# FGD signaling

The FGD TCP implements the specifications of the feature group D (FGD) switched access service. This service provides interconnection to the BOC (Bell operating companies) network for the provision of message telecommunications service/wide area telecommunications service (MTS/WATS) and MTS/WATS-type services. FGD service, which provides access to the trunk side of suitably equipped BOC switching systems, is available for termination and originating access.

If you used the FDI protocol in the past, you should now migrate to the new feature group D (FGD) protocol.

Feature group D signaling is derived from wink start signaling. Like wink start, FGD uses only two of the four bits per signaling direction supported by E1 channel associated signaling (CAS) framing. The signaling channels supporting the FGD 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 inbound side uses multiple winks to acknowledge reception of different series of incoming digits. The following table describes the line signaling for a typical call:

State | Outbound AfBf | Direction | Inbound AbBb
--- | --- | --- | ---
Idle | 00 |  | 00
Seizure | 11 |  | 00
Seizure acknowledged | 11 |  | 00-11-00 (wink)
The outbound side starts to send the address information using MF tones. Feature group D can transfer more than one digit field to speed up long distance calls. Every field starts with a KP tone (start of pulsing) and ends with an ST tone (end of pulsing). After each digits field the inbound side acknowledges the reception with a signaling bit wink.
Register signaling first field: digit spill | MF tones |  | 00
Acknowledgment of first series of digit |  |  | 00-11-00 (wink)
Register signaling second field: digit spill | MF tones |  | 00
Once all the address information has been transferred, the inbound side accepts the call by sending the off-hook signaling code or rejects the call by playing busy. If the call is rejected, the outbound side switches back to signaling AB = 00 (idle), clearing the line.
Clear forward and idle | 00 |  | 00
If the call is accepted, the inbound side answers the call by flipping both backward bits to 1.
Answer - conversation state | 11 |  | 11
Depending on which side hangs up the call first, a clear back signal or a clear forward signal is generated. Idle follows.
Inbound hangs up first: Clear back | 11 |  | 00
Outbound hangs up first: Clear forward | 00 |  | 00 or 11
Idle | 00 |  | 00