POTS Overview

Scope of POTS

Plain Old Telephone Service is an analog service delivered through the Public Switched Telephone Network. It provides bidirectional speech, call access, subscriber alerting, and exchange-fed power. A subscriber receives all of these functions through one two-wire line.

The speech path normally limits the useful spectrum to approximately$300\ \text{Hz}$ through $3400\ \text{Hz}$. This range reduces channel bandwidth while preserving speech intelligibility. It does not reproduce the full audio spectrum.

Local subscriber loop

Thelocal subscriber loop is usually a twisted copper pair. The same conductors carry DC feed, speech in both directions, and the alternating voltage used to ring the telephone.

Two-wire transmission uses a common path for sending and receiving. A hybrid circuit in the exchange separates this path into the four-wire directions used farther inside the network. Impedance mismatch between the hybrid and the line produces sidetone or echo.

Line power and telephone-state detection

An analog telephone can receive power from the exchange without a local supply. Many systems use a nominal value near $48\ \text{V DC}$. The voltage measured at the telephone depends on the line card, loop length, and telephone state.

• In the on-hook state, the DC circuit is open and the exchange treats the line as idle.
• Lifting the handset closes the circuit and causes loop current to flow. The off-hook state requests service or indicates that a subscriber has answered.
• Ringing uses an alternating voltage at a frequency below the speech band. Voltage and cadence depend on the national signaling plan and equipment.

Exchange line-card functions

The acronym BORSCHT groups the principal duties of an analog line card. These are interface functions rather than stages of call establishment.

• Battery feed supplies the loop.
• Overvoltage protection limits surge damage.
• Ringing applies the alerting signal.
• Supervision detects current and line-state changes.
• Codec performs analog-to-digital and digital-to-analog conversion.
• Hybrid separates transmission directions.
• Testing supports line measurement and fault location.

Parameters affecting reach and quality

Loop resistance increases with conductor length and decreases as wire diameter increases. Excess resistance limits off-hook current and can prevent reliable state detection. Distributed capacitance raises high-frequency attenuation, while pair imbalance reduces rejection of common-mode interference.

Insulation resistance, open or shorted conductors, ground contact, crosstalk, and foreign voltages also affect service. Line assessment requires several measurements. One resistance threshold cannot represent every telephone, line card, conductor gauge, and national network.

Conversion in a digital exchange

An analog subscriber loop does not require the complete call path to remain analog. The line-card codec filters the signal, samples it at$8\ \text{kHz}$, and encodes each sample according to the selected companding law. European PCM systems normally use A-law.

The resulting $64\ \text{kbit/s}$ channel can be switched and multiplexed in the digital network. A codec at the receiving side reconstructs an analog waveform for the other subscriber loop.

Local-loop diagnostics

Measurements are made between the conductors and from each conductor to ground. This separates cable faults from a defective telephone or line port.

Limitations and remaining applications

A conventional switched path remains reserved for the duration of a call. This gives predictable delay but consumes resources during silence. Narrow bandwidth limits audio quality and modem data rates.

Exchange-fed power remains useful during a local power failure when the operator network has backup power. Service request, dialing, alerting, and release are signaling functions and are covered in the analog signaling article.

Signaling within the POTS service

POTS requires telephone-state detection, digit collection, and call-progress information. The following diagram organizes the relationship among the telephone, local loop, and exchange. It does not define the timers of one particular switching system.

A number can be conveyed through loop-current pulses or DTMF tone pairs. The frequency matrix identifies a digit using one low-group tone and one high-group tone.

These diagrams identify functions included in POTS. On-hook and off-hook detection, DTMF frequencies, and call release are developed in the separate analog signaling article.