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Following are the essential elements of communication system.

  1. Information source: Source that produces a message
  2. Transmitter: An element that functions on the message to generate a signal which can be delivered through a medium/channel
  3. Communication Channel: that is the medium over which the signal (carrying the information that composes the message) is sent.
  4. Receiver: An element converts the signal back into the intended message.
  5. Destination: It can be a person/machine, for whom / which the message is intended.


  1. Signals are records waveforms as they are. Signal occupies the same order of spectrum as the analog data.
  2. In analog systems electronic circuits are used for transformation of signals.
  3. About Noise analog signals are more likely to get affected and results in reducing accuracy
  4. Data transmission is not of high quality


  1. Converts analog waveforms into set of numbers and records them. The numbers are converted into voltage stream for representation. In case of binary it is converted in 1’s and 0’
  2. In this transformation is done using logic circuits.
  3. Digital signals are less affected, because noise response are analog in nature
  4. Data transmission has high quality.

Asynchronous transmission has following advantages and disadvantages:

  1. Each individual character is complete unit, hence if there is an error in a character, other sequence of characters are not affected. However, Error in start and stop bit(s) may cause serious problems in data transfer.
  2. Doesn't require synchronization of both communication sides.
  3. It is cost effective
  4. The speed of transmission is limited.
  5. Large relative overhead, a high proportion of the transmitted bits are uniquely for control purposes

Synchronous transmission has following advantages and disadvantages:

  1. In comparison to asynchronous communication it has higher speeds, because the system has lesser possibility of error. But, if an error takes place, the complete set of data is lost instead of a single character.
  2. Serial synchronous transmission is principally used for high-speed communication between computers but is unsuitable where the characters are transferred at irregular intervals.
  3. Lower overhead and thus, greater throughput.
  4. Process is more complex.
  5. It is not very cost effective as hardware are more expensive.

Simplex Transmission

Simplex transmission is one-way transmission. As the name implies, is simple in term of process and hardware. It is also called unidirectional because the signal travels in only one direction. For example, Radio or TV broadcasting system, which are always in one direction from Radio/TV station to our radio or TV sets.

Half-Duplex Transmission

In half-duplex transmission data transmission can be take place in both directions, but not at the same time. This means that only one side can transmit at a time. For example, walky-talky devices used by security agencies are half-duplex as only one person can talk at one time.

Full-Duplex Transmission

Full-duplex (also known as Duplex) transmission can take place in both directions at the same time. For example, telephone or mobile conversation is an example of full-duplex communication, where both sender and receiver can hear each other at the same time.


  • Modulation is the Process by which a property or a parameter of a signal is varied in proportion to another signal.
  • The original signal is normally referred as the modulating signal and the high frequency signal, whose properties are changed, is referred as the carrier signal. The resulting signal is finally referred as the modulated signal.

Analog Modulation

Analog modulation is the simplest form of the modulation. In analog modulation, the  modulation is applied continuously in response to the analog information signal. The process of the analog modulation has been shown in the Figure 1, below. Here the original signal at the baseband frequency has been shifted to the broadband frequency (Fc) .      

Common analog modulation techniques are:

  1. Amplitude Modulation (AM): Here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal.
  1. Angle Modulation: Here the frequency or phase of the carrier signal is varied in accordance with the strength of the modulating signal. Consequently, the Analog Modulation has two forms
  • Frequency Modulation (FM): In this case, the frequency of the carrier signal is varied in accordance to the instantaneous frequency of the modulating signal).
  • Phase Modulation (PM): In this case, the phase of the carrier signal is varied in accordance to the instantaneous phase of the modulating signal.


Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a high frequency carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent.

Advantages of Amplitude Modulation

  1. Coverage area of AM Receiver is wider than FM because atmospheric propagation.
  1. AM is long distance propagation because of its high power.
  2. AM Circuit is the cheapest and least complex.
  3. AM can be easily demodulated using a Diode Detector.

Disadvantages of Amplitude Modulation

  1. Amplitude modulation is very much sensitive to noise and hence the performance is very weak.
  1. Signal of AM is not stronger than FM when it propagates through and obstacle.
  2. Only one sideband of AM transmits Information Signal, so it loses power on other sideband and Carrier. Hence the power efficiency of the Amplitude Modulation is very poor.
  1. Noise mixes AM Signal in amplitude when it propagates in free space that it makes it difficult to recover the original signal at receiver in a highly noisy environment.


  • Frequency modulation, FM is widely used for a variety of radio communications applications.
  • FM broadcasts on the VHF bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two way radio communications, and it is especially useful for mobile radio communications, being used in taxis, and many other forms of vehicle.
  • in view of its widespread use, frequency modulation FM, is an important form of modulation, despite many forms of digital transmission being used these days.


  • Digital communication is the process of communication in which, the signals are transferred in the form of discrete formats rather than the continuous analog forms.
  • Digital communication is very common in the present day communication systems and the signals are normally transmitted in binary formats.
  • It is always easy to process the digital information as compared to the analog signals, because of their discrete nature and hence they have become more popular in the electronic communication.

