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Digital data communications

  1. 1. Data Communication & Networking IV Sem BCA NetworksThe idea of networking is an old one. A network can be defined as "A collection of two or more deviceswhich are interconnected using common protocols to exchange data."Networks are large distributed systems designed to send information from one location to another. An endpoint is a place in a network where data transmission either originates or terminates. A node is a point inthe network where data travels through without stopping. Nodes are connected by channels, paths that dataflows down. Channels can be physical linear objects such as a wire or a fiber optic cable, or it can be lesstangible, like a wireless connection at a particular frequency.The cellular concept of space-divided networks was first developed in AT&T in the 1940s and 1950s.AMPS, an analog frequency division multiplexing network was first implemented in Chicago in 1983, andwas completely saturated with users the next year. The FCC, in response to overwhelming user demand,increased the available cellular bandwidth from 40Mhz to 50Mhz.Wireless GenerationsIt is often instructive to break the history of wireless networking up into several specific generations.First Generation (1G)The 1G wireless generation comprised of mainly analog signals for carrying voice and music. These wereone directional broadcast systems such as Television broadcast, AM/FM radio, and similar communications.Second Generation (2G)2G introduced concepts such as TDMA and CDMA for allowing bi-directional communications amongnodes in large networks. 2G is when some of the first cellular phones were made available, althoughcommunications were restricted to very low bitrates.The second generation is frequently divided into sub-sets as well. "2.5G" represented a significant increasein throughput capacity as digital communications techniques became more refined. "2.75G" is anothercommon pseudo-generation that saw an additional increase in speed and capacity among digital wirelessnetworks.Third Generation (3G)3G is the current generation, and represents the combination of voice traffic with data traffic, and the adventof high-bandwidth mobile devices such as PDAs and smartphones.Fourth Generation (4G)The 4G generation, which is a theoretical future generation, will see the ubiquity of broadband dataconnections and universal internet access. These networks, many of which are being designed around theWiMAX (IEEE 802.16) specification.K. Adisesha, 1Presidency College COPY: Jan 2009
  2. 2. Data Communication & Networking IV Sem BCABi-directional CommunicationsBi-directional communications means that data is flowing both to and from an end point. An end point canbe both a client and a server.Point-to-Point communicationSome channels are point-to-point -- they have only a single producer (at one end), and a single consumer (atthe far end).Many networks have "full duplex" communication between nodes, meaning they have 2 separate point-to-point channels (one in each direction) between the nodes (on separate wires or allocated to separatefrequencies).Some "mesh" networks are built from point-to-point channels. Since wiring every node to every other nodeis prohibitively expensive, when one node needs to communicate with a distant node, the "intermediate"nodes must pass through the information.Multiple AccessMultiple access networks are networks where multiple clients, multiple servers, or both are attempting toaccess the network simultaneously. Networks with one server and multiple clients are called "broadcastnetworks", "multicast networks", or "SIMO networks". "SIMO" stands for "Single Input Multiple Output".Networks with multiple clients and servers are known as "MIMO" or "Multiple Input Multiple Output"networks.Network TopologiesThe shape of a network and the relationship between the nodes in that network is known as the networktopology. The network topology determines, in large part, what kinds of functions the network can perform,and what the quality of the communication will be between nodes. Common TopologiesStar topology - A star topology creates a network by arranging 2 or more host machines around a centralhub. A variation of this topology, the star ring topology, is in common use today.The star topology is still regarded as one of the major network topologies of the networking world.A star topology is typically used in a broadcast or SIMO network, where a single information sourcecommunicates directly with multiple clients. An example of this is a radio station, where a single antennatransmits data directly to many radios.‘Tree topology’- A tree topology is so named because it resembles a binary tree structure from computerscience. The tree has a "root" node, which forms the base of the network. The root node then communicatesK. Adisesha, 2Presidency College COPY: Jan 2009
