Data Communication Data and Signals Behrouz A. Forouzan Data Communication - Data and Signals

Data Communication - Data and Signals Index ANALOG AND DIGITAL PERIODIC ANALOG SIGNALS DIGITAL SIGNALS TRANSMISSION IMPAIRMENT DATA RATE LIMITS PERFORMANCE Data Communication - Data and Signals

Data Communication - Data and Signals major functions of physical layer is to move data in the form of electromagnetic signals across a transmission medium data must be transformed to electromagnetic signals for transmission over medium Data Communication - Data and Signals

Data Communication - Data and Signals ANALOG AND DIGITAL Analog and Digital Data computer file is digital Can be transmitted through analog or digital signal sound is analog Analog and Digital Signal Analog signals have infinite number of values in a range; digital signals have only a limited number of values Data Communication - Data and Signals

Data Communication - Data and Signals ANALOG AND DIGITAL Data Communication - Data and Signals

ANALOG AND DIGITAL Periodic and Nonperiodic Signals Both analog and digital signals can be: periodic non-periodic repeats a pattern over identical periods The completion of one full pattern is called a cycle changes without exhibiting a pattern or cycle Data Communication - Data and Signals

ANALOG AND DIGITAL Periodic and Nonperiodic Signals In data communications, we commonly use periodic analog signals (because they need less bandwidth, Chapter 5) and nonperiodic digital signals (because they can represent variation in data, (Chapter 6) Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Simple cannot be decomposed into simpler signals Composite composed of multiple sine waves Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Sine Wave fundamental form of a periodic analog signal sine wave can be represented by three parameters: peak amplitude frequency phase Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Sine Wave Peak Amplitude absolute value of its highest intensity, proportional to the energy it carries, measured in volts. The power in your house can be 110 to 120V peak value of an AA battery is normally 1.5 V Period and Frequency Period : amount of time, in seconds, a signal needs to complete 1 cycle. (Second) Frequency : number of periods in 1 second (HZ) Phase Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Sine Wave Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Sine Wave Power at home has a frequency of 60 Hz Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Sine Wave Peak Amplitude Period and Frequency Phase position of the waveform relative to time 0 measured in degrees or radians Phase of 180° corresponds to a shift of one-half of a period Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Sine Wave Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Sine Wave Example: A sine wave is offset 1/6 cycle with respect to time 0. What is its phase in degrees and radians? 1/6 * 360 degree = 60 degree = Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Wavelength Distance a signal can travel in one period depends on both frequency and medium often used wavelength to describe transmission of light in an optical fiber in a vacuum, light speed is . speed is lower in air and even lower in cable Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Wavelength Example: In coaxial or fiber-optic cable, wavelength is shorter (0.5 micron) because propagation speed is decreased Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Time and Frequency Domains Time Domain: amplitude versus time plot Phase is not explicitly shown Frequency Domain: relationship between amplitude and frequency concerned with only peak value of the frequency Amplitude changes during one period are not shown frequency domain conveys the information in time domain plot The advantage of the frequency domain is that we can immediately see values of frequency and peak amplitude Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Time and Frequency Domains Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Time and Frequency Domains Example: frequency domain is more compact and useful when dealing with more than one sine wave Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Composite Signals Simple sine 60 Hz as energy A signal of danger Composite Sine Send composite signal to communicate data Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Composite Signals Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Composite Signals periodic composite signal can be decomposed into a series of simple sine waves with discrete frequencies not typical of those found in data communications nonperiodic composite signal can be decomposed into infinite number of simple sine waves with continuous frequencies having real values Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS periodic Composite Signals Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS periodic Composite Signals Fundamental frequency: (first harmonic) frequency of the composite signal In Previous example , composite signal fundamental frequency is f and it includes 1st, 3rd and 9th harmonic of frequency of f Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS non-periodic Composite Signals Example: signal created by a microphone or a telephone Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Bandwidth difference between the highest and the lowest frequencies contained in a composite signal Data Communication - Data and Signals

PERIODIC ANALOG SIGNALS Bandwidth AM Radio 530 to 1700 kHz each AM radio station is 10kHz bandwidth FM Radio ranges from 88 to 108 MHz each FM radio station is 200-kHz bandwidth black-and-white TV has 3.85 MHz bandwidth analog color TV channel has 6-MHz bandwidth Data Communication - Data and Signals

Data Communication - Data and Signals DIGITAL SIGNALS information can be represented by an analog or digital signal if a signal has L levels, each level needs (log L based2 ) bits Data Communication - Data and Signals

DIGITAL SIGNALS Bit Rate number of bits sent in 1 s, expressed in bps digital signals are nonperiodic, thus period and frequency are not appropriate Characteristics bit rate is used to describe digital signals. A digitized voice channel bit rate id 64kbps high-definition TV (HDTV) bit rate is 20 to 40Mbps Data Communication - Data and Signals

DIGITAL SIGNALS Bit Length wavelength for an analog signal = bit length in digital signal distance one bit occupies on the transmission medium Data Communication - Data and Signals

Digital Signal as a Composite Analog Signal Frequency components od the square wave is as follows usually rare Data Communication - Data and Signals

