Telephone comes from the Greek word tele, meaning from afar, and phone, meaning voice or voiced sound. Generally, a telephone is any device which conveys sound over a distance.
A standard dictionary defines the telephone as "an apparatus for reproducing sound, especially that of the voice, at a great distance, by means of electricity; consisting of transmitting and receiving instruments connected by a line or wire which conveys the electric current." Electricity operates the telephone and it carries your voice. With that important point established, let's look at how a telephone works and its history.
How it works
The Handset and phone
On a very simplistic level, the telephone works using a microphone and speaker system. As you speak into the mouthpiece, the sound waves move a membrane, causing a small electrical coil (or carbon granules) to move through a magnetic field. This causes an analogue current to flow through the wire (the process is called magnetic induction) and thus creates an electrical signal akin to the sound waves coming in.
The first telephones used this principle to send sound via an electrical cable and decoded at the other end by running the current through a wire coil in a magnetic field, which moves according to the current passing through it, thus moving the membrane attached and creating sound.
The best demonstration of just how simply your telephone works, is to build your own system. To do this, you need to buy a long phone cord, cut it to size and strip the wires. Put a cell (battery) and resistor in a series circuit with two handsets and when two people pick up the phones together, they can talk to each other. This sort of arrangement will work at distances of up to several miles apart before the signal will become too degraded to receive.
- - - - - - - - - - - - - - - - - -
| _____ |
- - - - - - - |_____| - - - - - - -
should create a current of about 30mAmps (for example, using a 9V cell and a 300Ω resistor). The only things this circuit can't achieve which your phone can are long distance communication (simply impractical and the signal degredation is high without being boosted), a dial tone and a ringing capability (telling the person on the other end to pick up). The "ring" signal is generally a 90V AC wave at 20Hz - and used to be generated (on some of the very first telephones) by using a hand crank attached to the side of the telephone.
On a more complex level, the telephone is much more than just a speaker, microphone and wires. The major problem with our simplistic phone is that when you talk, you will hear your voice through the speaker, so most phones contain a device called a duplex coil to block these signals. Add to that a bell (or speaker if you want something more complex than your average fire alarm), a touch tone keypad and a frequency generator and you have a basic modern phone. Modern phones also tend to use electronic microphones and speakers, replacing the carbon granules and loading coil.
Making a call
If you travel back to the days of a manual switchboard, it becomes easier to see how the complex systems used today to direct calls developed. Each town had its own switchboard and a pair of copper wires ran from every house in the town to the central switchboard. When PersonA picked up the phone to make a call, the hook switch would close, completing the circuit and lighting a small light on the switchboard. The operator would plug their headset in to the jack socket beneath the light, ask PersonA who they wanted to talk to and then send a ring signal to PersonB's house, causing PersonB's phone to ring. When PersonB picked up, the operator would connect the two jacksockets relating to the two telephones together and leave them to it.
When you pick up the handset or press your connection button a switch connects you to the network, this switch is generally called the hook switch - a long low tone will tell you that you're ready to dial (called, rather originally, a dial tone). The dial tone sound is a combination of a 350Hz tone and a 440Hz tone.
Most networks used to work using pulse dialling, using a swtich called a Stroger Switch, after its inventor. That is, that a number corresponded to a set number of rapid pulses. If you tapped a hook switch four times in quick sucession, the network should recognise it as the digit "4" - "0" is ten pulses. The old rotary dial telephones used a simple circuit which clicked as each number passed, with is why 0 was the furtherest around. Most networks still recognise pulse dialling, although the major networks have moved to tones which represent each number, allowing the clever people to play tunes. The different tones are, like the dial tone, made up of two frequencies played together as shown below :
697Hz 770Hz 852Hz 941Hz
1209Hz 1 4 7 *
1336Hz 2 5 8 0
1477Hz 3 6 9 #
When you dial (see above) the signal runs down the line to the phone jack and then down a pair of copper wires from a box at your house
(often called an entrance bridge) to one at the roadside. Along the road runs a thicker cable packed with copper pairs
, this oftens leads to a digital concentrator
(that box that is about the size of a fridge) at the end of the road. This concentrator digitises your voice at a rate of about 8 MHz and 8-bit resolution
. Sometimes the linking wires are underground
, sometimes they are carried on telegraph poles
, but the principle remains the same.
