Communication Systems and Clouding PDF

Communication Systems and Clouding COMMUNICATION SYSTEMS 2 Tyseer Alsamany Communications and Networks n Data Communications n Transmission of signals n Encoding, interfacing, signal integrity, multiplexing etc....

Communication Systems and Clouding COMMUNICATION SYSTEMS 2 Tyseer Alsamany Communications and Networks n Data Communications n Transmission of signals n Encoding, interfacing, signal integrity, multiplexing etc. n Networking n Topology & architecture used to interconnect devices n Networks of communication systems n Data communications: are the exchange of data between two devices via some form of transmission medium such as a wire cable. 3 8024305-3: Communication Networks Dr. Abdulfattah Noorwali Computer Network 8024305-3: Communication Networks Dr. Abdulfattah Noorwali 4  Communication Systems n Process describing transfer of information, data, instructions between one or more systems through some media n Examples n people, computers, cell phones, etc. n Computer communication systems n Signals passing through the communication channel can be Digital, or analog n Analog signals: continuous electrical waves n Digital signals: individual electrical pulses (bits) n Receivers and transmitters: desktop computers, mainframe computers, etc. Communication channel X T X R X X R R Amp/Adaptor Communication media 16 8024305-3: Communication Networks Dr. Abdulfattah Noorwali o The shown figure presents three typical communication systems: a wire-line telephone-cellular phone connection, a TV broadcasting system, and a wireless computer network. Because of the numerous examples of communication systems in existence, it would be unwise to attempt to study the details of all kinds of communication systems. o Instead, the most efficient and effective way to learn about communication is by studying the major functional blocks common to practically all communication systems. 3  A Communications Model 25 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  The key components of a communication system The source: the source originates a message, such as a human voice, a television picture, an e-mail message, or data. 4  A Communications Model n Source n generates data to be transmitted n Transmitter n Converts data into transmittable signals n Transmission System n Carries data n Receiver n Converts received signal into data n Destination n Takes incoming data 8024305-3: Communication Networks Dr. Abdulfattah Noorwali baseband signal (message signal): If the data is nonelectric (e.g., human voice, e-mail text, television video), it must be converted by an input transducer into an electric waveform referred to as the baseband signal or message signal through physical devices such as a microphone, a computer keyboard, or a CCD camera. The transmitter: the transmitter modifies the baseband signal for efficient transmission. The transmitter may consist of one or more subsystems: an A/D converter, an encoder, and a modulator. 5 The channel: the channel is a medium of choice that can convey the electric signals at the transmitter output over a distance. A typical channel can be a pair of twisted copper wires (telephone and DSL), coaxial cable (television), an optical fiber, or a radio link. 6 The receiver: the receiver reprocesses the signal received from the channel by reversing the signal modifications made at the transmitter and removing the distortions made by the channel. The receiver output is fed to the output transducer, which converts the electric signal to its original form-the message. The receiver may consist of a demodulator, a decoder, and a D/A converter. 7 Data Communications Model 26 8024305-3: Communication Networks Dr. Abdulfattah Noorwali Tyseer Alsamany Communications Tasks Transmission system utilization Addressing Interfacing Routing Signal generation Recovery Synchronization Message formatting Exchange management Security Error detection and correction Network management Flow control 27 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 2.15 Summary of layers 8024305-3: Communication Networks 2.46 Dr. Abdulfattah Noorwali  Figure 2.21 Port addresses 8024305-3: Communication Networks 2.58 Dr. Abdulfattah Noorwali Figure 1.1 Components of a data communication system 1.188024305-3: Communication Networks Dr. Abdulfattah Noorwali Tyseer Alsamany Communications Components n Basic components of a communication system: n Communication technologies n Communication devices n Communication channels n Communication software Components of a data communication system 28 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.2 Data flow (simplex, half-duplex, and full-duplex) 1.198024305-3: Communication Networks Dr. Abdulfattah Noorwali  Communication Technologies - Applications n Different technologies allowing us to communicate n Examples: