Electronic Communication LECTURES 01May 2003

DET610Electronic Communication Systems

Text books Kennedy, George. Electronic Communication

SystemsFrenzel, Louis. Principles of Electronic

Communication SystemsCarlston, Bruce A. Communication Systems

1Farrukh Aziz Bhatti

Communication

• It is a process of exchanging information.• Distance has been the main barrier in communication

since early times.• In early times long distance communication was

accomplished by sending simple signals like drum beats, smoke signals, waving flags etc.

• Communication took a dramatic leap forward in late nineteenth century, when electricity was discovered. Telegraph (1844), telephone (1876) and radio (1887) were discovered subsequently.


Communication Systems

Transmitter(TX)

Communication channel or medium

Receiver (RX)

Noise

Information (audio, video, computer data)

Recoveredinformation


Transmitter• First step is to convert the information into an electrical

form suitable for transmission. e.g. A microphone translates voice to audio signal. A camera converts the light information in the scene to an image or video signal. In computer systems the message typed from a keyboard is converted into binary codes and transmitted serially. Transducer converts physical quantities (temperature, pressure etc) into electrical signals.

• Transmitter is a collection of electronic components (oscillators, amplifiers, tuned circuits, filters etc) and circuits designed to convert the input electrical signal to a signal suitable for transmission over a given communication medium.

• The original signal modulates a high-frequency carrier, and the combination is raised in amplitude by power amplifiers.


Communication Channel• Communication channel is the medium by which the

electronic signal is sent from one place to another. E.g. Copper wires, fiber-optic cable, free space etc

• Electrical conductors: e.g. Coaxial cable, twisted pair cable (LAN)

• Optical Media: e.g. Fiber-optic cable used for internet communication etc

• Free Space: Radio communication using electromagnetic waves. Communication by visible or infrared light.

• Other Types of Media


Communication channel

• Other Mediums: Sonar (for SOund Navigation And Ranging) is a technique that uses sound propagation to navigate, communicate with or detect other vessels. Water is the medium.

• AC power lines are used as communication medium carrier current transmission. Used for some voice intercoms or to control electrical equipments.


Receiver

• A receiver is a collection of electronic components and circuits that accepts the transmitted signal from the medium and converts it back to a form understandable to humans.

• Transceivers: Most electronic communication is two way, so it has both transmitter and reciever


Gain

• Gain means amplification.• It is a ratio of the output to input.• Av= Vout/Vin

• Ap=Pout/Pin

• Exp• What is the voltage gain of an amplifier that produces an

output of 750mV for a 30 μV input?

• Gain of cascaded circuits.


Attenuation

• Attenuation is the loss introduced by circuit.• It has a gain <1• The reduction in the strength of the signal as it

propagates through the medium is called attenuation.• In practical mediums, attenuation is inevitable.

• Decibels


Electromagnetic Spectrum


Electromagnetic Spectrum


Communication applications

• Telephone• Radio• TV• Cellular network• Electronic mail• Pager• Remote control• Satellite Communication• Global Positioning System


Noise

• It is an unwanted signal or energy that interferes with the proper and easy reception and reproduction of wanted signals. E.g. noise may produce a hiss in the loudspeaker output in a radio, and in TV ‘snow’ becomes superimposed over video.

• External Noise: Noise created outside the receiver, e.g. Atmospheric noise, solar noise, cosmic noise

• Internal Noise: Noise created inside the receiver.


Tuned Circuits

• All communication equipment usually contains tuned circuits, made up of inductors and capacitors that resonate at specific frequencies.

• Reactive Components: Inductors (coils) and capacitors, they offer opposition to the alternating current, which is called reactance (expressed in ohms).

• Similar to resistance, but in this phase shift between current and voltage is also involved.

• Capacitance causes the current to lead the voltage.• Inductance causes the voltage to lead the current in the

circuit.


Capacitors

• A capacitor used in an ac circuit continually charges and discharges.

• It opposes voltage change across it, that causes an opposition to ac current known as capacitive reactance Xc.

• Xc = 1/2πfC• Example: Find reactance of a 100-pF capacitor at 2MHz


Inductors

• An inductor also called a choke or coil, is simply a winding of multiple turns.

• When current is passed through a coil, a magnetic field is produced around the coil. If the applied voltage and current are varying, the magnetic field alternately expands and collapses. This causes a voltage to be self-induced into the coil, which has the effect of opposing current changes in coil. This is inductance.