  3. 3. Data Communication & Networking IV Sem BCAwith a number of smaller nodes, and those in turn communicate with an even greater number of smallernodes. An example of a tree topology network is the DNS system. DNS root servers connect to DNSregional servers, which connect to local DNS servers which then connect with individual networks andcomputers. For your personal computer to talk to the root DNS server, it needs to send a request through thelocal DNS server, through the regional DNS server, and then to the root server.Ring topology - A ring topology (more commonly known as a token ring topology) creates a network byarranging 2 or more hosts in a circle. Data is passed between hosts through a token. This token movesrapidly at all times throughout the ring in one direction. If a host desires to send data to another host, it willattach that data as well as a piece of data saying who the message is for to the token as it passes by. Theother host will then see that the token has a message for it by scanning for destination MAC addresses thatmatch its own. If the MAC addresses do match, the host will take the data and the message will bedelivered. A variation of this topology, the star ring topology, is in common use today.The ring topology is still regarded as one of the major network topologies of the networking world.Mesh topology - A mesh topology creates a network by ensuring that every host machine is connected tomore than one other host machine on the local area network. This topologys main purpose is for faulttolerance - as opposed to a bus topology, where the entire LAN will go down if one host fails. In a meshtopology, as long as 2 machines with a working connection are still functioning, a LAN will still exist.The mesh topology is still regarded as one of the major network topologies of the networking world.Line topology - This rare topology works by connecting every host to the host located to the right of it. Mostnetworking professionals do not even regard this as an actual topology, as it is very expensive (due to itscabling requirements) and due to the fact that it is much more practical to connect the hosts on either end toform a ring topology, which is much cheaper and more efficient.Tree topology - A tree topology, similar to a line topology in that it is extremely rare and is generally notregarded as one of the main network topologies, forms a network by arranging hosts in a hierarchal fashion.A host that is a branch off from the main tree is called a leaf. This topology in this respect becomes verysimilar to a partial mesh topology - if a leaf fails, its connection is isolated and the rest of the LAN cancontinue onwards.‘Bus topology’ - A bus topology creates a network by connecting 2 or more hosts to a length of coaxialbackbone cabling. In this topology, a terminator must be placed on the end of the backbone coaxial cabling -in Michael Meyers Network+ textbook, he commonly compares a network to a series of pipes that watertravels through. Think of the data as water; in this respect, the terminator must be placed in order to preventthe water from flowing out of the network.The bus topology is still regarded as one of the major network topologies of the networking world.‘Hybrid topology’ - A hybrid topology, which is what most networks implement today, uses a combinationof multiple basic network topologies, usually by functioning as one topology logically while appearing asanother physically. The most common hybrid topologies include Star Bus, and Star Ring.Network Size DesignationsPersonal Area Network (PAN) Extremely small networks, often referred to as "piconets" that encompass an area around a single person. These networks, such as Bluetooth, have a range of only 1-5 meters, and tend to have very low power requirements, but also very low datarates. personal area network (PAN) - wireless PANLocal Area Network (LAN) LAN networks can encompass a building such as a house or an office, or a single floor in a multi- level building. Common LAN networks are IEEE 802.11x networks, such as 802.11a, 802.11g, and 802.11n. local area network (LAN) - wireless LANMetropolitan Area Network (MAN) These networks are designed to cover large municipal areas. Data protocols such as WiMAX (802.16) and Cellular 3G networks are MAN networks. metropolitan area network (MAN)K. Adisesha, 3Presidency College COPY: Jan 2009
  4. 4. Data Communication & Networking IV Sem BCAWide Area Network (WAN) Wide-Area Networks are very similar to MAN, and the two are often used interchangably. WiMAX is also considered a WAN protocol. Television and Radio broadcasts are frequently also considered MAN and WAN systems. wide area network (WAN)Regional Area Network (RAN) Large regional area networks are used to communicate with nodes over very large areas. Examples of RAN are satellite broadcast media, and IEEE 802.22.Sensor Area Networks These networks are low-datarate networks primarily used for embedded computer systems and wireless sensor systems. Protocols such as Zigbee (IEEE 802.15.4) and RFID fall into this category. Network ArchitectureNetwork TypesAnalog Networks • Circuit Switching Networks • Cable Television Network • Radio CommunicationsDigital Networks • Internet • Ethernet • Wireless InternetHybrid Networks • Analog and Digital TV • Analog and Digital Telephony • Analog and Digital RadioK. Adisesha, 4Presidency College COPY: Jan 2009