Voice mail, fax, email, instant message, chat rooms, news groups, telephony, GPS, and more n Voice mail: Similar to answering machine but digitized n Fax: Sending hardcopy of text or photographs between computers using fax modem n Email: electronic mail – sending text, files, images between different computer networks - must have email software n More than 1.3 billion people send 244 billion messages monthly! n Chat rooms: Allows communications in real time when connected to the Internet n Telephony: Talking to other people over the Internet (also called VoIP) n Sends digitized audio signals over the Internet n Requires Internet telephone software n Groupware: Software application allowing a group of people to communicate with each other (exchange data) n Address book, appointment book, schedules, etc. n GPS: consists of receivers connected to satellite systems n Determining the geographical location of the receiver n Used for cars, advertising, hiking, tracking, etc. 29 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  Communication Devices n Any type of hardware capable of transmitting data, instructions, and information between devices n Functioning as receiver, transmitter, adaptor, converter n Examples: n Dial-up modem: uses standard phone lines n Converts digital information into analog n Consists of a modulator and a demodulator n Can be external, internal, wireless n ISDN and DSL Modem: Allows digital communication between networks and computers n Requires a digital modem n Cable modem: a modem that transmits and receives data over the cable television (CATV) network n Also called broadband modem (carrying multiple signals) n Requires a cable modem n Network interface cards: Adaptor cards residing in the computer to transmit and receiver data over the network (NIC) n Operate with different network technologies (e.g., Ethernet) 30 8024305-3: Communication Networks Dr. Abdulfattah Noorwali Communication Channels n A channel is a path between two communication devices n Channel capacity: How much data can be passed through the channel (bit/sec) n Also called channel bandwidth n Consists of one or more transmission media n Materials carrying the signal n Two types: n Physical: wire cable T1 lines T1 n Wireless: Air destination lines network server T3 lines T1 lines 32 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  What is the effect of the channel? ❑ A channel is a physical medium that behaves partly like a filter that generally attenuates the signal and distorts the transmitted waveforms. ❑ Signal attenuation increases with the length of the channel, varying from a few percent for short distances to orders of magnitude in interplanetary communications. ❑ Signal waveforms are distorted because of physical phenomena such as frequency-dependent gains. example, a frequency-selective channel causes different amounts of attenuation and phase shift to different frequency components of the signal. 8 ❑ A square pulse is rounded or "spread out" during transmission over a low-pass channel. ❑ These types of distortion, called linear distortion, can be partly corrected at the receiver by an equalizer with gain and phase characteristics complementary to those of the channel. ❑ Channels may also cause nonlinear distortion through attenuation that varies with the signal amplitude. Such distortions can also be partly corrected by a complementary equalizer at the receiver. 9 ❑ Signals passing through communication channels not only experience channel distortions but also are corrupted along the path by undesirable interferences and disturbances lumped under the broad term noise. ❑ These interfering signals are random and are unpredictable from sources. ❑ External noise includes interference signals transmitted on nearby channels, human-made noise generated by faulty contact switches of electrical equipment, automobile ignition radiation, fluorescent lights or natural noise from lightning, microwave ovens, and cellphone emissions, as well as electric storms and solar radiation. 10 ❑ With proper care in system design, external noise can be minimized. ❑ Internal noise results from thermal motion of charged particles in conductors, random emission, and diffusion or recombination of charged carriers in electronic devices. Proper care can reduce the effect of internal noise but can never eliminate it. ❑ Noise is one of the underlying factors that limit the rate of telecommunications. 11 ❑ Thus, in practical communication systems, the channel distorts the signal, and noise accumulates along the path. ❑ Worse yet, the signal strength attenuates while the noise level remains steady regardless of the distance from the transmitter. ❑ Thus, the signal quality would continuously degrade along the length of the channel. ❑ Amplification of the received signal to make up for the attenuation is ineffective because the noise will be amplified by the same proportion, and the quality remains, at best, unchanged. ❑ These are the key challenges that we must face in designing modern communication systems. 