# STA signaling

The operator workstation protocol implements the PBX side of a loop start connection. The protocol talks with analog phones over a local loop link. It is implemented by the STA0 TCP.

This protocol requires an analog board to operate. The board must be capable of providing loop current and ring voltage to a passive terminal. AG 2000 boards have configurations that support this capability.

For information about transmitting an FSK caller ID sequence between the ring cycle while running the STA protocol, see Caller ID generation with the STA protocol.

The following tables describe operator workstation signaling. Two tables are necessary because the protocol changes depending on the side that starts the call.

## AG 2000 presents the call to the terminal equipment

The following table describes the case where the AG 2000 (for instance using the STA0 TCP) presents the call to the terminal equipment:

State | Outbound AG 2000 | Line/Direction | Inbound terminal
--- | --- | --- | ---
Idle | N/A | No loop current |
Ringing | Apply ringing voltage | Ringing voltage | (Telephone rings)
At this point, the call can be answered. If it is answered, the phone is picked up, and loop current flows in the circuit. If the call is not answered after a certain number of rings, the STA0 TCP abandons the call. The protocol does not support connect on proceeding because the outbound protocol must apply a ring signal to the inbound side. Connecting on proceeding would bypass this state and, therefore, the inbound side would not ring.
Answer - conversation state |  | Loop current, voice | Off-hook
AG 2000 clears first (optional) |  | Loop current interruption, or cleardown tone |
Clear |  | Loop current interruption | On-hook
Idle |  | No loop current |

## Terminal equipment places a call to the AG 2000

The following table describes the case where the terminal equipment places a call to the AG 2000:

State | Outbound terminal | Line/Direction | Inbound AG 2000
--- | --- | --- | ---
Idle |  | No loop current |
Seizure | Off-hook | Loop current |
Seizure acknowledge |  | Dial tone |
The outbound side starts to send the address information using DTMF tones or decadic pulses. If the method is decadic pulses, the loop current goes on (pulse on) and off (pulse off) repeatedly to signal the digits. When the address information has been completely received, the STA0 TCP presents the call to the application. The application must then decide if the call is to be accepted or rejected. If the call is accepted, the STA0 TCP plays a ring tone on the line.
Call progress tones |  | Ring tone |
If the call is rejected, the STA0 TCP plays a busy tone on the line. The terminal is expected to abandon the call and turn off loop current.
Answer - conversation state |  | Voice |
AG 2000 clears first (optional) |  | Loop current interruption, or cleardown tone |
Clear | On-hook | Loop current interruption |
Idle |  | No loop current |

## Loop start protocol on digital CAS trunks

The STA0 protocol can also be used to implement the network side of loop start protocol on digital CAS trunks. These can be provisioned on boards such as the AG 4040 and CG 6000 series boards.

For an example that shows the signaling bits for the loop start protocol on digital CAS trunks, refer to OPS signaling.

To use the STA0 protocol on a digital trunk, you must set the linetype and CDsignalbits parameters appropriately.