communucation system

Modern Electronic Communication

2011-08-21T23:46:00.001-07:00

Electronics is a subfield within the wider electrical engineering academic subject. An academic degree with a major in electronics engineering can be acquired from some universities, while other universitites use electrical engineering as the subject. The term electrical engineer is still used in the academic world to include electronic engineers. However, some people think the term 'electrical engineer' should be reserved for those having specialized in power and heavy current or high voltage engineering, while others believe that power is just one subset of electrical engineering (and indeed the term 'power engineering' is used in that industry) as well as 'electrical distribution engineering'. Again, in recent years there has been a growth of new separate-entry degree courses such as 'information engineering', 'systems engineering' and 'communication systems engineering', often followed by academic departments of similar name, which are typically not considered as subfields of electronics engineering but of electrical engineering.
Beginning in the 1980s, the term computer engineer was often used to refer to a subifeld of electronic or information engineers. However, Computer Engineering is now considered a subset of Electronics Engineering and computer science and the term is now becoming archaic.

Electronic engineering as a profession sprang from technological improvements in the telegraph industry in the late 19th century and the radio and the telephone industries in the early 20th century. People were attracted to radio by the technical fascination it inspired, first in receiving and then in transmitting. Many who went into broadcasting in the 1920s were only 'amateurs' in the period before World War I.^[9]
The modern discipline of electronic engineering was to a large extent born out of telephone, radio, and television equipment development and the large amount of electronic systems development during World War II of radar, sonar, communication systems, and advanced munitions and weapon systems. In the interwar years, the subject was known as radio engineering and it was only in the late 1950s that the term electronic engineering started to emerge.
The electronic laboratories (Bell Labs in the United States for instance) created and subsidized by large corporations in the industries of radio, television, and telephone equipment began churning out a series of electronic advances. In 1948, came the transistor and in 1960, the integrated circuit to revolutionize the electronic industry.^[11]^[12] In the UK, the subject of electronic engineering became distinct from electrical engineering as a university degree subject around 1960. Before this time, students of electronics and related subjects like radio and telecommunications had to enroll in the electrical engineering department of the university as no university had departments of electronics. Electrical engineering was the nearest subject with which electronic engineering could be aligned, although the similarities in subjects covered (except mathematics and electromagnetism) lasted only for the first year of the three-year course.

Signal Analysis and Transmission

2011-08-13T22:03:00.000-07:00

http://www.smtkmr.com/

Baseband bandwidth
A baseband bandwidth is equal to the highest frequency of a signal or system, or an upper bound on such frequencies,^[1] for example the upper cut-off frequency of a passband filter. By contrast, passband bandwidth is the difference between a highest frequency and a nonzero lowest frequency.

Baseband channel

A baseband channel or lowpass channel (or system, or network) is a communication channel that can transfer frequencies that are very near zero.^[2] Examples are serial cables and local area networks (LANs), as opposed to passband channels such as radio frequency channels and passband filtered wires of the analog telephone network. Frequency division multiplexing (FDM) allows an analog telephone wire to carry a baseband telephone call, concurrently as one or several carrier-modulated telephone calls.

Digital baseband transmission

Main article: Line code

Digital baseband transmission, also known as line coding,^[3] aims at transferring a digital bit stream over base-band channel, typically an unfiltered wire, as opposed to passband transmission, also known as carrier-modulated transmission.^[4] Passband transmission makes communication possible over a bandpass filtered channel, such as the telephone network local-loop or a band-limited wireless channel.
An unfiltered wire is intrinsically a low-pass transmission channel, while a line code is intrinsically a pulse wave signal that occupies a frequency spectrum of infinite bandwidth. According to the Nyquist theorem, error-free detection of the line code requires a channel bandwidth of at least the Nyquist rate, which is half the line code pulse rate.

Baseband transmission in Ethernet

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 and an unfiltered wire are used.
This is as opposed to 10PASS-TS Ethernet, where "PASS" implies passband transmission. 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 network access technologies are wireless networks and cable modems.

Baseband signal

A baseband signal or lowpass 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).^[5]
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. A baseband signal is a low frequency signal which when modulated is transmitted on various channels.
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.

Equivalent baseband signal

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 passband 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.

