Introduction to Digital Communication PDF

Introduction to digital communication Analog systems ◼ Analog systems use electrical signals that vary continuously, not having discrete values  Analog signals are electrical representations of signals from nature (pressure, light, sound, etc.)  Examples of analog systems: AM...

Introduction to digital communication Analog systems ◼ Analog systems use electrical signals that vary continuously, not having discrete values  Analog signals are electrical representations of signals from nature (pressure, light, sound, etc.)  Examples of analog systems: AM/FM radio, cassettes, telephone, VCR, standard television voltage Voltage (V) time Digital signals ◼ Binary digital signals use two discrete voltage levels to represent binary 1 or 0. ◼ Combining multiple bits into words permits us to represent larger values. ◼ Digital circuits operate on digital signals performing logic and arithmetic functions. voltage 1 0 1 1 0 1 5V 0V time Analog examples Digital systems ◼ Digital systems use electrical signals that represent discrete, binary values.  Digital signals are not representative of signals that occur in nature (pressure, light, sound, etc.).  Natural signals must be converted into digital format. ◼ Historically, signals in communications systems have been analog but a migration to digital systems has been underway for the last 25 years. Digital examples  Advantages of digital signals ◼ The most important advantage of digital communications is noise immunity. ◼ Receiver circuitry can distinguish between a binary 0 and 1 with a significant amount of noise. 1 0 1 1 0 1 Voltage (V) Time (sec) analog signal with noise digital signal with noise Advantages of digital signals ◼ Digital signals can be stripped of any noise in a process called signal regeneration. ◼ Consider a network of relay stations. microwave relay stations Advantages of digital signals ◼ An analog signal is received, amplified and retransmitted at each station. ◼ However, the noise is also amplified each link. original analog signal signal at repeater 1 microwave relay stations signal at repeater 2 signal at repeater 3 Advantages of digital signals ◼ An digital signal is received, regenerated, then retransmitted at each station. ◼ The noise can be eliminated at each repeater. original analog signal signal at repeater 1 microwave relay stations signal at repeater 2 signal at repeater 3 Advantages of digital signals ◼ Even if a digital signal does contain bit errors, many of these errors can be fixed at the receiver through the use of error correcting codes.  Error correcting codes allows CDs with minor scratches to be played without errors.  We will discuss such codes later. scratched CD Advantages of digital signals ◼ Digital signals are easier to multiplex. ◼ Multiplexing is the process of allowing multiple signals to share the same transmission channel. Advantages of digital signals ◼ Digital is the native format for computers. ◼ Computers permit the digital signal processing (DSP) and digital storage of communication signals.  DSP allows operations such as filtering, equalization and mixing to be done numerically without the use of analog circuits.  DSP also permits data compression. Transmission of digital data ◼ There are two ways to move bits from one place to another:  Transmit all bits of a word simultaneously (parallel transfer).  Send only 1 bit at a time (serial transfer). Serial transmission ◼ In serial transmission, each bit of a word is transmitted sequentially, one after another.  The least significant bit (LSB) is transmitted first, and the most significant bit (MSB) last.  Each bit is transmitted for a fixed interval of time. ◼ Serial data can be transmitted faster and over longer distances than parallel data. ◼ Serial buses are now replacing parallel buses in equipment where very high speeds are required. Parallel transmission ◼ Parallel data transmission is extremely fast because all the bits of the data word are transferred simultaneously.  There must be one wire for each bit of information to be transmitted. Multi-wire cable must be used. Parallel transmission ◼ Parallel data transmission is impractical for long-distance communication because of:  Cost of laying multi-wire cables.  Signal attenuation over long distances.  