UNIT-1

Syllabus

UNIT 1 - INTRODUCTION

Basic representation of communication system – Transmitter- Channel- Noise and

Receiver-Base band and band pass signal- Transmission media- bandwidth and

capacity-Electromagnetic spectrum- Bandwidth requirements- Spectra of sinusoidal

and non sinusoidal waves- Analog versus digital communication

____________________________________________________________________

CONTENTS:

Introduction

Model of Communication systems

Hybrid Communication Systems

Types of Communication systems

Basic representation of communication system

Electromagnetic spectrum

Bandwidth requirements

Analog versus digital communication

History of communication systems

INTRODUCTION

Communication System:

In telecommunication, a communications system is a collection of individual

communications networks, transmission systems, relay stations, tributary stations,

and data terminal equipment (DTE) usually capable of interconnection and

interoperation to form an integrated whole. The components of a communications

system serve a common purpose, are technically compatible, use common

procedures, respond to controls, and operate in unison. Telecommunications is a

method of communication (e.g., for sports broadcasting, mass media, journalism,…)

A communications subsystem is a functional unit or operational assembly that

is smaller than the larger assembly under consideration. Communication subsystem

basically consists of a receiver, frequency translator and a transmitter. It also contains

transponders and other transponders in it and communication satellite

communication system receives signals from the antenna subsystem.

Signal

Communications is invariably about sending information from one side to

another.

Information may be text, data, or signals.

A signal may be defined as any physical quantity that varies with time, space,

or any other variable(s).

Some signals can be described by `tractable' expressions. For example, a

Sinusoidal signal:

m(t) = sin(mt)

Carrier Modulation

Base band communications often refers to information transmission over

wirelines; e.g., telephone lines, and coaxial cables.

In baseband transmissions, low frequency regions are used.

In wireless transmissions such as radio, TV, mobile comm. & satellite comm.,

modulation (or carrier modulation) is required.

Modulation is a process that causes a shift in the range of frequencies of the

message signal. Demodulation is its inverse counterpart.

MODEL OF A COMMUNICATION SYSTEM

The purpose of a Communication System is to transport an information bearing

signal from a source to a user destination via a communication channel.

Basic Blocks

1. Transmitter

2. Channel

3. Receiver

Transmitter:

• Signal processing conditions the message signal

– Lowpass filtering to make sure that the message signal occupies a

specific bandwidth, e.g. in AM and FM radio, each station is assigned a

slot in the frequency domain.

– In a digital communications system, we might add redundancy to the

input bit stream

• Carrier circuits

– Convert baseband signal into a frequency band appropriate for the

channel

– Uses analog and/or digital modulation

Channel

• Transmission media:

– Wireline (twisted pair, coaxial, fiber optics)

– Wireless (indoor/air, outdoor/air, underwater, space)

• Propagating signals experience a gradual degradation over distance

• Boosting improves signal and reduces noise, e.g. repeaters

TRANSMITTERS AND RECEIVERS

Generalized Transmitters

– Transmitters generate the modulated signal at the carrier frequency fc

, from the modulating signal m(t).

– Any type of modulated signal could be represented by

g(t) is a function of the modulating signal m(t) .

The particular relationship that is chosen for g(t) in terms of m(t) defines

the type of modulation that is used, such as AM, SSB, or FM.

Generalized Receiver

• The receiver has the job of extracting the source information from the

received modulated signal that may be corrupted by noise.

• Often, it is desired that the receiver output be a replica of the modulating

signal that was present at the transmitter input.

• There are two main classes of receivers:

1. The Tuned Radio-Frequency (TRF) receiver and

2. The Superheterodyne receiver.

• Most receivers employ the Superheterodyne receiving technique.

Receiver Parameters

• Sensitivity

– The voltage that must be applied to the Rx I/P to give a standard O/P

• Selectivity

– Ability of the receiver to reject adjacent unwanted signals

• Image Frequency Rejection

– Rejection of the frequency which would generate the same IF when

mixed with the LO frequency fsi=fs+2fi = fo+fi

)( : PM

)](1[ : AM

g(m) Modulation of Type

tmjDc

c

peA

tmA

Wire line Channel Impairments

• Attenuation: linear distortion that is dependent on the frequency response

of the channel.