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 bandpass 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

power line communiation

2011-08-03T09:56:00.000-07:00

Power line communication or power line carrier (PLC), also known as Power line Digital Subscriber Line (PDSL), mains communication, power line telecom (PLT), power line networking (PLN), or Broadband over Power Lines (BPL) are systems for carrying data on a conductor also used for electric power transmission.
Electrical power is transmitted over high voltage transmission lines, distributed over medium voltage, and used inside buildings at lower voltages. Powerline communications can be applied at each stage. Most PLC technologies limit themselves to one set of wires (for example, premises wiring), but some can cross between two levels (for example, both the distribution network and premises wiring). Typically the transformer prevents propagating the signal, which requires multiple technologies to be used to form very large networks

Basics
Power line communications systems operate by impressing a modulated carrier signal on the wiring system. Different types of powerline communications use different frequency bands, depending on the signal transmission characteristics of the power wiring used. Since the power distribution system was originally intended for transmission of AC power at typical frequencies of 50 or 60 Hz, power wire circuits have only a limited ability to carry higher frequencies. The propagation problem is a limiting factor for each type of power line communications.
Data rates and distance limits vary widely over many power line communication standards. Low-frequency (about 100-200 kHz) carriers impressed on high-voltage transmission lines may carry one or two analog voice circuits, or telemetry and control circuits with an equivalent data rate of a few hundred bits per second; however, these circuits may be many miles long. Higher data rates generally imply shorter ranges; a local area network operating at millions of bits per second may only cover one floor of an office building, but eliminates installation of dedicated network cabling.

Utility
Power line carrier systems have long been a favorite at many utilities because it allows them to reliably move data over an infrastructure that they control. Many technologies are capable of performing multiple applications. For example, a communication system bought initially for automatic meter reading can sometimes also be used for load control or for demand response applications.
PLC is one of the technologies used for automatic meter reading. Both one-way and two-way systems have been successfully used for decades. Interest in this application has grown substantially in recent history—not so much because there is an interest in automating a manual process, but because there is an interest in obtaining fresh data from all metered points in order to better control and operate the system. PLC is one of the technologies being used in Advanced Metering Infrastructure (AMI) systems.
In a one-way (inbound only) system, readings "bubble up" from end devices (such as meters), through the communication infrastructure, to a "master station" which publishes the readings. A one-way system might be lower-cost than a two-way system, but also is difficult to reconfigure should the operating environment change.
In a two-way system (supporting both outbound and inbound), commands can be broadcast out from the master station to end devices (meters) -- allowing for reconfiguration of the network, or to obtain readings, or to convey messages, etc. The device at the end of the network may then respond (inbound) with a message that carries the desired value. Outbound messages injected at a utility substation will propagate to all points downstream. This type of broadcast allows the communication system to simultaneously reach many thousands of devices—all of which are known to have power, and have been previously identified as candidates for load shed. PLC also may be a component of a smart power grid.

Digital Signal

2011-08-02T22:25:00.000-07:00

A digital signal is a physical signal that is a representation of a sequence of discrete values (a quantified discrete-time signal), for example of arbitrary bit stream, or of a digitized (sampled and analog-to-digital converted) analog signal. The term digital signal can refer to

a continuous-time waveform signal used in any form of digital communication.
a pulse train signal that switches between a discrete number of voltage levels or levels of light intensity, also known as a a line coded signal, for example a signal found in digital electronics or in serial communications using digital baseband transmissionin, or a pulse code modulation (PCM) representation of a digitized analog signal.

A signal that is generated by means of a digital modulation method (digital passband transmission), produced by a modem, is in the first case considered as a digital signal, and in the second case as converted to an analog signal

WAVEFORM OF DIGITAL SIGNAL

In computer architecture and other digital systems, a waveform that switches between two voltage levels representing the two states of a Boolean value (0 and 1) is referred to as a digital signal, even though it is an analog voltage waveform, since it is interpreted in terms of only two levels.
The clock signal is a special digital signal that is used to synchronize digital circuits. The image shown can be considered the waveform of a clock signal. Logic changes are triggered either by the rising edge or the falling edge.
The given diagram is an example of the practical pulse and therefore we have introduced two new terms that are:

Rising edge: the transition from a low voltage (level 1 in the diagram) to a high voltage (level 2).
Falling edge: the transition from a high voltage to a low one.

Although in a highly simplified and idealised model of a digital circuit we may wish for these transitions to occur instantaneously, no real world circuit is purely resistive and therefore no circuit can instantly change voltage levels. This means that during a short, finite transition time the output may not properly reflect the input, and indeed may not correspond to either a logically high or low voltage.

passband signals

2011-08-02T22:21:00.000-07:00

In telecommunications and signal processing, baseband is an adjective that describes signals and systems whose range of frequencies is measured from close to 0 hertz to a cut-off frequency, a maximum bandwidth or highest signal frequency; it is sometimes used as a noun for a band of frequencies starting close to zero. Baseband can often be considered as a synonym to lowpass or non-modulated, and antonym to passband, bandpass, carrier-modulated or radio frequency (RF) signal.

