What is the PSTN (Public Switched Telephone Network)?

The PSTN carries a phone call by building a temporary, dedicated circuit between two telephones and keeping that circuit open until someone hangs up.[2] When you lift a handset, your phone forms a connection over a pair of copper wires, the local loop, to the nearest telephone exchange, also called a central office in North American usage.[1] That exchange reads the number you dial and either connects you to another line it serves directly, or hands the call up to higher-level switches that route it across the country or the world.[1] The path your voice takes is reserved for you alone for the length of the call, which is the defining trait of circuit switching and the main way the PSTN differs from the packet networks behind the internet.[2]

The network is arranged as a hierarchy so that it does not need a direct wire between every pair of exchanges.[1] Local exchanges connect to each other and to larger tandem and toll switches through shared links called trunks, and a long distance call climbs that hierarchy only as far as it needs to before it comes back down to the destination exchange.[1] This structure keeps local traffic local and reserves the costly long-haul circuits for calls that actually cross regions.

A worldwide system, not one company

No single organization owns the PSTN. It is the combined result of many national, regional, and local carriers interconnecting their networks under shared technical standards, most of them set by the International Telecommunication Union, so that a call placed in one country can reach a phone in another.[1][5] Those standards cover how numbers are formatted, how calls are signaled between networks, and how voice is encoded once it becomes digital, which is what lets equipment from different vendors and different countries work together as a single global network.

Although the local line to a home is often still analog, the core of the PSTN has been digital since the 1960s, when carriers began converting voice into a stream of numbers rather than a continuous electrical wave.[1] The conversion uses pulse code modulation, sampling the sound of a voice 8,000 times a second and coding each sample as an 8 bit value, which produces a single voice channel of 64 kbit/s.[3][7] The ITU-T standard that describes this encoding, G.711, still defines the baseline sound quality most people associate with a normal phone call.[3]

That 64 kbit/s channel, known as a DS0, is the basic unit the network multiplexes onto shared circuits.[7] In North America a T1 line combines 24 of these channels into a 1.544 Mbit/s signal, while much of the rest of the world uses an E1 that carries 32 timeslots at 2.048 Mbit/s, of which 30 carry voice.[7] Higher up, carriers aggregate thousands of calls onto fiber using synchronous transport systems such as SONET and SDH.[1] The switching stayed circuit based through all of this, so even a fully digital call still holds a fixed 64 kbit/s slot from end to end rather than sharing capacity the way internet traffic does.

The local loop is the physical line that connects a single telephone to its local exchange, and for most of the PSTN's history it has been a pair of copper wires running from the premises to the nearest central office.[1] This segment is often called the last mile, and it is the part of the network a subscriber actually holds a connection to. One practical feature of the traditional copper loop is that it carries its own electrical power from the exchange, which is why an old wired phone usually kept working during a local power cut.[9] As carriers replace copper with fiber and IP, that self-powered property goes away, so the replacement services need their own backup power to keep emergency calls working, a point regulators have made central to the switch off.[9]

A local exchange, called a central office in North American usage, is the building and switching equipment that terminates the local loops in an area and connects calls between them.[1] When you dial a number, this switch reads the digits and either completes the call to another line it serves or passes it toward the correct destination.[2] In the traditional numbering scheme the exchange is identified by the prefix in the middle of a phone number, which is why lines in the same neighborhood often share those digits.[6] Engineers historically called this a Class 5 switch, the lowest tier of the hierarchy and the one closest to the subscriber.[1] Because both the calling and called lines depend on their local exchanges, a call can only be completed when the exchanges at both ends are in service.

Trunks are the shared, high-capacity links that connect exchanges to each other, and they let the PSTN avoid running a separate wire between every possible pair of phones.[1] Each local exchange connects to a limited number of trunks, so a call that cannot be completed locally travels over them to higher-level switches.[1] Those higher tiers, historically called tandem switches for regional traffic and toll or Class 4 switches for long distance, route a call across the hierarchy only as far as it needs to go before it comes back down to the destination exchange.[1] Because many calls share the same trunks, carriers size them for expected simultaneous use rather than for the total number of phones, the same capacity logic that later carried over to counting concurrent call paths on IP trunks.[2] This tiered design lets one network handle both a call between neighbors and a call between continents, with each using only as much of the hierarchy as its distance requires.

