# OPS signaling

OPS protocols exist in both analog and digital variations. The NMS OPS TCP covers digital interfaces. The protocol can handle T1 or E1 digital trunks, of signaling types FX (foreign exchange) or SA (special access). The trunk and signaling type is determined using the trunktype parameter.

If you used LPS8 or LPS9 protocols in the past, you should now migrate to the off-premises station (OPS) protocol.

Although E1 channel associated signaling (CAS) framing supports four signaling bits per direction, only two of them are used for OPS line signaling. The signaling channels supporting the OPS line signaling protocol 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 other bits in either direction (the C and D bits) usually have fixed values. However, their values may change from network to network.

The following tables describe digital OPS signaling in the two cases of FX and SA. Two tables are necessary because the protocol changes depending on the side that started the call.

The following table describes the case in which the switch presents the call to the terminal equipment:

State | Outbound switch AfBf (STA) | Direction | Inbound terminal AbBb (OPS)
--- | --- | --- | ---
Idle | 01 (FX) 11 (SA) |  | 01 (FX) 00 (SA)
Ring on | 00 (FX) 10 (SA) |  | 01 (FX) 00 (SA)
Ring off | 01 (FX) 11 (SA) |  | 01 (FX) 00 (SA)
At this point, the incoming call is presented to the application. The application can answer or reject it. If the application answers it, the line code changes to the conversation state code.
Answer - conversation state | 01 (FX) 11 (SA) |  | 11 (FX) 10 (SA)
If the inbound side application rejects the call, the OPS TCP does not pick up the phone, and the calling party abandons the call.
If the switch side clears the call, a cleardown tone might be on the line. The terminal responds to this by hanging up the call.
Clear and idle | 01 (FX) 11 (SA) |  | 01 (FX) 00 (SA)

The following table illustrates the case in which the terminal equipment places the call:

State | Outbound terminal AfBf (OPS) | Direction | Inbound switch AbBb (STA)
--- | --- | --- | ---
Idle | 01 (FX) 00 (SA) |  | 01 (FX) 11 (SA)
Seizure | 11 (FX) 10 (SA) |  | 01 (FX) 11 (SA)
Seizure acknowledged | 11 (FX) 10 (SA) | Dial tone | 01 (FX) 11 (SA)
The outbound side starts to send the address information. This can be done using DTMF tones or decadic pulses. If the method is decadic pulses, the loop Af bit is switched off (pulse on) and on (pulse off) repeatedly to signal the digits.
Call progress tones | 11 (FX) 10 (SA) | Ring tone | 01 (FX) 11 (SA)
If the called switch rejects the call, the terminal detects the busy tone on the line and abandons the call. Or, if the called switch does not answer, the terminal abandons the call after a parameterized number of rings.
Answer - conversation state | 11 (FX) 10 (SA) | Voice | 01 (FX) 11 (SA)
If the switch side clears the call, a cleardown tone might be on the line. The terminal responds to this by hanging up the call.
Clear and idle | 01 (FX) 00 (SA) |  | 01 (FX) 11 (SA)