Digital Communication and Switching Lecture Notes PDF

EETE 4220 Digital Communication and Switching Semester I AY 2024-25 Class Lecture -2 Outcome #2&#3-Design of Digital Communication systems Course Lecturer :Lenin Joseph (LJ) E&...

EETE 4220 Digital Communication and Switching Semester I AY 2024-25 Class Lecture -2 Outcome #2&#3-Design of Digital Communication systems Course Lecturer :Lenin Joseph (LJ) E&E Section. Dept. of Engineering. ACT Dec 24, 2024 1 OUTCOME #2 & #3 2. Explain the general constraints and limitation in the design of communication systems. 3. Understand the problems and techniques involved in the baseband data transmission – ISI, Pulse Shaping, Eye diagram. EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 2 ering. ACT 2024 DIGITAL TRANSMISSION  Transmission of digital signals between two or more points in a communications system  Physical transfer of digital bit stream over a point-to-point or point to multipoint communication channel.  In digital transmission, the information transmitted may be: (a) a digital messages originating from a data source, for example a computer or a keyboard. (b) an analog signal such as a phone call or a video signal, digitized into a bit-stream using PCM or advance source coding (Analog to Digital Conversion and data compression) scheme. TRANSMISSION SCHEMES FOR ANALOG AND DIGITAL SIGNALS TYPES OF DIGITAL DATA TRANSMISSION Baseband Transmission  “Digital-over-digital" transmission  direct transmission of information signal without modulation (without frequency shifting)  Transmission of a sequence of electrical pulses or light pulses produced by means of a line coding scheme such as as RZ, NRZ, Manchester coding  This is typically used in serial cables, wired LAN such as Ethernet, and in optical fiber communication. TYPES OF DIGITAL DATA TRANSMISSION Passband Transmission  "digital-over-analog" transmission  transmission of a modulated sine wave signal representing a digital bit-stream.  The signal is produced by means of a digital modulation schemes such as PSK, FSK or QAM.  The modulation and demodulation is carried out by modem equipment.  This is used in wireless communication, and over telephone network local-loop and cable-TV networks. DIGITAL TRANSMISSION MODE Parallel Transmission  In telecommunications, parallel transmission is the simultaneous transmission of the signal elements of a character or other entity of data.  In digital communications, parallel transmission is the simultaneous transmission of related signal elements over two or more separate paths.  Multiple electrical wires are used which can transmit multiple bits simultaneously, which allows for higher data transfer rates than can be achieved with serial transmission.  This method is used internally within the computer, for example the internal buses, and sometimes externally for such things as printers. DIGITAL TRANSMISSION MODE Parallel Transmission  In transmitting a byte, all 8 bits are transmitted simultaneously over 8 lines during the time of one clock pulse. DIGITAL TRANSMISSION MODE Parallel Transmission  The disadvantage of this transmission mode is "skewing" because the wires in parallel data transmission have slightly different properties so some bits may arrive before others, which may corrupt the message.  A parity bit or error check bit can help to reduce the error.  Electrical wire parallel data transmission is less reliable for long distances. DIGITAL TRANSMISSION MODE Serial Transmission  In telecommunications, serial transmission is the sequential transmission of signal elements of a group representing a character or other entity of data.  Digital serial transmissions are bits sent over a single wire, frequency or optical path sequentially. DIGITAL TRANSMISSION MODE Serial Transmission  Eight clock pulses are required to transmit eight bit data.  Because it requires less signal processing and fewer chances for error than parallel transmission, the transfer rate of each individual path may be faster.  This can be used over longer distances with error check bit or parity bit sent along with the data. Two types of Serial Transmission (a) Asynchronous transmission  Sometimes referred to as start-stop transmission.  One character at a time is transmitted.  Each character or series of 8-bits is marked by a start bit and a stop bit. Two types of Serial Transmission (a) Asynchronous transmission  The first bit transmitted is the start bit which is always a logic-0. the character code bits are transmitted next beginning with the LSB through the MSB.  The parity bit or error check bit is transmitted directly after the MSB of the character.  The last bit transmitted is the stop bit, which is always logic 1. There can be 1 or 2 stop bits. Two types of Serial Transmission Synchronous transmission  Instead of using start and stop bits, synchronous transmission sends data in blocks.  A block, referred to as a frame or packet, may consist of hundreds or thousands of characters. Each frame is started and ended by 1 or more sync bytes.  