Start Signaling and Options

When a caller lifts the phone to make a call, they expect a dial tone to be there. Although the caller may not be aware of it, there's a process going on "behind the scenes"; the PBX notifies the Central Office (CO) that a trunk is needed.  The start method used and the start options are usually combined in the terminology.  For example one might say that ACME corporation is using "Loop/Wink start".

Start signaling is the name of this process, and it's one of the features of a PBX trunk line. The PBX sends a start signal to the CO, and the CO responds by sending a dial tone to the PBX.  There are two types of start signals, ground start and loop start.

Start Signaling

Loop start is shown below.  When the receiver is lifted (Off-Hook condition), the tip and ring wires are connected.  This completes the eletrical "loop", and the PBX recognizes this condition, sends a dial tone to the receiver, and waits for the user to dial the phone, sending DTMF (Dual-Tone Multi-Frequency) signals.

Ground Start is shown below.  When a user lifts the station set handset to make a call, the PBX closes the circuit and electricity from the battery flows through ring to ground.

Start Options

Loop and Ground start tell the PBX that the receiver is off-hook and ready to lace a call.  The PBX and the CO then mst communicate that a call is to be started, using one of the following options :

• Wink start - the PBX sends the CO an off-hook signal for a brief period, usually 140 milliseconds. After the start signal is sent, the CO sends digit pulses to the PBX

• Immediate start - the PBX sends the start signal, then the CO switch immediately sends digit pulses to the PBX

• Delay start - the PBX sends the start signal. The CO switch then waits 250 milliseconds before sending digit pulses to the PBX.