Baseband bandwidth
A baseband bandwidth is equal to the highest frequency of a signal or system, or an upper bound on such frequencies,^[1] for example the upper cut-off frequency of a passband filter. By contrast, passband bandwidth is the difference between a highest frequency and a nonzero lowest frequency.

Baseband channel

Digital baseband transmission

Main article: Line code

Baseband transmission in Ethernet

Baseband signal

Equivalent baseband signal

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)

where

ω

is the carrier angular frequency in rad/s.

Baseband Signals

2011-08-02T22:19:00.000-07:00

Baseband signal
A baseband signal or lowpass 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).^[5]
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. A baseband signal is a low frequency signal which when modulated is transmitted on various channels.
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.

Equivalent baseband signal

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)

where

ω

is the carrier angular frequency in rad/s.

communiucation channels

2011-08-02T22:17:00.000-07:00

In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel. A channel is used to convey an information signal, for example a digital bit stream, from one or several senders (or transmitters) to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second
In information theory, a channel refers to a theoretical channel model with certain error characteristics. In this more general view, a storage device is also a kind of channel, which can be sent to (written) and received from (read).

Examples
A channel can take many forms. Examples of communications channels include:

A connection between initiating and terminating nodes of a circuit.
A single path provided by a transmission medium via either
- physical separation, such as by multipair cable or
- electrical separation, such as by frequency-division or time-division multiplexing.
A path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths.
- A storage which can communicate a message over time as well as space
- The portion of a storage medium, such as a track or band, that is accessible to a given reading or writing station or head.
- A buffer from which messages can be 'put' and 'got'. See Actor model and process calculi for discussion on the use of channels.
In a communications system, the physical or logical link that connects a data source to a data sink.
A specific radio frequency, pair or band of frequencies, usually named with a letter, number, or codeword, and often allocated by international agreement.
Examples:
- Marine VHF radio uses some 88 channels in the VHF band for two-way FM voice communication. Channel 16, for example, is 156.800 MHz. In the US, seven additional channels, WX1 - WX7, are allocated for weather broadcasts.
- Television channels such as North American TV Channel 2 = 55.25 MHz, Channel 13 = 211.25 MHz. Each channel is 6 MHz wide. Besides these "physical channels", television also has "virtual channels".
- Wi-Fi consists of unlicensed channels 1-13 from 2412 MHz to 2484 MHz in 5 MHz steps.
- The radio channel between an amateur radio repeater and a ham uses two bands often 600 kHz (0.6 MHz) apart. For example, a repeater that transmits on 146.94 MHz typically listens for a ham transmitting on 146.34 MHz.
A room in the Internet Relay Chat (IRC) network, in which participants can communicate with each other.

All of these communications channels share the property that they transfer information. The information is carried through the channel by a signal.

Channel models

A channel can be modelled physically by trying to calculate the physical processes which modify the transmitted signal. For example in wireless communications the channel can be modelled by calculating the reflection off every object in the environment. A sequence of random numbers might also be added in to simulate external interference and/or electronic noise in the receiver.
Statistically a communication channel is usually modelled as a triple consisting of an input alphabet, an output alphabet, and for each pair (i, o) of input and output elements a transition probability p(i, o). Semantically, the transition probability is the probability that the symbol o is received given that i was transmitted over the channel.
Statistical and physical modelling can be combined. For example in wireless communications the channel is often modelled by a random attenuation (known as fading) of the transmitted signal, followed by additive noise. The attenuation term is a simplification of the underlying physical processes and captures the change in signal power over the course of the transmission. The noise in the model captures external interference and/or electronic noise in the receiver. If the attenuation term is complex it also describes the relative time a signal takes to get through the channel. The statistics of the random attenuation are decided by previous measurements or physical simulations.
Channel models may be continuous channel models in that there is no limit to how precisely their values may be defined.
Communication channels are also studied in a discrete-alphabet setting. This corresponds to abstracting a real world communication system in which the analog->digital and digital->analog blocks are out of the control of the designer. The mathematical model consists of a transition probability that specifies an output distribution for each possible sequence of channel inputs. In information theory, it is common to start with memoryless channels in which the output probability distribution only depends on the current channel input.
A channel model may either be digital (quantified, e.g. binary) or analog.

