Communication | Simplex | Full Duplex | Modulation and Multiplexing

Types of Electronic Communication

communication are classified according to whether they are (1) one-way (simplex) or two-way (full duplex or half duplex) transmissions and (2) analog or digital signals.

What is Simplex Communication?

The simplest way in which electronic communication is conducted is one-way communications, normally referred to as simplex communication. Examples are shown in Fig. 1-3. The most common forms of simplex communication are radio and TV broadcasting. Another example of one-way communication is transmission to a remotely controlled vehicle like a toy car or an unmanned aerial vehicle (UAV or drone).

What is Full Duplex Communication?

The bulk of electronic communication is two-way, or duplex communication. Typical duplex applications are shown in Fig. 1-4.

For example, people communicating with one another over the telephone can talk and listen simultaneously, as Fig. 1-4(a) illustrates. This is called full duplex communication.

What is Half Duplex Communication?

The form of two-way communication in which only one party transmits at a time is known as half duplex communication [see Fig. 1-4(b)]. The communication is two-way, but the direction alternates: the communicating parties take turns transmitting and receiving. Most radio transmissions, such as those used in the military, fire, police, aircraft, marine, and other services, are half duplex communication. Citizens band (CB), Family Radio, and amateur radio communication are also half duplex.

What is Analog Signals?

An analog signal is a smoothly and continuously varying voltage or current. Some typical analog signals are shown in Fig. 1-5. A sine wave is a single-frequency analog signal. Voice and video voltages are analog signals that vary in accordance with the sound or light variations that are analogous to the information being transmitted.

Digital Signals

Digital signals, in contrast to analog signals, do not vary continuously, but change in steps or in discrete increments. Most digital signals use binary or two-state codes.

Some examples are shown in Fig. 1-6. The earliest forms of both wire and radio communication used a type of on/off digital code. The telegraph used Morse code, with its system of short and long signals (dots and dashes) to designate letters and numbers. See Fig. 1-6(a). In radio telegraphy, also known as continuous-wave (CW) transmission, a sine wave signal is turned off and on for short or long durations to represent the dots and dashes. Refer to Fig. 1-6(b). Data used in computers is also digital. Binary codes representing numbers, letters, and special symbols are transmitted serially by wire, radio, or optical medium. The most commonly used digital code in communications is the American Standard Code for Information Interchange (ASCII, pronounced “ask key”). Fig. 1-6(c) shows a serial binary code. Many transmissions are of signals that originate in digital form, e.g., telegraphy messages or computer data, but that must be converted to analog form to match the transmission medium. An example is the transmission of digital data over the telephone network, which was designed to handle analog voice signals only. If the digital data is converted to analog signals, such as tones in the audio frequency range, it can be transmitted over the telephone network. Analog signals can also be transmitted digitally. It is very common today to take voice or video analog signals and digitize them with an analog-to-digital (A /D) converter. The data can then be transmitted efficiently in digital form and processed by computers and other digital circuits.

What is Modulation and Multiplexing?

Modulation and multiplexing are electronic techniques for transmitting information efficiently from one place to another. Modulation makes the information signal more compatible with the medium, and multiplexing allows more than one signal to be transmitted concurrently over a single medium. Modulation and multiplexing techniques are basic to electronic communication. Once you have mastered the fundamentals of these techniques, you will easily understand how most modern communication systems work.


 Multiplexing has been used in the music industry to create stereo sound. In stereo radio, two signals are transmitted and received— one for the right and one for the left channel of sound.

What is Baseband Transmission?

Before it can be transmitted, the information or intelligence must be converted to an electronic signal compatible with the medium. For example, a microphone changes voice signals (sound waves) into an analog voltage of varying frequency and amplitude. This signal is then passed over wires to a speaker or headphones. This is the way the telephone system works. A video camera generates an analog signal that represents the light variations along one scan line of the picture. This analog signal is usually transmitted over a coaxial cable. Binary data is generated by a keyboard attached to a computer. The computer stores the data and processes it in some way. The data is then transmitted on cables to peripherals such as a printer or to other computers over a LAN. Regardless of whether the original information or intelligence signals are analog or digital, they are all referred to as baseband signals. In a communication system, baseband information signals can be sent directly and unmodified over the medium or can be used to modulate a carrier for transmission over the medium. Putting the original voice, video, or digital signals directly into the medium is referred to as baseband transmission.

For example, in many telephone and intercom systems, it is the voice itself that is placed on the wires and transmitted over some distance to the receiver. In most computer networks, the digital signals are applied directly to coaxial or twisted-pair cables for transmission to another computer. In many instances, baseband signals are incompatible with the medium. Although it is theoretically possible to transmit voice signals directly by radio, realistically it is impractical. As a result, the baseband information signal, be it audio, video, or data, is normally used to modulate a high-frequency signal called a carrier. The higher- frequency carriers radiate into space more efficiently than the baseband signals themselves. Such wireless signals consist of both electric and magnetic fields. These electromagnetic signals, which are able to travel through space for long distances, are also referred to as radio-frequency (RF) waves, or just radio waves.

