William Stallings Data and Computer Communications 7 th Edition Chapter 3 Data Transmission

Terminology (1) Transmitter Receiver Medium (Transmission media may be classified as guided or unguided —Guided medium ( the waves are guided along a physical path) e.g. twisted pair, optical fiber —Unguided medium (also called wireless) e.g. air, water, vacuum

Terminology (2) Direct link —No intermediate devices Point-to-point —Direct link —Only 2 devices share link Multi-point —More than two devices share the link

Terminology (3) Simplex —One direction e.g. Television Half duplex —Either direction, but only one way at a time e.g. police radio Full duplex —Both directions at the same time e.g. telephone

5 Analog versus Digital Analog is a continuous waveform, with examples such as (naturally occurring) music and voice.

6 Analog versus Digital Digital is a discrete or non-continuous waveform with examples such as computer 1s and 0s.

Frequency, Spectrum and Bandwidth Time domain concepts —Analog signal Various in a smooth way over time.in other words, there are no breaks or discontinuities in the signal. —Digital signal Is one in which the signal intensity Maintains a constant for some period of time and then changes to another constant level. —Periodic signal Pattern repeated over time —Aperiodic signal Pattern not repeated over time

Analogue & Digital Signals

Periodic Signals The sine wave is the fundamental periodic signal. A general sine wave can be represented by three parameters: peak amplitude (A), frequency(f), and phase(  )

10 All Signals Have Three Components Amplitude Frequency Phase

11 Amplitude The amplitude of a signal is the height of the wave above or below a given reference point.

12 Frequency The frequency is the number of times a signal makes a complete cycle within a given time frame. Spectrum - The range of frequencies that a signal spans from minimum to maximum. Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal.

13 Data Communications and Computer Networks Chapter 2

14 Frequency For example, consider an average voice: The average voice has a frequency range of roughly 300 Hz to 3100 Hz. The spectrum would thus be Hz The bandwidth would be 2800 Hz

15 Phase The phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero. A change in phase can be any number of angles between 0 and 360 degrees. Phase changes often occur on common angles, such as 45, 90, 135, etc.

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Sine Wave Peak Amplitude (A) —maximum value or strength of signal over time. —Measured by volts Frequency (f) —Rate of change of signal —Hertz (Hz) or cycles per second —Period = time for one repetition (T) —T = 1/f Phase (  ) —Is a measure of the Relative position in time within a single period of a signal

Varying Sine Waves s(t) = A sin(2  ft +  ) Phase shift of bi/4 radians 2bi radians=360=1 period

Note: velocity is defined as the rate of change of position

Wavelength Distance occupied by one cycle Distance between two points of corresponding phase in two consecutive cycles Assuming that the signal is traveling with a velocity v, then the wavelength is related to the period as follows — = vT — f = v —Of particular relevance to this discussion is the case where v=c —c = 3*10 8 ms -1 (speed of light in free space)

Frequency Domain Concepts In practice, an electromagnetic signal will be usually made up of many frequencies Components are sine waves (In the next figure) Of frequencies (f) and (3f) Parts (a) and (b) of the figure shows these individual components Can be shown (Fourier analysis) that any signal is made up of component sine waves at various frequencies. For each signal there is a time domain functions s(f). That specifies the amplitude of the signal at each instant in time. Similarly, there is a frequency domain function S(f)

Addition of Frequency Components (T=1/f) The components of this signal are just sine waves of frequencies (f) and (3f). Part a and b show these individual components.

Frequency Domain Representations

Spectrum —range of frequencies contained in signal. For signal of figure in the slide 22 last one the spectrum extends from f to 3f. Absolute bandwidth —width of spectrum —For signal of figure in the slide 22 last one bandwidth is 2 f —Many signals, such as that in the slide 23 last one have an infinite bandwidth.

Spectrum & Bandwidth Effective bandwidth —Often just bandwidth —Narrow band of frequencies containing most of the energy DC Component —If the signal include a component of zero frequency, that component is a direct current (dc) —With no dc component, a signal has an average amplitude of zero, as seen in the time domain. With a dc component, it has a frequency term at f=0 and a nonzero average amplitude.