This is also the box where your phone signal is switched on and off from, that is, when you pick up your handset, this is where the dial tone generates from. If you are calling someone in your immediate area, the switch simply creates a loop and sends the signal to the phone in question, if you're calling out of the area, the signal is sent to the local exchange, which behaves pretty much like the operator of many years ago, causing the destination phone to ring and then just connecting the two telephones.
How phone numbers work
Phone numbers are a lot like addresses, but in reverse. If you could translate a phone number into words, instead of reading :
A N Other
25, Anywhere Street
A telephone number runs in reverse, narrowing down the area as it goes along...
A N Other's phone
So, in a full length phone number we have :
(00) (44) (161) (286) (8355)
| | | | |
International This This This This
Call Country County Town Phone
In theory (and often still in practice), phone numbers in the same area will run sequentially
. However, with the advent of people wanting to take their telephone number with them, new houses going up in the middle of old villages and multiple service providers needing their own set of numbers, the occurances of sequential telephone numbers are falling. In the smallest towns it still survives though and this stems from the time when their local exchanges were so local that only two, three or four digit local telephone numbers were necessary. With the coming of larger and more centralised exchanges, only needing to dial a three digit number has become a thing of the past.
In order to pack in as many calls as their cables can carry, phone companies restrict the bandwidth of the telephone calls to about 3,000Hz. All the frequenices between 400Hz and 3,400Hz are filtered out, thus reducing the "room" each call takes up without causing much detriment to the sound quality as most speech is within those frequencies.
The History of the Telephone
Some people might say that Francis Bacon
predicted the telephone in 1627 in his book New Utopia
. However, until the electrical principles needed to build a telephone were known in 1831, the telephone could not be built. In 1854, Bourseul
suggested transmitting speech electrically and after 22 years, in 1876, that the idea became a reality.
In 1753, an anonymous writer (possibly Charles Morrison) suggested that electricity might be used to convey messages in the The Scot's Magazine. He used separate wires to represent each letter and an electrostatic generator to electrify each line in turn, attracting the appropiate piece of paper by static charge at the other end (much like rubbing a balloon on your jumper and sticking it to the wall).
Progress was slow until 1800, many inventors worked alone, often misunderstanding earlier discoveries or repeating experiments. Static electricity could only take development so far, it couldn't provide the energy to communicate over long distances. So in 1800, when Alessandro Volta produced the first battery, it was a major step forward, although the principles behind transmitting speech came much later - they needed electricity and magnetism.
Christian Oersted first demonstated electromagnetism in 1820 and in 1821, Michael Faraday made a weak current flow in a wire, eventually publishing his results in 1831. Between them, they set up the process for turning mechanical energy into electrical energy - and if that were possible, then sending analogue signals produced as waves down an electrical wire was also possible. Professor Joseph Henry transmitted the first practical electrical signal in 1830. In his Albany Academy classroom, Henry created the forerunner of the telegraph.
Firstly, Henry built an electromagnet by winding an iron bar with several feet of wire. A steel bar mounted on a pivot was placed next to the electromagnet and on the other side of the magnet, a bell. A mile of wire lead around Henry's classroom and he completed the circuit by connecting the ends of the wires at a battery. When the circuit was completed the steel bar would swing towards the electromagent and strike the bell. Henry set up several circuits of this type during his life, including one from his office at Philosophical Hall to his home so that he might send signals to his wife. This gave a man called Samuel Finley Breese Morse a very good idea.
Morse's system used a key (a switch) to make or break the electrical circuit, a battery to produce power, a single line joining one telegraph station to another and an electromagnetic receiver or sounder that upon being turned on and off, produced a clicking noise. He applied for its patent in 1838, and was finally granted it in 1848. He completed the package by devising the Morse code system of dots and dashes. Telegraphy soon became big business.
In 1854, Charles Bourseul wrote about transmitting speech electrically. He described a flexible disk that would make and break an electrical connection to reproduce sound but never investigated his ideas further. Johann Phillip Reis took over where Boursel left off and built the first (albeit nowhere near perfect) telephone, based on his ideas. His transmitter and reciever used a cork, a knitting needle, a sausage skin, and a piece of platinum to transmit bits of music and certain other sounds. Reis' work failed where his telephone relied on the transmitter's diaphragm making and breaking contact with the electrical circuit. However, this approach does not work for analogue speech only discrete binary type signals - like morse - for speech, the transmitter should be in continuous contact with the circuit and varying the electrical current depending on the acoustic pressure. Reis' telephone may well have carried speech, but only accidentally and he did not continue development of the instrument.