• Unit of inductance is henry (H).• Depends on physical characteristics of coil, no of turns,

spacing in turns, length of coil, diameter of coil, type of magnetic core material.

• It opposes change in voltage, the inductive reactance XL is expressed as XL=2πfL (Ohm)


Capacitors and Inductors


Series Resonant Circuits

• Consists of inductance, capacitance and resistance

• Inductive and capacitive reactance depends on freq of applied voltage.

• Resonance occurs when XL=XC

• Total impedance of the circuit is given as

• At resonance the total circuit impedance is simply all resistances added up


Series Resonant Circuits

Resonant frequency is found by equating XL and XC


Bandwidth of series resonant circuit

BW=f2 – f1 BW=fr/Q


Filters

• Filters pass the desired frequencies and reject others.• The series and parallel resonant circuits are examples of

filters.• Types of filters: Low pass filter, High pass filter

Bandpass filter, Band-reject filter, All pass filter• Practical circuits have gradual transitions at cut-off

frequencies.


Low-pass filter

The cut-off frequency in this filter is the point where R=Xc

Filter has a voltage gain of -3dB at the cut-off point


High-pass filter

Decade means 10 times relationship e.g. From 100 to 1000 Hz there is an increase of a decade

Octave means doubling e.g. From 100 to 200 Hz lies a shift of one octave


Bandwidth

• It is the portion of spectrum occupied by a signal.• Bandwidth is the difference between the upper and lower

frequency limits of the signal.


The need for modulation

Modulation is a process of having a baseband signal like voice, video etc,modify another high-frequency signal ‘the carrier’. The information signal changes one of the parameters of the carrier frequency in proportion to theinformation signal.


The need for modulation• Modulated carrier transmitted• Problems with transmitting baseband signals

– Antennas difficult at low frequencies

– Noise and interference at low frequencies

– Can’t share with others• Easier to transmit carrier at higher frequency

– Can choose convenient frequency• Antennas can be smaller• May be useful propagation effects

– Fractional bandwidth much smaller (BW/Fc)• Antennas and other components easier to design• Can have many frequency channels


Modulation


Various Modulation schemes

Phase Shift keyingFrequency Shift keying


Amplitude Modulation


The peak value of the carrier is the reference point for modulating signal.




Farrukh Aziz Bhatti 31

The zero reference line of the modulating signal coincides with the peak value of unmodulated carrier

The amplitude of the modulating signal should be less than the amplitude ofcarrier signal

The instantaneous value of the modulating signal algebraically adds to peak value of the carrier.

Carrier waveform characteristic of AM



Example: If carrier voltage is 9Vand modulating signal is 7.5V, thenthe modulation index m is 0.8333and percentage of modulation is 0.8333 100 =83.33

Overmodulation in AM


Modulation index should be between 0 and 1

If the amplitude of modulating signal is higher than the carrier voltage m>1causing distortion

Ideal condition is m=1, which gives 100% modulation

Amplitude modulation


Frequency components in AM• Whenever a carrier is modulated by an information signal, new

signals at different frequencies are generated as a result. These new frequencies are called side-bands, and they occur directly above and below the carrier frequency.

• If the modulating signal is a simple sine wave then two sidebands are generated. But if the modulating signal is a complex signal like video, then a whole range of side-bands is generated.

Farrukh Aziz Bhatti 35carrier lower side-band upper side-band

Frequency components in AM wave


Power components in AM wave• The total transmitted power is the sum of the carrier power and the

power in two sidebands

• Vc and Vm are peak values, for power calculation rms values of voltages should be used. RMS value is obtained by dividing peak value by

• The power in carrier and side-bands can be calculated by using power formula where P is the output power, V is the rms output

voltage and R is the resistive part of the load impedance, which is usually an antenna


Power components in AM wave


Example: The carrier of an AM transmitter is 1000W andit is modulated 100 percent, find the total AM power.

Since is equal to the rms carrier power Pc

Single-Sideband Modulation

• In AM two-third of the transmitted power is in the carrier, which itself conveys no information. The real info is in sidebands

• One way to improve the efficiency is to suppress the carrier and eliminate one sideband.

• The first step in this is to suppress the carrier, leaving the upper and lower sidebands. This is called double-sideband suppressed carrier (DSSC or DSB). The benefit is that no power is wasted in carrier.