Advantages of Digital Communication

  1. Reliable communication
  2. Less sensitivity to changes in environmental conditions (temperature, etc.)
  3. Easy multiplexing
  4. Easy signaling
  5. Voice and data integration
  6. Easy processing like encryption and compression
  7. Easy system performance monitoring
  8. Quality of Service monitoring
  9. Better Signal to Noise Ratio
  10. Easy Regeneration of signals


  1. Increased bandwidth requirement for the communication channels.
  2. Need for precision timings (Bit, character, frame synchronization needed)
  3. Need for the Analog to Digital and Digital to Analog conversions
  4. Higher complexity of the system design

Analog to Digital Conversion

  • An analog-to-digital converter (ADC, A/D) is a device that converts the input continuous physical quantity to a digital number that represents the quantity's amplitude.
  • Instead of doing a single conversion, an ADC often performs the conversions ("samples" the input) periodically. The result is a sequence of digital values that have converted a continuous-time and continuous-amplitude analog signal to a discrete-time and discrete-amplitude digital signals.
  • The most commonly employed A/D converter is the Ramp based circuit. It uses a comparator to compare the voltage levels

Digital to Analog Conversion

  • Digital to analog converter is the electronic circuit, which takes digital input and converts this into an analog waveform.
  • A common use of digital-to-analog converters is generation of audio signals from digital information in music players.
  • Digital video signals are converted to analog in television and cell phones to display colors and shades.

Digital Modulation Schemes

  • Amplitude-shift keying (ASK), frequency-shift keying (FSK), and phase-shift keying (PSK) are digital modulation schemes.
  • ASK refers to a type of amplitude modulation that assigns bit values to discrete amplitude levels. The carrier signal is then modulated among the members of a set of discrete values to transmit information.
  • FSK refers to a type of frequency modulation that assigns bit values to discrete frequency levels. FSK is divided into no coherent and coherent forms. In no coherent forms of FSK, the instantaneous frequency shifts between two discrete values termed the "mark" and "space" frequencies. In coherent forms of FSK, there is no phase discontinuity in the output signal. FSK modulation formats generate modulated waveforms that are strictly real values, and thus tend not to share common features with quadrature modulation schemes.
  • PSK in a digital transmission refers to a type of angle modulation in which the phase of the carrier is discretely varied—either in relation to a reference phase or to the phase of the immediately preceding signal element—to represent data being transmitted.


  • Multiplexing (also known as MUXing) is a method by which multiple analog message signals or digital data streams are combined into one signal over a shared medium.
  • The basic aim of the Multiplexing is to share an expensive resource by putting-up multiple signals on the same channel.
  • For example, in telecommunications, several telephone calls may be carried using one wire.
  • Multiplexing originated in telegraphy in the 1870s, and is now widely applied in different streams of communications.
  • When several communication channels are needed between the same two points, significant economies may be realized by sending all the messages on one transmission facility – called multiplexing.
  • This opposite process is referred as demultiplexing.
  • Multiplexing refers to the ability to transmit data coming from several pairs of equipment (transmitters and receivers) called low-speed channels on a single physical medium (called the high-speed channel).
  • Whereas, a multiplexer is the multiplexing device that combines the signals from the different transmitters and sends them over the high-speed channel.
  • A demultiplexer is the device which separates signal received from a high-speed channel into different signal and sends them to receivers.

There are four basic multiplexing techniques:

  • Frequency division multiplexing (FDM)
  • Frequency division multiplexing (FDM) is the technique used to divide the available bandwidth into a number of smaller independent logical channels with each channel having a small bandwidth.
  • The method of using a number of carrier frequencies each of which is modulated by an independent speech signal is in fact frequency division multiplexing. Frequency division multiplexing (FDM) is also referred as the Wavelength division multiplexing (WDM), where we are using the optical communications focusing on the wavelength rather than the frequency.

Advantages of FDM:

  1. The users can be added to the system by simply adding another pair of transmitter modulator and receiver demodulators.
  1. FDM system support full duplex information (Both side simultaneous Communication) flow which is required by most of application.

Disadvantages of FDM:

  1. In FDM system, the initial cost is high. This may include the cable between the two ends and the associated connectors for the cable.
  1. A problem with one user can sometimes affect the others.
  2. Each user requires a precise carrier frequency for transmission of the signals.

Time division Multiplexing (TDM)

  • Time slots are allocated to message signals in an non overlapping manner in the time domain so that individual messages can be recovered from time synchronized switches)
  • It is the basis for broadcast radio.
  • Several stations can transmit simultaneously without interfering with each other provided they use separate carrier frequencies ( separate channels).
  • In data communications FDM is implemented by sending multiple carrier waves over the same copper wire.
  • At the receiver’s end, demultiplexing is performed by filtering out the frequencies other than the one carrying the expected transmission.
  • Any of the modulation methods discussed before can be used to carry bits within a channel.

Applications of TDM

The PDH (Plesiochronous Digital Hierarchy) system, also known as the PCM (Pulse Code Modulation) systems.

  • The synchronous digital hierarchy (SDH) / synchronous optical networking (SONET) network transmission standards.
  • TDM can be further extended into the time division multiple Channel (TDMA) scheme, where several stations connected to the same physical medium, for example sharing the same frequency channel, can communicate. Application examples include the widely used GSM telephone system

Advantages of TDM

  1. It uses a single link
  2. It does not require precise carrier matching at both end of the links.
  3. Use of the channel capacity is high.
  4. Each to expand the number of users on a system at a low cost.
  5. There is no need to include identification of the traffic stream on each packet.

Disadvantages of TDM

  1. The sensitivity to other user is very high and causes problems
  2. Initial cost is high
  3. Technical complexity is more


Code division Multiplexing (CDM)

  • Users occupy the same frequency band but modulate their messages with different codes TDMA FDMA CDMA when used for multiple access TDMA, FDMA, e.g., GSM, FM, AM, Wireless networks
  • CDMA uses spread-spectrum technology and
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