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Data communications Class notes


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Meant for Electronics and CSE students

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Data communications Class notes

  1. 1. Dr. Y. Narasimha Murthy Ph.D UNIT-I DATA COMMUNICATIONS Introduction: Data communications deals with the transmission (sending and receiving) of information between two locations by means of electrical signals. The two types of electrical signals may be either analog or digital. Digital Data communication is the name given to the communication where exchange of information takes place in the form of 0s and 1s over some transmission medium like a wire or wireless . The effectness of a data communication system depends on three fundamental characteristics .They are Delivery , Accuracy and Timeliness. (i) Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. (ii).Accuracy :The system must deliver the data accurately .Data that have been altered in transmission and left uncorrected must not be used. (iii).Timeliness: The system must deliver the data in time.The data delivered late are useless .For example in the case of audio and video ,timely delivery means ,delivering the data as they are produced in the same order of production and without significant delay.This type of delivery is called real-time transmission. Components : A data communication system is made up of five basic components .They are (i).Messages (ii).Sender (iii).Receiver (iv).Medium and (v).Protocol (i) Message: The message is the data information to be communicated. It can be either text, numbers or audio or video or any combination of these types. (ii).Sender: The sender is the device that sends the data message. It can be a computer ,workstation ,telephone handset ,video camera etc.. 1
  2. 2. Dr. Y. Narasimha Murthy Ph.D (iii) Receiver : The receiver is the device that receives the messages. It can be a computer, workstation , telephone handset ,video camera or television etc. (iv).Medium:The transmission medium is the physical path by which a message travels from sender to receiver.It should be a twisted –pair wire ,co-axial cable,fiber optic cable or Radio waves etc. (v).Protocol: A protocol is a set of rules that govern the data communications.It represents an arrangement between the communicating devices.Without a protocol ,two devices may be connected but can not communicate. Basic terms and concepts : In data communication mainly the data is being transmitted between two devices namely transmitter and receiver. To understand this entire phenomena we need to know about five basic concepts .They are Line configuration Topology Transmission mode Categories of Networks. Internetworks. LINE CONFIGURATION: It refers to the way by which two or more communication devices are connected to a link.A link is a physical communication pathway that transfers data from one device to another. Based on the requirements, there are two possible line configurations.Point to point and multipoint • Point- to-point communication: A point to point connection provides a dedicated link between two devices.The total capacity of the link is used for the communication between these two devices. In most point-to point actual length of wire or cable is used to connect the two devices. For example the use of an infrared remote to change the channel in a Television establishes a point-to-point contact between remote and T.V 2
  3. 3. Dr. Y. Narasimha Murthy Ph.D Similarly , in the case of voice communication using telephones, there is one calling party and one called party. Hence the communication is point-to-point. Multipoint : A multipoint or multi drop connection is one in which more than two specific devices share a single link. In a multi-point configuration the capacity of the channel is shared either spatially or temporally .If several devices are using the link simultaneously, they share it spatially on the other hand many users are using the same link by turn it is a time share connection.(means ,they share it temporally). • Broadcasting: In a broadcasting system, there is a central location from which information is sent to many recipients, as in the case of audio or video broadcasting. In a broadcasting system, the listeners are passive, and there is no reverse communication path. TOPOLOGY : The topology defines how the devices (computers, printers..etc) are connected and how the data flows from one device to another. There are two conventions while representing the topologies. The physical topology defines how the devices are physically wired. The logical topology defines how the data flows from one device to another. Topologies are broadly categorized into i) Bus ii ) Ring iii) Star iv) Mesh 3
  4. 4. Dr. Y. Narasimha Murthy Ph.D Mesh Topology : In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n - I nodes, node 2 must be connected to n – 1 nodes, and finally node n must be connected to n - 1 nodes. We need n(n - 1) physical links. However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh topology, we need n(n -1) /2 duplex-mode links. A mesh offers several advantages over other network topologies. First, the use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links are shared by multiple devices. Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system. 4