The start and end bit patterns (sync bytes) synchronize internal clocks in the transmitting and receiving device. Two types of Serial Transmission Synchronous transmission  Synchronous data have two SYN characters added to each message.  Due to absence of start and stop bits, the data transfer rate is quicker although more errors will occur as the clocks eventually get out of sync, and the receiving device would have the wrong time that had been agreed in the protocol for sending/receiving data, so some bytes could become corrupted.  Synchronization problem is solved by including re- synchronization of the clocks and use of check digits to ensure the byte is correctly interpreted and received. TRANSMISSION BANDWIDTH  The band of frequencies allowed for signal transmission  The band of frequencies at which we are allowed to use to transmit the data. B = fH - fL TRANSMISSION BANDWIDTH Example 3: If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the frequency spectrum, assuming all components have maximum amplitude of 10 V. B= 900Hz-100Hz=800 Hz TRANSMISSION BANDWIDTH Example 4: A periodic signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all frequencies of the same amplitude. fl= fh-B= 60 Hz-20Hz=40Hz  PARAMETERS IN DIGITAL TRANSMISSION INFORMATION CAPACITY  A measure of how much information can be propagated through a communication systems and is a function of bandwidth and transmission time.  Represents the number of independent symbols that can be carried through a system in a given unit of time.  The most basic digital symbol used to represent information is the binary digit, or bit. Therefore, it is often convenient to express the information capacity of a system as a bit rate or data rate.  Bit rate is simply the number of bits transmitted during one second and is expressed in bits per second (bps)  PARAMETERS IN DIGITAL TRANSMISSION HARTLEY’S LAW  In 1928, R. Hartley of Bell Telephone Laboratories developed a useful relationship among bandwidth, transmission time, and information capacity.  Hartley’s Law states that the amount of information that can be transmitted in a given time is directly proportional to bandwidth and transmission time. I  B t where: I = Information capacity in bps t = transmission time in seconds B = bandwidth in Hz  PARAMETERS IN DIGITAL TRANSMISSION SHANNON LIMIT FOR INFORMATION CAPACITY  1948, mathematician Claude E. Shannon published a paper in the Bell System Technical Journal relating the information capacity of a communications channel to bandwidth and signal to noise ratio.  The higher the signal to noise ratio, the better the performance and the higher the information capacity.  Also known as Shannon-Hartley Theorem which determines the maximum theoretical data rate for a given noise level, no matter how elaborately data is coded.  PARAMETERS IN DIGITAL TRANSMISSION SHANNON LIMIT FOR INFORMATION CAPACITY  Mathematically, the Shannon’s limit for information capacity is ) ) where: I = Information capacity in bps S/N = signal to noise ratio (as power ratio; not in decibels) B = bandwidth in Hz  PARAMETERS IN DIGITAL TRANSMISSION SHANNON-HARTLEY THEOREM  For multiple coding levels (multilevel signalling) and ignoring the effect of noise, the limit to the amount of data that can be sent in a given bandwidth is given as 𝑰 =𝟐 𝑩𝒍𝒐𝒈 𝟐 𝑴 where: I = Information capacity in bps M = number of different encoding levels B = bandwidth in Hz Note:  PARAMETERS IN DIGITAL TRANSMISSION SHANNON-HARTLEY THEOREM  For binary information with two possible levels only (binary 0 or binary 1), the information capacity is where: I = Information capacity in bps B = bandwidth in Hz PARAMETERS IN DIGITAL TRANSMISSION M-ARY ENCODING  In an M-ary encoding, M represents a digit that corresponds to the number of conditions, levels, or combination possible for a given number of binary variables.  The number of bits necessary to produce a given number of conditions is expressed mathematically as 𝑵 =𝒍𝒐𝒈 𝟐 𝑴 where: N = number of bits necessary to produce M levels M = number of conditions, levels, or combination possible with N bits PARAMETERS IN DIGITAL TRANSMISSION M-ARY ENCODING  From the equation above, the number of conditions possible with N bits can be expressed as 𝑵 𝟐 =𝑴 PARAMETERS IN DIGITAL TRANSMISSION Example 5: A typical dial-up telephone connection has a bandwidth of 3KHz and a signal to noise ratio of 30dB. Calculate the Shannon limit. PARAMETERS IN DIGITAL TRANSMISSION Example 6: A radio channel has a bandwidth of 10KHz and a signal to noise ratio of 15dB. What is the maximum data rate that can be transmitted: (a) using any system? (b) using a code with four possible states? PARAMETERS IN DIGITAL TRANSMISSION Example 7: Determine how many voltage levels will the following bits represent: (a) 1 bit (b) 2 bits (c) 3 bits N is given. a)M=21 =2 b) M=22 =4 c) M=23 =8 PARAMETERS IN DIGITAL TRANSMISSION Drill Problem #1: (a) If an eight-level encoding scheme is used in a 30KHz bandwidth system, what is the channel capacity in bits per second? (b) What is the information capacity in bps of a 15MHz channel with a signal to noise ratio of 28dB? PARAMETERS IN DIGITAL TRANSMISSION BIT RATE VS. BAUD RATE Bit rate  the number of bits transmitted per second and is expressed in bits per second (bps).  