  5. 5. Data Communication & Networking IV Sem BCAProtocolsProtocols are the rules by which computers communicate. Generally a "Network Protocol" defines howcommunications should begin and end properly, and the sequence of events that should occur during datatransmissions. At the transmitting computer the protocol is responsible for: • Breaking the data down into packets • Adding the address of the intended receiving computer • Preparing the data for transmission through the NIC and data-transmission media • At the receiving computer the protocol is responsible for • Collecting the packets off the data-transmission media through the NIC • Stripping off transmitting information from the packets • Copying only the data portion of the packet to a memory buffer • Reassembling the data portions of the packets in the correct order • Checking the data for errorsProtocol Architecture • Task of communication broken up into modules • For example file transfer could use three modules —File transfer application —Communication service module —Network access moduleStandardized Protocol Architectures • Required for devices to communicate • Vendors have more marketable products • Customers can insist on standards based equipment • Two standards: —OSI Reference model •Never lived up to early promises —TCP/IP protocol suite •Most widely used • Also: IBM Systems Network Architecture (SNA), FTPThe OSI Reference Model (Open Systems Interconnection)Developed by the ISO (International Standards Organization) in the early 1970s as a standard architecturefor the development of computer networks. It provides a structured and consistent approach for describing,understanding, and implementing networks. The OSI Model: • Provides general design guidelines for data-communications systems • Provides a standard way to describe how portions (layers) of data-communications systems interact • Divides communication problems into standard layers, facilitating the development of network products and encouraging "mix and match" interchangeability of network components • Promotes the development of a global internetwork in which disparate systems can freely share network data and resources • Is a tool for learning how networks functionK. Adisesha, 5Presidency College COPY: Jan 2009
  6. 6. Data Communication & Networking IV Sem BCA OSI Reference ModelThe OSI model allows for different developers to make products and software to interface with otherproducts, without having to worry about how the layers below are implemented. Each layer has a specifiedinterface with layers above and below it, so everybody can work on different areas without worrying aboutcompatibility. The Layers and their Responsibilities1. Application – Provides services that directly support user applications, such as the user interface, e-mail,file transfer, terminal emulation, database access, etc... Communicates through: Gateways and ApplicationInterfaces2. Presentation – Translates data between the formats the network requires and the computer expects.Handles character encoding, bit order, and byte order issues. Encodes and decodes data. Determines theformat and structure of data. Compresses and decompresses, encrypts and decrypts data. Communicatesthrough: Gateways and Application Interfaces3. Session – Allows applications on a separate computer to share a connection (called a session). Establishesand maintains connection. Manages upper layer errors. Handles remote procedure calls. Synchronizescommunicating nodes. Communicates through: Gateways and Application Interfaces4. Transport – Ensures that packets are delivered error free, in sequence, and without loss or duplication.Takes action to correct faulty transmissions. Controls the flow of data. Acknowledges successful receipt ofdata. Fragments and reassembles data. Communicates through: Gateway Services, Routers, and Brouters5. Network – Makes routing decisions and forwards packets (a.k.a. datagrams) for devices that could befarther away than a single link. Moves information to the correct address. Assembles and disassemblespackets. Addresses and routes data packets. Determines best path for moving data through the network.Communicates through: Gateway Services, Routers, and BroutersK. Adisesha, 6Presidency College COPY: Jan 2009