At high speeds, capacitance and inductance of multiple wires distorts the pulse signal. To reduce this, line lengths must be severely shortened.  To achieve clock speeds up to 400 MHz, line lengths must limited to a few inches ◼ Parallel data transmission by radio would be complex and expensive. ◼ One transmitter and one receiver for each bit. Serial-parallel conversion ◼ Because both parallel and serial transmission occur in computers and other equipment, there must be techniques for converting between parallel and serial and vice versa.  Such data conversions are usually taken care of by shift registers, sequential logic circuits made up of a number of flip-flops connected in cascade. Parallel outputs Q0 Q1 Q2 Q3 Data input D0 Q0 D1 Q1 D2 Q2 D4 Q4 C C C C Clock input Serial-parallel conversion Conversion from analog to digital ◼ Before we can use digital transmission, we must convert the signal of interest into a digital format. ◼ Translating an analog signal into a digital signal is called analog-to-digital (A/D) conversion, digitizing a signal, or encoding.  The device used to perform this translation is known as an analog-to-digital converter or ADC.  Conversion from analog to digital ◼ An analog signal is a smooth or continuous voltage or current variation.  Through A/D conversion these continuously variable signals are changed into a series of binary numbers. Voltage (V) 01101010100111001101010101111 Time (sec) A/D conversion ◼ The first step in A/D conversion is a process of sampling the analog signal at regular time intervals. sample points Voltage (V) sampling frequency 1 f = T sampling period (T) Time (sec) A/D conversion ◼ How often do we need to sample the signal?  How large does our sampling frequency f need to be in order to accurately represent the signal? Voltage (V) Voltage (V) Time (sec ) Time (sec ) high sampling rate Voltage (V) Voltage (V) Time (sec ) Time (sec ) low sampling rate Minimum sampling frequency ◼ The minimum sampling rate required in order to accurately reconstruct the analog input is given by the Nyquist sampling rate fN given f N  2 fm where fm is the highest frequency of the analog input.  The Nyquist rate is a theoretical minimum.  In practice, sampling rates are typically 2.5 to 3 times the Nyquist rate fN. A/D conversion ◼ The actual analog signal is smooth and continuous and represents an infinite number of actual voltage values.  Itis not possible to convert all analog samples to a precise binary number.  Therefore, samples are converted to a binary number whose value is close to the actual sample value. A/D conversion ◼ The A/D converter can represent only a finite number of voltage values over a specific range. ◼ An A/D converter divides a voltage range into discrete increments, each of which is represented by a binary number. A/D conversion ◼ The process of mapping the sampled analog voltage levels to these discrete, binary values is called quantization. ◼ Quantizers are characterized by their number of output levels. ◼ An N-bit quantizer has 2N levels and outputs binary numbers of length N.  Telephones use 8-bit encoding → 28 = 256 levels  CD audio use 16-bit encoding → 216 = 65,536 levels Digital Transmission Digital Transmission Methods to transmit data digitally Line coding Block coding Sampling Transmission modes Parallel Serial Synchronous Asynchronous 4123702 Data Communications System @YRU 2 Digital Signals Digital – have a limited number of defined values Use binary (0s and 1s) to encode information Less affected by interference (noise); fewer errors 4123702 Data Communications System @YRU 3 4.1 Line Coding Process of converting binary data to a digital signal 4123702 Data Communications System @YRU 4 Line Coding Characteristics Signal Level versus Data Level Pulse Rate versus Bit Rate DC Components Self-Synchronization 4123702 Data Communications System @YRU 5 Signal Level versus Data Level Signal level – number of different values allowed in a signal Data level – number of symbols used to represent data b.Three signal levels, two data levels 4123702 Data Communications System @YRU 6 Pulse Rate versus Bit Rate Pulse rate – defines number of pulses per second Pulse – minimum amount of time required to transmit a symbol Bit rate – defines number of bits per second Bit rate = Pulse rate × log 2 L When L is the number of data