# LPS signaling

The analog loop start protocol (LPS) is implemented by the LPS0 TCP. The analog variations act much like a telephone terminal, connected to a local switch or PBX through a local loop. These variations run on AG 2000/C analog line interface boards.

The following tables describe analog loop start signaling. Two tables are necessary, because the protocol changes depending on the side that started the call.

This topic describes LPS signaling in the following cases:

- The switch presents the call to the terminal equipment

- The terminal equipment places the call

## Switch presenting calls to terminal equipment

The following table describes the case where the switch presents the call to the terminal equipment:

State | Outbound switch | Line | Inbound terminal
--- | --- | --- | ---
Idle |  | No loop current |
Ringing | Apply ringing voltage | Ringing voltage | (Telephone rings)
After the first ring, the network can send caller ID information on the line using modem tones. The LPS TCP can detect caller ID, following specifications for USA, France, and Japan. At this point, the incoming call is presented to the application, which can answer it or reject it. If the call is answered, the LPS TCP picks up the phone.
Answer - conversation state |  | Loop current | Off-hook
If the inbound side application rejects the call, the LPS TCP does not pick up the phone, and eventually the calling party abandons the call. While the call is in conversation state, if it is connected to a PBX that supports this feature, the call can be transferred to a different extension. Different PBXs support transferring calls in different ways (the usual way is to send a flash, then dial the required extension). A parameterized sequence of actions is executed by the LPS TCP to transfer the call.
Clear |  | Loop current interruption or cleardown tone |
Idle |  | No loop current |

## Terminal equipment placing calls

The following table describes the case where the terminal equipment places the call:

State | Outbound terminal | Line | Inbound switch
--- | --- | --- | ---
Idle |  | No loop current |
Seizure | Off-hook | Loop current |
Seizure acknowledge |  | Dial tone |
The outbound side starts to send the address information through DTMF tones or by decadic pulses. If the method is decadic pulses, the loop current is switched on (pulse on) and off (pulse off) repeatedly to signal the digits.
Call progress tones |  | Ring tone |
If the called terminal rejects the call, the LPS TCP detects the busy tone on the line, and abandons the call. If the called terminal does not answer, the TCP abandons the call after a parameterized number of rings.
Answer - conversation state |  | Voice | (Someone has answered)
While the call is in conversation state, if it is connected to a PBX that supports this feature, the call can be transferred to a different extension. Different PBXs support transferring calls in different ways. The usual way is to send a flash, then dial the required extension. A parameterized sequence of actions is executed by the LPS TCP to transfer the call.
Clear |  | Loop current interruption, or cleardown tone |
Idle |  | No loop current |