Telecommunications –
for a layman
What is Communication? It can broadly be defined as two or
more persons speaking and hearing to each other. This involves mouth and ear. After
inhaling the air in our lungs when we pump out, our throat muscles modulate the
passing out air in a set pattern. This creates vibrations in the surrounding
air creating a wave pattern. This wave or vibration travels in the air and
reaches nearby ears. The receiving ear converts that waveform into
understandable intelligence.
Mouth is the transmitter and ear the receiver. Is that all?
Ok, then try the same process while swimming under water. Can you communicate?
No. That means there is one more factor involved other than transmitter mouth
and receiver ear. This 3rd factor is medium. Transmitter/ Receiver
working in one medium may or may not work in another medium. Humans and species
living in the air atmosphere are designed to communicate through air. Without
air medium we can’t communicate, neither under water nor in the space or on the
surface of moon.
Different mediums require different modes of communication.
The water animals have their own way of communication. Medium is very important
in Communication which opens a vast and interesting field called “Tele-Communication”,
means communication from a distance. Anything that involves communication
beyond ‘human mouth-ear range’ may be termed as Telecommunication. Till not
very long time, say just two centuries back, we were using distant
communications through pigeons carrying messages tied to their neck.
What are the possible mediums of communications other than
air? One we have already talked about above, that is water. Unfortunately, so
far we have not been able to exploit this medium much although over three
fourth of earth surface is occupied by water. Only limited communication is
available for short distances, mainly used in warfare, in the form of signals
to detect movement of enemy ships/ submarines etc. Like RADAR, we have SONAR
for underwater.
Human voice or speech doesn’t work beyond earth’s air
atmosphere. But sunlight is coming to us from millions of miles where no air
exists. That means light can travel in air as well as outer space or vacuum. Like
sound waves, light also travels in the form of waves. They are called
Electromagnetic Waves or simply EM Waves. This EM wave has very vast range out
of which only limited range is visible to human eye. Can we communicate in
visible range? Suppose you are standing in a hall where loud music is playing.
Can you talk with other people? No, because the loud music overpowers your weak
voice. Similarly in visible range, strong sunrays overpower all other optical
signals if one tries to communicate. Hence we devised methods to communicate
using non-visible range of EM waves, the details of which will be discussed
later.
In nineteenth century, there was big invention which changed
the history of mankind. It was electricity. One more medium ‘metallic wire’,
other than air water and space, was discovered. Initially it was used for the
transfer of electrical power. Do you know the basic difference between
‘Electrical’ and ‘Telecommunication’ engineering? Very simple. Electrical is
‘transfer of power’ whereas Telecom is ‘transfer of intelligence’. Hence
scientists were looking to exploit this new medium for ‘transfer of
intelligence’. How to do that?
Let me go back to the first para using “mouth-ear” analogy.
Mouth has to modulate air vibration in a particular pattern understandable to
the ear. If mouth doesn’t speak a language but shouts randomly. Or mouth speaks
Hindi but ear understands only Creole, it will make no sense. It will simply be
noise to the ear. So a scientist Samuel Morse tried to put intelligence
(language) on electricity. On one side he put a switch and at the far end an
electrically operated magnet. When he puts the switch “on”, the magnet on the
far end hits a bell and makes sound. But this was not enough. So he developed
an electrical language involving”dash (-)(long press)” and “dot (o)(short
press)”. In this language, A= -o (one
dash, one dot); B= -ooo; C= -o-o and so on. The person sitting at the far end
was able to decipher and note down. This language was known as “Morse Code”,
and became the first instance of telecommunication on electrical wire.
Suppose your wife is very angry after seeing a girl roaming
with you. She calls you home and starts yelling. Due to fear, you just keep listening
without saying a word. This is called one-way communication or “simplex mode”.
A leader standing high on a platform is addressing the huge
crowd. Only leader is speaking and many others are listening. This is called
one-to-many communication or “broadcast mode”. Radio channels, Televison,
Newspaper portals are some of the examples.
A boy and a girl are newly married. They both have a lot to
talk and share. Both are speaking and both listening. This is called two-way
communication or “duplex mode”. Telephone, Video conferencing, internet
chatting are some of the examples.