level of the signal 4123702 Data Communications System @YRU 7  Example 1 A signal has two data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows: 1 Pulse Rate = = 1000 pulses/s 110−3 Bit Rate = Pulse Rate × log2 L = 1000 × log2 2 = 1000 bps 4123702 Data Communications System @YRU 8  Example 2 A signal has four data levels with a pulse duration of 1 ms. We calculate the pulse rate and bit rate as follows: 1 Pulse Rate = 110−3 = 1000 pulses/s Bit Rate = Pulse Rate × log2 L = 1000 × log2 4 = 2000 bps 4123702 Data Communications System @YRU 9 DC Components Residual direct-current (dc) components or zero frequencies are undesirable Some systems do not allow passage of a dc component (such as a transformer); may distort the signal and create output errors DC component is extra energy residing on the line and is useless 4123702 Data Communications System @YRU 10 DC Component 4123702 Data Communications System @YRU 11 Self-Synchronization Digital signal includes timing information in the data being transmitted to prevent misinterpretation Figure 4.16 Lack of synchronization 4123702 Data Communications System @YRU 12  Example 3 In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 Kbps? How many if the data rate is 1 Mbps? Solution At 1 Kbps: 1000 bits sent 1001 bits received 1 extra bps At 1 Mbps: 1,000,000 bits sent 1,001,000 bits received 1000 extra bps 4123702 Data Communications System @YRU 13 Line Coding Schemes 4123702 Data Communications System @YRU 14 Unipolar Simplest method; inexpensive Uses only one voltage level Polarity (+ or -) is usually assigned to binary 1; a 0 is represented by zero voltage 4123702 Data Communications System @YRU 15 Unipolar Potential problems: DC component Lack of synchronization 4123702 Data Communications System @YRU 16 Polar Uses two voltage levels, one positive and one negative Alleviates DC component Variations Nonreturn to zero (NRZ) Return to zero (RZ) Manchester Differential Manchester 4123702 Data Communications System @YRU 17 Nonreturn to Zero (NRZ) Value of signal is always positive or negative NRZ-L (NRZ-Level) Signal level depends on bit represented; positive usually means 0, negative usually means 1 Problem : synchronization of long streams of 0s or 1s NRZ-I (NRZ-Invert) Inversion of voltage represents a 1 bit 0 bit represented by no change Allows for synchronization 4123702 Data Communications System @YRU 18 NRZ-L and NRZ-I Encoding 4123702 Data Communications System @YRU 19 Return to Zero (RZ) In NRZ-I, long strings of 0s may still be a problem May include synchronization as part of the signal for both 1s and 0s How? Must include a signal change during each bit Uses three values: positive, negative, and zero 1 bit represented by positive-to-zero 0 bit represented by negative-to-zero 4123702 Data Communications System @YRU 20 RZ Encoding 4123702 Data Communications System @YRU 21 RZ Encoding Disadvantage Requires two signal changes to encode each bit; more bandwidth necessary 4123702 Data Communications System @YRU 22 Manchester Uses an inversion at the middle of each bit interval for both synchronization and bit representation Negative-to-positive represents binary 1 Positive-to-negative represents binary 0 Achieves same level of synchronization with only two levels of amplitude 4123702 Data Communications System @YRU 23 Manchester Encoding 4123702 Data Communications System @YRU 24 Differential Manchester Inversion at middle of bit interval is used for synchronization Presence or absence of additional transition at beginning of interval identifies the bit Transition means binary 0; no transition means 1 Requires two signal changes to represent binary 0 but only one to represent 1 4123702 Data Communications System @YRU 25 Differential Manchester 4123702 Data Communications System @YRU 26 Bipolar Encoding Uses three voltage levels: positive, negative, and zero Zero level represents binary 0; 1s are represented with alternating positive and negative voltages, even when the 1 bits are not consecutive Two schemes Alternate mark inversion (AMI) Bipolar n-zero substitution (BnZS) 4123702 Data Communications System @YRU 27 Bipolar AMI Neutral, zero voltage represents binary 0 Binary 1s represented by alternating positive and negative voltages 4123702 Data Communications System @YRU 28 Bipolar n-zero substitution (BnZS) Solves problem of synchronizing sequential 0s, often occurring in long-distance transmission If n consecutive zeros occur, some of the bits in those n bits become positive or negative Substitution violates rules of AMI in a manner that receiver knows the bits are actually 0s and not 1s 4123702 Data Communications System @YRU 29 Other Schemes 2B1Q (two binary, one quaternary) uses four voltage levels One pulse can represent 2 bits; more efficient 4123702 Data Communications System @YRU 30 Other Schemes MLT-3 (multi-line transmission, three level) – similar to NRZ-I using three levels of signals; signal transitions occur at beginning of 1 bit, no transition at beginning of 0 4123702 Data Communications System @YRU 31 4.2 Block Coding Coding method to ensure synchronization and detection of errors Three steps: division, substitution, and line coding 4123702 Data Communications System @YRU 32 Steps in Transformation Step 1 Step 2 Step 3 4123702 Data Communications System @YRU 33 Transformation Steps Step 1: bit stream is divided into groups of m bits Step 2: substitute an m-bit code for an n-bit group Codes with no more than three consecutive 0s or 1s are used to achieve synchronization Since only a subset of blocks are used, if one or more bits are changed and an invalid code is received, a receiver can easily detect the error Step 3: line encoding scheme is then used to create the signal 4123702 Data Communications System @YRU 34 Common Block Codes 4B/5B – every 4 bits of data is encoded into a 5- bit code; NRZ-1 is usually used for line coding 8B/10B – group of 8 bits of data is substituted by a 10-bit code 8B/6T – each 8-bit group is substituted with a six- symbol code; uses less bandwidth since three signal levels may be used 4123702 Data Communications System @YRU 35  Figure 4.16 Substitution in block coding 4123702 Data Communications System @YRU 36  Table 4.1 4B/5B encoding Data Code Data Code 0000 11110 1000 10010 0001 01001 1001 10011 0010 10100 1010 10110 0011 10101 1011 10111 0100 01010 1100 11010 0101 01011 1101 11011 0110 01110 1110 11100 0111 01111 1111 11101 4123702 Data Communications System @YRU 37  Table 4.1 4B/5B encoding (Continued) Data Code Q (Quiet) 00000 I (Idle) 11111 H (Halt) 00100 J (start delimiter) 11000 K (start delimiter) 10001 T (end delimiter) 01101 S (Set) 11001 R (Reset) 00111 4123702 Data Communications System @YRU 38 Figure 4.17 Example of 8B/6T encoding 4123702 Data Communications System @YRU 39 4.3 Sampling Analog data must often be converted to digital format (ex: long-distance services, audio) Sampling is process of obtaining amplitudes of a signal at regular intervals 4123702 Data Communications System @YRU 40 Pulse Amplitude Modulation (PAM) Analog signal’s amplitude is sampled at regular intervals; result is a series of pulses based on the sampled data Pulse Coded Modulation (PCM) is then used to make the signal digital 4123702 Data Communications System @YRU 41  Note: Pulse amplitude modulation has some applications, but it is not used by itself in data communication. However, it is the first step in another very popular conversion method called pulse code modulation. 4123702 Data Communications System @YRU 42 Pulse Coded Modulation (PCM) First quantizes PAM pulses; an integral value in a specific range to sampled instances is assigned Each value is then translated to its 7-bit binary equivalent Binary digits are transformed into a digital signal using line coding 4123702 Data Communications System @YRU 43  Figure 4.19 Quantized PAM signal 4123702 Data Communications System @YRU 44 Figure 4.20 Quantizing by using sign and magnitude 4123702 Data Communications System @YRU 45 Figure 4.21 PCM 4123702 Data Communications System @YRU 46 Digitization of an Analog Signal 4123702 Data Communications System @YRU 47 Sampling Rate: Nyquist Theorem Accuracy of digital reproduction of a signal depends on number of samples Nyquist theorem: number of samples needed to adequately represent an analog signal is equal to twice the highest frequency of the original signal 4123702 Data Communications System @YRU 48  Note: According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency. 4123702 Data Communications System @YRU 49  Figure 4.23 Nyquist theorem 4123702 Data Communications System @YRU 50  Note: Note that we can always change a band-pass signal to a low-pass signal before sampling. In this case, the sampling rate is twice the bandwidth. 