Chapter 8 Fundamentals of Communications Information Technology in Theory By Pelin Aksoy and Laura DeNardis Chapter 8 Fundamentals of Communications

Information Technology in Theory Objectives Understand how binary streams are physically generated Learn how carriers are modulated to carry the binary streams Understand important transmission concepts, including attenuation, bandwidth, channel capacity, and multiplexing Information Technology in Theory

Objectives (continued) Learn the properties of different types of transmission media Identify sources of transmission errors and learn about error detection and correction techniques for digital transmission systems Information Technology in Theory

Information Technology in Theory Electrical Signaling Electrical signals transmitted through conducting materials, such as metal wires, effectively transmit both analog and digital information Metallic conductors, such as copper wires, comprise atoms with loosely attached electrons, or negatively charged particles, around their nuclei Information Technology in Theory

Electrical Signaling (continued) When a voltage/potential difference from an electrical source, such as a battery, is introduced between the two ends of a conductor, the electrons are stimulated to move within the metal from one atom to another The relationship between voltage (V), current (I), and resistance (R) is defined by Ohm’s Law: V = IR Information Technology in Theory

Electrical Signaling (continued) Information Technology in Theory

Analog and Digital Signaling When analog signals are transmitted across a conductor, a continuous voltage difference proportional to the amplitude of the analog signal is applied at the input of the communications circuit The current flowing through the circuit is proportional to the applied voltage according to Ohm’s law, as there is a resistance associated with the circuit Information Technology in Theory

Analog and Digital Signaling (continued) One method for sending digital information across a communication system is called binary signaling An alternative method to sending digital information across a communication system is 4-ary signaling The 4-ary signaling may be generalized to M-ary signaling The data rate (D) in bits per second for M-ary transmission can be calculated by the following: D = R log2M Information Technology in Theory

Analog and Digital Signaling (continued) Information Technology in Theory

Analog and Digital Signaling (continued) Information Technology in Theory

Analog and Digital Signaling (continued) Information Technology in Theory

Analog and Digital Signaling (continued) Problem: Calculate the data rate for a communication system that employs 8-ary signaling if the signal transmission rate is 1000 signals per second R = 1000 signals per second M = 8 According to the equation: D = R log2M = 1000 log28 = 1000 × 3 = 3000 bps = 3 Kbps The data rate is therefore 3 Kbps Information Technology in Theory

Radio Wave Communications Besides electrical energy transmitted over conductors, electromagnetic (EM) energy transmitted over air or a vacuum is also commonly used for analog and digital communications EM energy travels in the form of EM waves, such as radio waves, light waves (infrared, visible light, ultraviolet), x-rays, and gamma rays Information Technology in Theory

Radio Wave Communications (continued) The EM waves are used as carriers to wirelessly carry both analog and digital information by altering certain properties of the wave in proportion to the information signal These waves vary in a sinusoidal manner, taking on a range of frequencies Information Technology in Theory

Radio Wave Communications (continued) Information Technology in Theory

The Electromagnetic Spectrum Information Technology in Theory

The Electromagnetic Spectrum (continued) Information Technology in Theory

The Electromagnetic Spectrum (continued) Besides frequency, EM waves can also be characterized in terms of their wavelength (λ) λ=c/f EM energy can occur naturally or be generated artificially Information Technology in Theory

Information Technology in Theory The Radio Spectrum Information Technology in Theory

The Radio Spectrum (continued) Information Technology in Theory

The Radio Spectrum (continued) The Federal Communications Commission (FCC) allocates radio frequencies in the United States, and most countries have a corresponding organization that assigns frequencies Cordless telephones, walkie-talkies, and wireless network adapters are examples of other systems that have designated sets of operating frequencies but do not require permission for channel use Information Technology in Theory

Modulation/Demodulation Information Technology in Theory

Analog Modulation Techniques Information Technology in Theory

Analog Modulation Techniques (continued) Information Technology in Theory

Analog Modulation Techniques (continued) FM and PM have higher fidelity than AM AM is more vulnerable to noise, but systems that employ AM typically consume less power and have a wider coverage area FM and PM are also more expensive to implement because they require a slightly more complex demodulator Information Technology in Theory

Digital Modulation Techniques Information Technology in Theory

Digital Modulation Techniques (continued) Information Technology in Theory

Digital Modulation Techniques (continued) Information Technology in Theory

Digital Modulation Techniques (continued) The quality of FSK and PSK exceeds that of ASK, although ASK consumes less energy ASK is commonly used in fiber-optic communication systems, and PSK is commonly used in satellite communications, space exploration, modems, and computer networking FSK is regularly used in facsimile machines to transmit digital information across telephone lines Information Technology in Theory