Signal with DC Component The result of adding a dc component to the signal

Data Rate and Bandwidth Any transmission system has a limited band of frequencies. This limits the data rate that can be carried

Assignment - 1 What is the Relationship between data rate and bandwidth

Analog and Digital Data Transmission Data —Entities that convey meaning Signals —Electric or electromagnetic representations of data Transmission —Communication of data by propagation and processing of signals

Analog and Digital Data Analog —Continuous values within some interval —e.g. sound, video Digital —Discrete values —e.g. text, integers

Acoustic Spectrum (Analog)

Analog and Digital Signals Means by which data are propagated Analog —Continuously variable —Various media wire, fiber optic, space —Speech bandwidth 100Hz to 7kHz —Telephone bandwidth 300Hz to 3400Hz —Video bandwidth 4MHz Digital —Use two DC components

Advantages & Disadvantages of Digital Advantages Cheaper Less susceptible to noise Disadvantages Greater attenuation —Pulses become rounded and smaller —Leads to loss of information

Which is the preferred method of transmission?

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.

Attenuation of Digital Signals

Components of Speech Frequency range (of hearing) 20Hz-20kHz —Speech 100Hz-7kHz Easily converted into electromagnetic signal for transmission Sound frequencies with varying volume converted into electromagnetic frequencies with varying voltage Limit frequency range for voice channel — Hz

Conversion of Voice Input into Analog Signal

Video Components USA lines scanned per frame at 30 frames per second —525 lines but 42 lost during vertical retrace So 525 lines x 30 scans = lines per second —63.5  s per line —11  s for retrace, so 52.5  s per video line Max frequency if line alternates black and white Horizontal resolution is about 450 lines giving 225 cycles of wave in 52.5  s Max frequency of 4.2MHz

Binary Digital Data From computer terminals etc. Two dc components Bandwidth depends on data rate

Conversion of PC Input to Digital Signal

Data and Signals Usually use digital signals for digital data and analog signals for analog data Can use analog signal to carry digital data —Modem Can use digital signal to carry analog data —Compact Disc audio

Analog Signals Carrying Analog and Digital Data

Digital Signals Carrying Analog and Digital Data

Analog Transmission Analog signal transmitted without regard to content May be analog or digital data Attenuated over distance Use amplifiers to boost signal Also amplifies noise

In a communications system, 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. An analog signal is a continuously varying electromagnetic wave that may be propagated over a variety of media, depending on spectrum; examples are wire media, such as twisted pair and coaxial cable; fiber optic cable; and unguided media, such as atmosphere or space propagation.

analog signals can be used to transmit both analog data represented by an electromagnetic signal occupying the same spectrum, and digital data using a modem (modulator/demodulator) to modulate the digital data on some carrier frequency. However, analog signal will become weaker (attenuate) after a certain distance. To achieve longer distances, the analog transmission system includes amplifiers that boost the energy in the signal. Unfortunately, the amplifier also boosts the noise components.

With amplifiers cascaded to achieve long distances, the signal becomes more and more distorted. For analog data, such as voice, quite a bit of distortion can be tolerated and the data remain intelligible. However, for digital data, cascaded amplifiers will introduce errors.

Digital Signals Carrying Analog and Digital Data

Digital Transmission Concerned with content Integrity endangered by noise, attenuation etc. Repeaters used Repeater receives signal Extracts bit pattern Retransmits Attenuation is overcome Noise is not amplified

A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium; eg. a constant positive voltage level may represent binary 0 and a constant negative voltage level may represent binary 1.

digital signals can be used to transmit both analog signals and digital data. Analog data can converted to digital using a codec (coder-decoder), which takes an analog signal that directly represents the voice data and approximates that signal by a bit stream. At the receiving end, the bit stream is used to reconstruct the analog data. Digital data can be directly represented by digital signals.