• Spreading: the finite extent of each transmitted pulse increases, i.e. pulse

widens due to

– Transmit pulse length T s

– Channel impulse response length T h

– Resulting waveform due to convolution has duration T s + T h

• Phase jitter: the same sinusoid experiences different phase shifts in the

channel

• Additive noise: arises from many sources in the transmitter, channel, and

receiver

• Same as wire line channel impairments plus others

• Fading: multiplicative noise

– Example: talking on a cellular phone while driving a car when the

reception fades in and out

• Multiple propagation paths

– Multiple ways for transmitted signal to arrive at receiver

Receiver and Information Sinks

• Receiver

– Carrier circuits undo effects of carrier circuits in transmitter, e.g.

demodulate from a bandpass signal to a baseband signal

– Signal processing subsystem extracts and enhances the baseband

signal

• Information sinks

– Output devices such as computer screens, speakers,and TV screens

Hybrid Communication Systems

• Mixed analog and digital signal processing in the transmitter and receiver

– Ex: message signal is digital but broadcast over an analog channel

(compressed speech in digital cell phones)

• Signal processing in the transmitter

• Signal processing in the receiver

Types of Communication:

1. Analog Communication

2. Digital Communication

Digital Communication System - Block diagram

Digital Communication- Blocks

• Information Source

• Source Encoder and Decoder

• Channel Encoder and Decoder

• Modulator and Demodulator

• Channel

Block diagram with additional blocks

Additional Blocks

• Encryptor

• Decryptor

• Multiplexer

• Demultiplexer.

Digital Communication- Advantages

• Less Distortion, Low noise & interference.

• Regenerative Repeaters can be used.

• Digital Circuits are more reliable.

• Hardware implementation is more flexible.

• Secrecy of information.

• Low probability of error due to error detection and error correction.

• Multiplexing- ( TDM )

• Signal Jamming is avoided.

Digital Communication- Disadvantages

• Large Bandwidth

• Synchronization

Channels for Digital Communication

Channel Characteristics:

Bandwidth

Power

Linear or Non-linear

External interference

Types of Channels

1. Telephone Channels

2. Coaxial Cables

3. Optical fibers

4. Microwave radio

5. Satellite Channel

1. Telephone Channels

Provides voice grade Communication.

Good for data communication over long distances.

Frequency range: 300Hz – 3400Hz.

High SNR – about 30dB.

Flat amplitude response for voice signals.

For data & image transmissions EQUALIZERS are used.

Transmission rate = 16.8kb/s

2. Coaxial Cable

Single-wire conductor inside an outer Conductor with dielectric

between them.

Wide Bandwidth

Low external Interference.

Closely spaced Repeaters are required.

Transmission rate = 274 Mb/s.

3. Optical fibers

Communication is by light rays.

Fiber consists of Inner core and an outer core called CLADDING.

Refractive Index of Cladding is less.

Larger Bandwidth.

Immune to cross talk and EMI.

More secure.

Low cost.

Date rate = Terra bits/sec.

4. Microwave radio

Transmitter & Receiver With antennas.

Works on Line-of-sight principle.

Point to Multipoint communication.

Reliable & High Speed of Transmission.

Operating Frequency - (1 – 30)GHz

System Performance degrades due to meteorological variations.

5. Satellite Channel.

Repeater in the sky.

Placed in geo-stationary orbit.

Long distance transmission.

High Bandwidth.

Operates in microwave frequency.

Uplink frequency is more than down link frequency

Why Modulate?

Reduce noise and interference.

Channel assignment.

Multiplexing or transmission of several messages over a single channel.

Overcome equipment limitation.

Why Digital Communication?

Inexpensive digital circuits may be used.

Privacy by using data encryption.

Greater dynamic range.

In long-distance systems, noise does not accumulate from repeater to

repeater.

Errors may be corrected.

Base band and band pass signals

• Baseband: refers to the signals and systems before modulation,

which have frequencies/bandwidth much lower than the

carrier frequency

• Passband: refers to the signals and systems after (including)

modulation, which have frequencies/bandwidth around the

carrier frequency

• Baseband signal: is usually the message signal

• Passband signal: is usually the modulated signal, or transmitted

signal

• Base band signal is the original signal having the original

frequencies when delivered by transmitters.

• In base band communication, signals are transmitted without

modulation.

• Band pass signal is a signal which is modulated by one of the

modulation schemes.

