POTS Overview

Scope of POTS

Plain Old Telephone Service (POTS) is analog access over the Public Switched Telephone Network. At the exchange, the ITU-T Q.551 Z interface carries speech, voiceband analog data, and multifrequency push-button signals. It also provides DC feed, DC signaling, ringing, and metering where applicable. The subscriber side uses one two-wire line for these functions.

ITU-T Q.552 specifies voice-frequency interface characteristics over approximately $300\ \text{Hz}$ through $3400\ \text{Hz}$. This narrow band limits the channel bandwidth and preserves speech intelligibility. Full-band audio requires a wider passband.

Local subscriber loop

The local 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. ETSI TS 102 080 models a local line as connected cable sections that can have different conductor diameters, splices, and bridged taps.

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 source 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 a low-frequency alternating voltage. For European terminal detection, ETSI EN 300 001 specifies 32 to 75 V AC at 23 to 28 Hz with 0 to 85 V DC superimposed. National plans also use 25 Hz or 50 Hz variants, so voltage and cadence depend on the network.

Exchange line-card functions

BORSCHT is a mnemonic for the principal duties of an analog line card. ITU-T Q.551 describes the corresponding Z interface as a two-wire port for speech, voiceband data, multifrequency signals, DC feed, DC signaling, ringing, and metering where applicable.

• Battery feed supplies the loop.
• Overvoltage protection limits surge damage.
• Ringing applies the alerting signal.
• Supervision detects current and line-state changes.
• Coding covers analog-to-digital and digital-to-analog conversion in the codec.
• 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 8-bit sample with one of the companding laws defined by ITU-T G.711. 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.

Interpret the measurements against the requirements of the installed line card and the operator network. A universal resistance threshold ignores different feed voltages, detection thresholds, and conductor gauges.

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.