  5. 5. Dr. Y. Narasimha Murthy Ph.D Third, there is the advantage of privacy or security. When every message travels along a dedicated line, only the intended recipient sees it. Physical boundaries prevent other users from gaining access to messages. Finally, point-to-point links make fault identification and fault isolation easy. Traffic can be routed to avoid links with suspected problems. This facility enables the network manager to discover the precise location of the fault and aids in finding its cause and solution. The main disadvantages of a mesh are related to the amount of cabling and the number of I/O ports required. First, because every device must be connected to every other device, installation and reconnection are difficult. Second, the sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate. Finally, the hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. For these reasons a mesh topology is usually implemented in a limited fashion, for example, as a backbone connecting the main computers of a hybrid net work that can include several other topologies. One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office. Star Topology: In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. Unlike a mesh topology, a star topology does not allow direct traffic between devices. The controller acts as an exchange: If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device .A star topology is less expensive than a mesh topology. In a star, each device needs only one link and one I/O port to connect it to any number of others. This factor also makes it easy to install and reconfigure. Far less cabling needs to be housed, and additions, moves, and deletions involve only one connection: between that device and the hub. Other advantages include robustness. If one link fails, only that link is affected. All other links remain active. This factor also lends itself to easy fault identification and fault isolation. As long as the hub is working, it can be used to monitor link problems and bypass defective links. The main disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead. Although a star requires 5
  6. 6. Dr. Y. Narasimha Murthy Ph.D far less cable than a mesh, each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus).The star topology is used in local-area networks (LANs), Bus topology: In a bus topology all devices are connected to the transmission medium as backbone. There must be a terminator at each end of the bus to avoid signal reflections, which may distort the original signal. Signal is sent in both directions, but some buses are unidirectional. Good for small networks. Can be used for 10BASE5 (thick net), 10BASE2 (thin net) or 10BROAD36 (broad band) co-axial bus standards. Nodes are connected to the bus cable by drop lines and taps. A drop line is a connection running between the device and the main cable. A tap is a connector that either splices into the main cable or punctures the sheathing of a cable to create a contact with the metallic core. As a signal travels along the backbone, some of its energy is transformed into heat. Therefore, it becomes weaker and weaker as it travels farther and farther. For this reason there is a limit on the number of taps a bus can support and on the distance between those taps. Advantages of a bus topology include ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. In this way, a bus uses less cabling than mesh or star topologies. In a star, for example, four network devices in 6
  7. 7. Dr. Y. Narasimha Murthy Ph.D the same room require four lengths of cable reaching all the way to the hub. In a bus, this redundancy is eliminated. Only the backbone cable stretches through the entire facility. Each drop line has to reach only as far as the nearest point on the backbone. Disadvantages of bus topology are difficulties in reconnection and fault isolation. A bus is usually designed to be optimally efficient at installation. It can therefore be difficult to add new devices. Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable. Adding new devices may therefore require modification or replacement of the backbone. The other disadvantage is , a fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions. Bus topology was the one of the first topologies used in the design of early local area networks. Ethernet LANs can use a bus topology, but they are less popular. Ring Topology : In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along. A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors (either physically or logically). To add or delete a device requires changing only two connections. The only constraints are media and traffic considerations (maximum ring length and number of devices). In addition, fault isolation is simplified. Generally in a ring, a signal is circulating at all times. If one device does not receive a signal within a specified period, it can issue an alarm. The alarm alerts the network operator to the problem and its location. However, unidirectional traffic can be a disadvantage. In a simple ring, a break in the ring (such as a disabled station) can disable the entire network. This weakness can be solved by using a dual ring or a switch capable of closing off the break. 7