4123702 Data Communications System @YRU 51 Example 4 What sampling rate is needed for a signal with a bandwidth of 10,000 Hz (1000 to 11,000 Hz)? Solution The sampling rate must be twice the highest frequency in the signal: Sampling rate = 2 x (11,000) = 22,000 samples/s 4123702 Data Communications System @YRU 52 Example 5 (How many bit per sample) A signal is sampled. Each sample requires at least 12 levels of precision (+0 to +5 and -0 to -5). How many bits should be sent for each sample? Solution We need 4 bits; 1 bit for the sign and 3 bits for the value. A 3-bit value can represent 23 = 8 levels (000 to 111), which is more than what we need. A 2-bit value is not enough since 22 = 4. A 4-bit value is too much because 24 = 16. 4123702 Data Communications System @YRU 53  Example 6 (Bit rate) We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample? Solution The human voice normally contains frequencies from 0 to 4000 Hz. Sampling rate = 4000 x 2 = 8000 samples/s Bit rate = sampling rate x number of bits per sample 4123702 Data Communications System @YRU 54 4.4 Transmission Mode 4123702 Data Communications System @YRU 55 Parallel Transmission Bits in a group are sent simultaneously, each using a separate link n wires are used to send n bits at one time Advantage: speed Disadvantage: cost; limited to short distances 4123702 Data Communications System @YRU 56 Serial Transmission Transmission of data one bit at a time using only one single link Advantage: reduced cost Disadvantage: requires conversion devices Methods: Asynchronous Synchronous 4123702 Data Communications System @YRU 57 Asynchronous Transmission Transfer of data with start and stop bits and a variable time interval between data units Timing is unimportant Start bit alerts receiver that new group of data is arriving Stop bit alerts receiver that byte is finished Synchronization achieved through start/stop bits with each byte received 4123702 Data Communications System @YRU 58 Asynchronous Transmission 4123702 Data Communications System @YRU 59 Asynchronous Transmission Requires additional overhead (start/stop bits) Slower Cheap and effective Ideal for low-speed communication when gaps may occur during transmission (ex: keyboard) 4123702 Data Communications System @YRU 60 Synchronous Transmission 4123702 Data Communications System @YRU 61 Synchronous Transmission Requires constant timing relationship Bit stream is combined into longer frames, possibly containing multiple bytes Any gaps between bursts are filled in with a special sequence of 0s and 1s indicating idle Advantage: speed, no gaps or extra bits Byte synchronization accomplished by data link layer 4123702 Data Communications System @YRU 62 Credits All figures obtained from publisher-provided instructor downloads Data Communications and Networking, 3rd edition by Behrouz A. Forouzan. McGraw Hill Publishing, 2004 4123702 Data Communications System @YRU 63  Digital modulation – Is the transmittal of digitally modulated analog signals between two or more points in a communications system. – Can be propagated through Earth’s atmosphere and used in wireless communication system - digital radio. – Offer several outstanding advantages over traditional analog system. Ease of processing Ease of multiplexing Noise immunity  Applications: Low speed voice band data comm. modems High speed data transmission systems Digital microwave & satellite comm. systems PCS (personal communication systems) telephone  Why digital modulation? The modulation of digital signals with analogue carriers allows an improvement in signal to noise ratio as compared to analogue modulating schemes.  Important Criteria 1. High spectral efficiency 2. High power efficiency 3. Robust to multipath 4. Low cost and ease of implementation 5. Low carrier-to-co channel interference ratio 6. Low out-of-band radiation 7. Constant or near constant envelop 8. Bandwidth Efficiency Ability to accommodate data within a limited bandwidth Tradeoff between data rate and pulse width 9. Power Efficiency To preserve the fidelity of the digital message at low power levels. Can increase noise immunity by increasing signal power PULSE MODULATION  Pulse modulation includes many different methods of converting information into pulse form for transferring pulses from a source to a destination. Divided into two categories; 1. Analog Pulse Modulation (APM) 2. Digital Pulse Modulation (DPM)  PULSE MODULATION Sampling analog information signal Converting samples into discrete pulses Transport the pulses over physical transmission medium. Four (4) Methods 1. 