  7. 7. Data Communication & Networking IV Sem BCA6. Data Link – Provides for the flow of data over a single link from one device to another. Controls accessto communication channel. Controls flow of data. Organizes data into logical frames (logical units ofinformation). Identifies the specific computer on the network. Detects errors. Communicates through:Switches, Bridges, Intelligent HubsThe Data Link Layer contains 2 sub-layers: A. The LLC (Logical Link Control) – The upper sub-layer which establishes and maintains linksbetween communicating devices. Also responsible for frame error correction and hardware addresses. B. The MAC (Media Access Control) – The lower sub-layer which controls how devices share amedia channel. (Either through contention or token passing)7. Physical – Handles the sending and receiving of bits. Provides electrical and mechanical interfaces for anetwork. Specific type of medium used to connect network devices. Specifies how signals are transmittedon network. Communicates through: Repeaters, Hubs, Switches, Cables, Connectors, Transmitters,Receivers, MultiplexersLayers request the services of the layers below them and provide services to the layers above them. Thepoint of communication between layers is called the SAP (Service Access Point).TCP/IP Protocol Architecture • Developed by the US Defense Advanced Research Project Agency (DARPA) for its packet switched network (ARPANET) • Used by the global Internet • No official model but a working one. • This model has five layers —Application layer —Host to host or transport layer —Internet layer —Network access layer —Physical layer OSI v/s TCP/IPTCP•Usual transport layer is Transmission Control Protocol—Reliable connection•Connection—Temporary logical association between entities in different systems•TCP PDU—Called TCP segmentK. Adisesha, 7Presidency College COPY: Jan 2009
  8. 8. Data Communication & Networking IV Sem BCA—Includes source and destination port (c.f. SAP)•Identify respective users (applications)•Connection refers to pair of ports•TCP tracks segments between entities on each connection TCP/IP ConceptsUDP • Alternative to TCP is User Datagram Protocol • Not guaranteed delivery • No preservation of sequence • No protection against duplication • Minimum overhead • Adds port addressing to IP Some Protocols in TCP/IP SuiteK. Adisesha, 8Presidency College COPY: Jan 2009
  9. 9. Data Communication & Networking IV Sem BCAData TransmissionIn a communications system, data are propagated from one point to another by means of electromagneticsignals. Both analog and digital signals may be transmitted on suitable transmission media.An analog signal is a continuously varying electromagnetic wave that may be propagated over a variety ofmedia, depending on spectrum; examples are wire media, such as twisted pair and coaxial cable; fiber opticcable; and unguided media, such as atmosphere or space propagation. Figure 1 Figure 2Above figure 1, illustrates, analog signals can be used to transmit both analog data represented by anelectromagnetic signal occupying the same spectrum, and digital data using a modem(modulator/demodulator) to modulate the digital data on some carrier frequency.However, analog signal will become weaker (attenuate) after a certain distance. To achieve longer distances,the analog transmission system includes amplifiers that boost the energy in the signal. Unfortunately, theamplifier also boosts the noise components. With amplifiers cascaded to achieve long distances, the signalbecomes more and more distorted. For analog data, such as voice, quite a bit of distortion can be toleratedand the data remain intelligible. However, for digital data, cascaded amplifiers will introduce errors.A digital signal is a sequence of voltage pulses that may be transmitted over a wire medium; eg. a constantpositive voltage level may represent binary 0 and a constant negative voltage level may represent binary 1.As Figure 2, illustrates, digital signals can be used to transmit both analog signals and digital data. Analogdata can converted to digital using a codec (coder-decoder), which takes an analog signal that directlyrepresents the voice data and approximates that signal by a bit stream. At the receiving end, the bit stream isused to reconstruct the analog data. Digital data can be directly represented by digital signals.A digital signal can be transmitted only a limited distance before attenuation, noise, and other impairmentsendanger the integrity of the data. To achieve greater distances, repeaters are used. A repeater receives thedigital signal, recovers the pattern of 1s and 0s, and retransmits a new signal. Thus the attenuation isovercome.The principal advantages of digital signaling are that it is generally cheaper than analog signaling and is lesssusceptible to noise interference. The principal disadvantage is that digital signals suffer more fromattenuation than do analog signals. A sequence of voltage pulses, generated by a source using two voltagelevels, and the received voltage some distance down a conducting medium. Because of the attenuation, orreduction, of signal strength at higher frequencies, the pulses become rounded and smaller.Which is the preferred method of transmission? The answer being supplied by the telecommunicationsindustry and its customers is digital. Both long-haul telecommunications facilities and intra-buildingservices have moved to digital transmission and, where possible, digital signaling techniques, for a range ofreasons.The maximum rate at which data can be transmitted over a given communication channel, under givenconditions, is referred to as the channel capacity. There are four concepts here that we are trying to relate toone another.• Data rate, in bits per second (bps), at which data can be communicatedK. Adisesha, 9Presidency College COPY: Jan 2009