12 Tyseer Alsamany Wireless Transmission Media n Broadcast Radio n Distribute signals through the air over long distance n Uses an antenna n Cellular Radio n A form of broadcast radio used for mobile communication n High frequency radio waves to transmit voice or data n Microwaves n Radio waves providing high speed transmission n Used for satellite communication n Infrared (IR) n Wireless transmission media that sends signals using infrared light- waves 34 8024305-3: Communication Networks Dr. Abdulfattah Noorwali Physical Transmission Media Wireless channel capacity: 100 Mbps is how many bits per sec? Which is bigger: 10,000 Mbps, 0.01Tbps or 10Gbps? 35 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  The Digital Revolution in Communications ❑ It is no secret to even a casual observer that every time one looks at the latest electronic communication products, another newer and better “digital technology” is displacing the old analog technology. ❑ Most visibly in every household, digital video technology (DVD) and Blu-ray have made the analog VHS cassette systems obsolete. Digital iPod and MP3 players have totally vanquished the once popular audio-cassette players in consumer electronics. This begs the question: Why are digital technologies superior? 13  Why are digital technologies superior? The answer has to do with both economics and quality. o The case for economics is made by the ease of adopting versatile, powerful, and inexpensive high-speed digital microprocessors. o But more importantly at the quality level, one prominent feature of digital communications is the enhanced immunity of digital signals to noise and interferences. 14 ❑ Clearly, a digital communication system is more rugged than an analog communication system in the sense that it can better withstand noise and distortion (as long as they are within a limit). ❑ Digital signal can be recovered correctly as long as the distortion and the noise are within limits. In contrast, the waveform shape itself in an analog message carries the needed information, and even a slight distortion or interference in the waveform will show up in the received signal. 15 Consider a binary case: two symbols are encoded as rectangular pulses of amplitudes A and −A. The only decision at the receiver is to select between two possible pulses received; the fine details of the pulse shape are not an issue. 16 The digital message in Fig.(a) is distorted by the channel, as shown in Fig.(b). Yet, if the distortion is not too large, we can recover the data without error because we only need to make a simple binary decision: Is the received pulse positive or negative? In contrast, the waveform shape itself in an analog message carries the needed information, and even a slight distortion or interference in the waveform will show up in the received signal. 17 Distortionless Regeneration of Digital Signals ❑ One main reason for the superior quality of digital systems over analog ones is the viability of signal regeneration by repeaters. ❑ When directly communicating over a long distance, transmitted signals can be severely attenuated and distorted. For digital pulse signals used in digital communications, repeater nodes can be placed along the communication path at distances short enough to ensure that noise and distortion effects are minor such that digital pulses can be detected with high accuracy. 18 ❑ At each repeater node, the incoming digital pulses are detected such that new, “clean” pulses are regenerated for transmission to the next node along the path. This process prevents the accumulation of noise and distortion along the path by cleaning up the pulses at regular path intervals. We can thus transmit messages over longer distances with greater accuracy. ❑ In analog systems, however, signals and noise within the same bandwidth cannot be separated. Repeaters in analog systems are basically filters plus amplifiers and are not “regenerative.” It is therefore impossible to avoid in-band accumulation of noise and distortion along the path. 19 ❑ Consequently, the distance over which an analog message can be successfully received is limited by the first transmitter power. Despite these limitations, analog communication is simpler and was used widely and successfully in the past for short- to medium-range communications. ❑ In modern times, however, almost all new communication systems being installed are digital, although a small number of old analog communication technologies are still in use, such as those for AM and FM radio broadcasting. 