What is Broadband Transmission?

Modulation is the process of having a baseband voice, video, or digital signal modify another, higher-frequency signal, the carrier. The process is illustrated in Fig. 1-7. The information or intelligence to be sent is said to be impressed upon the carrier. The carrier is usually a sine wave generated by an oscillator. The carrier is fed to a circuit called a modulator along with the baseband intelligence signal. The intelligence signal changes the carrier in a unique way. The modulated carrier is amplifi ed and sent to the antenna for transmission. This process is called broadband transmission. Consider the common mathematical expression for a sine wave:

The three ways to make the baseband signal change the carrier sine wave are to vary its amplitude, vary its frequency, or vary its phase angle. The two most common methods of modulation are amplitude modulation (AM) and frequency modulation (FM). In AM, the baseband information signal called the modulating signal varies the amplitude of the higher-frequency carrier signal, as shown in Fig. 1-8(a). It changes the Vp part of the equation. In FM, the information signal varies the frequency of the carrier, as shown in Fig. 1-8(b). The carrier amplitude remains constant. FM varies the value of f in the first angle term inside the parentheses. Varying the phase angle produces phase modulation (PM). Here, the second term inside the parentheses (θ) is made to vary by the intelligence signal. Phase modulation produces frequency modulation; therefore, the PM signal is similar in appearance to a frequency-modulated carrier. Two common examples of transmitting digital data by modulation are given in Fig. 1-9. In Fig. 1-9(a), the data is converted to frequency-varying tones. This is called frequency-shift keying (FSK). In Fig. 1-9(b), the data introduces a 180º-phase shift. This is called phase-shift keying (PSK). Devices called modems (modulator-demodulator) translate the data from digital to analog and back again. Both FM and PM are forms of angle modulation. At the receiver, the carrier with the intelligence signal is amplified and then demodulated to extract the original baseband signal. Another name for the demodulation process is detection. (See Fig. 1-10.)

What is Multiplexing?

The use of modulation also permits another technique, known as multiplexing, to be used. Multiplexing is the process of allowing two or more signals to share the same medium or channel; see Fig. 1-11. A multiplexer converts the individual baseband signals to a composite signal that is used to modulate a carrier in the transmitter. At the receiver, the composite signal is recovered at the demodulator, then sent to a demultiplexer where the individual baseband signals are regenerated (see Fig. 1-12). There are three basic types of multiplexing: frequency division, time division, and code division. In frequency-division multiplexing, the intelligence signals modulate subcarriers on different frequencies that are then added together, and the composite signal is used to modulate the carrier. In optical networking, wavelength division multiplexing (WDM) is equivalent to frequency-division multiplexing for the optical signals. In time-division multiplexing, the multiple intelligence signals are sequentially sampled, and a small piece of each is used to modulate the carrier. If the information signals are sampled fast enough, sufficient details are transmitted that at the receiving end the signal can be reconstructed with great accuracy. In code-division multiplexing, the signals to be transmitted are converted to digital data that is then uniquely coded with a faster binary code. The signals modulate a carrier on the same frequency. All use the same communications channel simultaneously. The unique coding is used at the receiver to select the desired signal

A Survey of Communication Applications

The applications of electronic techniques to communication are so common and pervasive that you are already familiar with most of them. You use the telephone, listen to the radio, and watch TV. You also use other forms of electronic communication, such as cellular telephones, ham radios, CB and Family radios, home wireless networks for Internet access, texting, electronic mail, and remote-control garage door openers. Fig. 1-17 lists all the various major applications of electronic communication.