• Demodulation is the process of extracting the baseband

message from the carrier so that it may be processed and

interpreted by the intended receiver

MODULATION TECHNIQUES:

l List of modulation methods

– Amplitude modulation methods and applications

1. AM (amplitude modulation): AM radio, short wave radio

broadcast, CB radio

2. DSBSC (double sideband suppressed carrier AM): data

modem, Color TV’s color signals

3. SSB (single sideband AM): telephone

4. VSB (vestigial sideband AM): TV picture signal

Angle modulation methods and applications

1. FM (frequency modulation): FM radio broadcast, TV sound

signal, analog cellular phone

2. PM (phase modulation): not widely used, except in digital

communication systems (but that is different)

Electromagnetic spectrum:

The electromagnetic spectrum is the range of all possible frequencies of

electromagnetic radiation. The "electromagnetic spectrum" of an object is the

characteristic distribution of electromagnetic radiation emitted or absorbed by that

particular object.

The electromagnetic spectrum extends from below frequencies used for modern

radio through to gamma radiation at the short-wavelength end, covering

wavelengths from thousands of kilometers down to a fraction of the size of an atom.

The long wavelength limit is the size of the universe itself, while it is thought that

the short wavelength limit is in the vicinity of the Planck length, although in principle

the spectrum is infinite and continuous.

Range of the spectrum

EM waves are typically described by any of the following three physical properties:

the frequency f, wavelength λ, or photon energy E. Frequencies range from 2.4x1023

Hz (1 GeV gamma rays) down to tiny fractions of Hertz (nanohertz for astronomical

scale waves). Wavelength is inversely proportional to the wave frequency, so

gamma rays have very short wavelengths that are fractions of the size of atoms,

whereas wavelengths can be as long as the universe. Photon energy is directly

proportional to the wave frequency, so gamma rays have the highest energy around

a billion electron volts and radio waves have very low energy around femto electron

volts. These relations are illustrated by the following equations:

Where:

c = 299,792,458 m/s (speed of light in vacuum) and

h = 6.62606896(33)×10−34 J·s (Planck's constant).

Whenever electromagnetic waves exist in a medium (matter), their wavelength is

decreased. Wavelengths of electromagnetic radiation, no matter what medium they

are traveling through, are usually quoted in terms of the vacuum wavelength,

although this is not always explicitly stated.

Generally, EM radiation is classified by wavelength into radio wave, microwave,

infrared, the visible region we perceive as light, ultraviolet, X-rays and gamma rays.

The behavior of EM radiation depends on its wavelength. When EM radiation

interacts with single atoms and molecules, its behavior also depends on the amount

of energy per quantum (photon) it carries. Electromagnetic radiation can also be

divided into octaves, as sound waves are.

Spectroscopy can detect a much wider region of the EM spectrum than the visible

range of 400 nm to 700 nm. A common laboratory spectroscope can detect

wavelengths from 2 nm to 2500 nm. Detailed information about the physical

properties of objects, gases, or even stars can be obtained from this type of device.

Spectroscopes are widely used in astrophysics. For example, many hydrogen atoms

emit a radio wave photon which has a wavelength of 21.12 cm. Also, frequencies of

30 Hz and below can be produced by and are important in the study of certain stellar

nebulae and frequencies as high as 2.9×1027 Hz have been detected from

astrophysical sources.

Bandwidth:

Bandwidth is the information-carrying capacity of a communication channel. The

channel may be analog or digital. Analog transmissions such as telephone calls, AM

and FM radio, and television are measured in cycles per second (hertz or Hz). Digital

transmissions are measured in bits per second. For digital systems, the terms

"bandwidth" and "capacity" are often used interchangeably, and the actual

transmission capabilities are referred to as the data transfer rate (or just data rate).

Bandwidth Requirements and Ratings

In the United States, the Federal Communications Commission (FCC) is in charge of

allocating the electromagnetic spectrum and, thus, the bandwidth of various

communication systems. In the electromagnetic spectrum, sound waves occupy low

ranges, while microwaves, visible light, ultraviolet, and X-rays occupy upper ranges.

The bandwidths occupied by various communication technologies are described in

"Electromagnetic Spectrum."

The bandwidth requirements of various applications are listed in Table Bandwidth-1.

The rates are shown in bits/sec (bits per second), Kbits/sec (thousands of bits per

second), Mbits/sec (millions of bits per second), and Gbits/sec (billions of bits per

second). Compression and other techniques can reduce these requirements.

