What is POTS? The Foundation of Telephony

POTS (Plain Old Telephone Service) is the traditional, basic telephone service enabling analog voice transmission over switched telephone connections. It represents the original wired telephone system that preceded digital telephony and the Internet as we know them today. Despite technological changes, many POTS operating principles still find application in modern telecommunications systems.

The voice signal for POTS operates within the VF (Voice Frequency) band, typically from 300 Hz to 3400 Hz. This limited band is sufficient for speech intelligibility but restricts high-quality audio or data transmission.

Analog Subscriber Line (Local Loop)

The subscriber line, often called the local loop, is the critical connection between the subscriber's telephone and the telephone exchange. This line is typically a two-wire copper pair that carries signals in both directions: from subscriber to exchange and vice versa.

Electrical Characteristics of the Subscriber Line

Proper functioning of the analog telephone system depends on several electrical parameters of the subscriber line, which are constantly monitored and managed by the telephone exchange.

• Ringing Voltage: The calling system (ringing) typically operates with AC voltage from 40V to 90V at 25 Hz or 50 Hz frequency, sent from the exchange to the subscriber's telephone. Standard ringing voltage is approximately 80V (RMS) (according to ITU-T recommendations: 15-25 Hz, 90-150V).
• DC Supply Current: The subscriber's telephone is powered by DC current (typically around 50V) from the exchange, which powers the microphone and internal electronics. In Poland this is 48V, while in USA/Russia it's 60V/100V. The DC voltage is usually negative relative to ground to minimize corrosion of copper cables.
• Calling Insensitivity Threshold: The calling system (ringing) typically has an insensitivity range from 0V to 16V. This means that small voltage fluctuations do not trigger false signals.
• Loop Resistance: The total DC resistance of the subscriber loop (sum of both wire resistances) must not exceed a certain threshold (e.g., typically 600 Ω) for reliable operation. Too high loop resistance can prevent the exchange from detecting the "off-hook" state, making connection establishment impossible.

Analog Signaling: Call Establishment Process

Signaling is the exchange of control information between telecommunications network elements to establish, maintain, and disconnect connections. In analog telephony, these signals are typically electrical currents or tones.

Call Establishment Phases

1. Preselection (Idle State): The subscriber's telephone is initially in on-hook state, so no current flows through the subscriber loop.
2. Service Request (Lifting Handset): Lifting the handset switches the telephone to off-hook state, closing the circuit. The exchange detects the flowing DC current.
3. Exchange Ready Signal: The exchange responds with a ready signal (typically 400-450 Hz tone) to confirm readiness for number dialing.
4. Address Signaling (Number Dialing): The subscriber dials the desired number. In older systems, this was done through pulse dialing (interrupting the loop circuit). Modern systems use DTMF (Dual Tone Multi-Frequency) tones (two specific tones sent simultaneously for each digit). The exchange decodes these digits to route the call.
   
5. Call Progress Signals: As the call progresses, the exchange sends various signals to the calling subscriber:
   • Dial waiting tone (ringing tone): 400-450 Hz signal indicating waiting for the called party's answer (e.g., after long-distance connection, a routing signal may be sent – 50ms tone, 50ms silence).
   • Busy tone: 400-450 Hz tone when the called party's line is busy (fast interrupted rhythm).
   • Congestion tone: 400-450 Hz tone when network resources are unavailable (even faster interrupted rhythm).
6. Ringing to Subscriber: The exchange sends high-voltage AC current (e.g., 80V at 15-25 Hz) to activate the bell in the called party's telephone. Simultaneously, a ringback signal (400-450 Hz, interrupted rhythm) may be sent to the calling party. ITU-T recommends that ringing current be sent for 0.67 to 1.5s, followed by a pause of 3 to 5s.
7. Connection: When the called party lifts the handset (goes to off-hook state), the exchange stops the ringing current and establishes a dedicated voice path through the switching matrix.
8. Conversation: Analog voice signals are now transmitted between the two subscribers.

Call Disconnection (On-Hook)

The disconnection process begins when either subscriber hangs up the handset.

• Initiation: When a subscriber hangs up the handset (goes to on-hook state), the exchange detects the absence of DC current in the subscriber loop.
• Release Signal: The exchange sends a release signal through the network to free reserved resources. The exchange monitors the connection and waits about 90 seconds for the other subscriber to hang up. If the second subscriber doesn't hang up within this time, a busy signal may be sent or the connection may be forcibly disconnected.
• Call Termination: After both subscribers hang up, the connection is completely disconnected and all network resources associated with it are released.

Signaling Functions and POTS Problems

Signaling in telephone networks serves three main functions:

• Supervisory (line): Detects and implements line state changes (e.g., calling, exchange ready, disconnection).
• Selective (address): Transmits address information (numbers) and additional signals (e.g., request for next digit, acknowledgment).
• Management: Transmits information about equipment, billing, alarms, and network blocks.

Critical Problems of Analog Subscriber Lines

Despite their simplicity and ubiquity, analog telephone lines have several limitations:

• Poor transmission medium utilization: The link is occupied for the entire call duration, regardless of whether information is actually being transmitted, leading to low efficiency.
• High operating costs: Infrastructure maintenance and resource allocation are expensive.
• Low signaling efficiency: Long call setup and disconnection times (up to several seconds), limited number of available signaling signals, hindering introduction of new services.
• Transmission faults: Susceptibility to damage such as breaks, short circuits, insulation breakdowns. Low insulation capacitance ($< 0.54 \text{ µF}$ between wire and shield) or too high line capacitance ($> 6 \text{ µF}$) deteriorate audibility. Capacitance difference between A-Z and B-Z wires (capacitance asymmetry) $> 5 \%$ worsens quality.
• Ground influence: Poor line-to-ground insulation (e.g., resistance $0 \Omega \text{ to } 50 \text{ k}\Omega$ for short circuit, $100 \text{ k}\Omega \text{ to } 4 \text{ M}\Omega$ for breakdown) leads to faults.
• Crosstalk and interference: Analog lines are susceptible to interference from foreign voltages (DC/AC $> 5.0 \text{ V}$), causing crosstalk and reducing signal-to-noise ratio.