Light-Wave Communications Light-wave communication systems frequently use infrared, as in fiber-optic communication, and infrared/visible light, as in free-space optical communications, for carrying information Information Technology in Theory

Light-Wave Communications (continued) Information Technology in Theory

Light-Wave Communications (continued) Information Technology in Theory

Information Technology in Theory Attenuation When signals travel through any transmission medium, including fiber-optic cable, copper wire, or free space, they lose energy The loss of energy, called attenuation, is a significant factor that affects the quality and distance of communications Information Technology in Theory

Attenuation (continued) Information Technology in Theory

Attenuation (continued) Attenuation is measured in decibels (dB), and each transmission medium has its own attenuation figure, which is measured in dB per unit length In long-haul communication systems, electronic devices called repeaters serve as amplifiers, and are placed at certain intervals to amplify weak signals and relay them along the transmission line Information Technology in Theory

Attenuation (continued) Information Technology in Theory

Information Technology in Theory Bandwidth The primary factor that generally governs the choice of transmission media is its bandwidth In digital systems, the maximum number of bits per second (channel capacity, or C) that can reliably be carried over a channel depends on the bandwidth B (expressed in Hz) of the channel and a unitless ratio called the signal-to-noise ratio (SNR) The formula is: C = B log2(1+SNR) Information Technology in Theory

Bandwidth (continued) Sources of noise in communication systems are numerous An important type of noise is called thermal noise; it arises from random agitation of electrons of the conductor material due to heat Information Technology in Theory

Information Technology in Theory Multiplexing A single line can simultaneously transmit multiple information-carrying signals using a technique called multiplexing Multiplexing signals over a single transmission line uses one of several possible techniques: Time division multiplexing (TDM) Frequency division multiplexing (FDM) Statistical multiplexing Wavelength division multiplexing (WDM) Information Technology in Theory

Multiplexing (continued) Information Technology in Theory

Time Division Multiplexing Information Technology in Theory

Copper Transmission Media Twisted pair Unshielded Twisted Pair (UTP) Shielded Twisted Pair (STP) Coaxial Cable Information Technology in Theory

Copper Transmission Media (continued) Information Technology in Theory

Information Technology in Theory Twisted Pair Information Technology in Theory

Twisted Pair (continued) Information Technology in Theory

Information Technology in Theory Coaxial Cable Information Technology in Theory

Managing Errors in Digital Communication Systems Whenever digital information is sent across any communication channel, be it twisted pair, coax, air, or optical fiber, there is always a possibility that some bits will arrive at their destination with errors The incorrect detection of a binary digit is called an error Information Technology in Theory

Managing Errors in Digital Communication Systems (continued) Information Technology in Theory

Managing Errors in Digital Communication Systems (continued) By encoding a bit stream prior to transmission using an assortment of techniques called error-control coding (ECC), the receiver can detect and sometimes even correct errors that may occur at the receiver Some of these codes include: Block codes Convolutional codes Information Technology in Theory

Information Technology in Theory Block Codes Single parity checking Rectangular coding Cyclic redundancy checking (CRC) Information Technology in Theory

Single Parity Checking Information Technology in Theory

Information Technology in Theory Rectangular Coding Information Technology in Theory

Cyclic Redundancy Checking CRC is slightly more complex than parity checking or rectangular coding and is based on appending a stream of bits to the end of a data block The appended bits are generated by performing a simple mathematical operation on the original bit stream CRC is also used for verifying data integrity within the area of computer forensics Information Technology in Theory

Information Technology in Theory Convolutional Codes Information Technology in Theory

Digital Communications Scenario Information Technology in Theory

Digital Communications Scenario (continued) Information Technology in Theory

Digital Communications Scenario (continued) Information Technology in Theory

Information Technology in Theory Summary Binary signaling schemes communicate digital information over a transmission system by corresponding a 0 or a 1 to one of two discrete voltage values Signaling schemes may be extended to M-ary signaling, whereby log2M groups of bits may be sent at one time using M different signaling levels The capacity of a channel, expressed in bits per second, depends on the bandwidth and the signal-to-noise ratio of the channel Modulation techniques superimpose information signals onto a carrier wave, such as a radio wave or a light wave, by varying some of its properties, such as the wave’s amplitude, frequency, and phase Information Technology in Theory

Information Technology in Theory Summary (continued) Attenuation (measured in decibels) is the loss of energy occurring over a transmission line; it is a major factor that affects the quality and distance of communications Multiplexing techniques enable a single transmission line to simultaneously transmit multiple information-carrying signals Sources of transmission errors include electromagnetic interference, distortion, systems failures, and atmospheric conditions such as lightning and rain By encoding bit streams prior to transmission using error control coding (ECC) techniques, a receiver can detect and sometimes even correct errors that may occur at the receiver of a communication system Information Technology in Theory