In the 1870s, Alexander Graham Bell and Elisha Gray had both started out attempting to improve the number of messages a telegraph wire could carry, thus improving the efficiency of the telegraph. What they did do, however, was both invent something completely different.
The making of the telephone
While the principle behind the telephone was finally uncovered in 1874, it was the unique combination of voice and electricity that led to Alexander Graham Bell's actual invention of the telephone in 1876.
Bell was born on March 3, 1847, in Edinburgh, Scotland. He was the son and grandson of authorities in elocution and the correction of speech and soon followed in his father's footsteps. During the 1870s though, he changed tack and began to work on the conjecture that it was possible to transmit multiple telegraph messages over the same wire using musical tones of different frequencies for each message. By October 1874, he felt able to inform his future father-in-law (Boston attorney Gariner Greene Hubbard) about the possibility of a multiple telegraph. Hubbard (rather sensibly, as it turns out), backed Bell financially and allowed him to continue his work. What he didn't tell Hubbard or Thomas Watson (the young electrician whos' services he had enlisted) was that he was working on a new idea - a device that would transmit speech.
Bell's partners Gardiner Greene Hubbard and lawyer Thomas Sanders were not convinced about the potential for voice and in fact, often threatened to pull Bell's funding. Despite this, the trio were awarded US Patent No. 174,465, issued on March 3, 1876 for "Improvements in Telegraphy" - now widely recognised as the most lucrative patent ever issued. It was on March 10 (a week later), 1876, that Bell actually made his telephone work. Further to that, Bell didn't actually make his telephone, it was Thomas Watson (he of "Mr Watson, come here, I want you!" fame) who fashioned the device itself; a crude thing made of a wooden stand, a funnel, a cup of acid, and some copper wire.
Bell considered his invention's greatest advantage over every other form of electrical apparatus to be the fact that it could be used by anyone regardless of expertise or knowledge. Telegraphs required knowledge and use of morse, for example.
But Bell was nearly beaten to the patent office by Elisha Gray, who had independently developed a very similar invention. Gray filed a caveat just hours after Bell but neither man had actually built a working telephone, in fact, Bell took several methods outlined in Gray's Notice of Invention and used them himself to make his telephone work about three weeks later. Gray and Bell entered into a famous legal battle over the invention of the telephone, which Bell eventually won. Gray went on to cofound Western Electric, which became one of the Bell System's major competitors. Western Union was another major competitor, having already established itself as a communications provider with the telegraph system.
Aside from "stealing" ideas, Bell succeeded by building on older ideas and developments. He understood acoustics and sound - other inventors working on similar projects knew about electricity but not much about acoustics.
Gray wasn't alone in having his ideas whipped out from under his nose. Bell suffered a similar fate when Thomas Edison took advantage of Bell's failure to secure a patent in Britain for the Bell receiver and instead, patented his "electro-motograph" which required coninuous cranking.
However, by 1880 the Bell transmitter and the Edison receiver were combined and used throughout Britain.
Commercial service - 1880 - 1919
Commercial telephone service began in 1877 in the US and by 1878 the first exchange had been developed, enabling calls to be switched between destinations rather than requiring a direct line between the two - these exchanges were handled manually, initially by boys but later by the now famous women. In 1879, subscribers began to be designated by alphanumerics, rather than their family names.
The dial phone was invented in the 1880s by an undertaker called Almond Brown Stroger. Stroger was convinced that the Bell Telephone operator was sending calls for his business to the operator's brother-in-law and invented the dial phone in order to overcome this issue. He installed automatic telephone exchanges in both the US and Europe. By 1924, Bell Telephone System had licensed Stroger's invention and ditched their operators.
Western Union, Western Electric and Bell were in heavy competition with each other and their systems were incompatible with each other.
The common-battery system was developed by Hammond V. Hayes in 1888 after problems with disruptions to the supply to the system. This meant that the a central battery supplied a telephone system with power, rather than rely on each each unit's individual power. In 1900, the first coin telephone was installed in Hartford, Connecticut and party lines were soon developed to lower the cost of the telephone for people, especially those in rural locations.
In 1906, Dr Lee De Forest began working on a system to amplify the sound waves on a telephone system, thus allowing long-distance services to be improved. The system used audions, a three-element vacuum tube, which could amplify radio waves. He installed them at mid points along a wire to boost the signal. The Bell System bought the rights to De Forest's patents in 1913.
