Analog Transmission: Using some property of the transmission media to convey a signal. Examples:
- Phonograph
- Record players
- Braille
- Typical home telephone lines
Analog phone lines use the properties of electricity for voice transmission. As you speak into an analog phone, your voice is converted into electricity. The properties of the electricity are used to convey the properties of your voice.
ANALOG SIGNALING: LOOP AND GROUND START
Phone lines run in pairs. Tip and Ring. The ring wire is typically connected to the negative side of the battery/(electric charge) and when the receiver is on-hook, the circuit is broken. Lifting the handset completes the circuit and dial tone is sent from the Central Office (exchange) to the handset. This is known as Loop Start.
Loop Start is fine for domestic environments but not scalable. In the event of a PBX, it can lead to GLARE – someone picks up a line just as call comes in and the calls are bridged. Remedied by Ground Start – the phone sends a ground signal burst down one of the wires (usually the ring wire) which grounds out the line so the CO knows a line is required.
ANALOG SIGNALING: SUPERVISORY SIGNAL
Used to send the following signals over an analog line:
- On-hook – drop the receiver
- Off-hook – lift the receiver
- Ringing – use AC current rather than DC. DC current has to through the phone and back to the Exchange to complete a voice circuit. AC (live current) trips a special transistor that can be tripped to make the phone ring without having to loop back to the exchange.
ANALOG SIGNALING: INFORMATION SIGNAL
Used to send the following signals over an analog line:
- Dial Tone
- Busy
- Ringback
- Congestion
- Reorder
- Receiver off hook
- No such number
- Confirmation
These tones are generated by the phone company sending a specific electrical frequency down the phone to convey the information.
ANALOG SIGNALING: ADDRESS SIGNAL
Used to send the dialing information over an analog line:
- Pulse – breaks the circuit to the phone company on a set frequency – 30% broken, 70% connected.
- Dual Tone Multi Frequency (DTMF) – tone dial.