Transmission of nonperiodic Digital Signals Baseband Transmission sending a digital signal over a channel without changing the digital signal to an analog signal Case1: Low-Pass Channel with Wide Bandwidth (dedicated medium) Case 2: Low-Pass Channel with Limited Bandwidth Broadband Transmission (Using Modulation) changing the digital signal to an analog signal for transmission (band-pass channel) Data Communication - Data and Signals

Digital Signal Baseband Transmition A digital signal is a composite analog signal with an infinite bandwidth requires a low-pass channel, with a bandwidth that starts from zero to infinity Design based on medium (channel) property: Case 1: Low-Pass Channel with Wide Bandwidth (dedicated medium) Case 2: Low-Pass Channel with Limited Bandwidth Data Communication - Data and Signals

Digital Signal Baseband Transmition dedicated medium Example: LAN (medium channel is time-division between users) entire spectrum of a medium is required (however limited in spectrum) amplitudes of higher frequencies in digital signal is so small that they can be ignored So over a dedicated medium, such as a coaxial cable or fiber optic sending digital signals with very good accuracy is possible Data Communication - Data and Signals

Digital Signal Baseband Transmition dedicated medium Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium we approximate the digital signal with an analog signal The level of approximation depends on the bandwidth available Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium Let us assume that we have a digital signal of bit rate N roughly simulate this signal, we need to consider the worst case, a maximum number of changes in the digital signal 01010101 ... or the sequence 10101010· ... So analog signal of frequency f = N/2 however, just this one frequency cannot make all patterns; we need more components Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium Example: a digital signal with a 3-bit pattern is simulated by using analog signals. 000: frequency f =0 and a phase of 180° 111: frequency f =0 and a phase of 0° The two worst cases 010: frequenc of =NI2 and phases of 180° 101: frequenc of =NI2 and phases of 0°. other four cases: can only be simulated with an analog signal with f = NI4 and phases of 180°, 270°, 90°, and 0°. Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium rough approximation is referred to as using the first harmonic (NI2) frequency Bandwidth required is N/2 – 0 = N/2 Hz Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium Better Approximation To make the shape of the analog signal look more like that of a digital signal, we need to add more harmonics of the frequencies Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium Example: required bandwidth of a low-pass channel to send 1 Mbps by using baseband transmission? a. The minimum bandwidth, is B =: bit rate /2, or 500 kHz. b. A better result can be achieved by using the first and the third harmonics with the required bandwidth B =: 3 x 500 kHz =: 1.5 MHz. c. Still a better result can be achieved by using the first, third, and fifth harmonics with B =: 5 x 500 kHz =2.5 MHz Data Communication - Data and Signals

Digital Signal Baseband Transmition limited bandwidth medium Example: We have a low-pass channel with bandwidth 100 kHz. What is the maximum bit rate of this channel? The maximum bit rate can be achieved if we use the first harmonic. The bit rate is 2 times the available bandwidth, or 200 kbps Data Communication - Data and Signals

Broadband Transmission Modulation allows us to use a bandpass channel (a channel with a bandwidth that does not start from zero) This type of channel is more available than a low-pass channel Data Communication - Data and Signals

Broadband Transmission We have used a single-frequency analog signal (called a carrier); The result, is not a single-frequency signal; it is a composite signal, (Chapter 5) At the receiver, the received analog signal is converted to digital Data Communication - Data and Signals

Data Communication - Data and Signals Broadband Example: sending of computer data through a telephone subscriber line designed to carry voice (analog signal) with a limited bandwidth (frequencies between 0 and 4 kHz). Although this channel can be used as a low-pass channel, it is normally USED AS a bandpass channel. Because the bandwidth is so narrow (4 kHz), the maximum bit rate can be only 8 kbps Solution: is to consider the channel a bandpass channel. Data Communication - Data and Signals

Broadband Transmission Example: digital cellular telephone bandwidth allocated to a company providing digital cellular phone service is very wide, but bandpass Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT What is sent is not what is received causes of impairment are attenuation, distortion, and noise Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT Attenuation loss of energy in overcoming resistance of the medium. To compensate for this loss, amplifiers are used Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT Attenuation use the unit of the decibel to show lost or gained strength of a signal decibel numbers can be added (or subtracted) when we are measuring several points (cascading) decibel is negative if a signal is attenuated and positive if a signal is amplified A loss of 3 dB (-3 dB) is equivalent to losing one-half the power Gain of 10 dB (+10 dB) is equivalent to increasing 10 times the power Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT Attenuation dB=-3+7-3=+1 dBm = 10 log Pm where Pm is the power in milliwatts Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT Attenuation Example : If the signal at the beginning of a cable with -0.3 dBm/km has a power of 2 mW, what is the power of the signal at 5 km? Power in source = 2mW = +3 dBm Loss = 5 * (-0.3dBm) = -1.5dBm Power in dst = 3 – 1.5 = 1.5dBm = 1.4mW Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT Distortion signal changes its form or shape Differences in delay in a compound signal at different frequencies may create a difference in phase Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT Noise thermal noise, induced noise, comes from sources such as motors and appliances crosstalk, effect of one wire on the other impulse noise spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on Data Communication - Data and Signals