Application Rate

Personal communications 300 to 9,600 bits/sec or higher

E-mail transmissions 2,400 to 9,600 bits/sec or higher

Remote control programs 9,600 bits/sec to 56 Kbits/sec

Digitized voice phone call 64,000 bits/sec

Database text query Up to 1 Mbit/sec

Digital audio 1 to 2 Mbits/sec

Access images 1 to 8 Mbits/sec

Compressed video 2 to 10 Mbits/sec

Medical transmissions Up to 50 Mbits/sec

Document imaging 10 to 100 Mbits/sec

Scientific imaging Up to 1 Gbit/sec

Full-motion video 1 to 2 Gbits/sec

Table Bandwidth-1: Bandwidth requirements for various applications

The transmission rates of various communication systems are listed in Table

Bandwidth-2. Compression techniques and signal encoding are used to boost data

rates. For example, modems use the ITU V.42 bis data compression standard to

compress data at a ratio of over 3 to 1. V.42 bis compresses and decompresses on the

fly as data is sent and received by connected modems.

Type Rate

Dial-up modem connection 1,200 bits/sec to 56 Kbits/sec

Serial port file transfers 2,000 bits/sec

ISDN (Integrated Services Digital

Network)

64 Kbits/sec or 128 Kbits/sec

Fractional T1 digital WAN link 64 Kbits/sec

Parallel port 300 Kbits/sec

DirecPC (satellite) Internet downloads 400 Kbits/sec

DSL (Digital Subscriber Line) 512 Kbits/sec to 8 Mbits/sec

Cable (CATV) modems 512 Kbits/sec to 10 Mbits/sec (or

higher)

T1 digital WAN link 1.544 Mbits/sec

ARCNET LANs 2.5 or 20 Mbits/sec

Token ring LANs 4 or 16 Mbits/sec

Ethernet LANs 10, 100, 1,000 Mbits/sec

T3 digital WAN link 44.184 Mbits/sec

HSSI (High-Speed Serial Interface) 52 Mbits/sec

FDDI (Fiber Distributed Data Interface) 100 Mbits/sec

Fibre Channel 1 Gbit/sec

Gigabit Ethernet 1 Gbit/sec

10GE (10 Gigabit Ethernet) 10 Gbits/sec

SONET (Synchronous Optical Network) 51.9 Mbits/sec to 2.5 Gbits/sec

Optical (lambda) networks implementing

DWDM

Hundreds (or perhaps thousands) of

lambdas per fiber, each running at 2.5

Gbits/sec

Table Bandwidth-2: Transmission rates of various communication systems

γ= Gamma rays MIR= Mid infrared HF= High freq.

HX= Hard X-Rays FIR= Far infrared MF= Medium freq.

SX= Soft X-Rays Radio waves LF= Low freq.

EUV= Extreme ultraviolet EHF= Extremely high freq. VLF= Very low freq.

NUV= Near ultraviolet SHF= Super high freq. VF/ULF= Voice freq.

Visible light UHF= Ultra high freq. SLF= Super low freq.

NIR= Near Infrared VHF= Very high freq. ELF= Extremely low freq.

Freq=Frequency

Base band:

In telecommunications and signal processing, baseband is an adjective that describes

signals and systems whose range of frequencies is measured from zero to a

maximum bandwidth or highest signal frequency; it is sometimes used as a noun for

a band of frequencies starting at zero. It can often be considered as synonym to

lowpass, and antonym to pass band, band pass or radio frequency (RF) signal.

Spectrum of a baseband signal, amplitude as a function of frequency

Various uses:

A baseband bandwidth is equal to the highest frequency of a signal or

system, or an upper bound on such frequencies. By contrast, a non-baseband

(pass band) bandwidth is the difference between a highest frequency and a

nonzero lowest frequency.

A baseband signal or low pass signal is a signal that can include frequencies

that are very near zero, by comparison with its highest frequency (for

example, a sound waveform can be considered as a baseband signal, whereas

a radio signal or any other modulated signal is not).

A baseband channel or low pass channel (or system, or network) is a

channel (e.g. a telecommunications system) that can transfer frequencies that

are very near zero. Examples are serial cables and local area networks

(LANs).

Baseband modulation, also known as line coding, aims at transferring a

digital bit stream over a baseband channel, as an alternative to carrier-

modulated approaches.

An equivalent baseband signal or equivalent lowpass signal is – in analog

and digital modulation methods with constant carrier frequency (for example

ASK, PSK and QAM, but not FSK) – a complex valued representation of the

modulated physical signal (the so called pass band signal or RF signal). The

equivalent baseband signal is , where I(t) is the

inphase signal, Q(t) the quadrature phase signal, and j the imaginary unit. In a

digital modulation method, the I(t) and Q(t) signals of each modulation

symbol are evident from the constellation diagram. The frequency spectrum

of this signal includes negative as well as positive frequencies. The physical

passband signal corresponds to

where ω is the carrier angular frequency in rad/s.