  8. 8. Dr. Y. Narasimha Murthy Ph.D Ring topology was prevalent when IBM introduced its local-area network Token Ring. With the advent of higher-speed LANs this topology has become less useful. TRANSMISSION MODE :The term transmission mode is used to define the direction of signl flow between two linked devices.There are three types of transmission modes.They are Simplex , Half Duplex and Full-Duplex. Simplex: In simplex mode communication is unidirectional .i.e only one of the two devices on a link can transmit data and the other will only receive the data.As shown below a computer always sends the data and the monitor only receives the data. Similarly a key board always receives the data from the computer and computers send the data to the keyboard. Half Duplex: In half duplex both the devices can transmit and receive the data in the data communication. But not at the same time. When one device is sending the data the other will be receiving and vice versa. The use of a walkie-talkie by police and army personal is an example for this half-duplex transmission. The Half duplex method is like a one lane road with two directional traffic. While cars are travelling one direction , cars going the other direction must wait. In a half Duplex transmission the entire capacity of a channel is taken over by which ever of the two devices is transmitting at the time. Full Duplex : In full duplex mode (also called Duplex) both stations can transmit and receive data simultaneously. One common example is mobile phone or telephone line . 8
  9. 9. Dr. Y. Narasimha Murthy Ph.D The full duplex mode is like a two way street with traffic following in both directions at the same time .In full duplex ,the signals going in either direction share the capacity of the link. This sharing can occur in two ways. Either the link must contain two physically separate transmission paths .One for sending and the other for receiving .or the capacity of the channel is divided between signals travelling in both directions. MODEMS: The term modem stands for Modulator & De-Modulator .The most familiar type of Data Communication Equipment (DTE) is a Modem. The term modem is a composite word that refers to the two functional entities that make up the device: a signal modulator and a signal demodulator. A modulator converts a digital signal into an analog signal using ASK, FSK, PSK, or QAM. A demodulator converts an analog signal into a digital signal. While a demodulator resembles an analog-to-digital converter, it is not in fact a converter of any kind. It does not sample a signal to create a digital facsimile; it merely reverses the process of modulation-that is, it performs demodulation. The modem consists of • Interface circuitry for linking with the host PC • Circuits to prepare data for transmission by adding the proper start, stop, and parity bits. • The modulator that converts PCs digital signals into analog signals. • The user interface that gives you commands of the modem's operation. 9
  10. 10. Dr. Y. Narasimha Murthy Ph.D In the above diagram the two PCs are the DTEs i.e data Terminal Equipments. The Modems are DCEs. The demodulator takes theASK,FSK, PSK or QAM signal and decodes into a suitable form for a computer.It then relays the resulting digital signal to the receiving computer via the internet .Each DCE must be compatible with both its own DTE and other DTEs. Modem Standards The two modem standards are Bell modems and ITU· modems. Bell Modems The first commercial modems were produced by the Bell Telephone Company in early 1970s. As the first and, for a long time, lone manufacturer in the marketplace, Be defined the development of the technology and provided a de facto standard upon which subsequent manufacturers have built. Today there are dozens of companies producing hundreds of different types of modems worldwide. As complex and powerful as many models have become, they all evolved from the original and. relatively simple first models from Bell. The Bell 212 series modems have two speeds. The option of a second speed allows for compatibility with a wider number of systems. Both speeds operate in full-duplex mode over switched telephone lines. The slower speed, 300 bps, uses FSK The 201 series modems operate in either half-duplex mode over two-we switched lines or fullduplex mode over four-wire leased lines. The entire bandwidth over two-wire line is dedicated to a single direction of transmission. Four-wire lines allow for two completely separate channels, one in each direction, to be processed through single modem on each end. ITU-T Modem Standards Now a days most popular modems available are based on standards published by the ITU-T. These modems can be divided into two groups: those that are essentially equivalent to Bell series modems-for example, V.21 is similar to Bell modem 103-and those that are not. 10