20  Analog-to-Digital (A/D) Conversion for Digital Communications ❑ Digital communication systems can carry analog messages by first converting analog signals to digital signals. A key device in electronics, the analog-to-digital (A/D) converter, enables digital communication systems to convey analog source signals such as audio and video. ❑ Generally, analog signals are continuous in time and in range; that is, they have values at every time instant, and their values can be anywhere within the range. ❑ On the other hand, discrete digital signals exist only at discrete points of time, and they can take on only finite values. 21 Analog Continuous signal Digital Continuous signal Analog Discrete signal Digital Discrete signal 22 ❑ Two steps take place in A/D conversion: a continuous time signal is first sampled into a discrete time signal, whose continuous amplitude is then quantized into a discrete level signal. ❑ The sampling theorem states that if the highest frequency in the signal spectrum is B (in hertz), the signal can be reconstructed from its discrete samples, taken uniformly at a rate above 2B samples per second. This means that to preserve the information from a continuous-time signal, we only need to transmit its samples. 23 ❑ However, the sample values are still not digital because they lie in a continuous dynamic range. Here, the second step of quantization comes to the rescue. ❑ Through quantization, each sample is approximated, or “rounded off,” to the nearest quantized level. Since human perception has only limited sensitivity, quantization with sufficient granularity does not compromise the signal quality. ❑ A quantizer partitions the signal range into L intervals. Each sample amplitude is approximated by the midpoint of the interval in which the sample value falls. Each sample is now represented by one of the L numbers. The information is thus digitized. 24 ❑ Hence, after the two steps of sampling and quantizing, the A/D conversion is completed. The quantized signal is an approximation of the original one. ❑ We can improve the accuracy of the quantized signal to any desired level by increasing the number of levels L. 25 Pulse-Coded Modulation—A Digital Representation ❑ Once the A/D conversion is over, the original analog message is represented by a sequence of samples, each of which takes on one of the (L) preset quantization levels. The transmission of this quantized sequence is the task of digital communication systems. ❑ For this reason, signal waveforms must be used to represent the quantized sample sequence in the transmission process. ❑ Pulse-coded modulation (PCM) is a very simple and yet common mechanism for this purpose. 26 ❑ The idea of PCM is to represent each quantized sample by an ordered combination of two basic pulses: p1(t) representing 1 and p0(t) representing 0. ❑ Because each of the (L) possible sample values can be written as a bit string of length log2(L), each sample can therefore also be mapped into a short pulse sequence to represent log2(L) bits. For example, if L = 16, then, each quantized level can be described uniquely by 4 bits. If we use two basic pulses p1(t) = A and p0(t) = −A, respectively, to represent 1 and 0 for each bit, then a sequence of four such pulses gives 2×2×2×2 = 16 distinct patterns, as shown in figure. 27 28 ❑ Each quantized sample is now coded into a sequence of four binary pulses. This is the principle of PCM transmission, where signaling is carried out by means of only two basic pulses (or symbols). 29  Signal Bandwidth and Channel Capacity ❑ The bandwidth of a channel is the range of frequencies that it can carry with reasonable fidelity. For example, if a channel can carry with reasonable fidelity a signal whose frequency components vary from 0 Hz (dc) up to a maximum of 5000 Hz (5 kHz), the channel bandwidth B is 5 kHz. ❑ The faster a signal changes, the higher its maximum frequency is, and the larger its bandwidth is. Signals rich in content with quick changes (such as those for battle scenes in a video) have larger bandwidth than signals that are dull and vary slowly (such as those for a video of sleeping lions). 30 ❑ A signal transmission is likely successful over a channel if the channel bandwidth exceeds the signal bandwidth. ❑ To understand the role of B, consider the possibility of increasing the speed of information transmission by compressing the signal in time. Compressing a signal in time by a factor of 2 allows it to be transmitted in half the time, and the transmission speed (rate) doubles. ❑ Time compression of the signal by a factor of 2, however, implying that the frequencies of its components are doubled. 