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