# MELCAS signaling

The MEL TCP implements the Mercury exchange line (MEL) channel associated signaling (CAS) protocol. The protocol can be run on a PCM30 E1 trunk, or on a T1 trunk in ESF mode.

The following table describes the signaling states of a typical inbound call originating from the PBX. ABCD signaling bits are shown:

State | Network | Direction | Terminal
--- | --- | --- | ---
Idle | 1101 |  | 1101
Ring on | 1011 |  | 1101
Ring off | 1001 |  | 1101
| Optional FSK caller ID |  |
At this point an incoming call is presented to the application. The application can answer or reject it. If the application rejects the call, the inbound terminal does not respond to the ring signal. The calling party abandons the call with the signaling bits for the outbound side returning to the idle state.
Idle | 1101 |  | 1101
If the application answers the call, the line code changes to the call established state line code.
Answer | 1001 |  | 0101
Call established | 0101 |  | 0101

The following table describes the signaling states of a typical outbound call originating from the terminal (application):

State | Network | Direction | Terminal
--- | --- | --- | ---
Idle | 1101 |  | 1101
Seizure | 1101 Dial tone Call progress tone |  | 0101 DTMF address digits
Answer | 0101 |  | 0101

The following table describes the disconnect of established calls, regardless of which side originated the call.

State | Network | Direction | Terminal
--- | --- | --- | ---
The application disconnects the established call first:
Call established | 0101 |  | 0101
Clear back | 0101 |  | 1101
Disconnect clear | 0001 1101 |  | 1101
Idle | 1101 |  | 1101
The network disconnects the established call first:
Call established | 0101 |  | 0101
Disconnect clear | 0001 1101 |  | 0101
Clear back | 1101 |  | 1101
Idle | 1101 |  | 1101
Out of service blocking is also supported.
Network blocked | 1111 |  | 1101
Terminal blocked | 1101 |  | 1111