31 ❑ Many people have had firsthand experience of this effect when playing a piece of audiotape twice as fast, making the voices of normal people sound like the high-pitched speech of cartoon characters. Now, to transmit this compressed signal without distortion, the channel bandwidth must also be doubled. ❑ Thus, the rate of information transmission that a channel can successfully carry is directly proportional to B. ❑ More generally, if a channel of bandwidth B can transmit N pulses per second, then to transmit KN pulses per second by means of the same technology, we need a channel of bandwidth KB. 32 Channel Capacity: ❑ One of the most commonly encountered channels is known as the additive white Gaussian noise (AWGN) channel. The AWGN channel model assumes no channel distortions except for the additive white Gaussian noise and its finite bandwidth B. ❑ The band-limited AWGN channel capacity was dramatically highlighted by the equation owing to C. E. Shannon: 𝐶 = 𝐵𝑙𝑜𝑔2 1 + 𝑆𝑁𝑅 33 ❑ Here the channel capacity C is the upper bound on the rate of information transmission per second. In other words, C is the maximum number of bits that can be transmitted per second with arbitrarily small probability of error; that is, the transmission is as accurate as one desires. ❑ Conversely, it is also impossible to transmit at a rate higher than C without incurring a large number of errors. Shannon’s equation clearly shows the limit on the rate of communication jointly imposed by B and SNR. 34 As a practical example of trading SNR for bandwidth B, consider the scenario in which we meet a soft-spoken man who speaks a little bit too fast for us to fully understand. This means that as listeners, our bandwidth B is too low and therefore, the capacity C is not high enough to accommodate the rapidly spoken sentences. However, if the man can speak louder (increasing power and hence the SNR), we are likely to understand him much better without changing anything else. This example illustrates the concept of resource exchange between SNR and B. 35  DIGITAL SOURCE CODING AND ERROR CORRECTION CODING ❑ SNR and bandwidth are two factors that determine the performance of a given communication scheme. ❑ Unlike analog communication systems, digital systems often adopt aggressive measures to lower the source data rate and to fight against channel noise. ❑ In particular, source encoding is applied to generate the fewest bits possible for a given message without sacrificing its accuracy. 36 ❑ For digital signals, the overall transmission time is minimized if a message (or symbol) of probability P is assigned a code word with a length proportional to log2(1/P). ❑ On the other hand, to combat errors that arise from noise and interferences, redundancy needs to be introduced systematically at the transmitter such that the receivers can rely on the redundancy to correct errors caused by channel distortion and noise. This process is known as error correction coding by the transmitter and decoding by the receiver. 37  Advantages of Digital Communication 1. Digital communication can withstand channel noise and distortion much better than analog as long as the noise and the distortion are within limits. With analog messages, on the other hand, any distortion or noise, no matter how small, will degrade the received signal. 38 2. The greatest advantage of digital communication over analog communication, however, is the viability of regenerative repeaters in the former. In analog communications, a message signal becomes progressively weaker as it travels along the channel, whereas the cumulative channel noise and the signal distortion grow progressively stronger, ultimately overwhelming the signal. Amplification by analog repeaters offers little help since it enhances both the signal and the noise equally. Consequently, the distance over which an analog message can be transmitted is limited by the initial transmission power. 39 For digital communications, however, repeater stations can be set up along the signal path at intervals short enough to detect and recover digital signal pulses before the noise and distortion have a chance to accumulate sufficiently. At each repeater station the pulses are detected, and new, clean pulses are transmitted to the next repeater station, which, in turn, duplicates the same process. If the noise and distortion are within limits (which is possible because of the closely spaced repeaters), pulses can be detected correctly. This way the digital messages can be transmitted over longer distances with greater reliability. 40 3. Digital hardware implementation is flexible and permits the use of microprocessors, digital switching, and large- scale integrated circuits. 4. Digital signals can be coded to yield extremely low error rates and high fidelity. 5. Digital signals are easier to encrypt for security and privacy. 6. It is easier and more efficient to multiplex several digital signals. 41 7. Digital signal storage is relatively simple and inexpensive. It is also easier to index and search information in large electronic databases. 8. The cost of digital hardware continues to halve every two or three years, while performance or capacity doubles over the same period. 