In an equivalent baseband model of a communication system, the

modulated signal is replaced by a complex valued equivalent baseband signal

with carrier frequency of 0 hertz, and the RF channel is replaced by an

equivalent baseband channel model where the frequency response is

transferred to baseband frequencies.

A signal "at baseband" is usually considered to include frequencies from near

0 Hz up to the highest frequency in the signal with significant power.

In general, signals can be described as including a whole range of different

frequencies added together. In telecommunications in particular, it is often the case

that those parts of the signal which are at low frequencies are 'copied' up to higher

frequencies for transmission purposes, since there are few communications media

that will pass low frequencies without distortion. Then, the original, low frequency

components are referred to as the baseband signal. Typically, the new, high-

frequency copy is referred to as the 'RF' (radio-frequency) signal.

The concept of baseband signals is most often applied to real-valued signals, and

systems that handle real-valued signals. Fourier analysis of such signals includes a

negative-frequency band, but the negative-frequency information is just a mirror of

the positive-frequency information, not new information. For complex-valued

signals, on the other hand, the negative frequencies carry new information. In that

case, the full two-sided bandwidth is generally quoted, rather than just the half

measured from zero; the concept of baseband can be applied by treating the real and

imaginary parts of the complex-valued signal as two different real signals.

Baseband vs passband transmission in Ethernet and other network access

technology

The word "BASE" in Ethernet physical layer standards, for example 10BASE5,

100BASE-T and 1000BASE-SX, implies baseband digital transmission, i.e that a line

code is used, and that an unfiltered wire (i.e. a low-pass transmission channel) is

used. This is as opposed to 10PASS-TS Ethernet, where "PASS" implies passband

transmission. Passband transmission makes communication possible over a passband

filtered channel such as the telephone network local-loop or a wireless channel.

Passband digital transmission requires a digital modulation scheme, often provided

by modem equipment. In the 10PASS-TS case the VDSL standard is utilized, which is

based on the Discrete multi-tone modulation (DMT) scheme. Other examples of

passband transmission are wireless networks and cable modems.

Modulation

A signal at baseband is often used to modulate a higher frequency carrier wave in

order that it may be transmitted via radio. Modulation results in shifting the signal up

to much higher frequencies (radio frequencies, or RF) than it originally spanned. A

key consequence of the usual double-sideband amplitude modulation (AM) is that,

usually, the range of frequencies the signal spans (its spectral bandwidth) is

doubled. Thus, the RF bandwidth of a signal (measured from the lowest frequency

as opposed to 0 Hz) is usually twice its baseband bandwidth. Steps may be taken to

reduce this effect, such as single-sideband modulation; the highest frequency of

such signals greatly exceeds the baseband bandwidth.

Some signals can be treated as baseband or not, depending on the situation. For

example, a switched analog connection in the telephone network has energy below

300 Hz and above 3400 Hz removed by band pass filtering; since the signal has no

energy very close to zero frequency, it may not be considered a baseband signal, but

in the telephone systems frequency-division multiplexing hierarchy, it is usually

treated as a baseband signal, by comparison with the modulated signals used for

long-distance transmission. The 300 Hz lower band edge in this case is treated as

"near zero", being a small fraction of the upper band edge.

The figure shows what happens with AM modulation:

Comparison of the equivalent baseband version of a signal and its AM-modulated

(double-sideband) RF version, showing the typical doubling of the occupied

bandwidth.

The simplest definition is that a signal's baseband bandwidth is its bandwidth before

modulation and multiplexing, or after demultiplexing and demodulation.

The composite video signal created by devices such as most newer VCRs, game consoles

and DVD players is a commonly used baseband signal.

Analog Vs Digital Communication:

Analog Communication:

� A physical quantity that varies with “time”, usually in a smooth or continuous

fashion.

� Fidelity describes how close the received signal to the original signal is. Fidelity

defines acceptability.

Digital Communication:

� An ordered sequence of symbols selected from a finite set of discrete elements.

� When digital signals are sent through a communication system, degree of

accuracy within a given time defines the acceptability.