  11. 11. Dr. Y. Narasimha Murthy Ph.D Those ITU- T modems that are Bell series compatible are given below table with their Bell equivalents. ITU-T Bell Baud Rate Bit Rate Modulation V.21 103 300 300 FSK V.22 V.23 V.26 V.27 V.29 212 202 201 208 209 600 1200 1200 1600 2400 1200 1200 2400 4800 9600 4-PSK FSK 4-PSK 8-PSK 16-QAM Intelligent Modems Intelligent modems contain software to support a number of additional functions, such as automatic answering and dialing. Intelligent modems were first introduced by Hayes Microcomputer Products, Inc. More recently, other manufacturers have come out with Hayes compatible modems. Instructions in the Hayes and Hayes-compatible modems are called AT commands (AT is short for attention). 56K MODEMS : Traditional modems have a limitation on the data rate (maximum of 33.6 Kbps), However, new modems, with a bit rate of 56,000 bps, called 56K modems, are now on the market. These modems may be used only if one party is using digital signaling (such as through an Internet provider). They area symmetrical in that the downloading (flow of data from the Internet provider to the PC) is a maximum of 56 Kbps, while the uploading (flow of data from the PC to the Internet provider) can be a maximum of 33.6 Kbps. If one side is an Internet provider and the signal does not have to pass through a PCM converter, quantization is eliminated in one direction and the data rate can be increased to 56 Kbps . Encoding and Modulating : In the process of data communication the data must be transformed into signals to send them from one place to another. Data stored in a computer is in the form of 0s and 1s.To transform this data from one place to another place , it must be converted into digital signals.This is called Encoding digital data into digital signal or digital to digital conversion. The process of converting analog signal into digital signal is called analog to digital conversion or digitizing an analog signal. 11
  12. 12. Dr. Y. Narasimha Murthy Ph.D The process of converting the digital signal into analog signal is called digital to analog conversion or modulating a digital signal. The process of sending an analog signal over long distances using a high frequency carrier signal is called analog to analog conversion or modulating an analog signal. Digital to Digital Encoding: It is the representation of a digital information by a digital signal .In this process of encoding the binary 1s and 0s generated by a computer are translated into a sequence of voltage pulses that can be transmitted over a cable or wire. This process is shown in the diagram below. There are three types of digital encodings. Unipolar ,Polar and Bi-polar. Uni-polar : Unipolar encoding uses only one level of value. i.e in a uni-polar scheme, all the signal levels are on one side of the time axis, either above or below. 12
  13. 13. Dr. Y. Narasimha Murthy Ph.D The uni-polar encoding has two problems .One is DC component and the other is synchronization. Due to these limitations ,this encoding is not widely used. The DC component problem arises because the average amplitude of a uni-polar encoded signal is non- zero. This causes what is called a direct current component. When a signal contains a DC component, it cannot travel through the media that cannot handle DC components. The Synchronization problem in uni-polar encoding occur when ever the data stream includes a long uninterrupted series of 1s or 0s. Compared to polar encoding this scheme is very costly. Polar Schemes: In polar schemes, the voltages are on the both sides of the time axis. For example, the voltage level for 0 can be positive and the voltage level for I can be negative.The popular Polar encoding techniques are Non-return zero(NRZ),Return to Zero(RZ) and bi-phase. The bi-phase refers to two methods.One is Manchester used in Ethernet LAN and the second is Differential Manchester used by token rings LANs. Types of polar encodings is shown below. 13
  14. 14. Dr. Y. Narasimha Murthy Ph.D Non-Return-to-Zero (NRZ) : In polar NRZ encoding, two levels of voltage amplitude are used. There are two versions of polar NRZ: NRZ-Land NRZ-I, as shown in Figure below. The figure also shows the value of r, the average baud rate, and the bandwidth. In the first variation, NRZ-L (NRZ-Level), the level of the voltage determines the value of the bit. In the second variation, NRZ-I (NRZ-Invert), the change or lack of change in the level of the voltage determines the value of the bit. If there is no change, the bit is 0; if there is a change, the bit is 1. NRZ-L and NRZ-J both have a DC component problem. The synchronization problem (sender and receiver clocks are not synchronized) also exists in both the schemes. This problem is more serious in NRZ-L than in NRZ-I. While a long sequence of datas can cause a problem in both schemes, a long sequence of 1s affects only NRZ-L. Another problem with NRZ-L occurs when there is a sudden change of polarity in the system. For example, if twisted-pair cable is the medium, a change in the polarity of the wire results in all 0s interpreted as I s and all I s interpreted 0s . NRZ-I does not have this problem. Both schemes have an average signal rate of N/2 Bd. Return to Zero (RZ) The main problem with NRZ encoding occurs when the sender and receiver clocks are not synchronized. The receiver does not know when one bit has ended and the next bit is starting. Hence the Return-to-zero (RZ) scheme is used. This scheme uses three values: positive, negative, and zero. In RZ, the signal changes not between bits but during the bit. In Figure below we see that the signal goes to 0 in the middle of each bit. It remains there until the beginning of the next bit. The main disadvantage of RZ encoding is that it requires two signal changes to encode a bit and therefore occupies greater bandwidth. This scheme has no DC component problem 14
