About PSTN

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PSTN stands for Public Switched Telephone Network. It represents the world's entire circuit-switched telephone networks telephone system minus the private ones which have their own PBX. Also known as POTS (plain old telephone system), it provides data and voice over a circuit-switched network. Currently, this is the network that most end-users are connected to, for both national and international services. Some elements of PSTN have already been replaced with VoIP, which is cheaper and offers a lot of new services. In this chapter we will take a close look at what PSTN is and how it evolved.


At the very beginning, PSTN was a network of fixed-line analogue telephone systems. Now it is almost entirely digital, except for the last part that connects to the user.

There are a lot of standards for PSTN, created by ITU-T. It uses E.163/E.164 addresses, which is the standard for telephone numbers. Also, PSTN delivers Quality of Service guarantees.

Notice that POTS is not exactly the same thing as PSTN. POTS refers to the services that were available before introducing the electronic telephone exchange. These services, almost all available since the introduction of the telephone, include:

  • the voice frequency is limited in the range 300-3400Hz, it is bi-directional or duplex
  • subscriber dialing
  • operator services, like assistance for long distance and conferences
  • dial tone and ringing signals

When POTS was first implemented it meant Post Office Telephone Service or Post Office Telephone System, but, as the telephone services are not under control of the post offices, these names are not used any more. The next step was made once the computers appeared. More services were included. Among these are:

  • voice mail
  • caller ID
  • conferences
  • call waiting

The idea of PTSN came from the Bell Labs. A DS0 (digital signal 0) is the basic digital circuit. It is a 64-kilobit-per-second channel, digitized at an 8khz sample rate using 8-bit pulse code modulation. This is the way the call is placed from the caller to the called person. Next, the signal goes through telephone exchanges, which are situated between the two endpoints. But this is not all. These DS0’s could also be called timeslots, because they are multiplexed in time-division. So, this way, multiple DS0`s can be transmitted over only one wire.

About telephone exchanges

The telephone exchange (known as telephone switch in US) is a piece of hardware that handles connections. This term, exchange, is also used when speaking about an area server by a switch, or it can refer to the first three digits of the local number.

  • It was in 1878 when the first telephone exchange had opened. Only two simultaneous conversations were supported, but it was a good start. The next “version” consisted of o large board, filled with plugs. Anyone had to call there and ask the operator to manually make the proper connections.
  • After that, in 1891, Almon Strowger designed the Strowger switch, which replaced the operator by automating his work. This switch could handle up to 100 connections at a time, at a rate at about 10 pulses per second (pps). After that, the crossbar technology wanted to take the place of the Strowger switch. It was much faster, at about 20pps.
  • But, then the DTMF tone-signaling were introduced. Since then, these are the tones used in PSTN. Of course, there existed a period when special hardware had to be made to convert from the traditional Strowger or crossbar to DTMF pulses.

So, the telephone switches are the devices that take care of routing calls from a telephone to the other. These are used to connect two or more digital virtual circuits together. Digital switches encode the voice in real-time. That is, many times per second it takes just a little piece of the speech and encodes it. This results in a short delay in the conversation. But there is nothing to worry, as this cannot be sensed. Usually, these telephone switches are owned by the local telephone service provider, but nothing prevents large companies to purchase their own. In this case, they are called PBX, or Private Branch Exchange.

What is circuit switching?

It is the main technology used in PSTN. It provides a method to route the voice and data traffic between the endpoints (in a conversation), from end-users to other switching centers. A continuous electrical circuit goes through the wires for as long as the conversation takes place, until it is closed by one of the participants. Switching centers know pieces of information about the other ones and, based on these, they can choose an optimal route for a certain call.

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