# NEC PBX signaling

The NEC PBX TCP implements the specifications of the 30 DLI (digital line interface) using PA-30DTS package, as specified in Annex 303-15-B 2/2 from NEC, NEC Australia PTY, LTD.

The NEC PBX protocol is generally a single B-bit signaling protocol. The exception to this is the blocking signal, which uses both A and B bits. The protocol is asymmetric between the TE side (user or application side) and the NT side (network or PBX side). The signaling channels supporting the line signaling of these protocols are referred to as Af and Bf in the forward direction, and Ab and Bb in the backward direction. The forward channel indicates the condition of the outbound switch equipment and reflects the condition of the calling party's line. The backward channel indicates the condition of the called party's line (the inbound equipment). The C and D bits are never used. Their value is fixed at 0 and 1 respectively.

The following table describes the signaling states of a typical inbound call originating from the PBX:

State | Outbound PBX AfBf | Direction | Inbound terminal AbBb
--- | --- | --- | ---
Idle | 00 |  | 00
Ring on | 01 |  | 00
Ring off | 00 |  | 00
At this point an incoming call is presented to the application. The application can answer or reject it. If the application answers the call, the line code changes to the conversation state line code after the outbound side re-asserts ring on B signaling bit.
Ring on | 01 |  | 00
Answer - conversation state | 01 |  | 01
If the application rejects the call, the protocol does not respond to the ring signal. The calling party eventually abandons the call with the signaling bits for the outbound side remaining in the idle state. The application disconnects the call, as shown:
Clear back | 01 |  | 00
Idle | 00 |  | 00

The following table illustrates the signaling states of a typical outbound call placed to the connecting PBX:

State | Outbound terminal AfBf | Direction | Inbound PBX AbBb
--- | --- | --- | ---
Idle | 00 |  | 00
Seizure | 01 |  | 00
Seizure acknowledged | 01 | Dial tone | 00
Dialing | 01 | DTMF digits dialed | 00
At this point the remote switch either answers or rejects the call. If the call is answered, the answer signal AB = 01 is asserted. The switch may provide ring tone before answering the call. The outbound TCP does call progress detection at this phase.
Answer - conversation state | 01 |  | 01
The application disconnects the call, as shown:
Clear forward | 00 |  | 01
Idle | 00 |  | 00