  15. 15. Dr. Y. Narasimha Murthy Ph.D . But the problem is the complexity : RZ uses three levels of voltage, which is more complex to create . As a result of all these deficiencies, the scheme is not used today. Instead, the betterperforming schemes like Manchester and differential Manchester schemes are used. Biphase: The best solution for the problem of synchronization is the Bi-phase encoding.This bi-phase encoding consists of two important encodings namely Manchester and Differential Manchester encodings . Manchester scheme is the resultant of the idea of RZ (transition at the middle of the bit) and the idea of NRZ-L . In Manchester encoding, the duration of the bit is divided into two halves. The voltage remains at one level during the first half and moves to the other level in the second half. The transition at the middle of the bit provides synchronization. On the other hand Differential Manchester, combines the ideas of RZ and NRZ-I. There is always a transition at the middle of the bit, but the bit values are determined at the beginning of the bit. If the next bit is 0, there is a transition; if the next bit is 1, there is none. Figure below shows both Manchester and differential Manchester encodings. 15
  16. 16. Dr. Y. Narasimha Murthy Ph.D In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization. The Manchester scheme overcomes several problems associated with NRZ-L, and differential Manchester overcomes several problems associated with NRZ-I. First, there is no baseline wandering. There is no DC component because each bit has a positive and negative voltage contribution. The only drawback is the signal rate. The signal rate for Manchester and differential Manchester is double that for NRZ. The reason is that there is always one transition at the middle of the bit and maybe one transition at the end of each bit. Manchester and differential Manchester schemes are also called bi-phase schemes. Channel capacity The maximum rate at which data can be transmitted over a communication channel under given conditions is referred as the channel capacity. Four important parameters are related in the evaluation of channel capacity. • Data rate: The rate at which data can be transmitted. Measured in bits per second • Bandwidth: The bandwidth of the transmitted signal. Measured in cycles per second (Hz). • Noise: The average level of unwanted signals over communication path. Expressed as the ratio between signal and noise. • Error rate: The rate at which error can occur. The Channel capacity (in cycles per second) is given by Shannon’s theorem. According to Shannon’s theorem C = B log2 (1+SNR) Where C is channel capacity in Cycles per second and this is error free capacity ; B is the bandwidth in Hertz and SNR = 10 log10 (Signal power/Noise power) known as signal to noise ratio. Here the SNR ratio assumes only white noise (thermal noise) where as other noise like impulse noise, attenuation noise and delay noise are not taken into account. Hence the actual value of channel capacity differs from the estimated value TRANSMISSION IMPAIRMENT: When signals travel through imperfect transmission media, the imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the 16
  17. 17. Dr. Y. Narasimha Murthy Ph.D signal at the end of the medium. What is sent is not what is received. The impairment of signals is mainly caused due to attenuation, distortion, and noise in the medium. Attenuation Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm . Some of the electrical energy in the signal is converted to heat. To compensate for this loss, amplifiers are used to amplify the signal. Distortion : Distortion means that the signal changes its form or shape. Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed (see the next section) through a medium and, therefore, its own delay in arriving at the final destination. Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration. In other words, signal components at the receiver have phases different from what they had at the sender. The shape of the composite signal is therefore not the same. Figure below shows the effect of distortion on a composite signal. 17