• This signal is algebraic sum of two sidebands.• The time domain DSB signal is a sine wave at carrier freq, varying

in amplitude.• It has phase transitions at zero crossings


SSB signals

• Conserves the spectrum• All the power previously going into carrier and other

sideband is channel into single sideband.• SSB occupies small BW, so amount of noise is reduced• Less fading over long distances• Main disadvantage of DSB and SSB is that they are

harder to recover. Carrier has to be regenerated for demodulation. The regenerated carrier must have same phase and frequency.


Application of DSB and SSB• DSB signals are used in FM and TV broadcasting to transmit two-

channel stereo signals.• Video signal also uses AM. It contains both video and audio signals.

Video info contains frequencies as high as 4.2MHz. A fully amplitude modulated signal would then contain 2(4.2)=8.4MHz. This is an excessive amount of BW, so to reduce it part of the lower sideband is suppressed reducing it to 6MHz BW.

Farrukh Aziz Bhatti 41TV signal spectrum

Types of Amplitude Modulators

• Amplitude modulators are generally of two types: low level or high level.

• Low-level modulators generate AM with small signals and thus must be amplified considerably if they are to be transmitted. E.g. Diode Modulator, Transistor Modulator, PIN diode Modulator, Differential Amplifier Modulator etc

• High-level modulators produce AM at high power levels, usually in the final amplifier stage of a transmitter. E.g. Collector Modulator (has a high power class C amplifier in its output stage), Series Modulator.

AM Modulation circuits

• Different circuits for AM are available• Diode modulator• Transistor modulator• Differential amplifier modulator• Today most AM circuits come in IC (integrated circuit)

form


Diode based Amplitude Modulator


Diode base Amplitude Modulator• The carrier and modulating signal are applied across the input

resistors. The sum appears across Rsum. This causes algebraic summing of the carrier and modulating signal.

• Modulation is multiplication not summation.

• The composite waveform is applied to diode rectifier.

• The current through diode is a series of positive going pulses whose amplitude varies in proportion to modulating signal.

• These pulses are applied to parallel tuned LC circuit, which are resonant at carrier frequency.

• Each time diode conducts, a pulse of current flows through LC circuit, the coil and capacitor repeatedly exchange energy, causing an oscillation at resonant frequency. The oscillation of tuned circuit creates one negative half cycle for every positive input pulse.

• The resulting output waveform is an AM signal.


Amplitude Demodulation Circuits

• Demodulators are the circuits that accept the modulated signals and recover the original modulating (or information) signal. They are radio receiver circuits.

• Simplest demodulator is the diode detector.• AM signal is transformer-coupled and applied to a half wave rectifier

(diode and resistor). The diode conducts during the positive half cycles of the AM signal. During the negative half cycles the diode is reverse biased and no current flows through it.

• As a result the voltage across Rd is a series of positive pulses whose amplitude varies with modulating signal.

• A capacitor CF connected across Rd filters out the carrier and recovers the original modulating signal.


Amplitude Demodulation Circuits


Fundamentals of Frequency Modulation

• A sine wave can be modulated by varying its amplitude , frequency or phase .

• When the frequency of the carrier is varied in proportion to the modulating signal then it is called frequency modulation (FM).

• When the amplitude (A) of the modulating signal increases the frequency of the carrier increases, when A decreases the carrier frequency decreases.

• The reverse relationship could also be implemented.

• The amount of change in carrier frequency produced by the modulating signal is known as frequency deviation fd.

• The frequency of the modulating signal determines the frequency deviation rate. E.g. a 500 Hz modulating signal, shifts the carrier frequency above and below the centre frequency 500 times per sec.


Frequency Modulation

• Assume a carrier freq of 150 MHz. If the peak amplitude of modulating signal causes a maximum shift of 30 kHz, the carrier freq will deviate upto 150.03 MHz and down to 149.97 MHz. The frequency deviation will be represented as 30kHz.

• The frequency of modulating signal has not affect on the amount of deviation, which is a function of amplitude of the modulating signal.


Concept of PM

• When the amount of phase shift of a constant-frequency carrier is varied in accordance with a modulating signal, the resulting output is a phase modulation (PM) signal.

• During PM as the modulating signal goes positive, the amount of phase lag, and thus the delay of the carrier output, increases with the amplitude of the modulating signal. The result, is as if the carrier signal has been stretched out or its frequency lowered.