2. 3. PAM PWM PPM  Analog Pulse Modulation 4. PCM  Digital Pulse Modulation  PULSE MODULATION :Sampling What is sampling? Sampling is the process of taking periodic sample of the waveform to be transmitted. “the more samples that are taken, the more final outcome looks like the original wave. However if fewer samples are taken, then other kinds information could be transmitted.”  PULSE MODULATION :Sampling Sampling theorem (Nyquist’s theorem) - is used to determine minimum sampling rate for any signal so that the signal will be correctly restored at the receiver. Nyquist’s theorem states that, “The original information signal can be reconstructed at the receiver with minimal distortion if the sampling rate in the pulse modulation system is equal to or greater than twice the maximum information signal frequency” PULSE MODULATION :Sampling 2) sampling at fs> 2fm(max) This sampling rate creates a guard band between fm(max) and the lowest frequency component (fs- fm(max)) of the sampling harmonics. Therefore a more practical LPF can be used to restore the modulating signal. Figure 4.2 Sampling at fs> 2fm(max)  PULSE MODULATION :Sampling Sampling at fs < 2fm(max) When the sampling rate is less than the minimum value, distortion will occurs. This distortion is called aliasing. Figure 4.3 Sampling at fs < 2fm(max)  Aliasing effect can be eliminated by using an anti-aliasing filter prior to sampling and using a sampling rate slightly higher than Nyquist rate (fs=2W). Anti-aliasing Sampler g ( kTs ) g (t ) Filter  ANALOG PULSE MODULATION (APM) In APM, the carrier signal is in the form of pulse waveform, and the modulated signal is where one of the characteristic (either amplitude, width or position) is changed according to the modulating/audio signal The three common techniques of APM are: Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM) and Pulse Position Modulation (PPM). The waveforms of APM are shown in figure 4.4 Pulse Amplitude Modulation (PAM) The simplest form of pulse modulation The amplitude of a constant width, constant position pulse (carrier signal) is varied according to the amplitude of the modulating signal. Basically the modulating signal is sampled by the digital train of pulses and the process is based upon the sampling theorem Fig.4.4 waveform for PAM,PWM & PPM Pulse Width Modulation (PWM) The technique of varying the width of the constant amplitude pulse proportional to the amplitude of the modulation signal. Also known as Pulse Duration Modulation (FDM). Either the leading edge, trailing edge or both may be varied by the modulating signal. Pulse Width Modulation (PWM) PWM gives better signal to noise performance than PAM. PWM has advantage, when compared with PPM, that is its pulse are of varying width and therefore of varying power content. PWM still works if synchronization between transmitter and receiver fails, whereas PPM does not.  Pulse Position Modulation (PPM) PPM is when the position of a constant-width and constant-amplitude pulse within prescribed time slot is varied according to the amplitude of the modulating signal. PPM has the advantage of requiring constant transmitter power output, but the disavantage of depending on transmitter-receiver synchronization. PPM has less noise due to amplitude changes, becaused the received pulses may be clipped at the receiver, thus removing amplitudeschanges caused by noise. Pulse Amplitude Modulation (PAM) Modulation in which the amplitude of pulses is varied in accordance with the modulating signal Pulse Width Modulation (PWM) Modulation in which the duration of pulses is varied in accordance with the modulating signal Pulse Width Modulation (PWM) Pulse Position Modulation (PPM) Modulation in which the temporal positions of the pulses are varied in accordance with some characteristic of the modulating signal. How to encode analog waveforms ? (from analog sources into baseband digital signals) Natural Sampling Flat-top Sampling