Used express the information capacity of a system.  Number of bits transferred between devices per second  If each bit is represented by a pulse of width Tb, then the bit rate, fb is PARAMETERS IN DIGITAL TRANSMISSION BIT RATE VS. BAUD RATE Example 8: A binary data source that generates bits. Each bit is represented by a pulse of width Tb = 0.1 ms. Calculate the bit rate for the source. Bit rate fb= 1/Tb =1/0.1 ms PARAMETERS IN DIGITAL TRANSMISSION Baud rate  Also referred to as symbol rate.  The number of symbols transmitted per second through the communication channel.  Refers to the rate at which the signaling elements are transmitted, i.e. number of signaling elements per second.  The Baud which is the encoded rate of change is related to the bit rate by the following equation: where: 𝒇𝒃 𝒇𝒃 𝑩𝒂𝒖𝒅= = fb = bit rate in bps 𝒍𝒐𝒈𝟐 𝑴 𝑵 Baud = symbol rate (baud per second) N = number of bits encoded into a signaling level M = number of discrete signal or voltage level PARAMETERS IN DIGITAL TRANSMISSION BIT RATE VS. BAUD RATE Example 9: A binary data source transmits binary data, the bit duration is 1µsec, Suppose we want to transmit symbols rather than bits, if each symbol is represented by four bits. What is the baud rate? PARAMETERS IN DIGITAL TRANSMISSION BIT RATE VS. BAUD RATE Example 10: A modulator transmits symbols, each of which has sixty-four different possible states, 10,000 time per second. Determine the baud rate and bit rate. PARAMETERS IN DIGITAL TRANSMISSION BIT RATE VS. BAUD RATE Drill Problem #2: (a) How fast can a 1526 byte block of data be transmitted on a 10Mbps Ethernet (IEEE802.3) packet? (b) A binary data source transmits binary data with a baud rate of 1Mbaud. Compare the bit rate for one-bit transmission and 2-bit symbol transmission. NYQUIST MINIMUM BANDWIDTH  Usually, symbols are used to transmit data when the transmission bandwidth is limited. For example, we need to transmit a data at high rate and the bit duration Tb is very small; to overcome this problem, we take a group of more than one bit, say 2 bits or 4 bits.  By transmitting symbols rather than bits we can reduce the spectrum of the transmitted signal. Hence, we can use symbol transmission rather than bit transmission when the transmission bandwidth is limited. NYQUIST MINIMUM BANDWIDTH Using multi-level signalling, the minimum Nyquist bandwidth necessary to pass M-ary digitally modulated carriers is given as where: fb = bit rate in bps B = minimum Nyquist bandwidth in Hertz N = number of bits encoded into a signaling element M = number of discrete signal or voltage levels PARAMETERS IN DIGITAL TRANSMISSION Drill Problem #3: What is the minimum allowable bandwidth that can transmit a 2-bit binary signal with a bit rate of 3Mbps? PROBABILITY OF ERROR (Pb), BIT ERROR RATE (BER) and ENERGY PER BIT (Eb)  In digital communication system, errors occur as a result of noise.  The error probability (Pb) is the number of errors per total number of bits received.  For instance, if 1 error bit per 100,000 bit occurs, the bit error probability (Pb) is expressed as 1/100,000 or 10-5.  The acceptable error probability in communication system ranges from about 10-5 up to 10-12. PROBABILITY OF ERROR (Pb), BIT ERROR RATE (BER)  The average number of errors in a transmission m bits long can be calculated as Average number of errors = m * Pb  The most common method of expressing quality of a digital communication system is its Bit Error Rate (BER).  The BER is the number of bit errors that occurs for a given number of bits transmitted.  The BER can be considered as an approximate estimate of the bit error probability. PROBABILITY OF ERROR (Pb), BIT ERROR RATE (BER) Example 11: Calculate the error probability (Pb) in a system that produces 7 error bits out of 5,700,000 total bits. Example 12: A digital transmission has an error probability, Pb of 10-6, and 107 bits are received. Calculate the expected number of errors. REFERENCES Text Book Frenzel, L., Principles of Electronic Communication Systems, 3rd Ed. McGrawHill, 2008 Book References Sklar, Benard, “Digital Communications: Fundamentals and Application”, Prentice Hall, USA, 2001 Glover, Ian A / Grant, Peter M, “ Digital Communications”, Prentice Hall, UK, 2004 Proakis,John G,“ Digital Communications”, McGraw Hill Inc, USA, 2000 Wesolowski,Krzysztof, “Introduction to Digital Communication Systems”,2009 Web References\ e-library(s) https://ebookcentral.proquest.com/lib/momp/detail.action?docID=3011330 571, CD/DVD, Electronic Communication Systems: A complete Course, ACT Store  THAN K YOU EETE 4220 Digital Communication & Switching -E&E Section. Dept. of Engine Dec 24, 44 ering. ACT 2024