• During increase or decrease of the modulating signal a varying frequency is produced. However during constant amplitude positive and negative peaks, no frequency change takes place.


FM and PM comparison


Phase Modulation Frequency Modulation

Sidebands of FM signal• Any modulation process produces sidebands.• In FM and PM, as in AM, some and difference sidebands frequencies

are produced. In addition, a large number of pairs of upper and lower sidebands are generated. As a result the spectrum of FM/PM signal is usually wider than that of an equivalent AM signal.

• The sidebands are spaced from the carrier and from one another by a frequency equal to the modulating frequency fm.

• As the amplitude of modulating signal varies, the frequency deviation changes. The number of sidebands produced, their amplitude and spacing, depend on the frequency deviation and modulating frequency.

• FM signal is a summation of sidebands, the sideband amplitudes must vary with frequency deviation and modulating frequency if their sum is to produce a constant amplitude.


FM sidebands

• Theoretically FM produces infinite sidebands. However only those sidebands with largest amplitudes are considered. Typically sidebands with amplitude less than 1% of the un-modulated carrier is considered insignificant.


Modulation index in FM

• The ratio of frequency deviation to the modulating frequency is known as modulation index.

• In most communication systems using FM, limits are put on both frequency deviation and modulating frequency. In standard FM broadcast max freq deviation allowed is 75kHz. Max permitted

modulating freq is 15kHz. This gives a modulation index of 5.


Bessel Functions• Given the modulation index, the number and amplitudes of the

significant sidebands can be determined by solving the basic equation of an FM signal.

• The term whose coefficient is mf is the phase angle of the carrier, and it is expressed in terms of sine wave of modulating signal.

• This equation is solved with a complex mathematical process called Bessel functions (out of scope). The result of this is

•


FM solution with Bessel functions• FM wave is expressed as a composite of sine waves of different

frequencies and amplitudes, which add up to give FM signal in time domain.

• The first term is the carrier with an amplitude given by a Jn coefficient. The coefficient for the carrier is J0. The next term represents a pair of upper and lower side frequencies (or sidebands) . The amplitude of these side frequencies is J1. Similarly the next pair of side frequencies have J2 amplitude.

• The amplitudes (Jn) of the side frequencies are calculated by a complex formula that considers mf. However you do not need to know or calculate these amplitudes, as they have been calculated and put in tables for different values of mf (modulation index).


Finding FM sidebands with Bessel functions


pairs

Calculating FM sidebands


Any FM signal with modulation index less than 1.57 is called narrowband FM (NBFM). As it has a single pair of sidebands (like AM).

Narrowband FM

• Common FM mobile radios use a maximum deviation of 5 kHz, with a max voice frequency of 3 kHz, giving a modulation index mf= 5kHz/3kHz=1.667. This lie close to NBFM.

• Example: State the amplitudes of the carrier and first four sidebands of an FM signal with a modulation index of 4. (Use Bessel functions table)


FM Signal Bandwidth (BW)

• The higher the modulation index in FM, the greater is the number of significant sidebands and the wider is the FM signal bandwidth.

• The total BW of an FM signal is determined by knowing the modulation index and using the Bessel functions table. The BW is determined by the simple formula

• For example, if the highest modulating freq is 3kHz and max frequency deviation is 6 kHz. Then fm=6kHz/3kHz=2. Referring to Bessel table, we can this produces four significant pairs of sidebands. So the BW = 2(3kHz)4 = 24kHz.


FM versus AM• Advantages of FM

• FM is generally considered superior to AM because of some benefits it offers.• Noise immunity. This is the main benefit of FM over AM. FM has clipper limiter

circuits in rxr, which effectively strip off all the noise variations, leaving a constant amplitude.

• Capture Effect. Because of amplitude limiter circuits and the demodulating methods used by FM rxr, a phenomenon known as capture effect takes place when two or more FM signals occur at same freq. If one signal is stronger than the other by a certain extent (1 dB for modern FM rxrs) the stronger signal captures the channel, totally eliminating the weaker signal. Whereas if this happens in AM then the weaker signal could still be heard in the background and is not totally eliminated.

• Transmitter Efficiency. In AM txrs, it must produce both very high RF and modulating signal power. The AM signal are generated at lower levels and then amplified with linear amplifiers to produce final RF signal. Linear amplifiers (class A, B) are used to avoid distortion and they are far less efficient than class C amplifiers. FM signals have a constant amplitude, and it is therefore not necessary to use linear amplifiers, so class C amplifiers are directly used in them, hence giving more efficiency.