Digital channel models

In a digital channel model, the transmitted message is modelled as a digital signal at a certain protocol layer. Underlying protocol layers, such as the physical layer transmission technique, is replaced by a simplified model. The model may reflect channel performance measures such as bit rate, bit errors, latency/delay, delay jitter, etc. Examples of digital channel models are:

Binary symmetric channel (BSC), a discrete memoryless channel with a certain bit error probability
Binary bursty bit error channel model, a channel "with memory"
Binary erasure channel (BEC), a discrete channel with a certain bit error detection (erasure) probability
Packet erasure channel, where packets are lost with a certain packet loss probability or packet error rate
Arbitrarily varying channel (AVC), where the behavior and state of the channel can change randomly

Analog channel models

In an analog channel model, the transmitted message is modelled as an analog signal. The model can be a linear or non-linear, time-continuous or time-discrete (sampled), memoryless or dynamic (resulting in burst errors), time-invariant or time-variant (also resulting in burst errors), baseband, passband (RF signal model), real-valued or complex-valued signal model. The model may reflect the following channel impairments:

Noise model, for example
- Additive white Gaussian noise (AWGN) channel, a linear continuous memoryless model
- Phase noise model
Interference model, for example cross-talk (co-channel interference) and intersymbol interference (ISI)
Distortion model, for example a non-linear channel model causing intermodulation distortion (IMD)
Frequency response model, including attenuation and phase-shift
Group delay model
Modelling of underlying physical layer transmission techniques, for example a complex-valued equivalent baseband model of modulation and frequency response
Radio frequency propagation model, for example
- Log-distance path loss model
- Fading model, for example Rayleigh fading, Ricean fading, log-normal shadow fading and frequency selective (dispersive) fading
- Doppler shift model, which combined with fading results in a time-variant system
- Ray tracing models, which attempt to model the signal propagation and distortions for specified transmitter-receiver geometries, terrain types, and antennas
- Mobility models, which also causes a time-variant system

Types of communications channels

Digital (discrete) or analog (continuous) channel
Baseband and passband channel
Transmission medium, for example a fibre channel
Multiplexed channel
Computer network virtual channel
Simplex communication, duplex communication or half duplex communication channel
Return channel
Uplink or downlink (upstream or downstream channel)
Broadcast channel, unicast channel or multicast channel

Communication System

2011-08-02T21:59:00.001-07:00

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 etc.).
A communications subsystem is a functional unit or operational assembly that is smaller than the larger assembly under consideration. Examples of communications subsystems in the Defense Communications System (DCS) are (a) a satellite link with one Earth terminal in CONUS and one in Europe, (b) the interconnect facilities at each Earth terminal of the satellite link, and (c) an optical fiber cable with its driver and receiver in either of the interconnect facilities. Communication subsystem (b) 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

What is Electronics

2011-06-18T10:43:00.003-07:00

What is electronics

The branch of physics and technology concerned with the design of circuits using transistors and microchips, and with the behavior and movement of electrons in a semiconductor, conductor, vacuum, or gas

Basic Electronics

The goal of this chapter is to provide some basic information about electronic circuits. We make the assumption that you have no prior knowledge of electronics, electricity, or circuits, and start from the basics. This is an unconventional approach, so it may be interesting, or at least amusing, even if you do have some experience. So, the first question is ``What is an electronic circuit?'' A circuit is a structure that directs and controls electric currents, presumably to perform some useful function. The very name "circuit" implies that the structure is closed, something like a loop. That is all very well, but this answer immediately raises a new question: "What is an electric current?" Again, the name "current" indicates that it refers to some type of flow, and in this case we mean a flow of electric charge, which is usually just called charge because electric charge is really the only kind there is. Finally we come to the basic question:

Current

Charge is mobile and can flow freely in certain materials, called conductors. Metals and a few other elements and compounds are conductors. Materials that charge cannot flow through are called insulators. Air, glass, most plastics, and rubber are insulators, for example. And then there are some materials called semiconductors, that, historically, seemed to be good conductors sometimes but much less so other times. Silicon and germanium are two such materials. Today, we know that the difference in electrical behavior of different samples of these materials is due to extremely small amounts of impurities of different kinds, which could not be measured earlier. This recognition, and the ability to precisely control the "impurities" has led to the massive semiconductor electronics industry and the near-magical devices it produces, including those on your Robot Board. We will discuss semiconductor devices later; now let us return to conductors and charges

