# EL7 signaling

The EL7 TCP implements the application computer variant of the EL7 protocol found on an Ericsson MD110 PBX. The protocol can run on a PCM30 E1 trunk or on a T1 trunk in ESF mode. The protocol is not symmetric, and only the terminal (application computer) side of the protocol is supported.

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
Seizure (internal) | 0110 |  | 1101
Seizure (external) | 0100 |  | 1101
Seizure (call back) | 0111 |  | 1101
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 | 01xx |  | 0101
Answer acknowledge | 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 |  | 0101
If the network cannot place the outbound call, it rejects it:
Seizure rejected | 0011 |  | 0101
If the network can place the outbound call:
Seizure acknowledge | 0101 Dial tone Call progress tones |  | 0101 DTMF address digits
The network may indicate a congestion/reorder or busy condition at this point:
Congestion/reorder | 0101 0100 0101 |  | 0101
Busy | 0101 0111 0101 |  | 0101
Otherwise, the call proceeds. If the called party answers, an answer indication may be given:
Answer | 0101 0001 0101 |  | 0101
On an established call, the application may request a register recall in order to transfer a call:
Call established | 0101 |  | 0101
Register recall | 0101 |  | 0101 0001 0101
If the network can place the outbound call,
Recall acknowledge | 0101 0111 0101 Dial tone |  | 0101
After which, the outdialing for the second call continues.

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
On hook | 0101 |  | 1101
Return to idle | 1101 |  | 1101
The network disconnects the established call first:
Call established | 0101 |  | 0101
Disconnect clear | 0101 1101 0101 |  | 0101
On hook | 0101 |  | 1101
Return to idle | 1101 |  | 1101