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The PSTN: How the Public Phone Network Works

The PSTN: How the Public Phone Network Works

The Public Switched Telephone Network (PSTN) is the global interconnected network of telephone switches, transmission links, and signaling systems that has provided voice communications for over a century. While VoIP is steadily replacing it, the PSTN remains operational and serves as the foundation that modern voice networks build upon — and interconnect with.

Network Hierarchy

The PSTN is organized in a hierarchical switching structure:

Class 5 Switch (Local / End Office)

The Class 5 switch is the workhorse of the PSTN. It is the switch that directly serves subscribers — connecting their telephone lines to the rest of the network. Every landline phone number is ultimately served by a Class 5 switch in a central office (CO).

A Class 5 switch:

  • Provides dial tone to subscribers
  • Collects dialed digits
  • Performs digit analysis and routing decisions
  • Connects local calls (subscriber to subscriber within the same switch)
  • Routes calls to tandem switches or directly to other Class 5 switches
  • Generates billing records (CDRs)
  • Provides custom calling features (call waiting, caller ID, voicemail)

Each Class 5 switch serves a geographic area corresponding to one or more rate centers. The NXX codes assigned to a rate center identify the switch (or switches) serving that area.

Class 4 Switch (Tandem / Transit)

Class 4 switches (tandem switches) route calls between Class 5 switches and between carriers. They do not serve subscribers directly. Instead, they act as intermediaries — receiving calls from originating switches and forwarding them toward the destination.

Tandem switches are essential for scalability. Without them, every Class 5 switch would need a direct trunk to every other Class 5 switch — an impractical N-squared problem. Instead, switches connect to a tandem, and the tandem routes traffic between them.

Types of tandems:

  • Local tandem: Routes calls between Class 5 switches within a LATA
  • Access tandem: Routes calls between local switches and long-distance carriers
  • Inter-toll: Routes long-distance traffic between regions (largely replaced by IP transit)

Historical Hierarchy

The original AT&T network had a five-level hierarchy (Class 1 through Class 5), with Class 1 regional centers at the top. This rigid hierarchy was gradually flattened as dynamic routing and IP transit made the upper levels unnecessary. Today, the network is effectively two-level: end offices (Class 5) and tandems (Class 4), plus IP-based transit.

The Central Office

A central office (CO) is the physical building housing a Class 5 switch and its associated equipment. The term dates from the early telephone era when operators worked at a central location to connect calls.

A central office contains:

  • The switch: The digital (or legacy analog/electromechanical) switching equipment
  • Distribution frame: Where outside plant cables (the copper or fiber lines running to subscribers) terminate and connect to the switch
  • Power systems: Battery backup and generators for reliability (central offices are designed to operate through extended power outages)
  • Carrier equipment: Multiplexers, fiber terminals, and transmission equipment connecting to other offices

Central offices are identified by their CLLI code (Common Language Location Identifier) — an 11-character code that encodes the city, state, and building. CLLI codes appear in carrier routing data and are used industry-wide for identifying telecom locations.

The Access Network (Last Mile)

The access network connects subscribers to their serving central office. This is the “last mile” — and historically the most expensive and least upgradeable part of the network.

Copper (POTS)

Traditional Plain Old Telephone Service (POTS) runs over twisted copper pairs from the central office to the subscriber’s premises. This is the technology that has served telephone customers since the late 1800s. The copper pair carries both voice and signaling (ringing voltage, dial tone, DTMF digits) as analog electrical signals.

Range is limited to roughly 3-5 miles from the central office (depending on wire gauge and conditions). For subscribers beyond this range, remote terminals with digital loop carrier (DLC) equipment extend the reach.

Fiber

Modern deployments use fiber optic cables for the access network:

  • FTTH (Fiber to the Home): Fiber all the way to the subscriber. Supports voice, broadband, and video.
  • FTTC (Fiber to the Curb): Fiber to a neighborhood node, with copper for the final few hundred feet.
  • FTTN (Fiber to the Node): Fiber to a remote terminal, with DSL over existing copper for the last mile.

Voice over fiber-based access networks typically uses VoIP (the analog phone signal is converted to IP at the subscriber’s ONT — optical network terminal).

Signaling

PSTN calls are controlled by signaling protocols:

  • SS7 (Signaling System 7): The primary signaling protocol between switches. SS7 runs on a separate network and handles call setup/teardown, number portability lookups, and database queries.
  • ISDN (Q.931): Used for signaling on PRI trunks between PBXs and central offices.
  • Analog signaling: On POTS lines, signaling uses electrical states — on-hook/off-hook detection, dial tone, DTMF tones, ringing voltage.

The PSTN Today

The PSTN is shrinking but not gone:

  • Millions of copper POTS lines remain in service, particularly in rural areas
  • Wireless networks (which account for the majority of voice calls) interconnect with the PSTN for calls to wireline numbers
  • VoIP networks interconnect with the PSTN via media gateways that convert between IP and TDM
  • Carrier TDM infrastructure is being decommissioned in phases — AT&T, Verizon, and Lumen all have active TDM retirement programs

The NPA/NXX data on this site reflects both PSTN and IP-based assignments. Many NXX codes originally assigned to ILEC Class 5 switches now serve subscribers through IP-based platforms, even though the NXX assignment data still references the original rate center and LATA.

Further Reading

Colocation Facilities and Carrier HotelsVoIP Network Architecture