A digital signal can be transmitted only a limited distance before attenuation, noise, and other impairments endanger the integrity of the data. To achieve greater distances, repeaters are used. A repeater receives the digital signal, recovers the pattern of 1s and 0s, and retransmits a new signal. Thus the attenuation is overcome.

Advantages of Digital Transmission Digital technology —Low cost LSI/VLSI technology Data integrity —Longer distances over lower quality lines Capacity utilization —High bandwidth links economical —High degree of multiplexing easier with digital techniques Security & Privacy —Encryption Integration —Can treat analog and digital data similarly

Transmission Impairments Signal received may differ from signal transmitted Analog - degradation of signal quality Digital - bit errors Caused by —Attenuation and attenuation distortion —Delay distortion —Noise

With any communications system, the signal that is received may differ from the signal that is transmitted due to various transmission impairments. For analog signals, these impairments can degrade the signal quality. For digital signals, bit errors may be introduced, such that a binary 1 is transformed into a binary 0 or vice versa.

Attenuation where signal strength falls off with distance depends on medium received signal strength must be: —strong enough to be detected —sufficiently higher than noise to receive without error so increase strength using amplifiers/repeaters is also an increasing function of frequency so equalize attenuation across band of frequencies used —eg. using loading coils or amplifiers

Attenuation Attenuation is where the strength of a signal falls off with distance over any transmission medium. For guided media, this is generally exponential and thus is typically expressed as a constant number of decibels per unit distance. For unguided media, attenuation is a more complex function of distance and the makeup of the atmosphere

Attenuation Attenuation introduces three considerations for the transmission engineer. 1.First, a received signal must have sufficient strength so that the electronic circuitry in the receiver can detect the signal. 2.Second, the signal must maintain a level sufficiently higher than noise to be received without error. 3.Third, attenuation varies with frequency

Attenuation The first and second problems are dealt with by attention to signal strength and the use of amplifiers or repeaters. The third problem is particularly noticeable for analog signals. To overcome this problem, techniques are available for equalizing attenuation across a band of frequencies. This is commonly done for voice-grade telephone lines by using loading coils that change the electrical properties of the line; the result is to smooth out attenuation effects. Another approach is to use amplifiers that amplify high frequencies more than lower frequencies.

Delay Distortion only occurs in guided media propagation velocity varies with frequency hence various frequency components arrive at different times particularly critical for digital data since parts of one bit spill over into others causing intersymbol interference

Delay distortion occurs because the velocity of propagation of a signal through a guided medium varies with frequency. For a bandlimited signal, the velocity tends to be highest near the center frequency and fall off toward the two edges of the band. Thus various frequency components of a signal will arrive at the receiver at different times, resulting in phase shifts between the different frequencies. Delay distortion is particularly critical for digital data, because some of the signal components of one bit position will spill over into other bit positions, causing intersymbol interference. This is a major limitation to maximum bit rate over a transmission channel.

Noise (1) Additional signals inserted between transmitter and receiver Thermal —Due to thermal agitation of electrons —Uniformly distributed —White noise Intermodulation —Signals that are the sum and difference of original frequencies sharing a medium

Noise (2) Crosstalk —A signal from one line is picked up by another Impulse —Irregular pulses or spikes —e.g. External electromagnetic interference —Short duration —High amplitude

Channel Capacity Data rate —In bits per second —Rate at which data can be communicated Bandwidth —In cycles per second of Hertz —Constrained by transmitter and medium

Nyquist Bandwidth If rate of signal transmission is 2B then signal with frequencies no greater than B is sufficient to carry signal rate Given bandwidth B, highest signal rate is 2B Given binary signal, data rate supported by B Hz is 2B bps Can be increased by using M signal levels C= 2B log 2 M

Shannon Capacity Formula Consider data rate,noise and error rate Faster data rate shortens each bit so burst of noise affects more bits —At given noise level, high data rate means higher error rate Signal to noise ration (in decibels) SNR db = 10 log 10 (signal/noise) Capacity C=B log 2 (1+SNR) This is error free capacity

Required Reading Stallings chapter 3