Signaling System No. 7, or SS7, is the separate control network the PSTN uses to set up, manage, and end calls, and it runs on its own channels rather than on the line that carries your voice.[4] Standardized by the ITU-T in its Q.700 series, SS7 does the behind-the-scenes work of a call: it passes the dialed number between switches, selects a route, rings the far end, and releases the circuit once the call is over.[4] Keeping this signaling out of band, on a dedicated data network, let calls set up faster and freed the voice path to carry only the conversation.[1] The same signaling network also made possible services callers now expect, such as caller ID and number portability, because switches can query shared databases before a call connects.[1]

Every telephone on the PSTN is reachable through a structured number, and the master framework for those numbers is ITU-T Recommendation E.164, the international public telecommunication numbering plan.[5] E.164 limits an international number to a maximum of 15 digits and begins it with a country code of one to three digits that identifies the destination country or global service.[5] Within that framework each region runs its own national plan. North America uses the North American Numbering Plan, a shared scheme covering the United States, Canada, and 18 other countries and territories under a single country code of 1, with local numbers written as a three digit area code, a three digit central office prefix, and a four digit line number.[6] A neutral administrator assigns these numbering resources so that no two lines collide and so that any switch can route a call using only the digits dialed.[6]

Yes. Telephone companies around the world are retiring the circuit-switched PSTN and moving voice onto internet protocol networks, because the older equipment is increasingly expensive to maintain and staff.[2][9] The clearest deadline is in the United Kingdom, where BT Group plans to retire its PSTN by 31 January 2027, after which landline calls run over broadband as Voice over IP.[8] Openreach, which operates the physical network most UK providers rely on, has been withdrawing the products that depend on the old network and has placed most exchanges under a stop sell that blocks new copper-based orders.[8] The UK government describes the analog network as having reached the end of its serviceable life and frames the migration as an industry-led program expected to be substantially complete by that date.[9]

What changes for a phone line

For most people the phone number and handset stay the same, and the call simply travels over a broadband connection instead of a dedicated copper circuit.[9] The difference regulators stress is power. Because a digital line does not draw power from the exchange the way copper did, providers in the UK are required to make sure customers can still reach emergency services during a power cut, which usually means offering a battery backup unit that supports at least an hour of calling.[9] In the United States the same shift is underway under the general heading of the technology transition, with carriers replacing copper and legacy time-division equipment with IP and fiber over a longer, less fixed timeline.[2]

The PSTN is the traditional telephone network, not the internet and not any single company's phone service, and a few common mix-ups are worth separating.

It is not VoIP

Voice over IP carries calls as data packets over the internet or a private IP network, while the PSTN reserves a dedicated circuit for each call.[2] The two connect through gateways, so a VoIP call can reach a PSTN number and the reverse, but they are different technologies, and the industry is now moving traffic from the first onto the second.[2]

It is not the mobile network

Cellular networks are their own radio-access systems, though they interconnect with the PSTN and share its numbering.[5] A mobile call may travel partly over the traditional network when it reaches a landline, but the wireless portion uses different technology and, on modern phones, often carries voice as IP data inside the carrier's network.

It is not just copper wires

People picture the PSTN as copper phone lines, and the local loop often is copper, but the core has long been digital fiber and microwave carrying millions of calls as 64 kbit/s channels.[1][7] Retiring the PSTN is really about replacing the circuit-switched method of connecting calls, not simply pulling out copper.

It is not the same as POTS

Plain old telephone service, or POTS, refers to the basic analog voice service delivered to a subscriber over the local loop, which is one service the PSTN provides rather than a synonym for the whole network.[1]

The PSTN's strengths and weaknesses both come from its circuit-switched design, and naming them explains why it lasted so long and why it is now being replaced.

Reliability and voice quality

Reserving a dedicated path for each call gives the PSTN very consistent voice quality and call setup, because the capacity for a conversation is guaranteed the moment the call connects rather than shared with other traffic.[2] The traditional copper line also carried its own power from the exchange, so it typically kept working during a local blackout, which is the property the digital replacements have to recreate with backup batteries.[9]

Efficiency and flexibility

Holding a full circuit open for every call, including the silences, uses network capacity less efficiently than packet switching, which sends data only when there is something to send.[2] The PSTN was also built for voice, so adding new features usually meant changing the switches themselves, a slow process compared with software-based IP systems.[1]

Cost of maintenance

The equipment at the heart of the network is decades old, and the pool of engineers who maintain it is shrinking, which is a large part of why carriers have decided that replacing it is cheaper than keeping it running.[9] For an organization still on the PSTN, the practical implication is that the timing of a migration is increasingly set by the carrier's retirement schedule rather than left entirely to the customer.[8]