By 1918, ten million Bell System telephones were in service.
Expansion and developments - 1920 - 1960
Automatic switching of calls was made possible in 1921, with the introduction of "phantom circuits", which allowed three conversations to be conducted on two pairs of wires. The "French" phone, with the transmitter and receiver in a single handset, had been developed by Bell System around 1904 but was not released until 1927, the same year that the first transatlantic service (from New York to London and transmitted by radio waves) became operational.
1946 saw the introduction of coaxial cables, which resulted in a major improvement in services as they were less likely to be interrupted by electrical interference.
The same year brought transmission via coaxial cables, resulting in a major improvement in service as they were less likely to be interrupted by other electrical interference. Microwave radio transmission was used for the first time in long-distance telephony in 1947. In the same year, the transistor was invented at Bell Labs.
The laying of transatlantic telephone cables began in 1955. Great care was taken to ensure that the repeaters would be of the highest quality and it was hoped that they would last at least 20 years.
In the latter half of the 1950s, ANC (All Number Calling) was introduced because the demand for telephones was restricted by the alpha-numeric combinations which had been in place for decades.
The space age - 1960-
The world's first communications satellite, Telstar, went into orbit on 10th July, 1962 as a result of collaboration between NASA and the Bell System. Stationed in a geosychronous orbit around the earth, Telstar (and many others like it) was used for long distance calls.
The 1980s brought the development of fibre optic cables, which allowed greater volumes of calls to be carried than satellite or microwave links. The signal is translated into a pulse of laser light which is bounced down extremely thin lengths of glass fibres. In fact, Bell could be credited for aniticipating fibre optics as he had spent time working on a "photophone" which transmitted sound using a beam of light.
The mobile / cell phone era
Mobile phones are basically two way radios that use radio frequency waves (a kind of electromagnetic field), much like a CB radio. Unlike a CB radio though, mobile (or cellular phones) operate on a duplex system (that is that they use two frequencies, one for transmitting data and one for recieving data). The first commerical mobile telephone service was put into service in 1946, by linking moving vehicles to the telephone networks by radio. The radio transmitter in the vehicle had to be strong enough to transmit 50 miles as each city would have one radio antenna with perhaps 25 channels available. This was a severely limited (but highly sucessful) system. The first commercial mobile phone network was opened for business in 1979 in Tokyo. A similar system was started in the United States of America in 1983; this system was called AMPS (Advanced Mobile Phone System) and used a range of frequencies between 824MHz and 894MHz.
At this point, the US goverment required the presence of two carriers in every market, known as the A and B carriers. One of them was normally the LEC (local exchange carrier) in order to encourage competition. Each carrier was assigned 832 frequencies (790 for voice and 42 for data), each frequency was 30kHz wide, giving a comparable quality to a landline phone. The system used today is still very similar.
The first in car (or mobile) phone weighed in the region of 40kg and relyed on proximity to one antenna. Today's mobile phones weigh in the region of 200g and rely on a network of "cells", each with its own central radio transmitter and receiver and each mobile phone has its own transmitter-receiver. Cells are generally taken to be hexagonal in shape and each has a base station. The size of of the cell varies but generally in the country there is one base station every 10km, in a densely populated area, the distance between base stations can be as little as 500m, this allows for seamless transfer between cells.
Each base station operates on a different frequency in order to differentiate it from its neighbours. To avoid interferance, the cells are assigned into clusters, in which each base station operates at a different frequency - the pattern of frequencies is then repeated in each cluster so that no two neighbouring cells are using the same frequency.
Each cell phone carrier typically gets 832 radio frequencies to use in a city and with 2 frequencies used per call (the duplex system) and 42 channels being used for control channels, there are typically 56 voice channels per cell. With digital transmission methods, this number of available channels dramatically increases.
There are three common technologies used by companies for transmitting mobile information:
Frequency Division Multiple Access (FDMA) - puts each call on a different frequency and is mainly used for analogue transmission as it isn't a very efficient use of bandwidth.
Time Division Multiple Access (TDMA) - assigns each call a certain period of time on a frequency, generally there are three calls happening on each frequency with infomation being sent in bursts from each call in turn - Each call gets the line for a third of the time. Called narrow band transmission, this is made possible because the digital information being sent is compressed to the degree that using a full channel to transmit and recieve that information would be very inefficient.