Analog communication systems, amplitude modulation (AM) radio being a typifying

example, can inexpensively communicate a band limited analog signal from one

location to another (point-to-point communication) or from one point to many

(broadcast). Although it is not shown here, the coherent receiver provides the

largest possible signal-to-noise ratio for the demodulated message. An analysis of

this receiver thus indicates that some residual error will always be present in an

analog system's output.

Although analog systems are less expensive in many cases than digital ones for the

same application, digital systems offer much more efficiency, better performance,

and much greater flexibility.

Efficiency: The Source Coding Theorem allows quantification of just how

complex a given message source is and allows us to exploit that complexity by

source coding (compression). In analog communication, the only parameters

of interest are message bandwidth and amplitude. We cannot exploit signal

structure to achieve a more efficient communication system.

Performance: Because of the Noisy Channel Coding Theorem, we have a

specific criterion by which to formulate error-correcting codes that can bring

us as close to error-free transmission as we might want. Even though we may

send information by way of a noisy channel, digital schemes are capable of

error-free transmission while analog ones cannot overcome channel

disturbances.

Flexibility: Digital communication systems can transmit real-valued discrete-

time signals, which could be analog ones obtained by analog-to-digital

conversion, and symbolic-valued ones (computer data, for example). Any

signal that can be transmitted by analog means can be sent by digital means,

with the only issue being the number of bits used in A/D conversion (how

accurately do we need to represent signal amplitude). Images can be sent by

analog means (commercial television), but better communication

performance occurs when we use digital systems (HDTV). In addition to

digital communication's ability to transmit a wider variety of signals than

analog systems, point-to-point digital systems can be organized into global

(and beyond as well) systems that provide efficient and flexible information

transmission. Computer networks, explored in the next section, are what we

call such systems today. Even analog-based networks, such as the telephone

system, employ modern computer networking ideas rather than the purely

analog systems of the past.

Consequently, with the increased speed of digital computers, the development of

increasingly efficient algorithms and the ability to interconnect computers to form a

communications infrastructure, digital communication is now the best choice for

many situations.

Transmission Media:

1. Transmission medium

Physical path between transmitter and receiver

May be guided (wired) or unguided (wireless)

Communication achieved by using em waves

2.Characteristics and quality of data transmission:

Dependent on characteristics of medium and signal

Guided medium

*Medium is more important in setting transmission parameters

Unguided medium

*Bandwidth of the signal produced by transmitting antenna is

important in setting transmission parameters

* Signal directionality

Lower frequency signals are omni directional

Higher frequency signals can be focused in a directional beam

3. Design of data transmission system

Concerned with data rate and distance

Bandwidth

*Higher bandwidth implies higher data rate

Transmission impairments

*Attenuation

*Twisted pair has more attenuation than coaxial cable which in turn is

not as good as optical fiber

Interference

*Can be minimized by proper shielding in guided media

Number of receivers

*In a shared link, each attachment introduces attenuation and

distortion on the line

Guided transmission media

Transmission capacity (bandwidth and data rate) depends on distance and

type of network (point-to-point or multipoint)

Twisted pair

* Least expensive and most widely used

*Physical description

Two insulated copper wires arranged in regular spiral

pattern

Number of pairs are bundled together in a cable

Twisting decreases the crosstalk interference between

adjacent pairs in the cable, by using different twist length

for neighboring pairs

*Applications

Most common transmission media for both digital and analog

signals

Less expensive compared to coaxial cable or optical fiber

Limited in terms of data rate and distance

Telephone network

*Individual units (residence lines) to local exchange

* Subscriber loops

*Supports voice traffic using analog signaling

*May handle digital data at modest rates using modems

Communications within buildings

*Connection to digital data switch within a building

*Allows data rate of 64 kbps

Transmission characteristics

*Requires amplifiers every 5-6 km for analog signals

*Requires repeaters every 2-3 km for digital signals

*Attenuation is a strong function of frequency

. Higher frequency implies higher attenuation

*Susceptible to interference and noise

* Improvement possibilities

. Shielding with metallic braids or sheathing reduces interference

. Twisting reduces low frequency interference

. Different twist length in adjacent pairs reduces crosstalk

Unshielded and shielded twisted pairs

* Unshielded twisted pair (utp)

. Ordinary telephone wire

. Subject to external electromagnetic interference

* Shielded twisted pair (stp)

