Tip positive with respect to ring. 0x04 Tip negative with respect to ring. 0 C bit N/A Reserved (should be ignored). Reserved (should be ignored). D bit N/A Reserved (should be ignored). Reserved (should be ignored).

Bit	Hex Bitmask	If line is off-hook	If line is on-hook
A bit	`0x08`	Detects loop current: Value Description `0` No loop current. `0x08` Current is flowing.	A bit toggles with ring frequency. Idle state = 0.
B bit	`0x04`	Loop current direction: Value Description `0` Tip positive with respect to ring. `0x04` Tip negative with respect to ring.	`0`
C bit	N/A	Reserved (should be ignored).	Reserved (should be ignored).
D bit	N/A	Reserved (should be ignored).	Reserved (should be ignored).

Subscriber Loop Line Interfaces

There are three states to which the subscriber loop module may be set: State Description Off The A and B bits of the transmitted stream are set to 0 (off). No battery is applied to the line and no ringing is applied. Audio can not be passed through the system. Idle or Active The A bit is set to 1. This is required for the audio path to function. The B bit is set to 0. The board is active and applying battery to the line. The loop current detector is active and audio passes through the system. This is the normal operation state for the subscriber loop module. Ringing The A bit and the B bit are set to 1. This state is used to close the ringing relay when it is desired to ring the attached line/phone. The B bit must set the ring signal cadence. Ring-trip is done automatically by the hardware.

State	Description
Off	The A and B bits of the transmitted stream are set to 0 (off). No battery is applied to the line and no ringing is applied. Audio can not be passed through the system.
Idle or Active	The A bit is set to 1. This is required for the audio path to function. The B bit is set to 0. The board is active and applying battery to the line. The loop current detector is active and audio passes through the system. This is the normal operation state for the subscriber loop module.
Ringing	The A bit and the B bit are set to 1. This state is used to close the ringing relay when it is desired to ring the attached line/phone. The B bit must set the ring signal cadence. Ring-trip is done automatically by the hardware.

Subscriber Loop Transmit Signaling

Figure 23 shows transmit signaling for subscriber loop line interfaces: Figure 23. Subscriber Loop Transmit Signaling
This table summarizes the transmit signaling for subscriber loop line interfaces: Bit Hex Bitmask Description A bit 0x08 Puts battery voltage on tip/ring pair. Value Description 0x08 Apply battery. 0 No battery. B bit 0x04 0x00 Applies ringing voltage/signal to tip/ring. No ring. C bit N/A Reserved (should be 0). D bit N/A Reserved (should be 0).

Bit	Hex Bitmask	Description
A bit	`0x08`	Puts battery voltage on tip/ring pair. Value Description `0x08` Apply battery. `0` No battery.
B bit	`0x04` 0x00	Applies ringing voltage/signal to tip/ring. No ring.
C bit	N/A	Reserved (should be 0).
D bit	N/A	Reserved (should be 0).

Subscriber Loop Receive Signaling

On the receive signaling path, the A bit indicates whether or not the loop current is flowing. When the current flows on the line and the port is in the active state or the idle state, the A bit will be high. When the port is in the ringing state and ring-trip occurs, the A bit will go high.
Figure 24 shows receive signaling for subscriber loop line interfaces: Figure 24. Subscriber Loop Receive Signaling
This table summarizes the receive signaling for subscriber loop line interfaces: Bit Hex Bitmask Description A bit 0x08 Denotes loop current is flowing. This bit is used to determine when the far-end is off-hook. Value Description 0x08 Current is flowing (off-hook). 0 No current is flowing (on-hook). B bit 0x04 Same as A bit. C bit N/A Reserved (should be ignored). D bit N/A Reserved (should be ignored). Note: Off-hook conditions during ringing is detected in 50 to 150 ms.

Bit	Hex Bitmask	Description
A bit	`0x08`	Denotes loop current is flowing. This bit is used to determine when the far-end is off-hook. Value Description `0x08` Current is flowing (off-hook). `0` No current is flowing (on-hook).
B bit	`0x04`	Same as A bit.
C bit	N/A	Reserved (should be ignored).
D bit	N/A	Reserved (should be ignored).

DID Line Interface Signaling

There are no dependencies between DID transmit signaling and DID receive signaling.

DID Transmit Signaling

Figure 25 shows transmit signaling for DID line interfaces: Figure 25. DID Transmit Signaling
This table summarizes the transmit signaling for DID line interfaces: Bit Hex Bitmask Description A bit 0x08 Reverse battery polarity Value Description 0 Normal polarity, tip positive 0x08 Reverse polarity, tip negative B bit 0x04 Must be 0. C bit N/A Reserved. D bit N/A Reserved.

Bit	Hex Bitmask	Description
A bit	`0x08`	Reverse battery polarity Value Description `0` Normal polarity, tip positive `0x08` Reverse polarity, tip negative
B bit	`0x04`	Must be 0.
C bit	N/A	Reserved.
D bit	N/A	Reserved.

DID Receive Signaling

Figure 26 shows receive signaling for DID line interfaces: Figure 26. DID Receive Signaling
This table summarizes the receive signaling for DID line interfaces: Bit Hex Bitmask Description A bit 0x08 Loop current detector Value Description 0 No current detected 0x08 Current detected B bit 0x04 Same as A bit. C bit N/A Reserved. D bit N/A Reserved.

Bit	Hex Bitmask	Description
A bit	`0x08`	Loop current detector Value Description `0` No current detected `0x08` Current detected
B bit	`0x04`	Same as A bit.
C bit	N/A	Reserved.
D bit	N/A	Reserved.

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