How the Public Switched Telephone Network Works

“A long time ago in a galaxy far, far away…”

Oh wait, that’s not the case at all. It was Alexander Graham Bell, in the U.S., with a patent. While that may read more like winning recipe to the game Clue, it’s actually what happened in 1876 with Bell’s invention of the telephone. If you’ve ever wondered what the public switched telephone network (PSTN) is and how it works, then you’ve found the post you’re looking for.

Over the better part of the ensuing century following Bell’s patent the PSTN network evolved into a communications behemoth, where by the 1970s AT&T owned and operated virtually the entire Bell system spanning the U.S. and Canada. What follows details primarily the United States PSTN, however, the pieces and functions of the components are largely the same in other countries, although the actual configurations may vary.

Good ole Ma Bell’s monopoly on the PSTN in North America eventually led to a lawsuit from the U.S. government. This resulted in the breakup of the Bell network in 1984 into regional corporations in an effort to increase competition for long distance service.

As carriers entered the picture, they each carved out pieces of the PSTN pie for themselves. When thinking about how telephone technology works today it’s important to remember that significant variances exist between the capabilities and infrastructure of different carriers.

So what exactly is the PSTN?

Originally the PSTN was “designed to support only continuous, real-time voice communications.” The system didn’t set out to provide the backbone for our modern communications infrastructure. In fact, it was designed for an average call duration of three minutes or less, relying on limited bandwidth–a mere 64 Kbps over a twister-copper-pair wire. This analog system is commonly referred to as Plain Old Telephone Service (POTS).

Although there have been a lot of upgrades to bring the components of the PSTN into the digital realm, the most common access method for landline telephones remains this analog, copper wire connection. The cost for telco operators of replacing “the last mile” of copper wire with fiber optic cable or other higher bandwidth media is prohibitive, especially with the progress that has been made in wireless technology in recent years.

The decline of the landline has made headlines in recent years as people forego their home phones for mobile smartphones. In 2008, the number of homes without a landline stood at around 25% in the United States, but that number increased to 40% by 2014. That percentage is even higher among those living in urban areas or in their late 20s.

Voice communications continue to rely heavily on the PSTN.

On the surface, the drop in landlines may suggest a corresponding decline in the relevance of the PSTN. Actually, the opposite it true. Sure, the analog “last mile” component isn’t an issue with mobile phones, but aside from the method used to connect to PSTN exchanges, everything else about how calls are processed and routed remain the same. Mobile phones can’t function without the PSTN.

The modern PSTN still has plenty of copper wire in it, but it also includes fiber optic cables, cellular networks, communication satellites, and undersea cables. These transmission media have much more bandwidth available and can accommodate much more than just voice communications. Video, for example, is one type of media that requires much more bandwidth than the 64Kbps that twisted-pair connections provide. Unless, of course, you’re a big fan of waiting for Netflix to buffer.

So how does the PSTN work? Someone picks up the phone, dials, and…

The circuit-switched PSTN opens up a continuous connection between two phones, that begins with a dial tone and ends when the phone is hung up.

To start you can have an individual subscriber, or a group of subscribers, like a business that requires multiple access lines. Individual subscribers connect directly to the local exchange, while businesses often use a private branch exchange (PBX) to manager all their connections. So the call starts with the actual phone and either connects directly to the Local Exchange or to the PBX and then to the Local Exchange, if the call comes from a business with multiple lines.

From the local exchange network, depending on where the call is going, it is pushed to international carriers, interexchange carriers, cellular providers, or internet service providers.

The number of layers of technology a call passes through varies depending on where the call is destined. This is one of the reasons that telephony can be so confusing, the sheer number of variables involved in making a “simple” phone call.

System Structure

Taking a step back and really looking at the pieces that comprise the PSTN puzzle makes it abundantly clear just how far from “simple” telephony is.

The PSTN is comprised of a complex web of interconnection nodes and transmissions links. Different infrastructures exist at the local, regional, and national levels, but regardless of how each section is configured the pieces function in the same way. There are four different types of nodes: customer premises equipment (CPE), transmission, service, and switching. Transmission links constitute the physical wires or fiber, as may be the case nowadays, that interconnect the various nodes.

CPE

The CPE node is the equipment on site where the call originates. That could be an individual subscriber line or a PBX.

Transmission

The transmission node consists of the equipment and media that carry information between nodes of a network. This can include things like amplifiers, repeaters, multiplexers, digital cross-connect systems, and digital loop carriers.

Service

The service node is responsible for signaling. This means determining when to setup, hold, charge, and release connections, and getting that information to the correct outlets that maintain and bill for each section of the network.

Switching

The real meat of the PSTN are the components of the switching node. In a PSTN setup there are four different types of switches.

1. The Local Exchange has already been alluded to, and is the component of the network that physically connects subscribers (the CPE node) to the rest of the PSTN. This is where carriers terminate customer lines and keep the equipment that interconnects those lines. A single exchange traditionally had the capacity for 10,000 lines (0000 to 9999) and a local exchange consists of one or more of these exchanges.Imagine you’re heading out on a road trip but you don’t have a map handy. Now think of a phone call as the route to your destination, and the different components of the system as gas stations where you can stop and ask for directions.If you’re calling a neighbor across the street the call likely doesn’t need to leave the local exchange.Everyone in the U.S. should be familiar with seven digital phone numbers. The first three digits designate the local exchange and local switch where each number resides. So in the movies when someone starts a number with 555, that’s the local exchange designation. The last four digits identify the individual subscriber line within that exchange.Nowadays, with so many phone lines in operation, ten digit dialing, which includes the local area code, is oftentimes required as well. Prior to the introduction of all-number calling (ANC), which started to roll out in 1958 but wasn’t completed for several decades, phone numbers consisted of a combination of letters and numbers. The letters were derived from local exchange names. If you’ve ever wondered what The Marvelettes’ classic Motown song Beechwood 4-5789 refers to, there’s your answer.
2. The Tandem Office (or junction network) primarily serves a metropolitan area with many local exchanges and handles the switching between them. A city like New York has multiple local exchanges. So if you live in Queens and want to call someone in Brooklyn your call will most likely be switched through a junction network.
3. The Toll Office is the switch where national long-distance connections are made. For example, a call placed in Florida intended for a Washington number passes through the Toll Office switch.
4. Finally, the International Gateway (or centre de transit [CT]) is what connects calls that originate domestically to international telephone systems. The International Telecommunications Union coordinates global communications standards that ensure compatibility between systems.

This is the structure of the PSTN, broadly speaking, in the United States. As mentioned above, other countries may configure their systems differently (like those using eight-digit phone numbers).

Call Routing

The ownership relationship between all of these nodes and transmission lines can be equally confusing. Different companies own different exchanges, and they may or may not also own the physical lines that link these pieces together.

Someone in New York trying to call San Francisco would go from their CPE, to the local exchange in New York on a trunk line, to the toll office on yet another trunk line, then back to a local exchange in San Francisco on a different trunk line, and finally to the CPE of their California contact along one more trunk line. In this situation, it’s possible for every trunk line and node to be owned by a different company.

Similarly, a call destined for an international number would again originate at the caller’s CPE, travel along a trunk line to their Local Exchange, and then be passed to the international gateway along another trunk line. Once the call is passed to the correct country’s PSTN, it then travels along a similar path in that country to reach the called party.

Therefore, if you have an automated voice solution and only callers from a specific area are unable to connect to your system, there’s a good chance that your technology is working just fine, but that there is an issue at the carrier level in that area.

The PSTN remains a critical and integral piece in the modern global communications network. As the system continues to see technological innovation and bandwidth increases its centrality to modern communications will only increase.