  18. 18. Dr. Y. Narasimha Murthy Ph.D Noise Noise is another cause of impairment. Several types of noise, such as thermal noise, induced noise, crosstalk, and impulse noise, may corrupt the signal. Thermal noise is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna. Crosstalk is the effect of one wire on the other. One wire acts as ascending antenna and the other as the receiving antenna. Impulse noise is a spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on. Signal-to-Noise Ratio (SNR) The signal-to-noise ratio is defined as SNR = average signal power / average noise power. SNR is actually the ratio of what is wanted (signal) to what is not wanted (noise). A high SNR means the signal is less corrupted by noise; a low SNR means the signal is more corrupted by noise. Because SNR is the ratio of two powers, it is often described in decibel units SNR dB, defined as SNRdB = 10 log 10SNR. 18
  19. 19. Dr. Y. Narasimha Murthy Ph.D Broadband System A "broadband" system has the capability for transmitting a broad range of analog frequencies over a single cable, Broadband LANs work in a similar way that cable television works. Broadband LANs transmit multiple radio frequency signals on the same cable, usually coaxial or sometimes fiber. This ability to send many types of communication simultaneously over the same cable, including voice, video, and data, distinguishes broadband LANs from the other common baseband LANs. For this operation, broadband LANs use the technique called frequency division multiplexing. Each channel in a broadband system is separated from the adjacent channel by a guard band, a frequency range over which no data will be transmitted. The simplest broadband LAN will be split into two channels, one for inbound data, one for outbound. Each of these gross channels may be split into two or many more channels. Broadband LANs use a tree-and-branch topology. The root of the tree is the head end, or central re- transmission facility. The trunk cable is attached to this root. Baseband System: Baseband methodology allows several devices to share a cable by means of a time-sharing technique. Each device is assigned a specific time slot some thousandths or millionths of a second in length in which to transmit. Only one device can transmit in a given time interval.This technique is called time division multiplexing. Baseband transmission is associated with specific network and performance characteristics. (a) Many types of media can be used, such as twisted pair, coaxial cable, and fiber optics. (b).There must be a protocol included in the network to assign time slots and provide access to the network. (c) . Modems are not required to connect devices to the cable when communicating digital information. (d).Any number of devices can be used for transmission or reception of information, without the addition of complex equipment, as long as there are enough slots available. Comparison Broadband (analog) transmission is generally regarded as a higher throughput technique because of the ability to simultaneously transmit multiple signals over a single cable. 19
  20. 20. Dr. Y. Narasimha Murthy Ph.D Baseband is a highly efficient way of moving data from one piece of digital equipment to another. Hundreds of millions of data bits per second can move along the cable, with no slowdowns for analog conversion. These data bits, however, must move single file, and there is a finite limit to the total number of bits that can be transmitted during any given time period. The transmission rate advantage of broadband transmission is rapidly disappearing, however, with recent advances in digital technology, particularly in fiber optics, allowing digital baseband transmissions to rival, and even exceed broadband analog transmission speed. Baseband technology has benefitted substantially from continuing advances in integrated circuit technology. Installation, device connection, and control of communication traffic are all less expensive for digital networks than for analog networks. Broadband cable per foot is less expensive than baseband. The additional hardware needed (modems and directional couplers) to modulate the data signal for analog transmission from point to point and the more complex design engineering required will however raise the cost of installing broadband. MULTIPLEXING : Multiplexing is the techniques that allows the simultaneous transmission of multiple signals across a single data link .As data and telecommunications use increases, so does traffic. The figure below shows the basic format of a multiplexed system. The lines on the left direct their transmission streams to a multiplexer (MUX), which combines them into a single stream (many-to one).At the receiving end, that stream is fed into a de-multiplexer (DEMUX), which separates the stream back into its component transmissions (one-to-many) and directs them to their corresponding lines. In the figure, the word link refers to the physical path. The word channel refers to the portion of a link that carries a transmission between a given pair of lines. One link can have many (n) channels. There are three basic multiplexing techniques: frequency-division multiplexing, wavelengthdivision multiplexing, and time-division multiplexing. The first two techniques designed for analog signals, the third one is , for digital signals 20
