How Many Volts Is A Residential Phone Line

Evidently Ordinary Telephone Service ( POTS )

In the manufacture, basic telephone service is known every bit Evidently Ordinary Telephone Service ( POTS ), and as well sometimes referred to as individual residence line service or dial tone.

Figure 1: POTS service scheme

Equally shown in ﬁgure one to a higher place, this service comprises a rotary dial telephone that is continued to a line card in the phone switch via a copper loop with 2 wires.

A telephone in POTS systems consists of the following main components:

Microphone
Speaker
Hybrid converter
Hook switch
Dial switch
Ringer
Protection

Tip and Ring

The two wires of the loop are sometimes still known every bit the tip and band. These names come from the ﬁrst phone switches that consisted of boards with female jacks. Trunks and loops were continued to these boards.

Operators had to plug ane end of a patch string into the jack for the body and the other end into the jack for the loop to connect a trunk to a loop.

The patch string connectors were designed in such as way that ane of the wires was attached to the metal ring below the tip, while the other was attached to the metal tip of a male person connector.

The two wires that made upwardly the loop were continued when the connector was plugged into the jack.

Twisted Pair

When ii-wire circuits are continued, they deed like loop antennas. The quantity of energy picked upward past a loop antenna is proportional to its area, and with a telephone, a loop with a diameter of 3 miles (5 km) is used!

The wires are covered in plastic and then twisted together to minimize the corporeality of noise picked up.

Read more near twisted pairs, in a series of articles here.

Although the wires still human activity similar ane large current loop with plastic covering from an electrical viewpoint, from an antenna point of view they deed as a series of small loops. The area of the small loops is less than the big loop, thus minimizing the antenna result of the wires.

Did you know? Y'all can extend POTS telephones a fashion much father than 3 miles (5 km) – to 20 km, or even 120 km abroad using Advertizement-net's Cobweb PCM multiplexers.

And then, what basically happens here is that you start catechumen analogue betoken to optical calorie-free, and then ship it to cobweb using fiber multiplexers. Scheme will usually and so await similar this:

Extending POTS lines beyond copper distance limitations

Figure 2: Extending POTS lines beyond copper distance limitations using our fiber muxes.

As ii wires are twisted together, they are known every bit twisted pairs. Twisted pairs is used for almost all cabling, including data cabling, inside wiring and telephone wires on poles. LAN cables uses 4 twisted pairs.

Effigy three: Screened twisted-pair (ScTP)

Line Carte

A twisted-pair loop is terminated on a line card on the network side. A line carte du jour consists of a minor ﬁberglass lath that is populated with components, including connectors and integrated circuits. The line card is plugged into a slot in a drawer that goes in a shelf in a rack. All of this is role of a traditional phone switch or PABX.

In newer systems, the line card is ofttimes part of a gateway that converts between POTS and Vocalisation over IP (VOIP).

The line bill of fare has a number of functions, and is sometimes referred to by the acronym BORSCHT: battery, overvoltage protection, ringing, supervision, codec, hybrid and testing.

Microphone and Speaker

A microphone is a transducer that creates a voltage from sound pressure waves. The value of this voltage is a representation of the forcefulness of the sound pressure waves coming from a speaker's throat.

This voltage is carried from the telephone over the loop to the line card at the near finish. Information technology is and so digitized by the codec and transported by the telephone network, where it is reproduced by the far-cease line card and then sent to the far-end telephone past the far-end loop.

A speaker works to a microphone as it uses voltage received to create audio pressure waves that are sent to the user's ear.

Figure 4: POTS service provision diagram illustrating rotary dial, hook switch, protection, ringer and tip & band wires.

Counterbalanced Signaling

Voltage is always measured as the difference of voltages between 2 objects. One object is often the earth, while the other is a wire, so the voltage measured is with respect to footing. With a phone loop, this is not the case equally the voltage is measured between the 2 wires of the loop, and not betwixt the wires and world.

In this case, counterbalanced signaling is used. This means that if the voltage on one wire with respect to earth is positive, the voltage on the other wire with respect to world will be the same value, but negative.

Since noise added will be the same on the 2 wires, the signal is doubled and the dissonance is canceled when the voltage between the ii wires is measured at the receiver.

Ii-Way Simultaneous

The loop's two wires are used to transmit data in both directions simultaneously. Both the line carte and the telephone create voltage analogs of audio across the two wires of the loop. The voltages from the devices at each finish are therefore added.

Hybrid Transformer

A device inside the telephone called the hybrid separates the voltage for each direction. The hybrid has 2 circuits on ane side and the 2-wire loop on the other side. There is one excursion for the speaker and some other for the microphone. A similar organisation can be found on the line card, connecting the loop to the receive and transmit pins of the codec.

Battery

Apart from the voltage analog of audio, which might be seen as an AC (or varying) betoken, the line card also places a DC (or steady) voltage beyond the 2 loop wires.

In industry, this voltage is called the battery and it is used to power the telephone. It is nominally -48 volts, measured from ring to tip.

Lightning Protection

The diagram above likewise shows a protection circuit across the loop. This is to protect the phone user from being electrocuted by either a high-voltage electrical transmission wire touching the loop, or lightning striking the loop.

Iii levels of protection are in fact built into the organisation:

A fuse on the POTS line card will blow if as well much current passes through it;
Circuitry on the demarcation point where the telephone company'south wires connect to the customer's wires will connect to ground if the voltage is besides high;
If the voltage across the loop is too loftier, a circuit inside the telephone will brusk-circuit the loop;

Supervision

The ringer and hook switch in the telephone are used for supervision.

Supervision means y'all must indicate to the other end of your loop that you want to start using the organization, irrespective of whom you wish to speak to, or what you are going to say to them.

The hook switch in the telephone is ordinarily open, disconnecting the 2 wires that brand upwardly the loop, and no current or electricity ﬂows in the loop.

The user must pick up the handset (goes off-hook) to initiate communications.

This closes the hook switch and connects the 2 wires, which allows the line voltage to push electric current effectually the loop.

This type of supervision is called loop start signaling as the two wires are continued to form a loop and allows current to ﬂow.

The current is detected by the line card on the telephone switch and it acknowledges with a dial tone.

Variations on this theme are used in other applications such every bit POTS PBX switches. These include:

Flash offset signaling where line voltage are reversed for a short interval, so returned to normal
Reverse bombardment signaling where negative and positive line voltages are reversed
Footing kickoff signaling where ane of the wires is continued ground and then the current ﬂows along one wire and and then back through the ground

For supervision in the opposite management, the switch indicates it wants to initiate communications past using the line card to put a ringing bespeak on the loop.

This is a different voltage that varies 20 times per second. It is applied to the line in a repeating wheel of 2 seconds on and and then iv seconds off.

When a POTS phone rings, it is on-claw with the hook-switch open. The ringing point current ﬂows through the ringer every bit indicated in ﬁgure beneath.

Original ringers consisted of two brass bells with a clapper between them that would oscillate xx times per 2nd for 2 seconds then residual for four seconds. The user accepts a call past going off-hook.

off-hook POTS

Figure five: Off-hook on POTS scheme

The line voltages are nominally equally follows:

Off-Hook: -vii to -12 Volts DC.
Ringing: -48 Volts DC, plus 100 Volts RMS @ 20 Hz
On Hook: -48 Volts DC

Call Progress Tones

A dial tone is i blazon of call progress tone. Others include busy, fast busy, congestion, band-back, howler and sounder tones. The telephone call progress tones are generated by the switch to inform the user of unlike atmospheric condition. Busy signals are generated by the far-end switch, while some tones, such as punch and fast busy tones are generated by the near-end switch.