KEY TERMS
Bell
telephone
transmit
sound

SEARCH MATERIALS

Bell, Alexander Graham
(Scotland, Mar. 3, 1847 - Aug. 2, 1922)

He was the inventor of the TELEPHONE
and a number of other devices.

In addition
to his own great drive and energy,
a number of external factors
aided him in his work on the telephone,
an invention that was
to revolutionize the field
of communications almost immediately.

Born in
to a family deeply interested
in the areas of speech and deafness,
he was trained by his father
and grandfather in public speaking
and in teaching the deaf
to speak.

He taught
and experimented along these lines,
and he also studied anatomy and physiology,
which provided him
with a biological basis
for his great invention.

He was also trained in the field of music.

After Bell's two brothers died of tuberculosis,
and he was threatened by the disease,
his family moved in 1870
to a more healthful location in Ontario,
Canada--his father sacrificing
a successful career in London in the process.

There,
Bell and his father
continued their work along the same lines,
but during the next few years
Bell's own work necessitated a move
to the United States.

He became a professor
of vocal physiology at Boston University.

At this time,
he was already nurturing
the idea of transmitting speech
by electrical means.

An increasing interest
in electricity led him
to try
to develop a harmonic TELEGRAPH,
which would make it possible
to transmit several telegraph messages
over the same line.

In 1875,
he discovered the principle
that made the telephone possible.

The basic patent,
granted Mar. 7, 1876,
was later challenged many times,
but his priority was always upheld.

on Mar. 10, 1876,
Bell transmitted the first message
ever sent by telephone--"Mr. Watson,
come here,
I want you"--
to his assistant,
who was linked by wire and receiver
to the sending device in Bell's office.

In 1877,
Bell married Mabel Hubbard,
who was deaf (as was his mother),
and in 1882 he became a U.S. citizen.

Bell was active in many fields
and invented a number of other devices,
including one that could transmit speech
via light waves.

He had much
to do
with the founding (1880)
of the important magazine Science,
was president of the National Geographic Society
from 1898 to 1904,
and was made a regent
of the Smithsonian Institution in 1898.

After 1895 his technical interests turned
to AVIATION.

He invented the tetrahedral KITE,
and under his patronage
a group called the Aerial Experiment Association
was formed in 1907.

The group developed the HYDROFOIL,
a new form of speedboat
that in 1919 set a world speed record
of 114.04 km/h (70.86 mph).

The hydrofoil principle is still in use today.

telephone
Alexander Graham BELL
is generally acknowledged
as the inventor (1876) of the telephone,
although a number of other inventors
in his day contributed
to various aspects of the device.

Bell's most important contribution,
however,
was his view of the telephone
as a means of communication over real distances,
using human speech instead of such nonvocal codes as smoke signals,
tomtoms,
or Morse code.

Bell and others discovered that the sounds of speech could be converted
to an electrical signal,
transmitted over copper wires,
and converted back
to sound at the receiving device.

Today's telephone system links the entire globe.

The telephone industry in the United States alone generates about $200 billion yearly in revenues
is the largest segment of the giant communications industry.

System Elements
There are four major elements of the elaborate network that carries the telephone signal:

(1) the telephone instruments themselves
other apparatus
for voice,
data,

image TELECOMMUNICATION that are connected
to the network;
(2) the various transmission media used
to carry telephone signals over distance;
(3) the switching machines
technologies used
to switch signals along the way;
and (4) the devices used
to control the operation of the network.

Telephones and Other Station Apparatus
The telephone itself is a rather simple appliance.

A microphone,
called the transmitter,
and an earphone,
called the receiver,
are contained in the handset.

The microphone converts speech in
to its direct electrical analog,
which is transmitted as an electrical signal;
the earphone converts received electrical signals back
to sound.

The switch hook determines whether current flows
to the telephone,
thereby signaling the central office that the telephone is in use.

The ringer responds
to a signal sent by the central office that causes the telephone
to ring.