TRANSMISSION IMPAIRMENT Noise Signal-to-Noise Ratio (SNR) SNR= (average signal power / average noise power) SNR dB(m) = lOlog SNR (m)W The values of SNR and SNRdB for a noiseless channel are which never achieve this ratio in real life Data Communication - Data and Signals

Data Communication - Data and Signals DATA RATE LIMITS Data rate depends on three factors: The bandwidth available The level of the signals we use The quality of the channel (the level of noise) calculate the data rate: Nyquist for a noiseless channel. Shannon for a noisy channel Data Communication - Data and Signals

DATA RATE LIMITS Noiseless Channel: Nyquist Bit Rate Maximum BitRate = 2 x bandwidth x 10g2 L bandwidth is the bandwidth of the channel, L is number of signal levels used to represent data, BitRate is the bit rate in bits per second Increasing levels of a signal may reduce the reliability of the system it can be applied to baseband transmission and modulation not just baseband transmit ion previously described Data Communication - Data and Signals

DATA RATE LIMITS Noisy Channel: Shannon Capacity Highest Capacity =bandwidth X log2 (1 +SNR) Bandwidth is the bandwidth of the channel, Hz SNR is the signal-to-noise ratio, in unit of W not dB capacity is the capacity of the channel in bits per second bps no matter how many levels we have, we cannot achieve a data rate higher than the capacity of the channel Data Communication - Data and Signals

DATA RATE LIMITS Noisy Channel: Shannon Capacity Example: Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero. C=B log2 (1 + SNR) =B log2 (l + 0) =0 Example: regular telephone line. Normally has a bandwidth of 3000 Hz (300 to 3300 Hz). The signal- to-noise ratio is usually 3162. C =B log2 (1 + SNR) =3000 log2 (l + 3162) = 34,860 bps highest bit rate for a telephone line is 34.860 kbps Data Communication - Data and Signals

DATA RATE LIMITS Noisy Channel: Shannon Capacity when the SNR is very high C = B * (SNR dB/3) Example: SNRdB = 36 and channel bandwidth is 2 MHz. C= 2 MHz X 36/3 =24 Mbps Data Communication - Data and Signals

DATA RATE LIMITS Using Both Limits Example: Bandwidth = 1MHz, SNR=63 Shanon for upper limit: C= B log2 (l + SNR) =6Mbps For better performance we choose something lower, 4 Mbps use the Nyquist formula to find the number of signal levels. 4Mbps=2x 1 MHz x log2 L  L=4 Data Communication - Data and Signals

Data Communication - Data and Signals PERFORMANCE Measure performance of the network-how good is it? Bandwidth Throughput Latency (Delay) Bandwidth-Delay Product Jitter Data Communication - Data and Signals

PERFORMANCE Bandwidth Bandwidth in Hertz range of frequencies contained in a signal range of frequencies a channel can pass Bandwidth in Bits per Seconds number of bits per second that a channel can transmit increase in bandwidth in hertz means an increase in bandwidth in bps depends on whether we have baseband transmission or transmission with modulation (Chapter 4 & 5) Data Communication - Data and Signals

PERFORMANCE Throughput bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data both in bps Because of congestion or sender limitation Data Communication - Data and Signals

PERFORMANCE Latency (Delay) how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source Latency =propagation time +transmission time +queuing time + processing delay Data Communication - Data and Signals

PERFORMANCE Latency (Delay) Propagation Time Propagation time = (Distance/Propagation-speed) propagation speed of electromagnetic signals depends on the medium and frequency of the signaL in a vacuum, light is propagated with a speed of 3 x 108 m/s Data Communication - Data and Signals

PERFORMANCE Latency (Delay) Transmition Time time between the first bit and the last bit leaving the sender Transmission time = (Message size /Bandwidth) Message size in bits , Bandwidth in bps Data Communication - Data and Signals

PERFORMANCE Latency (Delay) Queuing Time time needed for each intermediate /end device to hold the message before it can be processed The queuing time is not a fixed factor When there is heavy traffic on the network, the queuing time increases Data Communication - Data and Signals

PERFORMANCE Bandwidth-Delay Product in data communications product of the two (Bandwidth and Delay) is important maximum number of bits that can fill the link Data Communication - Data and Signals

PERFORMANCE Bandwidth-Delay Product Example: bandwidth of 1 bps, link delay is 5 s  no more than 5 bits at any time on the link Data Communication - Data and Signals

PERFORMANCE Bandwidth-Delay Product Data Communication - Data and Signals

PERFORMANCE Bandwidth-Delay Product This measurement is important in sending data in bursts and wait for the acknowledgment of each burst before sending the next one. To use the maximum capability of the link, we need to make the size of our burst 2 times the product of bandwidth and delay; we need to fill up the full-duplex channel The sender should send a burst of data of (2 x bandwidth x delay) bits then waits for receiver acknowledgment Data Communication - Data and Signals

Data Communication - Data and Signals PERFORMANCE Jitter Jitter is a problem if different packets of data encounter different delays It is important for time-sensitive applications (audio and video) Discussed in multimedia section Data Communication - Data and Signals