Fixed Line Telephones

by Jürgen Kock — last modified Mar 15, 2013 05:33 AM
This module discusses the features and the various stages of the development of fixed line telephones, its early history, the basic principle of a fixed line telephone system, plain old telephone service, digital telephones, cordless phones to today's features of fixed line telephones.

Early History

The telephone is one of the ground-breaking inventions of the 19th century. More than 150 years  have passed since Alexander Graham Bell was awarded the patent for the first electric telephone in 1876. Other pioneers worth mentioning are Johann P. Reiss, Antonio S. G. Meucci and Elisha Gray who all laid the foundation of what is even in today’s age of emails and internet one of the most important forms of communication.

Basic Principle of a Fixed Line Telephone System

The landline or fixed line telephone system basically consists of two end points and a transmission  medium. The endpoints are the telephone sets and the transmission medium the telephone line. In  a telephone set (also just called phone) sound waves are converted via a microphone into electric  waves, which are transmitted over a pair of twisted wires to the far end phone. Here the reverse happens. The electric waves are converted back into sound waves with the help of a speaker.

The first telephone consisted of a metal diaphragm, a bar magnet with a coil around it. The voice or audio waves on the sender side caused vibrations on the diaphragm, which changed the magnetic flow, inducing an electric signal in the coil. These signals were transmitted over an attached wire to the receiver side, where the same elements were used for the inverse process. The electric signal changed the magnetic flow, which caused vibrations on the diaphragm. These vibrations let to acoustic waves.

Ringer or Bell

Beside the transmission of voices or sounds, the phones also need a system to alert the user at the far end. This is why a phone has a ringer or bell.

Switch Hook

The telephone operates in two modes: on-hook and off-hook. When the phone is on-hook it reacts to alternating currents coming from the line, indication that the phone should ring. The user goes off-hook when he wants to initiate a call or to answer a call.


Though today there are solutions for multiparty telephone conferences, telephones are for point to point communication. To avoid having a direct wire to all possible contacts, the switch board was invented. At the beginning this was operated manually. If someone wanted to make an outgoing call, he alerted the operator of a switchboard and the operator connected the user to the desired destination.

Remark: Picture needs to be exchanged with a license free photo The fast success of the telephone system and growing number of users made it necessary to find an automated process to establish connections.

The fast success of the telephone system and growing number of users made it necessary to find an automated process to establish connections.


To allow subscribers to initiate telephone connections on their own, Almon Strowger started the development of an automatic telephone exchange, which got patented in 1891. Subsequently the finger-wheel was invented for dialing. The number of wires from the exchange to the subscriber was reduced to two. Siemens & Halske won in 1913 the patent for a number switch, which was the basis for the so called pulse dialing, a procedure which is still supported by today’s telephone networks.

However, the most common dialing system used by analogue phones today is called Dual Tone Multi Frequency (DTMF), introduced by Bell Telephone Laboratories in 1955. Everyone knows DTMF from push-button phones, where each number button, when pressed, generates a different tone. Actually, these tones are a mixture of two pure sine waves or sounds. So the name dual tone comes from the combination of two pure tones. To generate different sounds for all digits, multiple frequencies or pure sine waves are needed.

Plain Old Telephone Service - POTS

The term POTS came up, when innovations like the digitization of telephone network evolved. It still describes the technology, which more or less exists from the early days of telephony and includes:

  • A standard compliant analog telephone interface (the 2 wire telephone line)
  • Bi-directional full duplex speech channels, which means that users can talk and listen at the same time. Walkie-Talkies for example are only half duplex. Users can only talk or listen at a time.
  • Transmission of the limited frequency range of 300 to 3400 Hz which is suitable for the human voice.
  • Call progress tones like, dial tone, ringing signal, busy tone, etc.
  • Subscriber (self) dialing
  • Operator services  such as directory assistance or conference calling assistance

Digital Telephones – ISDN

In the 1970s the standardization body “Comité Consultatif International Téléphonique et Télégraphique” (CCITT), the predecessor of the International Telecommunication Union (ITU), started working on technical specifications for a digital telecommunication network. In 1980 the first standards were released for the so called Integrated Services Digital Network (ISDN).

The main difference to analog connections is the digital transmission of signals between the telephones. Audio signals are transformed into digital information and then transmitted over the telephone line. This improved the speech quality as the impact of noise is reduced.  With ISDN a more efficient usage of the telephone lines was achieved, allowing a user to have two simultaneous connections over the same line. In addition, a number of services (telephone, fax, data services), which required earlier separate lines or even networks could be combined. One disadvantage compared to analog phones is the need of a network terminator (NTBA – Network Terminator Basic Access). This network terminator is not power feed by the power coming from the telephone line. It needs its own power connector.

For more information about the difference between analogue and digital technology please refer to Module 2.4.2 Digitization.

Cordless Phones – DECT

In the late 1960s the cordless telephone was invented. Only in the 1980s they became more popular. A cordless portable phone replaces the handset cord with a wireless radio connection.  A base station is connected to the fixed telephone line. One or more handsets communicate with the base station over a limited range of usually less than 50 meters indoor or below 300m outdoor.

DECT (Digital Enhanced Cordless Telecommunications) defined by ETSI in EN 300 175 is a recognized standard for cordless phones.

Today’s Features of Fixed Line Telephones

While the basic principle of telephone calls hasn’t changed too much over the years, there is a range of features which make telephony more convenient.

Caller ID There are several features to display or suppress the telephone numbers of the calling and called party.

Call Forwarding or Call Diversion This allows a user to forward calls to another telephone number. This can apply to all calls or only when the subscriber is busy or doesn’t answer a call.
Call Waiting and Call Hold   
When a user is having an active call, he gets an indication when someone else calls. He can put the active call on hold and switch to the other call.
Speed Dial Many telephone sets allow the user to store frequently dialed numbers and assign a short code.
Screening Features Screening features are used to allow or block calls to or from certain numbers.
Voice Mail Allows to record voice messages from callers

Next Innovations for Fixed Line Telephones

The booming internet causes a threat to the traditional telephone connections. Data and voice networks will converge. Speech connections will just be one of many services transported over IP.

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