  • AM and FM radio broadcasting. Stations broadcast music, news, weather reports, and programs for entertainment and information. It includes shortwave.
  • Digital radio. There is both satellite and terrestrial. Radio programming is transmitted in digital format.
  • TV broadcasting. Stations broadcast entertainment, informational, and educational programs by radio.
  • Digital television (DTV). Radio transmission of television programming is performed by digital methods, both satellite and terrestrial, e.g., highdefi nition television (HDTV) and Internet Protocol Television (IPTV).
  • Cable television. Movies, sports events, and other programs are distributed to subscribers by fiber-optic and coaxial cable.
  • Facsimile. Printed visual material is transmitted over telephone lines. A facsimile, or fax, the machine scans a document and converts it to electronic signals that are sent over the telephone system for reproduction in printed form by another fax machine. Faxes can also be sent from a computer.
  • Wireless remote control. This category includes a device that controls any remote item by radio or infrared. Examples are missiles, satellites, robots, toys, and other vehicles or remote plants or stations. A remote keyless entry device, garage door opener, and the remote control on your TV set are other examples. 8. Internet of Things (IoT). The monitoring or control of remote devices, appliances, and other items in a home, offi ce or other facility is usually accomplished by a combination of wireless and Internet connectivity.
  • Navigation and direction-finding services. Special stations transmit signals that can be picked up by receivers for the purpose of identifying exact location (latitude and longitude) or determining direction and/ or distance from a station. Such systems employ both land-based and satellite stations. The services are used primarily by boats and ships or airplanes, although systems for cars and trucks are being developed. The Global Positioning System (GPS) which uses 24 satellites is the most widely used.
  • Telemetry. Measurements are transmitted over a long distance. Telemetry systems use sensors to determine physical conditions (temperature, pressure, fl ow rate, voltages, frequency, etc.) at a remote location. The sensors modulate a carrier signal that is sent by wire or radio to a remote receiver that stores and/or displays the data for analysis. Examples are satellites, rockets, pipelines, plants, and factories.
  • Radio astronomy. Radio signals, including infrared, are emitted by virtually all heavenly bodies such as stars and planets. With the use of large directional antennas and sensitive high-gain receivers, these signals may be picked up and used to plot star locations and study the universe. Radio astronomy is an alternative and supplement to traditional optical astronomy.
  • Surveillance. Surveillance means discreet monitoring or “spying.” Electronic techniques are widely used by police forces, governments, the military, business and industry, and others to gather information for the purpose of gaining some competitive advantage. Techniques include phone taps, tiny wireless “bugs,” clandestine listening stations, and reconnaissance airplanes and satellites.
  • Music services. Continuous background music is transmitted for doctors’ offi ces, stores, elevators, and so on by local FM radio stations on special high-frequency subcarriers that cannot be picked up by conventional FM receivers.
  • Internet radio and video. Music and video are delivered on a computer via the Internet.


  • Telephones. One-on-one verbal communication is transmitted over the vast worldwide telephone networks employing wire, fi ber optics, radio, and satellites.
  • Cordless telephones provide short-distance wireless communication for cord-free convenience.
  • Cell phones provide worldwide wireless communications via handsets and base stations and the wired telephone system. In addition to voice communications, cell phones facilitate e-mail, Internet access, instant message service, video, and games.
  • Internet telephones, known as voice over the Internet protocol (VoIP) phones, use high-speed broadband services (cable, DSL, wireless, fi ber) over the Internet to provide digital voice communications.
  • Satellite phones use low-earth-orbit satellites to give worldwide voice service from any remote location on earth.
  • Two-way radio. Commercial, industrial, and government communication is transmitted between vehicles, handheld units, and base stations. Examples include police, fire, taxi, forestry service, trucking companies, aircraft, marine, military, and government.
  • Radar. This special form of communication makes use of refl ected microwave signals for the purpose of detecting ships, planes, and missiles and for determining their range, direction, and speed. Most radar is used in military applications, but civilian aircraft and marine services also use it. Police use radar in speed detection and enforcement.
  • Sonar. In underwater communication, audible baseband signals use water as the transmission medium. Submarines and ships use sonar to detect the presence of enemy submarines. Passive sonar uses audio receivers to pick up water, propeller, and other noises. Active sonar is like an underwater radar with which reflections from a transmitted ultrasonic pulse are used to determine the direction, range, and speed of an underwater target.
  • Amateur radio. This is a hobby for individuals interested in radio communication. Individuals may become licensed “hams” to build and operate two way radio equipment for personal communication with other hams.
  • Citizens radio. Citizens band (CB) radio is a special service that any individual may use for personal communication with others. Most CB radios are used in trucks and cars for exchanging information about traffi c conditions, speed traps, and emergencies.
  • Family Radio Service. This is a two-way personal communication with handheld units over short distances (, 2 mi).
  • The Internet. Worldwide interconnections via fi beroptic networks, telecommunications companies, cable TV companies, Internet service providers, and others provide World Wide Web (WWW) access to millions of websites and pages and electronic mail (e-mail).
  • Wide-Area Networks (WANs). Worldwide fi ber-optic networks provide long-distance telephone and Internet services.
  • Metropolitan-area networks (MANs). Networks of computers transmit over a specifi c geographic area such as a college campus, company facility, or city. Normally they are implemented with fi ber-optic cable, but may also be coaxial cable or wireless.
  • Local-area networks (LANs). Wired (or wireless) interconnections of personal computers (PCs), laptops, servers, or mainframe computers within an office or building for the purpose of e-mail, Internet access, or the sharing of mass storage, peripherals, data, and software.

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