Data and Computer Communications

Data and Computer Communications Chapter 3 – Data Transmission 8th and 9th editions

Data Transmission • Quality of transmitted signal • Transmission Terminology. • Frequency, Spectrum and Bandwidth. • Analogue & Digital Signals. • Frequency, Spectrum and Bandwidth • Periodic and Aperiodic Signals. • The sine wave As the fundamental periodic signal. • Sine wave representation. • Wave length • Amplitude Lambda () • Frequency Domain Concepts

Data Transmission • Addition of Frequency Components. • Frequency Domain Representations. • Spectrum & Bandwidth. • Data Rate and Bandwidth. • Analog and Digital Data Transmission Terms. • What is Decibels dB? • Acoustic Spectrum (Analog). • Advantages and Disadvantages o Digital signals • Transmission Impairments • Noise and its types • Channel Capacity

Data Transmission The successful transmission of data depends principally on two factors: • The quality of the signal being transmitted. • The characteristics of the transmission medium

Transmission Terminology • data transmission occurs between a transmitter & receiver via some medium • Transmission media may be classified as guided or unguided • guided medium • eg. twisted pair, coaxial cable, optical fiber • waves are guided along a physical path • unguided / wireless medium • eg. air, water, vacuum • Provide means for transmitting electromagnetic waves but do not guide them; In both cases, communication is in the form of electromagnetic waves

Transmission Terminology • direct link • propagate directly from transmitter to receiver with no intermediate devices (except for repeaters and amplifiers) • point-to-point (guided transmission medium) • direct link • only 2 devices share link • multi-point • more than two devices share the link

Transmission Terminology Transmission modes • simplex • one direction (one station is transmitter and the other is receiver). Example: television • half duplex : both stations may transmit, but only one at a time. • either direction Example: police radio • full duplex • both directions at the same time Example: telephone

Frequency, Spectrum and Bandwidth The signal is a function of time, but it can also be expressed as a function of frequency.. Will see soon Viewed as a function of time, an electromagnetic signal can be either analog or digital • time domain concepts • analog signal • various in a smooth way over time • digital signal • maintains a constant level then changes to another constant level • periodic signal (The simplest sort of signal) • pattern repeated over time • aperiodic signal • pattern not repeated over time

Analogue & Digital Signals The continuous signal might represent speech, The discrete signal might represent binary 1s and 0s.

Periodic Signals • The signal consists of components of different frequencies. • frequency domain view of a signal is more important to an understanding of data transmission than a time domain view. • Frequency = Cycles per period of time the signal consists of components of different frequencies

Sine Wave The sine wave is the fundamental periodic signal. A general sine wave can be represented by three parameters: • peak amplitude (A) • maximum strength of signal over time • typically measured volts • frequency (f) • rate of change of signal • Hertz (Hz) or cycles per second • period = time for one repetition (T) • T = 1/f • phase () • relative position in time

Varying Sine Wavesgeneral sine wave can be written as s(t) = A sin(2ft +) sinusoid function

Wavelength There is a simple relationship between the two sine waves, one in time and one in space. • () is distance occupied by one cycle • between two points of corresponding phase in two consecutive cycles • assuming signal velocity v have  = vT (distance =speed *time) T = 1/f • or equivalently f = v because ( = v * 1/f) • especially when v = c • c = 3*108 ms-1 (speed of light in free space)

Frequency Domain Concepts • It turns out that the frequency domain view of a signal is more important to an understanding of data transmission than a time domain view. • signal are made up of many frequencies (cycles) • components are sine waves • Fourier analysis discipline can show that any signal is made up of component sine waves • can plot frequency domain functions

Addition of FrequencyComponents(T=1/f) • c is sum of f & 3f By adding together enough sinusoidal signals, each with the appropriate amplitude, frequency, and phase, any electromagnetic signal can be constructed. the components of this signal are just sine waves of frequencies f and 3f, as Shawn in parts (a) and (b).