. Shielded with a metallic braid or sheath

. Reduces interference

. Better performance at higher data rates

. More expensive and difficult to work compared to utp

Category 3 and Category 5 utp

*Most common is the 100-ohm voice grade twisted pair

*Most useful for lan applications

*Category 3 utp

. Transmission characteristics specified up to 16 MHz

. Voice grade cable in most office buildings

. May have data rates up to 16 Mbps over limited distances

. Typical twist length 7.5 to 10 cm

*Category 4 utp

. Transmission characteristics specified up to 20 MHz

*Category 5 utp

. Transmission characteristics specified up to 100 MHz

. Data grade cable in newer buildings

. May have data rates up to 100 Mbps over limited distances

. Much more tightly twisted, with typical twist length 0.6 to 0.85 cm,

for better performance

Coaxial cable

*Physical description

o Consists of two conductors with construction that allows it to

operate over a wider range of frequencies compared to

twisted pair

o Hollow outer cylindrical conductor surrounding a single inner

wire conductor

o Inner conductor held in place by regularly spaced insulating

rings or solid dielectrical material

o Outer conductor covered with a jacket or shield Diameter

from 1 to 2.5 cm

o Shielded concentric construction reduces interference and

crosstalk

o Can be used over longer distances and support more stations

on a shared line than twisted pair

* Applications

. Most common use is in cable tv

.Traditionally part of long distance telephone network

. Can carry more than 10,000 voice channels simultaneously

using frequency-division multiplexing

. Short range connections between devices

* Transmission characteristics

. Used to transmit both analog and digital signals

. Superior frequency characteristics compared to twisted pair

. Can support higher frequencies and data rates

. Shielded concentric construction makes it less susceptible to

interference and crosstalk than twisted pair

. Constraints on performance are attenuation, thermal noise,

and intermodulation noise

. Requires amplifiers every few kilometers for long distance

Transmission

. Usable spectrum for analog signaling up to 500 MHz

. Requires repeaters every few kilometers for digital

transmission

. For both analog and digital transmission, closer spacing is

necessary for higher frequencies/data rates

Optical fiber

* Thin, flexible material to guide optical rays

* Cylindrical cross-section with three concentric links

1. Core

Innermost section of the fiber

One or more very thin (dia. 8-100 µm) strands or fibers

2. Cladding

Surrounds each strand

Plastic or glass coating with optical properties different from core

Interface between core and cladding prevents light from escaping the core

3. Jacket

Outermost layer, surrounding one or more claddings

Made of plastic and other materials

Protects from environmental elements like moisture, abrasions, and crushing

Comparison with twisted pair and coaxial cable

*Capacity

Much higher bandwidth

Can carry hundreds of Gbps over tens of kms

*Smaller size and light weight

Very thin for similar data capacity

Much lighter and easy to support in terms of weight (structural

properties)

Digital Communication advantages:

Reliable communication; less sensitivity to changes in environmental

conditions (temperature, etc.)

Easy multiplexing

Easy signaling-

Hook status, address digits, and call progress information

Voice and data integration

Easy processing like encryption and compression

Easy system performance monitoring

QOS monitoring

Integration of transmission and switching

Signal regeneration, operation at low SNR, superior performance

Integration of services leading to ISDN

Digital Communication System Disadvantages:

Increased bandwidth64 KB for a 4 KHz channel, without compression

(However, less with compression)

Need for precision timing

Bit, character, frame synchronization needed

Analogue to Digital and Digital to Analogue conversions

Very often non-linear ADC and DAC used, some performance degradation

Higher complexity

HISTORY OF COMMUNICATION SYSTEMS:

1831 Samuel Morse invents the first repeater and the telegraph is born

1837 Charles Wheatstone patents "electric telegraph"

1849 England to France telegraph cable goes into service -- and fails after 8

days.

1850 Morse patents "clicking" telegraph.

1851 England-France commercial telegraph service begins. This one uses

gutta-percha, and survives.

1858 August 18 - First transatlantic telegraph messages sent by the Atlantic

Telegraph Co. The cable deteriorated quickly, and failed after 3 weeks.

1861 The first transcontinental telegraph line is completed

1865 The first trans-Atlantic cable goes in service

1868 First commercially successful transatlantic telegraph cable completed

between UK and Canada, with land extension to USA. The message rate is 2

words per minute.

1870 The trans-Atlantic message rate is increased to 20 words per minute.

1874 Baudot invents a practical Time Division Multiplexing scheme for

telegraph. Uses 5-bit codes & 6 time slots -- 90 bps max. rate. Both Western

Union and Murray would use this as the basis of multiplex telegraph systems.