FM versus AM

• Advantages of FM

• Excessive Spectrum use. Greatest disadvantage of FM is that they use more spectrum space (high BW). BW usage can be minimized by reducing frequency deviation, as in commercial broadcast the max allowed deviation is 5kHz, with a maximum modulating freq of 3kHz. Since FM takes large BW so it is normally used at higher frequencies above 30MHz, where spectrum space is availale.

• Circuit Complexity. FM txr and rxrs have higher circuit complexity. But today this advantage has almost disappeared because of IC’s. Though FM IC’s are still more complex than AM.


Application of FM• WIDEBAND FM (WFM)

• FM BROADCASTING

Commercial FM broadcasts occur in the VHF range, between 88 and 108 MHz. The carrier frequencies start at 88.1 MHz and are separated by 200 KHz intervals. The maximum audio bandwidth allowed is 15 KHz and the deviation is limited to +/- 75 KHz. Limiting the deviation to this value leaves a 25 KHz guard band at each end of the channel that limits inter-channel interference.

The DR for commercial FM broadcasting is 75/15 = 5.0. This is clearly a wideband FM signal.

• TELEVISION AUDIO

The audio portion of an analog (NTSC) television signal is sent using FM. The maximum permitted frequency deviation is 25 KHz, and the maximum audio frequency is 15 KHz. The DR for commercial FM broadcasting is 25/15 = 1.67. This is a wideband FM signal.

POINT-TO-POINT MICROWAVE

Telephone companies have made extensive use of microwave systems for medium and long distance trunk circuits, although many of these microwave systems are being replaced by optical fiber. A typical point-to-point digital microwave system such as the ATT DR6-30 operated with a carrier frequency of 6 GHz and carried 9408 3 KHz voice channels in a bandwidth of 210 MHz. The effective DR is 210 MHz/(3KHz*9408) = 7.44.


Application of FM

• NARROW BAND FM (NBFM)

• NBFM is widely used in business and public service communications. The DR for NBFM is restricted to values between 0.5 and 1.0. By holding the DR to such small values, only the carrier and the first sideband are of significant amplitude. When only one sideband and the carrier are transmitted, the NBFM signal occupies the same bandwidth as an AM signal. This overcomes one of the drawbacks of wideband FM, the large bandwidth required.

• The FCC permits the bandwidth of NBFM signals to be from 10 to 30 KHz, depending on the assigned carrier frequency and the type of operation authorized.


Digital Communication Systems

• Digital Communication techniques

• Multiplexing and Demultiplexing

• Binary data in the communication systems


Digital Transmission of Data

• The term data refers to information to be communicated.• Data is in digital form if it comes from a computer. Voice, video etc

are typically analog signals.• Digital communication was originally limited to the transmission of

data between computers e.g Local Area Network (LAN).• PC’s communicating via the telephone line (using modem) is

another example.• Since the spread of computers in 1970’s, the need for efficient

exchange of data was felt (using diskettes was inefficient). Hundreds of millions of PC’s are now used across the world, and digital communication techniques have been used for enabling efficient communication between PC’s and from PC’s to other devices like telephones.


Digital Transmission of DataSome Applications• Some examples of computer data communication are File transfer. Transfer of files, databases, company records, bank records etc

Electronic mail. Excessively used for individual communication

Computer peripheral links. PC to printer

Internet access. Most effective source of info

Local-area Networks (LANs). Group of computers in a setup are linked

TV remote control. Binary signals generated by pushbuttons modulate an infrared light beam for TV control

Garage door opener. A unique binary code modulates VHF or UHF signal for opening door.


Benefits of Digital Communication

• Noise immunity• Error Detection and Correction• Compatibility with time division multiplexing• Digital IC’s• Digital signal processing

• Disadvantages • More bandwidth needed for digital signals• More complexity


Parallel and Serial Data transmission

• Parallel transfer. All the bits of a code word are transferred simultaneously.

• The binary word to be transmitted is loaded into a register(temporary memory location) containing one flip-flop for each bit. Each flip-flop is connected to a wire.

• Parallel communication is much faster but not suitable for long distance communication. Data bus is typically used inside PC’s for parallel data transfer.


