PSTN Signaling

Signaling

Ground Start – The station/PBX will ground both ring and tip to request a dial tone.

Loop Start – When a phone is on hook the loop is open, when taken off hook the station will close the loop to the exchange to request a dial tone. Typically used in home environments as this is susceptible to glare.

Glare –If an incoming call happens at the same time as an outgoing line is requested in a PBX environment, they can become connected causing confusion to the outgoing caller.

Supervisory signaling

On-hook – When the phone is on-hook, the connection between the tip and ring wires is broken and no electrical signal passes between them.

 Off-hook – When the phone is off-hook, the phone connects the tip and ring wires, completing the circuit and allowing electrical signal to pass.

Ringing – To cause an analogue phone to ring, the phone company sends an alternating current (AC). 

Informational signaling

It is a way of conveying some kind of information to you, while you are on phone.

Dial tone – Indicates the phone company is ready to receive digits.

Busy – Indicates the remote phone is already in use.

Ring back – Indicates the remote phone is currently ringing.

Congestion – Indicates the long-distance telephone network is not able to complete the call.

Reorder – Indicates the local telephone company is not able to complete the call.

Receiver off-hook – Indicates the local receiver has been off-hook for an extended period of time.

No such number – Indicates the dialed number is invalid.

Confirmation – Indicates the telephone company is attempting to complete the call. 

Address signaling

This is a way of signaling digits that you dial over an analog line. One of the initial forms of address signaling was Pulse. What it would do is that it would literally break and reconnect the electrical circuit between the phone and telephone company (Tip and Ring wire) in a certain rhythmic rhythm that is of 30 % break and 70 % connected (with rotary phones).  

That is one of the initial forms that are out there and still this is a consideration as not everybody around the world supports DTMF (Dual Tone Multi-frequency) dialing. 

There are some phone companies that still only allow pulse dialing and you will see when we will configure our Cisco router for VoIP that Cisco routers are capable of signaling using Pulse.

DTMF is newer form of dialing in which the digits are assigned certain electric frequency. It actually plays two frequencies at once (Dual factor). The combination of these frequencies tells the phone company what numbers we are dialing.