42 Networks n Collection of computers and devices connected together n Used to transfer information or files, share resources, etc. n What is the largest network? n Characterized based on their geographical coverage, speed, capacities n Networks are categorized based on the following characteristics: n Network coverage: LAN, MAN, WAN n Network topologies: how the computers are connected together n Network technologies n Network architecture 36 8024305-3: Communication Networks Dr. Abdulfattah Noorwali Categories of Networks n Local Area Networks (LANs) n Short distances n Designed to provide local interconnectivity n Wide Area Networks (WANs) n Long distances n Provide connectivity over large areas n Metropolitan Area Networks (MANs) n Provide connectivity over areas such as a city, a campus 1.378024305-3: Communication Networks Dr. Abdulfattah Noorwali Network Topologies n Configuration or physical arrangement in which devices are connected together n BUS networks: Single central cable connected a number of devices n Easy and cheap n Popular for LANs n RING networks: a number of computers are connected on a closed loop n Covers large distances n Primarily used for LANs and WANs n STAR networks: connecting all devices to a central unit n All computers are connected to a central device called hub n All data must pass through the hub 38 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.4 Categories of topology 1.398024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.5 A fully connected mesh topology (five devices) 1.408024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.6 A star topology connecting four stations 1.418024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.7 A bus topology connecting three stations 1.428024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.8 A ring topology connecting six stations 1.438024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.9 A hybrid topology: a star backbone with three bus networks 1.448024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.10 An isolated LAN connecting 12 computers to a hub in a closet 1.458024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.11 WANs: a switched WAN and a point-to-point WAN 1.468024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.12 A heterogeneous network made of four WANs and two LANs 1.478024305-3: Communication Networks Dr. Abdulfattah Noorwali Tyseer Alsamany Network Architecture n Refers to how the computer or devices are designed in a network n Basic types: n Centralized – using mainframes n Peer-2-Peer: n Each computer (peer) has equal responsibilities, capacities, sharing hardware, data, with the other computers on the peer-to-peer network n Good for small businesses and home networks n Simple and inexpensive n Client/Server: n All clients must request service from the server n The server is also called a host n Different servers perform different tasks: File server, network server, etc. client client client laser printer server 48 8024305-3: Communication Networks Dr. Abdulfattah Noorwali  (Data) Network Technologies n Vary depending on the type of devices we use for interconnecting computers and devices together n Ethernet n Token Ring: a local area network in which a node can transmit only when in possession of a sequence of bits (called the token) that is passed to each node in turn n Alternative technologies: n Circuit switching n Packet switching n Frame relay n Asynchronous Transfer Mode (ATM) 49 8024305-3: Communication Networks Dr. Abdulfattah Noorwali 1-3 THE INTERNET The Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time. The Internet is a communication system that has brought a wealth of information to our fingertips and organized it for our use. Topics discussed in this section: Organization of the Internet Internet Service Providers (ISPs) 1.508024305-3: Communication Networks Dr. Abdulfattah Noorwali  Figure 1.13 Hierarchical organization of the Internet 1.518024305-3: Communication Networks Dr. Abdulfattah Noorwali  1-4 PROTOCOLS A protocol is synonymous with rule. It consists of a set of rules that govern data communications. It determines what is communicated, how it is communicated and when it is communicated. The key elements of a protocol are syntax, semantics and timing Topics discussed in this section: § Syntax § Semantics § Timing 1.528024305-3: Communication Networks Dr. Abdulfattah Noorwali  Elements of a Protocol n Syntax n Structure or format of the data n Indicates how to read the bits - field delineation n Semantics n Interprets the meaning of the bits n Knows which fields define what action n Timing n When data should be sent and what n Speed at which data should be sent or speed at which it is being received. 1.538024305-3: Communication Networks Dr. Abdulfattah Noorwali