  21. 21. Dr. Y. Narasimha Murthy Ph.D Frequency-Division Multiplexing : Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link. Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. These bandwidth ranges are the channels through which the various signals travel. Channels can be separated by strips of unused bandwidth-guard bands-to prevent signals from overlapping. In addition, carrier frequencies must not interfere with the original data frequencies. A very common application of FDM is AM and FM radio broadcasting. Radio uses the air as the transmission medium. A special band from 530 to 1700 kHz is assigned to AM radio. All radio stations need to share this band. Each AM station needs 10kHz of bandwidth. Each station uses a different carrier frequency, which means it is shifting its signal and multiplexing. The signal that goes to the air is a combination of signals. A receiver receives all these signals, but filters (by tuning) only the one which is desired. Without multiplexing, only one AM station could broadcast to the common link, the air. The same concept is also used in FM broadcasting. But the , FM has a wider band of 88 to 108 MHz , because each station needs a bandwidth of 200 kHz. Another common use of FDM is in television broadcasting. Each TV channel has its own bandwidth of 6 MHz. The first generation of cellular telephones (still in operation) also uses FDM Wavelength-Division Multiplexing: Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of fiber-optic cable. The optical fiber data rate is higher than the 21
  22. 22. Dr. Y. Narasimha Murthy Ph.D data rate of metallic transmission cable. Using a fiber-optic cable for one single line will waste the available bandwidth. So, multiplexing is used which allows to combine several lines into one. WDM is conceptually the same as FDM, except that the multiplexing and de-multiplexing involve optical signals transmitted through fiber-optic channels. The idea is combining different signals of different frequencies. The difference is that the frequencies are very high .Figure below gives a conceptual view of a WDM multiplexer and de-multiplexer. Very narrow bands of light from different sources are combined to make a wider band of light. At the receiver, the signals are separated by the de-multiplexer. Although WDM technology is very complex, the basic idea is very simple. Multiple light sources are combined into one single light at the multiplexer and do the reverse at the de-multiplexer.A multiplexer can be made to combine several input beams of light, each containing a narrow band of frequencies, into one output beam of a wider band of frequencies. A de-multiplexer can be made to reverse the process. The application of WDM is the SONET network in which multiple optical fiber lines are multiplexed and de-multiplexed. A new method, called dense WDM (DWDM), can multiplex a very large number of channels by spacing channels very close to one another. It achieves a greater efficiency than ordinary WDM. 22
  23. 23. Dr. Y. Narasimha Murthy Ph.D Demultiplexing Process : The de-multiplexer uses a series of filters to decompose the multiplexed signal into its constituent component signals. The individual signals are then passed to a demodulator that separates them from their carriers and passes them to the output lines. Error Detection and Correction : During the transmission of data from one point to other there is every possibility that the data may be corrupted and errors may occur. The data transfer through a Network must be always with acceptable accuracy. So, these data errors must be detected and corrected for a good data communication. Some applications can tolerate a small level of error. For example, random errorsin audio or video transmissions may be tolerable, but when we transfer text, we expect a very high level of accuracy. Types of Errors The errors in data communication are divided into two types.(i)Single Bit Errors and (ii) Burst errors. Single Bit Errors: Whenever bits flow from one point to another, they are subject to unpredictable changes due to interference. This interference can change the shape of the signal. In a single-bit error, a 0 is changed to a 1 or a 1 to a 0. The term single-bit error means that only 1 bit of a given data unit (such as a byte, character, or packet) is changed from 1 to 0 or from 0 to 1. The figure above shows the effect of a single-bit error on a data unit. To understand the impact of the change, imagine that each group of 8 bits is an ASCII character with a 0 bit added to the left. In the above figure 10.1,00000010 (ASCII STX) was sent, meaning start of text, but 00001010 (ASCII LF) was received, meaning line feed. Single-bit errors are the least likely type of error in serial data transmission. For example let us , imagine data sent at 1 Mbps. This means that each bit lasts only 1/1,000,000 s, or 1µs. For a single-bit error to occur, the noise must have a duration of only 1µs, which is very rare because noise normally lasts much longer than this. 23