  10. 10. Data Communication & Networking IV Sem BCA• Bandwidth, as constrained by the transmitter and the nature of the transmission medium, expressed incycles per second, or Hertz• Noise, average level of noise over the communications path• Error rate, at which errors occur, where an error is the reception of a 1 when a 0 was transmitted or thereception of a 0 when a 1 was transmittedAll transmission channels of any practical interest are of limited bandwidth, which arise from the physicalproperties of the transmission medium or from deliberate limitations at the transmitter on the bandwidth toprevent interference from other sources. Want to make as efficient use as possible of a given bandwidth. Fordigital data, this means that we would like to get as high a data rate as possible at a particular limit of errorrate for a given bandwidth. The main constraint on achieving this efficiency is noise.Nyquist Signaling rate:Consider a noise free channel where the limitation on data rate is simply the bandwidth of the signal.Nyquist states that if the rate of signal transmission is 2B, then a signal with frequencies no greater than B issufficient to carry the signal rate. Conversely given a bandwidth of B, the highest signal rate that can becarried is 2B. This limitation is due to the effect of intersymbol interference, such as is produced by delaydistortion.If the signals to be transmitted are binary (two voltage levels), then the data rate that can be supported by BHz is 2B bps. However signals with more than two levels can be used; that is, each signal element canrepresent more than one bit. For example, if four possible voltage levels are used as signals, then each signalelement can represent two bits. With multilevel signaling, the Nyquist formulation becomes: C = 2B log2 M, where M is the number of discrete signal or voltage levels.So, for a given bandwidth, the data rate can be increased by increasing the number of different signalelements. However, this places an increased burden on the receiver, as it must distinguish one of M possiblesignal elements. Noise and other impairments on the transmission line will limit the practical value of M.Shannon Channel Capacity:Consider the relationship among data rate, noise, and error rate. The presence of noise can corrupt one ormore bits. If the data rate is increased, then the bits become "shorter" so that more bits are affected by agiven pattern of noise. Mathematician Claude Shannon developed a formula relating these. For a given levelof noise, expect that a greater signal strength would improve the ability to receive data correctly in thepresence of noise. The key parameter involved is the signal-to-noise ratio (SNR, or S/N), which is the ratioof the power in a signal to the power contained in the noise that is present at a particular point in thetransmission. Typically, this ratio is measured at a receiver, because it is at this point that an attempt is madeto process the signal and recover the data. For convenience, this ratio is often reported in decibels. Thisexpresses the amount, in decibels, that the intended signal exceeds the noise level. A high SNR will mean ahigh-quality signal and a low number of required intermediate repeaters. SNRdb=10 log10 (signal/noise) Capacity C=B log2(1+SNR)The signal-to-noise ratio is important in the transmission of digital data because it sets the upper bound onthe achievable data rate. Shannons result is that the maximum channel capacity, in bits per second, obeysthe equation shown. C is the capacity of the channel in bits per second and B is the bandwidth of the channelin Hertz. The Shannon formula represents the theoretical maximum that can be achieved. In practice,however, only much lower rates are achieved, in part because formula only assumes white noise (thermalnoise). The successful transmission of data depends principally on two factors: the quality of the signal beingtransmitted and the characteristics of the transmission medium. Data transmission occurs betweentransmitter and receiver over some transmission medium. Transmission media may be classified as guidedK. Adisesha, 10Presidency College COPY: Jan 2009