What is Direct Current

Direct current electronics involves working with electrical systems where the flow of electrical current is constant through a conducting medium, generally a wire. Current and voltage in a direct-current circuit are conserved according to Kirchhoff's Current & Voltage Laws

Major Innovations

For years, electricity was used as part of parlor tricks, creating impressive showers of sparks. Benjamin Franklin is known as perhaps one of the earliest experimenters in electricity, whose invention of the lightning rod was one of the first practical applications. Throughout the 1800's the practical and theoretical concepts of electrical circuitry were developed

Electronic Engineering involves the design, manufacture and application of electronic components, in particular, semiconductor microchips. It also is central to industrial automation and encompasses areas such as robotics, artificial intelligence and satellite launching and position control. The knowledge required for a job in the field of electronics is very special and includes a good grounding in the fundamentals of electricity and electronics with their application to computer systems, communications systems, modern automobiles (including Formula One!), aerospace, and many other fields.

The modulation process

2011-06-18T10:40:00.001-07:00

The modulation process

As discussed earlier, the purpose of a communication system or to deliver a message signal from an information source in recognizable form to a user destination, with the source and the user being physically separated from each other.

To do this the transmitter modified the message signal into a form which is suitable for transmission over channel. This modification is achieved by means of a process as modulation, which involves varying some perameter of a carrier wave in accordance with the message signal. Also the receiver recreate the original message signal from a degraded version of the transmitted signal after propagation through the channel. The recreation is achieved by using a process called as demodulation.

Modulation may be defined as the process by which some characteristic of a signal called carrier is varied in accordance with the instantaneous value of another signal called modulation signal.

Signal containing information or intelligence are referred as modulation signal. This information bearing signal is also called baseband signal. The carrier frequency is greater them the modulation frequency. The signal resulting from the process of modulation is called modulation signal.

Classification of Communication

2011-06-17T11:17:00.001-07:00

Classification of communication

(i) Line Communication

(ii) Wire or Radio Communication

Line communication

In line communication the medium of transmission is a pair of conductor called transmission line. This is also called as line channel. This means that in the communication, the transmitter and the receiver are connected through a wire or line.the installation and maintenance of a transmission line is not only costly and complex but also over crowds the open source. Apart from this its message transmission capability is also limited.

Wire or Radio Communication

In Wireless or radio communication, a message is transmitted through open space by electromagnetic waves called a radio waves. Radio Waves are radiated from the transmitter in open space through a device called antenna. A receiving antenna intercepta the radio waves at the receiver. All the radio, TV and satellite broadcasting are wireless or radio communication. The advantage of wireless communication are cost effectiveness, possible long distance communication and simplicity.

Communication Process

2011-06-15T12:35:00.000-07:00

The Communication Process

1. Information Process

We know that a communication system serves to communication a message or information. This message or information originates in the information source. In general there can be various messages in the form of words, group of words, code, symbols, sound signal etc. however out of these messages only the desired message is selected and conveyed or communicated.

2. Input Transducer

A transducer is a device which converts one form of energy into another form. The messages from the information source may not be electrical in nature. In a case when the message produced by the information source is not the electrical in nature, an input transducer is used to convert it into a time varying electrical signal. For example in case of radio broadcasting, a microphone converts the information or messages which is in the form of sound waves into corresponding electrical signal.

3. Transmitter

The function of the transmitter is to process the electrical signal from different aspects. For example in radio broadcasting signal obtained from signal, is processed to restrict its range of audio frequencies and is often amplified. In wire telephony no real processing is needed, however in long distance radio main function of the transmitter, in modulation the message signal processing such as restriction of range of audio frequency, amplified, and modulation are achieved. All these processings of the message signal are done just to case the transmission of the signal through the channel.

4. The channel and the Noise

With reference to the block diagram of a communication system the term channel means the medium through which the message travels fom the transmitter to the receiver. In other words, we can say that the function of the channel is to provide a physical connection between the transmitter and the receiver.

There are two types of channels, namely point to point channels and broadcast channels. Examples of point to point channels are wirelines, microwaves link and optical fibres . Wirelines operated by guided electromagnetic waves and they are used for local telephonic transmission. In case of microwaves links are used in long distance telephone transmission. an optical fibre is a low loss, well controlled, guided optical differently, they all provides a physical medium for the transmission of signals from one point to another point. For these channels the term point to point is used.

