Data Services Unit 1 Introduction
Introduction to Communications
1.
2.
3.
4.
Communication System ............................................................................................................. 2
Communication Modes .............................................................................................................. 3
Communication mediums.......................................................................................................... 4
Analogue ..................................................................................................................................... 4
4.1
Analogue signals .................................................................................................................. 4
4.2
Disadvantage ........................................................................................................................ 5
5. Digital .......................................................................................................................................... 5
6. Binary Numbers and Arithmetic .............................................................................................. 6
6.1
Decimal Number System ..................................................................................................... 6
6.2
Binary Number System ........................................................................................................ 7
6.3
Binary to Decimal conversion.............................................................................................. 8
6.4
Decimal to Binary conversion.............................................................................................. 8
7. Bit and Bit-rates ......................................................................................................................... 8
8. Attenuation ................................................................................................................................. 8
9. Noise ............................................................................................................................................ 9
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Data Services Unit 1 Introduction
1. Communication System
The purpose of a telecommunications system is to transport information from user to user. A
communications system consists of a Signal Source, Transmitter, Channel, Receiver and Signal
Sink.
Generic transmission system
Signal
Source
Transmitter
Channel
Receiver
Signal
Sink
Aerial
Speaker
E.g. Radio
Microphone
Radio Mast
Airwaves
E.g. USB link
Memory
card
Electrical
Transmitter
Copper
wire
Electrical
Receiver
PC
The function of the transmitter is to take in an outside signal (from the signal source e.g. mic,
computer, cd player, magnetic tape) and convert it into a form appropriate to the Channel and send
it on to the medium (modulate the signal). The function of the receiver is to take in the signal from
the medium, if possible remove any noise or distortion introduced by the transmitter and/or the
medium and to convert it into a form suitable for the signal sink (i.e. for a speaker, digital signal for
a computer, signal to power a motor).
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2. Communication Modes
These are generic modes of communication. They are not mutually exclusive of each other e.g.
some systems can be said to be both Point-to-Point and Serial or both Duplex and Parallel.
Point-to-Point Where one user wishes to communicate with one other user, or with a small group
of nominated users - such as the telephone network or e-mail. Normally two way.
Broadcast
Where one sender communicates with all capable receivers who cannot respond so
that it is normally one way.
Multicast
One sender communicates with a nominated set of receivers who cannot respond.
Simplex
Data are transferred in one direction only. The receiver cannot communicate with the
sender
Duplex
Data transmission can take place in both directions simultaneously.
Half-Duplex Data transmission can take place in both directions, but not at the same time
Serial
The transmission if bits, one behind the other, over one transmission medium.
Parallel
The use of several transmission leads (channels) to allow the simultaneous
transmission of several bits at one time. A parallel data bus with 8 leads can process
8 bits at a time. Parallel transmission is much faster than serial transmission.
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3. Communication mediums
Copper




Telephone network
PCB
USB cable
Local area networks
Optical Fibre
 Telephone network
 Multimedia digital connections
 Plastic optical fibre in car networks
o Saab, BMW and Mercedes
 Local area networks
Wireless (radio and optical)
 Bluetooth
 Microwave telecommunications (public network)
 Wireless LAN
 Remote control
4. Analogue
In the past most signals were generated, transmitted and received in analogue form i.e. as a sine
wave or as a more complex signal which could be made up from a series of sine waves. This was
done because speech is an analogue signal and it was easier to implement analogue electronic
circuitry than digital. In a very simple system it is still easier to build in analogue (see early
structure of telephone network). It has disadvantages:
 It is inflexible, in that to make any changes to the system all of the changes have to be made in
hardware. This becomes more difficult and expensive as the system grows in size and
complexity.
 It is prone to noise and distortion, which cannot be removed.
 Control and manipulation of signals is difficult.
The mathematical treatment of analogue signals is relatively straightforward. An analogue signal is
considered to have the form of a sine wave, or a combination of sine waves, the treatment of which
is well established. See Fourier methods.
4.1 Analogue signals
From Wikipedia, the free encyclopaedia
An analog or analogue signal is any variable signal continuous in both time and amplitude. It differs
from a digital signal in that small fluctuations in the signal are meaningful. Analog is usually
thought of in an electrical context, however mechanical, pneumatic, hydraulic, and other systems
may also convey analog signals.
