GDS signaling

Ground start protocols exist in both analog and digital variations. The Dialogic 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:

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)