There is fourth category also “Semi-duplex”. While transmitting,
the ear gets closed. Police walkie-talkie is an example where they use press to
talk.
Above example of Morse code communication is simplex. The
sender doesn’t know whether the receiver at far end has got all correct messages
or no message at all. There could be snapping of wire or the far end magnet
might have missed some dashes and dots. To overcome this, he duplicated the
arrangements in a complimentary manner. Far end also has a switch and near end
a magnet. Upon receiving a message, the far end sends back the acknowledgement
or same message to the initiator. This Morse experiment graduated to be known
as Telegraphy after several improvements and the machines were called
teleprinters.
Human race is over 3 million years old. Out of this only last
200 yrs saw all the inventions of modern science, after the invention of
electricity in the nineteenth century. Isn’t it strange? No modern invention
can work without electrical current. We are so lucky to have taken birth in
this era.
What is electricity? We all have seen high-rise water tanks
wherein water is stored. This tank is connected to various houses through pipes
for distribution of water. How much water is to be released is controlled by a
valve. Same analogy can be applied in electricity which broadly consists of
three components Voltage, Resistance and Current. Water stored in tank at
higher level is ‘Voltage’. Regulator valve is ‘Resistance’. And amount of water
flowing out is ‘Current’. For a given
voltage, current will increase if we reduce the resistance and vice versa. In a
battery, electric is stored in the form of charge and we can measure the
voltage across it. Every battery has two poles +ve and –ve. Current flows from
+ve to –ve through a wire joining both the poles. Torch bulb is inserted as
resistance between the poles.
There is another interesting fact. If a magnet is moved
inside a coil of wire, electricity is produced. Similarly the reverse happens.
That means, if electricity flows in a coil, it produces magnetic field. There
is an interesting story behind this. Michael Faraday was working on producing
electricity with magnet. He was thinking if by flowing electricity through a
coil magnetic field is produced, the reverse phenomena also must happen. He
kept on working for many years but did not succeed. Finally he got fed up and
threw his magnet inside the coil in anger. But that produced deflection in the
current meter and he got what he was searching for. He threw again and meter
deflected again. That means the current is produced only when the magnet moves
inside the coil and not when it is stationary. That is how dynamo was invented
to produce electricity.
After transmitting short messages over electric wire using
Morse code, the next challenge comes how to transmit voice. It was achieved in
the late nineteenth century by Graham Bell. A set of microphone and speaker is
used on both sides. Microphone has a diaphragm connected to a magnet. Around
this magnet is a coil through which electricity flows. When a person speaks,
the diaphragm vibrates which in turn moves the magnet to & fro. Movement of
magnet changes the flow of current in coil. This coil is connected to the far
end speaker through a metallic wire. In a speaker, just the reverse happens.
Change of current in coil moves the magnet to & fro, which vibrates the
attached diaphragm to produce sound waves.
This invention of Graham Bell changed the history and
revolutionized the world. He may rightly be called the father of telecom. This
primitive telephone system underwent several improvements and subsequently
entire world was wired with telephone network.
But wiring the world was not so easy. Suppose you want a
message to be conveyed to a place 1 km away. But your shout can be heard only
up to 200 mtr distance. What to do? Put 1 person at every 200 mtr distance on
that 1 km stretch who keep re-shouting the same message onward. Same technique
was deployed in telephone system. The electrical voice signal in metallic wire
becomes weak after 2 or 3 km and can’t then be retrieved back. Hence at every 2
or 3 km these signals are amplified and retransmitted onwards. They are called
repeaters.
Thus far we have analyzed communication in air and
electrical wire. Third mode was discussed in short previously which involves EM
waves. Light rays also come under EM waves. As already stated earlier it’s
difficult to communicate in open space using light waves. Hence waves other
than optical range were looked for, generally called Radio waves. Marconi is
called the father of wireless. However, many dispute it. Jagadish Chandra Basu,
an Indian scientist successfully demonstrated wireless communication in
Calcutta much before Marconi, but he was not interested in getting it patented.
