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. |
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Idle |
1101 |
|
1101 |
If the application answers the call, the line code changes to the call established state line code. |
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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: |
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Seizure rejected |
0011 |
|
0101 |
If the network can place the outbound call: |
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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: |
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Congestion/reorder |
0101 |
|
0101 |
Busy |
0101 |
|
0101 |
Otherwise, the call proceeds. If the called party answers, an answer indication may be given: |
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Answer |
0101 |
|
0101 |
On an established call, the application may request a register recall in order to transfer a call: |
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Call established |
0101 |
|
0101 |
Register recall |
0101 |
|
0101 0001 0101 |
If the network can place the outbound call, |
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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: |
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Call established |
0101 |
|
0101 |
Disconnect clear |
0101 1101 0101 |
|
0101
|
On hook |
0101 |
|
1101 |
Return to idle |
1101 |
|
1101 |