1875 Typewriter invented.

1876 Alexander Graham Bell and Elisa Grey independently invent the

telephone (although it may have been invented by Antonio Meucci as early as

1857)

1877 Bell attempts to use telephone over the Atlantic telegraph cable. The

attempt fails.

1880 Oliver Heaviside's analysis shows that a uniform addition of inductance

into a cable would produce distortionless transmission.

1883 Test calls placed over five miles of under-water cable.

1884 - San Francisco-Oakland gutta-percha cable begins telephone service.

1885 Alexander Graham Bell incorporated AT&T

1885 James Clerk Maxwell predicts the existence of radio waves

1887 Heinrich Hertz verifies the existence of radio waves

1889 Almon Brown Strowger invents the first automated telephone switch

1895 Gugliemo Marconi invents the first radio transmitter/receiver

1901 Gugliemo Marconi transmits the first radio signal across the Atlantic

1901 Donald Murray links typewriter to high-speed multiplex system, later

used by Western Union

1905 The first audio broadcast is made

1910 Cheasapeake Bay cable is first to use loading coils underwater

1911 The first broadcast license is issued in the US

1912 Hundreds on the Titanic were saved due to wireless

1915 USA transcontinental telephone service begins (NY-San Francisco).

1924 The first video signal is broadcast

1927 First commercial transatlantic radiotelephone service begins

1929 The CRT display tube is invented

1935 Edwin Armstrong invents FM

1939 The blitzkrieg and WW II are made possible by wireless

1946 The first mobile radio system goes into service in St. Louis

1948 The transistor is invented

1950 Repeatered submarine cable used on Key West-Havana route.

1956 The first trans-Atlantic telephone cable, TAT-1, goes into operation. It

uses 1608 vacuum tubes.

1957 The first artificial satellite, Sputnik goes into orbit

1968 The Carterphone decision allows private devices to be attached to the

telephone

1984 The MFJ (Modification of Final Judgement) takes effect and the Bell

system is broken up

1986 The first transAtlantic fiber optic cable goes into service

Summary:

Communication System is to transport an information bearing signal from a

source to a user destination via a communication channel.

Basic blocks of communication systems are transmitter, channel and

receiver.

Digital communication systems are communication systems that use such a

digital sequence as an interface between the source and the channel input.

Modulation is a process that causes a shift in the range of frequencies of the

message signal. Demodulation is its inverse counterpart.

Baseband: refers to the signals and systems before modulation, which have

frequencies/bandwidth much lower than the carrier frequency.

Passband: refers to the signals and systems after (including) modulation,

which have frequencies/bandwidth around the carrier frequency

Bandwidth is the information-carrying capacity of a communication channel.

Objective type questions:

(1) The most common modulation system used for telegraphy is

(a) FSK (b) PSK (c) PCM (d) single tone modulation (e) Two tone modulation

(2)VSB is an abbreviation of vestigal sideband, is derived by filtering

(a) DSB (b) AM (c) either (a) or (b) (d) PM

(3) To send audio signal over long distance, carrier wave of higher frequency is used.

Why? __________________.

(4) Sky wave transmission of electromagnetic wave cannot be used for TV

transmission why?

(5) The advantage of digital communication is ____________.

(a) Voice and data integration (b) Increased bandwidth (c) Higher complexity

(6) ____________ is the information-carrying capacity of a communication channel.

(a) Multiplexer (b) Bandwidth (c) information capacity (d) digital communication

(7) ____________refers to the signals and systems before modulation.

(a) Pass band (b) Bandwidth (c) Base band (d) Multiplexing

(8) ____________refers to the signals and systems before modulation.

(a) Pass band (b) Bandwidth (c) Base band (d) Multiplexing

TWO MARKS:

1. Define Modulation.

2. Draw the model of the basic communication systems.

3. Write down the difference between base band and pass band.

4. List out the advantages of digital communication systems.

5. Compare analog communication systems with digital communication

systems.

6. Brief about receiver parameters

7. Define image frequency.

8. Define Low level Modulation.

9. Define High level Modulation.

10. List out the modulation methods.

16- Marks

1. Write notes on electro magnetic spectrum.

2. Explain the basic representation of a communication system.

3. Compare analog communication with digital communication.

4. Explain receiver parameters

5. Describe high level transmitters with a block diagram.

6. Describe low level transmitters with a block diagram.

7. Describe medium power modulator with circuit diagram.