  11. 11. Data Communication & Networking IV Sem BCAor unguided. In both cases, communication is in the form of electromagnetic waves. With guided media,the waves are guided along a physical path; examples of guided media are twisted pair, coaxial cable, andoptical fiber. Unguided media, also called wireless, provide a means for transmitting electromagneticwaves but do not guide them; examples are propagation through air, vacuum, and seawater.In the case of guided media, the medium itself is more important in determining the limitations oftransmission. For unguided media, the bandwidth of the signal produced by the transmitting antenna is moreimportant than the medium in determining transmission characteristics. One key property of signalstransmitted by antenna is directionality. In general, signals at lower frequencies are omnidirectional; that is,the signal propagates in all directions from the antenna. At higher frequencies, it is possible to focus thesignal into a directional beam. In considering the design of data transmission systems, key concerns are datarate and distance: the greater the data rate and distance the better.Transmission Characteristics of Guided Media Frequency Typical Typical Delay Repeater Range Attenuation Spacing Twisted pair (with 0 to 3.5 kHz 0.2 dB/km @ 1 50 µs/km 2 km loading) kHz Twisted pairs 0 to 1 MHz 0.7 dB/km @ 1 5 µs/km 2 km (multi-pair cables) kHz Coaxial cable 0 to 500 MHz 7 dB/km @ 10 4 µs/km 1 to 9 km MHz Optical fiber 186 to 370 THz 0.2 to 0.5 dB/km 5 µs/km 40 km Twisted PairBy far the most common guided transmission medium for both analog and digital signals is twisted pair. Itis the most commonly used medium in the telephone network (linking residential telephones to the localtelephone exchange, or office phones to a PBX), and for communications within buildings (for LANsrunning at 10-100Mbps). Twisted pair is much less expensive than the other commonly used guidedtransmission media (coaxial cable, optical fiber) and is easier to work with. A twisted pair consists of two insulated copper wires arranged in a regular spiral pattern. A wire pairacts as a single communication link. Typically, a number of these pairs are bundled together into a cable bywrapping them in a tough protective sheath. The twisting tends to decrease the crosstalk interferencebetween adjacent pairs in a cable. Neighboring pairs in a bundle typically have somewhat different twistlengths to reduce the crosstalk interference. On long-distance links, the twist length typically varies from 5to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm.K. Adisesha, 11Presidency College COPY: Jan 2009
  12. 12. Data Communication & Networking IV Sem BCA Coaxial CableCoaxial cable, like twisted pair, consists of two conductors, but is constructed differently to permit it tooperate over a wider range of frequencies. It consists of a hollow outer cylindrical conductor that surroundsa single inner wire conductor (Figure). The inner conductor is held in place by either regularly spacedinsulating rings or a solid dielectric material. The outer conductor is covered with a jacket or shield. Asingle coaxial cable has a diameter of from 1 to 2.5 cm. Coaxial cable can be used over longer distances andsupport more stations on a shared line than twisted pair. Coaxial cable is a versatile transmission medium, used in a wide variety of applications, including:• Television distribution - aerial to TV & CATV systems• Long-distance telephone transmission - traditionally used for inter-exchange links, now beingreplaced by optical fiber/microwave/satellite• Short-run computer system links• Local area networksCoaxial cable is used to transmit both analog and digital signals. It has frequency characteristics that aresuperior to those of twisted pair and can hence be used effectively at higher frequencies and data rates.Because of its shielded, concentric construction, coaxial cable is much less susceptible to interference andcrosstalk than twisted pair. The principal constraints on performance are attenuation, thermal noise, andintermodulation noise. The latter is present only when several channels (FDM) or frequency bands are inuse on the cable. For long-distance transmission of analog signals, amplifiers are needed every few kilometers, withcloser spacing required if higher frequencies are used. The usable spectrum for analog signaling extends toabout 500 MHz. For digital signaling, repeaters are needed every kilometer or so, with closer spacingneeded for higher data rates. Optical FiberAn optical fiber is a thin (2 to 125 µm), flexible medium capable of guiding an optical ray. Various glassesand plastics can be used to make optical fibers. An optical fiber cable has a cylindrical shape and consists ofthree concentric sections: the core, the cladding, and the jacket. The core is the innermost section andconsists of one or more very thin strands, or fibers, made of glass or plastic; the core has a diameter in therange of 8 to 50 µm. Each fiber is surrounded by its own cladding, a glass or plastic coating that has opticalK. Adisesha, 12Presidency College COPY: Jan 2009