An analog signal uses some property of the medium to convey the signal's information. For
example, an aneroid barometer uses rotary position as the signal to convey pressure information.
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Electrically, the property most commonly used is voltage followed closely by frequency, current,
and charge.
Any information may be conveyed by an analog signal, often such a signal is a measured response
to changes in physical phenomena, such as sound, light, temperature, position, or pressure, and is
achieved using a transducer.
For example, in an analog sound recording, the variation in pressure of a sound striking a
microphone creates a corresponding variation in the voltage amplitude of a current passing through
it. An increase in the volume of the sound causes the fluctuation of the current's voltage amplitude
to increase while keeping the same rhythm.
Since most natural data is analog before the digital conversion required to get a digital signal,
resolution of analog recording and transmitting technology has been higher until recent times. For
practical reasons such as memory conservation and the cost of phasing out older digital recordings,
the resolution of some digital signals may remain lower than most analog signals. For this reason,
some audiophiles prefer analog technology. However, in many cases, the difference is too minimal
to be noticed.
4.2 Disadvantage
The primary disadvantage of analog signalling is that any system has noise – i.e., random variation.
As the signal is copied and re-copied, or transmitted over long distances, these random variations
become dominant. Electrically, these losses can be diminished by shielding, good connections, and
several cable types such as coaxial or twisted pair.
The effects of noise make signal loss and distortion impossible to recover, since amplifying the
signal to recover attenuated parts of the signal amplifies the noise as well. Even if the resolution of
an analog signal is higher than a comparable digital signal, in many cases, the difference is
overshadowed by the noise in the signal.
5. Digital
Computers deal in 1s and 0s. Therefore communication between computers is a matter of
transferring digital sequences between machines. The next step was to convert speech and other
analogue signals into a digital format to permit a combined network. These days digital electronic
circuitry is cheaper than analogue circuitry for the implementation of complex functions. It has
advantages:
 Normally, large scale digital systems are software controlled so that it is possible to make
changes to the system in software and remotely, without touching the hardware.
 It is less prone to noise or distortion: a 1 remains as a 1 and will not be mistaken for a 0, unless
there is an extreme level of distortion.
 If noise or distortion does occur, methods exist to determine that this has happened, and if
appropriate to correct the error which has occurred.
 It is relatively easy to manipulate signals - see for example the speaking clock which is made up
of sequences of digitised speech that are assembled in the correct order to tell the time every 10
seconds.
 More complex signal processing is possible - for example, speech recognition.
The mathematical treatment is not as straightforward as for analogue.
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6. Binary Numbers and Arithmetic
In order to perform functions such as comparison and arithmetic, it is necessary to represent
numbers in binary. This unit introduces binary numbers, outlines their relationship with the
decimal number system and proceeds to introduce binary arithmetic. The decimal number system is
reviewed first to help understand the structure of other number systems.
6.1 Decimal Number System
It is not surprising that human anatomy helps describe the most common number system used
today. A human hand is characterised by four fingers and a thumb, thus this gives a range of ten
numbers from 0 to 9. This does not mean however that we are limited to a scale of ten numbers, as
the position of these numbers can be used to indicate their magnitude and thus can be assigned a
weight.
10n
…..
102
101
100

10-1
10-2
…..
10-n
Decimal point
For example:
3410 = 30 + 4
3
4
4 * 100 = 4
3 * 101= 30
Sum=3410
As can be seen, the number 3 has a weighting of 10 (101) due to its position and the number 4 has a
weighting of 1 (100) due to its position
For example:
123.4510 = 100 + 20 + 3 + (4/10) + (5/100)
= (1 * 102) + (2 * 101) + (3 * 100) + (4 * 10-1) + (5 * 10-2)
Note, a number with a subscript of 10 (i.e. Base 10) indicates that this is a decimal number.
However, decimal is the default number system, so usually the subscript is not included for decimal
numbers.
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6.2 Binary Number System
The binary system has two symbols. This is just another way of counting, and it is less complicated
than decimal as it is composed of only two binary digits (bits), 0 and 1.
Counting in binary is very similar to counting in decimal. As previously stated, the decimal system
only contains ten digits in total but this does not limit us to counting only from 0 to 9. When the
number 9 is reached, another column (to the left) is started and the counting continues from 10 to
99, where the process is repeated.