• Serial Transfer. • Data transfer in communication systems are made

serially; each bit is transmitted one after the other. Least significant bit (LSB) is transmitted first and MSB at last.

• There are 8 bits in a byte.• The transfer rate is affected by the type and length of

medium. E.g copper wire, fibre optic.


Analog to Digital (A/D) conversion

• Conversion from analog to digital and vice-versa is very important in digital communication.

• Three important steps in A/D conversion are– Sampling– Quantizing– Encoding

A key factor in sampling process is ‘sampling frequency’. It should be at least twice the highest frequency component of the analog signal. This minimum sampling frequency is known as Nyquist frequency.


A/D conversion


Example: The output of an FM radioIs to be digitized. The maximum audioFrequency is 15kHz. To ensure that the highest frequency is restored, thesampling rate must be at least 2 times15kHz , i.e. 30kHz. But in practice itIs kept at around 3 times. 3 times 15kHz Gives 45kHz sampling frequency.

• The higher the number of bits per sample, lower is the quantization error.

• Example. Assume a 10bit A/D converter, with voltage levels or 1024-1=1023 increments. Assume the input voltage range from 0 to 6V. The minimum voltage step increment then is 6/1023=5.865 mV. This is the maximum quantization error that can occur. Average error is half of this.


D/A Conversion

• From computer memory, digital data has to reproduce the original analog signal (e.g) listening to music on PC.

• The output of D/A has a stairstep characteristic.• It is then passed through a Low-pass filter with an

appropriate cut-off frequency. This makes the output smooth.

• If the digital signal contains more bits/sample, than the analog output is more closer.


Aliasing• If the sampling frequency is less than the Nyquist frequency, an

undesired phenomenon called aliasing occurs. The analog signal cannot be properly retrieved because of that.


Multiplexing• Multiplexing is the phenomenon of simultaneously

transmitting two or more individual signals over a single communication channel (wire, fiber optic or wireless).

• The greatest use of multiplexing is in the telephone system, where millions of calls are multiplexed on cables, long distance fiber-optic lines, satellites and wireless paths. Multiplexing increases the telephone carrier’s ability to handle more calls while minimizing spectrum usage.

• Multiplexing is also used for FM stereo broadcast and for transmission of colour and stereo sound in TV signal.


Multiplexing

• Multiplexing is done using an electronic circuit called multiplexer.• At the other end of the communication link a demultiplexer is used

to process the composite signal to recover individual signals.


Types of Multiplexing techniques

• The most common types of multiplexing are• Time Division Multiplexing (TDM)• Frequency Division Multiplexing (FDM)• Code Division Multiple Access (CDMA)• Space Division Multiplexing (SDM)

• TDMA (time division multiple access) and FDMA (frequency division multiple access) are extensions of TDM and FDM.

.


Frequency Division Multiplexing• Individual signals to be transmitted are assigned a different frequency

within a common bandwidth. All channels have specific BW e.g. coaxial cable has a BW of about 1GHz.

• Each signal to be transmitted is modulated by a different carrier frequency called a subcarrier. These subcarriers are equally spaced in the given BW. The resulting spectrum is shown in this figure below.

• Any kind of modulation can be used like AM, FM, PM etc• The modulator outputs are added algebraically added in a linear

mixer; no modulation or generation of sidebands takes place here.• The resulting composite signal can be sent on a wired channel or can

modulate a transmitter for radio transmission.


Transmitter and Receiver of an FDM system


Applications of FDM• Telephone Systems. Telephone systems used FDM to

send multiple telephone conversations over a minimum cables. The original voice signal is in 300 – 3kHz range, and it modulates a subcarrier. LSB (SSB) AM was used. Each subcarrier is on a different frequency. The multiplexing process is used at different levels so that upto 10800 phone calls can be carried on a single channel.

• Cable TV. Cable TV also uses FDM. Each TV channel has a BW of 6MHz, many TV channels are multiplexed on a coaxial cable or fiber optic cable. The TV receiver can select the desired frequency channel from the composite signal.

• FM Stereo. This uses two microphones to generate two separate audio signals that pick up sound from common source. These are multiplexed using FDM and transmitted as a composite signal.


Time Division Multiplexing

• In general it is used for digital information, though TDM is also sometimes used for analog information, because A/D and D/A techniques are so common.

• The individual signals are transmitted on entire bandwidth of the channel, however each signal only uses a specific time slot.