  13. 13. Data Communication & Networking IV Sem BCAproperties different from those of the core and a diameter of 125 µm. The interface between the core andcladding acts as a reflector to confine light that would otherwise escape the core. The outermost layer,surrounding one or a bundle of cladded fibers, is the jacket. The jacket is composed of plastic and othermaterial layered to protect against moisture, abrasion, crushing, and other environmental dangers. Optical fiber already enjoys considerable use in long-distance telecommunications, and its use inmilitary applications is growing. The continuing improvements in performance and decline in prices,together with the inherent advantages of optical fiber, have made it increasingly attractive for local areanetworking. Five basic categories of application have become important for optical fiber: Long-haul trunks,Metropolitan trunks, Rural exchange trunks, Subscriber loops & Local area networks.The following characteristics distinguish optical fiber from twisted pair or coaxial cable:• Greater capacity: The potential bandwidth, and hence data rate, of optical fiber is immense; datarates of hundreds of Gbps over tens of kilometers have been demonstrated. Compare this to the practicalmaximum of hundreds of Mbps over about 1 km for coaxial cable and just a few Mbps over 1 km or up to100 Mbps to 10 Gbps over a few tens of meters for twisted pair.• Smaller size and lighter weight: Optical fibers are considerably thinner than coaxial cable orbundled twisted-pair cable. For cramped conduits in buildings and underground along public rights-of-way,the advantage of small size is considerable. The corresponding reduction in weight reduces structuralsupport requirements.• Lower attenuation: Attenuation is significantly lower for optical fiber than for coaxial cable ortwisted pair, and is constant over a wide range.• Electromagnetic isolation: Optical fiber systems are not affected by external electromagnetic fields.Thus the system is not vulnerable to interference, impulse noise, or crosstalk. By the same token, fibers donot radiate energy, so there is little interference with other equipment and there is a high degree of securityfrom eavesdropping. In addition, fiber is inherently difficult to tap.• Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of error. Theperformance of optical fiber systems from this point of view has been steadily improving. Repeater spacingin the tens of kilometers for optical fiber is common, and repeater spacings of hundreds of kilometers havebeen demonstrated.Figure shows the principle of optical fiber transmission. Light from a source enters the cylindrical glass orplastic core. Rays at shallow angles are reflected and propagated along the fiber; other rays are absorbed bythe surrounding material. This form of propagation is called step-index multimode, referring to the varietyof angles that will reflect. With multimode transmission, multiple propagation paths exist, each with adifferent path length and hence time to traverse the fiber. This causes signal elements (light pulses) to spreadout in time, which limits the rate at which data can be accurately received. This type of fiber is best suitedfor transmission over very short distances. When the fiber core radius is reduced, fewer angles will reflect. By reducing the radius of the core tothe order of a wavelength, only a single angle or mode can pass: the axial ray. This single-modepropagation provides superior performance for the following reason. Because there is a single transmissionK. Adisesha, 13Presidency College COPY: Jan 2009
  14. 14. Data Communication & Networking IV Sem BCApath with single-mode transmission, the distortion found in multimode cannot occur. Single-mode istypically used for long-distance applications, including telephone and cable television. Finally, by varying the index of refraction of the core, a third type of transmission, known asgraded-index multimode, is possible. The higher refractive index (discussed subsequently) at the centermakes the light rays moving down the axis advance more slowly than those near the cladding. Rather thanzig-zagging off the cladding, light in the core curves helically because of the graded index, reducing itstravel distance. The shortened path and higher speed allows light at the periphery to arrive at a receiver atabout the same time as the straight rays in the core axis. Graded-index fibers are often used in local areanetworks.Unguided transmissionUnguided transmission techniques commonly used for information communications include broadcast radio,terrestrial microwave, and satellite. Infrared transmission is used in some LAN applications. Three generalranges of frequencies are of interest in our discussion of wireless transmission. Frequencies in the range of