# GDS signaling

Ground start protocols exist in both analog and digital variations. The NMS ground start 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 were using GST8 or GST9 protocols, you should migrate to the digital ground start (GDS) protocol.

Although E1 channel associated signaling (CAS) framing supports four signaling bits per direction, only two of them are used for digital ground start line signaling. Thus the signaling channels supporting the digital ground start 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 ground start 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.

This topic provides information about GDS signaling in the following cases:

- The switch presents the call to the terminal equipment

- The terminal equipment places the call

## Switch presenting calls to terminal equipment

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

State | Outbound switch AfBf | Direction | Inbound terminal AbBb
--- | --- | --- | ---
Idle | 11 (FX) 01 (SA) |  | 01 (FX) 00 (SA)
Seizure | 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, after the specified number of rings the connected code is put on the line.
Ring on | 00 (FX) 10 (SA) |  | 01 (FX) 00 (SA)
Ring off | 01 (FX) 11 (SA) |  | 01 (FX) 00 (SA)
Answer - conversation state | 01 (FX) 11 (SA) |  | 11 (FX) 10 (SA)
If the inbound side application rejects the call instead, the TCP does not pick up the phone, and eventually the calling party abandons the call.
In conversation state, 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.
Inbound disconnects first | 01 (FX) 11 (SA) |  | 01 (FX) 00 (SA)
Outbound disconnects | 11 (FX) 01 (SA) |  | 11 or 01 (FX) 10 or 00 (SA)
Idle | 11 (FX) 01 (SA) |  | 01 (FX) 00 (SA)

## Terminal equipment placing calls

The following table describes the case in which the terminal equipment places calls:

State | Outbound terminal AfBf | Direction | Inbound switch AbBb
--- | --- | --- | ---
Idle | 01 (FX) 00 (SA) |  | 11 (FX) 01 (SA)
Seizure | 00 (FX) 01 (SA) |  | 11 (FX) 01 (SA)
Seizure acknowledged | 00 (FX) 01 (SA) |  | 01 (FX) 11 (SA)
Off hook | 11 (FX) 10 (SA) |  | 01 (FX) 11 (SA)
Proceed to send | 11 (FX) 10 (SA) | dial tone | 01 (FX) 11 (SA)
Here the outbound side starts to send the address information. This can be done by means of DTMF tones, or by decadic pulses. If the method is decadic pulses, the A-bit is switched off (pulse on) and on (pulse off) repeatedly to signal the digits. If the call is accepted, the network (or the PBX) plays ring on the line; otherwise it plays an appropriate call progress tone.
Answer - conversation state | 11 (FX) 10 (SA) | voice | 01 (FX) 11 (SA)
Inbound disconnects first | 11 (FX) 10 (SA) |  | 11 (FX) 01 (SA)
Outbound disconnects | 01 (FX) 00 (SA) |  | 11 or 01 (FX) 01 or 11 (SA)
Idle | 01 (FX) 00 (SA) |  | 11 (FX) 01 (SA)