The touchtone dialer sends unique combinations of two single-frequency tones
to the central office
to indicate the particular digits dialed.

(Older rotary dialers actually interrupt the flow of the direct current
to the telephone,
thereby encoding the dialed digits as dial pulses.)
The telephone network offers enormous flexibility because many kinds of terminals in addition
to telephones can be connected
to it.

for example,
a MODEM transmits computer data over the telephone network;
a FACSIMILE machine sends data in the form of electrically-coded visual images;
a codec digitally encodes images from a television camera and converts them
to electrical signals that are carried over the telephone network.

Transmission and Multiplexing
Telephone signals from a home or business are carried over a twisted pair of copper wires,
called the local loop,

to a centrally located local office.

Hundreds and even thousands of wire pairs are carried together in a single large cable,
either buried underground in a conduit or fastened aboveground
to telephone poles.

At the central office each pair of wires is connected
to the local switching machine.

It would be very uneconomical
to assign a single pair of wires
to carry each telephone signal over the great distances of a long-distance telephone call.

Hence,
many telephone signals are combined,
or multiplexed,
together
to share a transmission medium.

Only a few years ago such multiplexing was accomplished by shifting each telephone signal
to its own unique FREQUENCY-band.

As many as 3,600 telephone signals were multiplexed together using such frequency-division multiplexing.

with today's time-division multiplexing,
each telephone signal is converted
to a digital representation.

That representation is inserted in
to a stream of bits carrying many digitized telephone signals,

with the overall stream operating at a very high bit rate.

(A single signal requires 64,000 bits per second in digital form.)
The multiplexed signals can be transmitted over a variety of transmission media.

The oldest digital multiplexing system,
called T1,
operates over a pair of copper wires carrying 24 telephone signals,
at an overall bit rate of 1.544 million bits per second.

First installed in 1962,
the system is still used today between central offices.

Very-high-frequency radio,
operating in the microwave bands,
carries multiplexed telephone signals across the United States between microwave towers located about every 42 km (26 mi) apart.

A COMMUNICATIONS SATELLITE is,
in effect,
a microwave tower located in geosynchronous orbit 35,900 km (22,300 mi) above the Earth's surface.

It takes time
for the radio signal
to complete its journey from Earth
to the satellite,
and hence a round-trip delay of one-half second occurs.

Since the delay is not acceptable
to most telephone users,
telephoning via satellite is used today only
to reach otherwise inaccessible parts of the world and
to carry one-way television signals.

with optical fiber (SEE FIBER OPTICS),
a beam of light is transmitted through a very thin,
highly pure glass fiber.

The light travels in parallel rays along the axis of the fiber.

Many telephone signals are time-division-multiplexed together,
and the light source is simply tuned on and off
to encode the ones and zeros of the digital signal.

A single strand of optical fiber used in today's telecommunication systems has a capacity in the order of 2 gigabits per second (2,000,000,000 bits per second) and can carry 30,000 telephone signals.

The history of transmission media and multiplexing shows an ever-increasing progression of the total number of telephone signals that can be carried over a specific generation of the technology.

The more signals carried,
the lower the cost per call.

The L1 coaxial-cable system,
first installed across the United States in 1946,
had a total capacity of 1800 two-way voice circuits;
this capacity had increased
to 132,000 circuits
with 1978's L5E system.

Similar progressions occurred
with other transmission media.

for example,
the first transatlantic submarine cable system,
TAT-l,
installed in 1958,
had a maximum capacity of 36 voice circuits.

The newest transatlantic system,
TAT-10 (1992),
utilizes optical fiber
has a capacity of 80,000 voice circuits.

Switching
The telephone network is a switched network:

that is,
the path,
or circuit,
needed
to connect one telephone
to another is created
maintained only
for the duration of each individual telephone call.

The earliest form of switching was performed manually,
by operators who used patch cords
to connect one telephone line
to another.

Switching was later automated,
using electromechanical relays
and switches that were controlled by the dialed digits.

Electromechanical switching reduced the need
for operators,
but was not flexible and needed considerable maintainance.