Wave is defined by two broad terms, Frequency and
Wavelength. Wavelength is defined as two successive crests or troughs of the
wave. Frequency is number of cycles of wave in one second. The behavior of
radio waves change with change in frequency. Therefore radio frequencies are
divided into many groups: 3 MHz-30MHz (HF); 30MHz-300MHz (VHF); 300MHz-3GHz
(UHF); 3GHz-30GHz (Microwave)
Radio waves in the range of VHF and above travel in straight
line similar to light. Since earth is round and not flat, the radio rays will
go away from the earth and get lost into space after travelling some distance.
That is why you might have seen antennas installed on high towers to achieve
greater range of coverage. To gain more height, they are installed on hill
peaks as well. To gain more range of coverage, we can install antenna repeaters
(like telephone described earlier) at regular intervals at a height before it
moves away from earth. Repeaters receive the signal, amplify it and then
forward ahead.
However, antennas looking into space to communicate with
satellites have no such restriction and they can be installed at ground level.
This was the basic communication concept for a layman. More
telecom techniques like Microwave, satellite, optical fiber etc shall be
discussed in next edition.
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Concluding part-2
In previous edition we discussed the basics of Telecom.
Broadly there are two ways to achieve telecommunication; wireline and wireless.
Suppose you have a road between 2 cities A & B. It
doesn’t make sense to use that road just for one vehicle commuting between A
& B. To make it cost effective, this road must be shared by maximum
possible number of vehicles. But at the same time, they need to follow traffic
rules to avoid accidents & collisions.
Suppose there are 4 political parties desiring to take out
procession from A to B with large fleet of vehicles. There are 2 possible ways
to achieve this. (1) Give one lane to each party on 4-lane road. All will move
simultaneously within their allocated lanes and reach the destination. (2) Give
all 4 lanes to each party one by one in different time-slots.
Similar to road we have frequency band in telecom called
bandwidth. This bandwidth can be used by many users in two ways. (1) We can assign
unique frequency slots to each user within the same bandwidth. All information (voice
or data) flows at the same time on different frequencies. This is called FDM (Frequency
Division Multiplexing). Alternatively like (2), we can assign entire bandwidth
to each user one by one on time sharing basis. This is called TDM or Time
Division Multiplexing. These 2 technologies, FDM and TDM, are the basic
building blocks of telecom.
In rural areas, we have just one narrow road. Same road is
used for the traffic in both the directions. Since the traffic is thin, vehicle
flow is manageable without much congestion. But if we move to urban areas, the
road widens. It becomes 2-lane, one each for both side of traffic. If we move
further up into bigger metro cities, the road may widen to 4 or 6 lanes.
Between two big cities, there are wide and fast express-highways.
Similarly in telecom, we have different type of transmission
systems:-
·
Small capacity copper telephone lines are used
from house to telephone exchange, like rural road above. It can send small
telecom traffic 2Mb upto 3 km (ADSL). This part of network is called ‘’Access
network’’ or ‘’last mile connectivity’’.
·
Coaxial cable is used between small exchanges to
carry little high traffic. They can send 10Mb upto 4 km (using cable modem).
·
Microwave is used for little bigger exchanges.
They can send 70Mb upto 30km.
·
Optical fiber Cable (OFC) is used as telecom
super highways between big cities. They typically carry 10Gb upto 80km or even
more. In fact the bandwidth of optical fiber is enormous with speed reaching
into terabits.
However, above network topology is old and has undergone sea
changes now. With the advancement of technology and optical fiber having
several advantages over copper and coaxial, like (i) cheaper in cost (ii)
lightweight hence easy to handle (iii) no resale value hence no pilferage (iv)
no metal content hence no interference/noise (iv) enormous bandwidth, it is
being used extensively in all areas right upto the level of each subscriber
(customer). The hunger for data requirement is increasing for each user with
many new technologies coming in, like triple play i.e., audio/video/internet on
the same line. We can say telecom highways are reaching to each subscriber
(FTTH). OFC has come a long way since its commercial induction in the year 1988
or so. It has eaten away copper and coaxial.