TDM• Sampling an analog signal produces Pulse Amplitude Modulation (PAM). The

analog signal is converted to a series of constant width pulses whose amplitude follows the shape of the analog signals.

• Shown below is the simple rotary switch multiplexer. The rotary arm rotates and momentarily stops on each contact. Allowing each channel to pass through for a fixed duration. The remaining channels are sampled in a similar way. The result is that the individual channels are sampled and interleaved with one another. The sampling rate is proportional to the speed of rotation of switch.


Binary Data in the Communication System

• Since the mid 1970’s when digital communication started to expand, the need for efficient methods of transmission, conversion and reception of digital data was felt.

• As with analog data, the amount of digital data that can be sent on a given bandwidth is finite and depends upon some important factors.

• The data stored in computers is in the form of codes. A code consists of 1 and 0. Different codes have been developed like Morse codes in the early times of communication.

• For modern data communication data is represented by a 7 bit binary code called American Standard Code for Information

Interchange (ASCII).


ASCII Codes



Serial transmission of ASCII characters

• Transmission rate

• Bit rate= baud rate bits per symbol


Digital Carrier Systems

• The most widespread use of TDM is in telephone system. All modern telephone systems use digital transmission via Pulse Code Modulation (PCM) and TDM.

• All local and long distance calls use digital signals.• The T-carrier system is used across United States and many other

countries. • The physical implementation of this is referred to as T-1, T-2 T-3

and T-4. The digital signals they carry are called DS1, DS2, DS3 and DS4.

• It begins with T1 that multiplexes 24 basic DS1 digital voice signals, that are then multiplexed (TDM) into larger and faster DS2, DS3 and DS4 signals.



T-1 line can also transmit a single source of computer data at 1.544 Mbps


96 voice channels

96*7=672Voice channels

Network Communications• A network is a communication system with two or more stations that

can communicate with one another. When it is desired to have each computer communicate with two or more additional computers, the interconnections can become complex.

• The number of links L required between N PCs (nodes) is determined as:

• Example: An office with 20 PCs is to be

be wired so that any computer can

Communicate with any other. How many

interconnecting wires (L) are needed?


Types of Network• Wide-Area Networks (WANs). The wide area network, often

referred to as a WAN, is a communications network that makes use of existing technology to connect local computer networks into a larger working network that may cover both national and international locations. This is in contrast to both the local area network and the metropolitan area network, which provides communication within a restricted geographic area. The worldwide fiber-optic networks (setup since mid 90s) carry internet traffic are also WANs.

• Metropolitan-Area Networks (MANs). A (MAN) is a large computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area network (LANs) using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks and the Internet. Cable TV systems are MANs.


Types of Network• Local area network (LAN). is a computer network covering a small

physical area, like a home, office, or small group of buildings, such as a school, or an airport. The defining characteristics of LANs, in contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased telecom lines. PCs in a lab make a LAN.

• Personal Area Network (PAN).The reach of a PAN is typically a few meters. PANs can be used for communication among the personal devices themselves (intrapersonal communication), or for connecting to a higher level network and the Internet (an uplink). E.g. A mobile phone links with a laptop using blue tooth.

• storage area network (SAN) is an architecture to attach remote computer storage devices (such as disk arrays, tape libraries, and optical jukeboxes) to servers in such a way that the devices appear as locally attached to the operating system. A SAN typically has its own network of storage devices that are generally not accessible through the regular network by regular devices.


Networking hierarchy


Network topologies

• Star Topology. It has a controlling

node, called server. All nodes are linked

to it. Any communication between two

nodes passes through the server. It is

simple and straight forward.

• Ring Topology. In this the server and

all other nodes (often PCs) are attached

in a single closed loop. Usually data is

transferred around in one direction passing

through all nodes. Its easy to implement

low cost, easily expandable.


Network Topologies

• Bus Topology. A bus is a

common cable to which all the nodes

are attached. Bus is bi-direcitonal.

Signal can be destined to a single

node or all nodes.• Mesh Topology. In this each node

Is connected to all other nodes.

It is more costly. Benefit is that it

provides redundancy for broken

paths, so gives more reliable network

communication.


Local Area Network

• Software. Network Operating Software (NOS)

• Hardware. Hubs, connectors, routers etc


NOS, what is it for?

• Proper support of a local area network requires hardware, software, and miscellaneous support devices.

• A network operating system is the most important software component.