  24. 24. Dr. Y. Narasimha Murthy Ph.D Burst Error The term burst error means that 2 or more bits in the data unit have changed from 1 to 0 or from 0 to 1.Here a burst of data is being corrupted and not a single bit. In the figure below the effect of a burst error on a data unit is shown. In this case, 100010001000011 was sent, but 0101110101100011 was received. Note that a burst error does not necessarily mean that the errors occur in consecutive bits. The length of the burst is measured from the first corrupted bit to the last corrupted bit. Some bits in between may not have been corrupted. A burst error is more likely to occur than a single-bit error. The duration of noise is normally longer than the duration of 1 bit, which means that when noise affects data, it affects a set of bits. The number of bits affected depends on the data rate and duration of noise. For example, if we are sending data at I kbps, a noise of 11100 s can affect 10 bits; if we are sending data at 1 Mbps, the same noise can affect 10,000 bits. Redundancy: The important concept in detecting or correcting errors is redundancy. To be able to detect or correct errors, we need to send some extra bits with our data. These redundant bits are added by the sender and removed by the receiver. Their presence allows the receiver to detect or correct corrupted bits. The correction of errors is more difficult than the detection. In error detection, we are looking only to see if any error has occurred. The answer is a simple yes or no. We are not even interested in the number of errors. A single-bit error is the same for us as a burst error. In error 24
  25. 25. Dr. Y. Narasimha Murthy Ph.D correction, we need to know the exact number of bits that are corrupted and more importantly, their location in the message. The number of the errors and the size of the message are important factors. If we need to correct one single error in an 8-bit data unit, we need to consider eight possible error locations; if we need to correct two errors in a data unit of the same size, we need to consider 28 possibilities. The difficulty of the receiver in finding 10 errors in a data unit of 1000 bits can be clearly understood from the above example. There are two main methods of error correction. Forward error correction is the process in which the receiver tries to guess the message by using redundant bits. This is possible, as we see later, if the number of errors is small. Correction by retransmission is a technique in which the receiver detects the occurrence of an error and asks the sender to resend the message. Resending is repeated until a message arrives that the receiver believes is error-free. Coding Redundancy is achieved through various coding schemes. The sender adds redundant bits through a process that creates a relationship between the redundant bits and the actual data bits. The receiver checks the relationships between the two sets of bits to detect or correct the errors. The ratio of redundant bits to the data bits and the robustness of the process are important factors in any coding scheme. Figure below shows the general idea of coding. We can divide coding schemes into two broad categories: block coding and convolution coding. Block Coding In block coding, the message is divided into blocks, each of k bits, called data words. And r redundant bits are added to each block to make the length n = k + r. The resulting n-bit blocks are called code words. With k bits, a combination of 2k data words are created ; with n bits, we can create a combination of 2n code words. Since n > k, the number of possible code words is larger than the number of possible data words.The block coding process is one-to-one ; the same data word is always encoded as the same code word. This means that 2n - 2k code words are not used which are invalid or illegal. 25
  26. 26. Dr. Y. Narasimha Murthy Ph.D Error Detection : In block coding the receiver can detect a change in the original code word and the error can be detected if the following conditions are satisfied. The receiver has (or can find) a list of valid code words. The original codeword has changed to an invalid one. The sender creates code words out of data words by using a generator that applies the rules and procedures of encoding . Each codeword sent to the receiver may change during transmission. If the received codeword is the same as one of the valid code words, the word is accepted; the corresponding data word is extracted for use. If the received code word is not valid, it is discarded. However, if the codeword is corrupted during transmission but the received word still matches a valid codeword, the error remains undetected. This type of coding can detect only single errors. Two or more errors may remain undetected. Cyclic Redundancy Check (CRC): CRC is a category of error correction in data transmission using cyclic codes ,which is called the cyclic redundancy check (CRC) .This CRC is used in networks such as LANs and WANs. One of 26
  27. 27. Dr. Y. Narasimha Murthy Ph.D the advantages of a cyclic code is that the encoder and decoder can easily and inexpensively implemented in hardware by using a handful of electronic devices. Also, a hardware implementation increases the rate of check bit and syndrome bit calculation. Reference: Data Communications and Networking, Second Edition ,by Behrouz A. Forouzan. 27