Last mile connectivity or Access network was always a
problem in telecom. Snapping of wires, loose connections, bad behavior of
linesmen made it unpopular. So when mobile technology stepped in, traditional
copper landlines started shrinking. It was thought at one time that wired
landline had gone into oblivion. But with the arrival of OFC, wired line got
fresh lease of life. Mobile technology also has many limitations. It has shadow
areas where the signal coverage is unavailable. It is dependent on limited
frequency spectrum shared among many telecom operators, whereas OFC line has no
such limitation and can entirely be dedicated to a customer.
Which type of telecom system to deploy depends on many
factors. If it is plain terrain with road connectivity, laying of Optical Fiber
Cable (OFC) along the roadside is easy. But if we have mountainous hills and we
need to connect two hills having no proper road, then microwave is the option.
But if the mountainous villages are scattered very far apart (say 400 km) and
reaching there with OFC is not possible. Deploying microwave with repeater at
every 40 km for thin traffic will not be cost effective. In case of ocean islands,
the microwave is not at all feasible due to difficulty in placing repeaters
(towers) enroot. In such cases we should think of connecting through satellite
(VSAT).
What is OFC and why has it become so popular? Why has it
remained invincible and non-replaceable since last 3 decades? There are more
than many factors. It is very thin, as thin as human hair. One cable can
accommodate from 1 fiber to several hundred fibers depending on the
requirement. Silica (sand, available in abundance) glass is used as raw
material to manufacture optical fibers. This bare fiber is very weak and
fragile, hence other materials are added to give it strength and protection.
Now the next question is, how does it work to give enormous speed?
In the previous edition, we discussed various EM
frequencies. HF, VHF, UHF, Microwave, Infrared, Visible light (optical). These
are in the ascending order of rising frequency or descending order of
wavelength. Higher the frequency more is the carrying capacity (bandwidth or
speed) and also the quality. Microwave the highest radio frequency, ends at 30
GHz, whereas optical range starts at 430,000,000 GHz (700 nano meter wavelength).
Optical fibers communicate at near infrared range, just below the visible
range, hence undetectable to human eye. Typical wavelength is 800/ 1310/ 1550
nm. At such high frequency or low wavelength, fibers are able to possess
enormous capacity. One hair size thin fiber can carry over 13 million
simultaneous telephone calls or a speed of over 1 terabit. At this speed one
can transmit 100,000 books from coast to coast in 1 second.
OFC, copper and coaxial were the wireline communications. In
wireless we have microwave for terrestrial (along the earth surface) and
satellite for space in long distance communication. Microwave is slowly being
removed by fast and high capacity OFC, except hilly regions where cable laying
routes may be difficult. However, electrical HT towers are also being shared
for carrying OFC. Since OFC doesn’t have any metal content, it remains unaffected
by the nearby high voltage electrical wires running in parallel on same
pole/towers.
Satellite communication is unique and can’t be replaced by
OFC or any other wireline technology. Moon is the natural satellite of earth. How
does it work and why not go away from earth? Take a thin rope, tie a piece of
stone at the end and start rotating in a circle around your body. Stone will
try to go away from you but the rope is applying force to keep it held around
you. Same thing happens in satellites. Gravitational pull of earth doesn’t
allow it to go away from its orbit. However, if it comes nearer than specified
distance, earth will pull it down. If it goes farther away than the specified
distance, gravitational pull will not be enough and the object will get lost
into space. This min and max distance to keep it rotating is called earth’s
orbit. A satellite nearer to earth will rotate faster. There are three types of
orbits (i) Low earth orbit (LEO) (ii) Medium earth orbit (MEO) (iii) High earth
orbit.
LEO satellites are used for weather and military purposes.
Communication satellites are mostly placed in geo-stationary orbit at 36,000
km. At this height the satellite rotation and earth rotation gets synchronized
and satellite looks stationary at one point with respect to earth. Three geo
satellites are required to achieve the coverage of entire 360 degree of earth.
Communication satellites are nothing but simple amplifiers (repeaters) for
greater range of earth coverage. LEO satellites for weather and military have
high resolution cameras.