Frequency Domain Representations for each signal, there is a time domain function s(t) that specifies the amplitude of the signal at each instant in time • Similarly, there is a frequency domain function S(f) that specifies the peak amplitude of the constituent frequencies of the signal.

Spectrum & Bandwidth • Spectrum: range of frequencies contained in signal (Fig 3.4c, it extends from f to 3f) • absolute bandwidth • width of spectrum eg is 2f in Fig 3.4c (3f – f = 2f) • Many Signals such as that of Figure 3.5b, have an infinite bandwidth. • effective bandwidth or just bandwidth • narrow band of frequencies containing most energy • DC Component • component of zero frequency

Data Rate and Bandwidth • Any transmission system has a limited band of frequencies • This limits the data rate that can be carried • Square wave have infinite components and hence bandwidth • But most energy in first few components • limited bandwidth increases distortion and the greater the potential for error by the receiver • The greater the bandwidth transmitted, the greater the cost • There is a direct relationship between data rate & bandwidth: the higher the data rate of a signal, the greater is its required effective bandwidth.

By defining the terms we will be able understand the terms Analog and digital as applied to data , signals and transmission Analog and Digital Data Transmission Terms • Data • Entities that convey information • Signals • Electric or Electromagnetic representations of data • signaling • Physically propagates along a medium • Transmission • Communication of data by propagation and processing of signals

Sound sensibility is different in humans than animals. Hint, dogs are used in rescue operations when earthquakes The decibel (abbreviated dB) • dB is the unit used to measure the intensity of a sound • The decibel scale is a little odd because the human ear is incredibly sensitive. • Your ears can hear everything from your fingertip brushing lightly over your skin to a loud jet engine. In terms of power, the sound of the jet engine is about 1,000,000,000,000 times more powerful than the smallest audible sound. That's a big difference! • On the decibel scale, the smallest audible sound (near total silence) is 0 dB. • A sound 10 times more powerful is 10 dB. A sound 100 times more powerful than near total silence is 20 dB.

Decibels dB • A sound 1,000 times more powerful than near total silence is 30 dB. • Here are some common sounds and their decibel ratings: • Near total silence - 0 dB • A whisper - 15 dB • Normal conversation - 60 dB • A lawnmower - 90 dB • A car horn - 110 dB • A rock concert or a jet engine - 120 dB • A gunshot or firecracker - 140 dB

Db and distance relationship • You know from your own experience that distance affects the intensity of sound -- if you are far away, the power is greatly diminished. All of the ratings above are taken while standing near the sound. • Any sound above 85 dB can cause hearing loss, and the loss is related both to the power of the sound as well as the length of exposure. • You know that you are listening to an 85-dB sound if you have to raise your voice to be heard by somebody else. • Eight hours of 90-dB sound can cause damage to your ears; any exposure to 140-dB sound causes immediate damage (and causes actual pain).

Acoustic Spectrum (Analog) Analog data take on continuous values in some interval, the most familiar example being audio, which, in the form of acoustic sound waves, can be perceived directly by human beings. Figure 3.9 see slide 22 shows the acoustic spectrum for human speech and for music (note log scales). Frequency components of typical speech may be found between approximately 100 Hz and 7 kHz, and has a dynamic range of about 25 dB (a shout is approx 300 times louder than whisper). Another common example of analog data is video, as seen on a TV screen.

Acoustic Spectrum (Analog)

Audio Signals • The most familiar example of analog information is audio/acoustic sound wave information, eg. human speech • freq range 20Hz-20kHz (speech 100Hz-7kHz) • easily converted into electromagnetic signals • varying volume converted to varying voltage • can limit frequency range for voice channel to 300-3400Hz narrower bandwidth will produce acceptable voice reproduction. The standard spectrum for a voice channel is 300 to 3400 Hz

Digital Data • As generated by computers (1s and 0s).then converted into digital voltage pulses for transmission. • Has two dc components • Bandwidth depends on data rate The greater the bandwidth of the signal, the more faithfully it approximates a digital pulse stream.