Today's electronic switches are controlled by programmable computers.

Many of today's switching machines switch signals that are in digital format.

In fact,
all of the switching machines in AT&T's long-distance network and over half of the local machines are digital,
and all local switching machines will be converted
to digital by the end of the century.

Digital switching,
which uses digital electronics in integrated circuits,
interfaces well
with the time-division-multiplex technology of today's transmission systems and requires very little maintainance.

Signaling
Some of the control signals used in the telephone network--such as the telephone ring and busy tone--directly involve the telephone user.

Other control signals are sent between switching offices,
over circuits that are distinct from the voice-traffic channel,
in order
to
to transmit the called and calling numbers and
to indicate the availability of transmission circuits.

The control signals used within this separate network are called common channel signaling,
or CCS.

CCS is facilitating new "intelligent" services
for telephone users,
such as knowing the telephone number of the caller before answering ( "caller-ID" ),
or instructing the system
to forward calls
to other numbers automatically or
to take messages if the telephone line is in use.

Today's advanced technology can also create new services,
such as universal telephone numbers that will remain the same no matter where their users move.

Cellular Mobile Telephone Service
Cellular mobile telephone service uses two-way,
low-power,
RADIO,
rather than wired electric current,

to transmit telephone calls (see CELLULAR RADIO).

An urban area is divided in
to a number of "clusters," each cluster consisting of seven "cells."

Each cell has its own radio transmitter,

with about 120 2-way radio channels.

The same channels are reused in neighboring clusters.

As the cellular user travels along a roadway,
the radio signal between the vehicle and the fixed transmitter becomes weaker.

Finally,
as the signal is about
to be lost,
it is switched automatically
to a new radio channel being served by the adjacent cell.

When not being used
for an actual call,
the cellular telephone monitors a shared data channel
to determine if it is being paged
for an incoming call.

The same shared data channel is used
for dialing an outgoing call.

Cellular mobile telephone service was initially intended
for use in automobiles,
but small,
portable cellular telephones are now in use nearly everywhere.

Cellular telephone service is just one form of mobile telephone service
for people on the move.

The cordless telephone uses radio over very short distances in the home or office.

Telephones are now common in most commercial aircraft.

Miniaturized pagers use radio
to alert us
to a telephone call or
to display a short message.

The Telephone Industry
In the past,
most telephone service in the United States was provided by the Bell System under the ownership and control of the American Telephone and Telegraph Company,
or AT&T.

In this vertically integrated monopoly,
telephone equipment was designed and developed by Bell Laboratories and manufactured
for the Bell System by Western Electric Company.

Local telephone service was provided by local Bell operating companies,
and long-distance service was provided by the Long Lines Division of AT&T.

All this changed in 1984
with the break-up of the Bell System,
which separated the local Bell operating companies from AT&T and organized them in
to seven regional,
private companies.

Competition is now a permanent feature of telephone service in the United States.

The telephone user purchases and owns the various telephones and other station apparatus connected
to the telephone line.

A number of long-distance companies compete fiercely
with each other.

(Although long-distance rates continue
to plummet,
this is more the result of advances in technology than of competition.) The one remaining monopoly is in the provision of local service,
but here,
too,
some competition exists in the form of the cellular-radio alternative.

The Future of Telephone Service
The signals from voice,
text,
numbers,
images,
and music all appear the same once they have been converted in
to the bit streams of the digital format.

Some people foresee a day when optical fiber will replace the copper wire of the local loop,

with the fiber carrying digitized voice,
data,
and television signals
to every telephone.

When all signals are digitized,
an Integrated Services Digital Network,
or ISDN,
could form a global,
digital network,
which would deliver basic voice and data service from everywhere in the world
to a home phone outlet.

The proposed (1993) merger of Bell Atlantic,
a regional phone company,

with Tele-Communications,
Inc.,
the giant U.S.

cable system operator,
was the largest in a year of such deals and an unmistakable sign of things
to come.