The system of binary counting is similar. Binary being a two-digit number system it starts at 0 and
then goes to 1. To continue counting, as with the decimal systems, another digit must be added to
the left, i.e. continue counting from 10 to 11. The next column is 100 101 110 111.
The subscript 2 is used to indicate that a number is binary, for example, 102 is a binary number.
COUNT
zero
one
two
three
four
five
six
seven
eight
nine
ten
eleven
twelve
thirteen
fourteen
fifteen
DECIMAL
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
BINARY
02
12
102
112
1002
1012
1102
1112
10002
10012
10102
10112
11002
11012
11102
11112
As can be seen from the table above, it takes four bits to count sixteen decimal numbers. A simple
formula is used to calculate how far it is possible to count with n bits, i.e. 2n-1, where n is the
number of bits available.
For Example:
n =4
24-1 = 16 - 1
= 15
Therefore, it is possible to count to the decimal number fifteen with four binary digits. To count to
sixteen a fifth binary digit (bit) is needed.
Computers use binary numbers to select memory locations. Each location is assigned a unique
number (called an address). The Pentium Microprocessor has 32 address lines, which means it has
232 = 4,294,967,296
unique locations.
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6.3 Binary to Decimal conversion
Like the decimal number system, the binary number system is also a weighted system, i.e. the
position of a 1 or a 0 indicates its weighting. Using the weighted system, any binary number can be
divided into parts that are to the power of two. Then, using their weighting the binary number can
be converted to decimal.
Ex 1: Convert the binary number 10012 to a decimal number.
Ex 2: Convert the binary number 1110112 to a decimal number.
[910]
[5910]
6.4 Decimal to Binary conversion
We can also convert a decimal number into a binary number by a successive division process. The
decimal number is divided continuously by 2 and the remainders indicate the equivalent binary
number. The binary number is read from the last remainder upwards to the first.
Ex 1: Convert the decimal number 123 to a binary number.
[11110112]
Ex 2: Convert the decimal number 79 to a binary number.
[10011112]
Ex 3: Convert the decimal number 46 to a binary number.
[1011102]
7. Bit and Bit-rates
A character used to represent one of the two digits in the numeration system with a base of two, and
with only two possible states. 1 or 0
1 bit
1,000 bits = 1 kbit
1,000,000 bits = 1,000 kbits = 1Mbit
1,000,000,000 bits = 1,000,000 kbits = 1,000 Mbits = 1 Gbit
1,000,000,000,000 bits = 1,000,000,000 kbits = 1,000,000 Mbits = 1,000 Gbit = 1 Tbit
1 kbit = 1 kilo bit
1Mbit = 1 Mega bit
1 Gbit = 1 Giga bit
1 Tbit = 1 Tera bit
8. Attenuation
Attenuation is the reduction in amplitude and intensity of a signal with respect to distance travelled
through a medium. Attenuation can also be understood to be the opposite of amplification.
Attenuation is an important property in fibre optics, microwave telecommunications and wired
telecommunications because of its importance in determining signal strength as a function of
distance. Attenuation is often measured in units of decibels per kilometre of medium.
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9. Noise
In the receiver of any telecommunications system the received signal consists of two parts:
 the desired, or the information signal,
 all other signals added to the desired signal, including distortion introduced by the
telecommunications system itself, interference between systems and noise.
Of all the added signals the most important is noise. The others can be reduced by proper design of
the telecommunications system, but the noise can never be reduced below a level decided by
fundamental physical laws. In addition, every stage of amplification increases the noise, so that its
effect cannot be removed by amplifying the signal.
The basic problem in a telecoms receiver is - how to recover the information signal without
error in the presence of noise, using the lowest possible signal power level.
Noise is any random undesirable signal energy that interferes with a desired signal. Sources
include:
 Environmental noise: due to cosmic and solar radiation, lightening discharges etc.
 Man made noise: from electric arc discharges from spark plugs, drills, motors, switching
lights on and off, mains hum, etc.
 Internal noise: This type of noise is inherent in all electrical and electronic circuits, and
will be considered in some detail because ultimately it places a limitation on the
performance of electronic circuits from the point of view of limiting the smallest desired
signal that can be distinguished from noise.
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