Data are propagated from one point to another by means of electromagnetic signals. Both analog and digital signals may be transmitted on suitable transmission media Analog Signals

A sequence of voltage pulses that may be transmitted over a wire medium. Digital Signals • Digital signals can be used to transmit both analog signals and digital data. • Analog data can converted to digital using a codec (coder-decoder), . A repeater receives the digital signal, recovers the pattern of 1s and 0s, and retransmits a new signal. Thus the attenuation is overcome. A digital signal can be transmitted only within a limited distance before attenuation, noise, and other impairments

Advantages & Disadvantages of Digital Signals Attenuation: Reduction in strength • Cheaper • Less susceptible to noise • But greater attenuation than Analog • Digital signals are now the preferred choice Because of the attenuation, or reduction, of signal strength at higher frequencies, the pulses become rounded and smaller.

Analog OR Digital? Which is the preferred method of transmission? The answer being supplied by the telecommunications industry and its customers is digital. • Both long-haul telecommunications facilities and intra-building services have moved to digital transmission and, where possible, digital signaling techniques, for a range of reasons.

Transmission Impairments • Signal received may differ from signal transmitted causing: • Analog - degradation of signal quality • Digital - bit errors • Most significant impairments are • Attenuation and attenuation distortion • Delay distortion • Noise

Attenuation • Where signal strength falls off with distance • Depends on medium • For unguided media, attenuation is a more complex function of distance and the makeup of the atmosphere. Attenuation introduces three considerations for the transmission engineer • Received signal strength must be strong enough to be detected • Sufficiently higher than noise to receive without error • Attenuation varies with frequency causing distortion • Increase strength using amplifiers/repeaters (first and second problems) Note: This also an increasing function of frequency. See next slide for Distortion

Delay Distortion • Only occurs in guided media • Propagation velocity varies with frequency • Various frequency components arrive at different times resulting in phase shifts between the different frequencies. • The velocity tends to be highest near the center frequency and fall off toward the two edges of the band • Particularly critical for digital data • Parts of one bit spill over into others causing intersymbol interference

Techniques to equalizing attenuation • Using loading coils: changes the properties of the electrical signal on the line • Using amplifiers: Amplifies higher frequencies more than lower ones These techniques result in smoothing the attunation effect on the transmitted signal

Noise. • What is Nose? Additional signals inserted between transmitter and receiver. • Types of Noise: • Thermal (static Noise) • Due to thermal agitation of electrons • Uniformly distributed ( often referred to as white noise) • Intermodulation Noise • Signals that are the sum and difference of original frequencies sharing a medium. • Pproduced by nonlinearities in the transmitter, receiver, and/or intervening transmission medium.

Types of Noise.. continue • Crosstalk • A signal from one line is picked up by another Occurs by electrical coupling between nearby twisted pairs or, rarely, coax cable lines carrying multiple signals. It can also occur when microwave antennas pick up unwanted signals; • Impulse • Irregular pulses or spikes • eg. external electromagnetic interference • short duration • high amplitude It is generated from a variety of causes, including external electromagnetic disturbances, such as lightning, and faults and flaws in the communications system. • A minor annoyance for analog signals • A major source of error in digital data, a noise spike could corrupt many bits

Channel Capacity • Max possible data rate on communication channel • There are four concepts here that we are trying to relate to one another: • Data rate - in bits per second at which data can be communicated • Bandwidth – dictated by the transmitter and the medium in cycles per second or Hertz • Noise – Average of noise on communications link • Error rate - of corrupted bits 0 to 1 or 1 to 0 There are limitations due to physical properties for all transmission channels

Summary • looked at data transmission issues • Frequency, spectrum & bandwidth • Analog vs Digital signals • Transmission impairments

Data and Computer Communications