• Application programs are also required to support users on a LAN.

• Support devices such as hubs, switches, routers, servers, modems, power supplies, and more are also necessary.


• An operating system manages all applications and resources in a computer.

• A multitasking operating system supports the execution of multiple processes at one time.

• A network operating system is a large, complex program than manages the resources common on most local area networks.

• Besides performing standard operating system functions, a network operating system is called upon for additional functions, such as …



LAN Hardware components

Transmission Media

Interface Cards

Repeaters

Hubs

Bridges

Switches

Routers

Gateways

Firewalls


LAN Transmission Media


Twisted Pair

Coaxial Cable

Woven Metal Shield

Plastic insulating jacket Non-conducting insulator

Central Copper Conduit Plastic Insulating Jacket

Central Copper Conduit

Fiber OpticGlass or Plastic Fiber Cable

Photodiode ReceiverLED or Laser Transmitter

High BW, high data rates. It is shielded, so it is protectedfrom external noise.

Two insulated copper wires twisted together. Connectindividual telephones to centraloffice.

Used for very high data rates.Made up of glass or plastic core

Repeaters


Repeater

Repeaters

• Repeaters repeat a signal from one port to another.• Repeaters pass all traffic through without error checking or filtering.. • Repeaters pass collisions, too.• Repeaters are used primarily to overcome maximum segment length restrictions.

telnet

Hubs


Hub

Hubs…

• Hubs propagate a signal received on one port to all other ports..• Hubs propagate errors and collisions across ports, too. • Hubs simplify the addition and removal of nodes on a LAN.• Hubs are also used to connect network segments cabled with different media types.

telnet

Hubs make itvery easy to addand remove hostson a network.

Bridges


Bridge

Hub

Bridges

• Bridges provide all the functionality of a hub, PLUS ...• Bridges filter frames by destination MAC, and segment a LAN into multiple collision

domains.• Bridges filter signal and timing errors.• Bridges can be used to connect segments operating at different speeds.

telnet telnet

HubBridges make itpossible to segmentyour network intoseparate collisiondomains to minimize collisions and improve performance.

Separate Collision Domains

Switches


Switches

• Switches provide all the functionality of a bridge PLUS ...• Switches typically offer more ports than bridges. • Switches allow for multiple, parallel channels of communication between ports.• Switches sometimes offer “full-duplex” functionality.• Switches are replacing both bridges and hubs in many modern networks.

Switch

telnet telnet

Routers and Gateways


Router

Router Router Router

Routers and Gateways

• Routers use IP addresses to route data between networks.• Routers can be used to connect different network types.• Routers don’t forward broadcast packets; broadcast packets are dropped.• Gateways are used to connect dissimilar networks over all 7 OSI layers

Gateway

Mainframe

Firewalls


Internet Firewall

Firewalls

• Firewalls determine what traffic is allowed in and out of your network.• Firewalls may filter packets by IP or port number.• Firewalls may log what packets are sent to and from whom.• Firewalls use these and many other features to improve network security.

Firewalls make it possible to control access to and fromyour local area network.

Putting all together makes ‘LAN’


Bridge(chicago office)

Hub(sales)

Hub(research)

Router

Firewall

Switch(london office)

Router

Internet

Gateway

Mainframe

Transmission Lines

• Transmission lines in communication carry telephone signals, computer data in LANs, TV signals in cable TV systems, and signals from a transmitter to an antenna or from an antenna to a receiver. Transmission lines are critical links in any communication systems. They are more than pieces of wires or cables.

• Their electrical characteristics are critical and must be matched to the equipment for successful communication to take place.

• Transmission lines are also circuits. At very high frequencies, they act as resonant circuits and even reactive components.

• At VHF, UHF and microwave frequencies, most tuned circuits and filters are implemented with transmission lines.



Wavelength of a 60 Hz power AC line signal is



• An RF (Radio Frequency) generator connected to a transmission line sees an impedance that is a function of the inductance, resistance and capacitance in the circuit , this is called Characteristic Impedance (Z0).

• If we assume the length of line to be infinite then Z0 is resistive.



(Conductance)

Standing wave


Standing Waves


Standing Wave Ratio


Stub


Impedance matching by using stub



Electronic Communication LECTURES 01May 2003

Documents

radio communication

internet communication

given communication

electronic signal

reciever farrukh aziz

input electrical signal

gain gain

audio signal