Source of information

2011-06-15T12:28:00.000-07:00

Source of information.

As a matter of fact, The telecommunication environment is dominated by the following four important sources of information.

(i) Speech

(ii) Television

(iii) Facsimile and

(iv) Personal computer

A source of nformation may be characterized in term of signal which carrier the information. Further, a signal is defined as a signal valued function of the time that play the role of the depedent variable. At every instant of time, the function has a unique value.

Speech

(a) Production: an intended message in the speaker mind is represented by the speech signal that consist of sounds generated inside the speaker mouth and whose arrangement is governed by the rule of language.

(b) Propagation: The siund waves propagates through the air, reaching the listener’s ears.

(c) Perception: The incoming sounds are deciphered by the listener into a received message, and thus completing the chain of events that results in the transfer of information from the speaker to the listener.

Television

The second source of information, television refers to the transmission of picture in motion by means of electrical signal. To accomplished this transmission, each complete picture has to be sequentially scanned. The scanning process is carried out in a TV camera. In a black and white TV, the camera contains optics designed to focus an image on a photocathode consisting of a large number of photosensitive elements.

The charge pattern so generated on the photosensitive surface is scanned by an electron beam, and thus producing an output current which vaies temporally in accordance with the way in which the brightness of the original picture varies spatially form one point to another. This resulting output current is called a video signal.

The type of scanning used in television is a form spatial sampling called raster scanning. The purpose of raster scanning is basically to convert a two dimensional image intensity into a one dimension waveform.

Facsimile

The purpose of the third source of information, facsimile machine, is to transmit still picture over a communication channel a telephone. Such a machine provide a highly popular facility for the transmission of hand written or printed text from one point to another. Further, transmitting text by facsimile is treated simply like transmitted a picture.

The basic principal employed fo signal generation in a facsimile machine is to scan an original document and use an image sensor to convert the light to an electrical signal.

Personal Computer

As a matter of fact, personal computer are becoming increasingly an important part of our daily lives. We use them for electronic mail, exchange of software, and sharing of resource. It is estimated that over 30 percent of the personal computer in use today are already networked and the number is increasing rapidely. The text transmitted by a PC is usually encoded using the American standard code for information interchange, ASCII which is the first code developed specifically for computer communication.

Each character in ASCII is represented by seven data bits constituting a unique binary pattern made up of 0s and 1s. hence a total of seven power of two = 128 different characters can be represented in ASCII. The character symbols are various lowercase and uppercase letters, numbers, special control symbols, and punctuation symbols commonly used such as @,$ and %. Some of the special “control” symbols such as BS ( back space ) and CR (carriage return), are used to control the printing of characters on a page Other symbols, such as ENQ (enquiry) and ETB (end of transmission block), are used for communication purpose.

The even data bits are ordered starting with the most significant bit b7 down to the least significant bit b1.

At the end of the data bits, an extra bit b8 is appended for the purpose of error detection. This error detected bit is called a parity bit.

Communication System

2011-05-11T10:50:00.000-07:00

Communication System

Information Source : Comminication system serve to communication a message or information. This message or information originates in the information source. In general, there can be various message in the form of words, group of words, code, symbols, sound signal etc. however out of these message, only the desired message is selected and conveyed or communicted.

Input Transducer : A Transducer is a device which converts one from of energy into another form. The message from the information source may or may not be electrical in nature. Transuder converts non electrical signal into electrical signals. for examples, a microphone converts sound signal as sound waves into is electrical signal.

Transmitter : The function of the transmitter is to process the electrical signal from different aspects. for examples, in redio broadcasting the electrical signal obtained from sound signal is processed to restrict its range of audio frequencies and amplified.

The Channel and the Noise : The channel means the medium through which the message travels from the transmitter to the receiver. The function of the channel is to provide a signal connnection between the transmitter and the receiver. Microwave link are used in long distance telephone transmisssion
Noise is an unwanted signl which tend to interfere with the required signal. noise signal is always random in character. Noise is an unwanted signal which tend to interfere with the required signal. Noise signal at any point in a communication system.

Receiver : The main function of the receiver is to receiver is to reproduce the message signal in electrical form from the dstrorted received signal. This reprodulation of the original signal is accomplished by a process known as demodulation or detected. Demodulation is the reverse process of modulation carried out in transmitter.

Destinaton : It is the final stage which is used to convert an electrical message signal into its original form. For examples, in radio broadcasting the destination is a loudspeaker which works as a transducer i.e it converts electrical signal into sound signal