TDMA is also used as the access technology for Global System for Mobile communications (GSM), however, GSM uses ecryption to make calls more secure. GSM is the standard in Europe, Australia and much of Asia and Africa.
Code Division Multiple Access (CDMA) - gives a unique code to each call and spreads it over the available frequencies, meaning multiple calls are overlaid on each other across the channel. Between 8 and 10 calls can be carried in the same channel space as one analogue AMPS call. CDMA technology is the basis for Interim Standard 95 (IS-95).
Multiple Access simply means that more than one user can use the cell at any one time.
Unfortunately, the GSM phones used in the USA are not compatible with the international system used by Europe and Australia. Most phones are now compatible with most systems (often called tri-band phones because of the three different systems used).
All cell phones have special codes associated with them, which are used to identify the phone, the phone's owner and the service provider.Electronic Serial Number (ESN) - a unique 32-bit number programmed into the phone when it is manufactured
Mobile Identification Number (MIN) - a 10-digit number derived from your phone's number
System Identification Code (SID) - a unique 5-digit number that is assigned to each carrier by the communications organisation for each country
While the ESN is considered a permanent part of the phone, both the MIN and SID codes are programmed into the phone when you purchase a service plan and have the phone activated.
On a basic level, a cell phone works on a similar basis to a landline phone, the signal is transmitted from your microphone to the recievers speaker and vice versa. However, a cell phone also needs to translate the information from analogue to digital for transmission and digital to analogue when recieving, transmit and recieve that information via the Electro Magnetic spectrum, carry its own power supply and display information on a LCD screen.
Short Message Service or SMS was added to mobile phones as a last minute idea. Short text messages of about 252 charcaters can be sent using the same technology as phone calls without having to disturb the person receiving the message. The service has really taken off in some countries while others rely more on calling one another. As many as 30 billion messages are sent a month.
Using a cell phone
As previously mentioned, cell phones are basically radio transmitters and recievers and operate on a very similar level. The transmitters in a cell phone are low power, broadcasting a background signal at 0.6 watts and transmitting a signal at 3 watts when in use - for comparason a CB radio transmits at 4 watts. Low power transmitters have two main advantages, the signals of both the base stations and mobile phones do not travel far out of the cells, meaning that calls should not be accidentally picked up by another base station or unit and also the power consumption of both the handset is much lower, meaning smaller batteries and thus, a more mobile unit.
Most cell phones now use subscriber indentification module (SIM) cards. These are small removable disks, usually used in GSM phones, which hold connection and identification data. Some SIMs also can store a small amount of SMS messages and phone numbers for the user (usually 10 messages and 99 phone book entries).
When you turn on a phone, it listens for a SID on a control channel, if it can't find one, it knows it is out of range and will display a "no signal" message. When it recives a SID, the phone compares it to the one programmed into the phone, if they match, the phone knows that the base station it is communicating with is part of its home system and can accept and transmit information. Along with the SID, the handset will also transmit a registration request with the MTSO (Mobile Telephone Switching Office - the central distribution point for calls in your geographical area), this way, the MTSO knows where you are when it wants to ring your phone.
Recieving a call is easy once the phone has been registered with the MTSO. Now it knows which cell you are in, it picks a frequency pair that your phone will use in that cell to take the call. The MTSO then communicates with your phone over the control channel, telling it which frequencies to use and connects the call. If you move whilst taking the call and need to change cells, the base station you are using will have noted that your signal strength is decreasing and the one you are moving towards notes that your signal strength is increasing (all base stations measure signal strength on all frequencies, not just its own set), the two base stations communicate with each other through the MTSO - which then sends a signal to your phone via the control channel giving it a new frequency to use within the new cell.
The revolutionary phone
This one solitary invention completely changed the way humans communicate, the social and historical influence it has had is so wide and deep that it is impossible to judge what our society would be like without it. With the invention of the telephone, easy and instant two-way communication over long distances was suddenly possible and plausible. It facilitated the breakup of the nuclear family because it was suddenly easier to keep in touch, wars were easier to fight as communications were suddenly quick, portable and two-way, news can now be seconds instead of days old when it arrives, it revoluntionised trade & industry and preceeded & seeded the Internet as a form of global communication.
It is, perhaps, alongside the invention of the computer, the most influential and important invention of man. Think of where your life would be without the telephone, would you even be here?