about 1 to 40 GHz are referred to as microwave frequencies. At thesefrequencies, highly directional beams are possible, and microwave is quite suitable for point-to-pointtransmission. Microwave is also used for satellite communications. Frequencies in the range of 30 MHz to 1 GHz are suitable for omni directional applications. Werefer to this range as the radio range. Another important frequency range is the infrared portion of thespectrum, roughly from 3 × 1011 to 2 × 1014 Hz. Infrared is useful to local point-to-point and multipointapplications within confined areas, such as a single room. For unguided media, transmission and receptionare achieved by means of an antenna. An antenna can be defined as an electrical conductor or system ofconductors used either for radiating electromagnetic energy or for collecting electromagnetic energy. For transmission of a signal, radio-frequency electrical energy from the transmitter is converted intoelectromagnetic energy by the antenna and radiated into the surrounding environment. For reception of a signal, electromagnetic energy impinging on the antenna is converted into radio-frequency electrical energy and fed into the receiver. In two-way communication, the same antenna can be and often is used for both transmission andreception. This is possible because antenna characteristics are essentially the same whether an antenna issending or receiving electromagnetic energy. An antenna will radiate power in all directions but, typically,does not perform equally well in all directions. A common way to characterize the performance of anantenna is the radiation pattern, which is a graphical representation of the radiation properties of an antennaas a function of space coordinates. The simplest pattern is produced by an idealized antenna known as the isotropic antenna. Anisotropic antenna is a point in space that radiates power in all directions equally. The actual radiationpattern for the isotropic antenna is a sphere with the antenna at the center.An important type of antenna is the parabolic reflective antenna, which is used in terrestrial microwaveand satellite applications. A parabola is the locus of all points equidistant from a fixed line (the directrix)and a fixed point (the focus) not on the line, as shown in Figure above. If a parabola is revolved about itsaxis, the surface generated is called a paraboloid. Paraboloid surfaces are used in headlights, optical and radio telescopes, and microwave antennasbecause: If a source of electromagnetic energy (or sound) is placed at the focus of the paraboloid, and if theparaboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of theparaboloid; as shown in Figure b above. In theory, this effect creates a parallel beam without dispersion. Inpractice, there will be some dispersion, because the source of energy must occupy more than one point. Thelarger the diameter of the antenna, the more tightly directional is the beam. On reception, if incoming wavesare parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus.K. Adisesha, 14Presidency College COPY: Jan 2009
  15. 15. Data Communication & Networking IV Sem BCA Parabolic Reflective AntennaAntenna gain is a measure of the directionality of an antenna. Antenna gain is defined as the power output,in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna(isotropic antenna). For example, if an antenna has a gain of 3 dB, that antenna improves upon the isotropicantenna in that direction by 3 dB, or a factor of 2. The increased power radiated in a given direction is at theexpense of other directions. In effect, increased power is radiated in one direction by reducing the powerradiated in other directions. It is important to note that antenna gain does not refer to obtaining more outputpower than input power but rather to directionality.The primary use for terrestrial microwave systems is in long haul telecommunications service, as analternative to coaxial cable or optical fiber. The microwave facility requires far fewer amplifiers or repeatersthan coaxial cable over the same distance, (typically every 10-100 km) but requires line-of-sighttransmission. Microwave is commonly used for both voice and television transmission. Anotherincreasingly common use of microwave is for short point-to-point links between buildings, for closed-circuitTV or as a data link between local area networks. The most common type of microwave antenna is the parabolic "dish”, fixed rigidly to focus a narrowbeam on a receiving antenna A typical size is about 3 m in diameter. Microwave antennas are usuallylocated at substantial heights above ground level to extend the range between antennas and to be able totransmit over intervening obstacles. To achieve long-distance transmission, a series of microwave relaytowers is used, and point-to-point microwave links are strung together over the desired distance. Microwave transmission covers a substantial portion of the electromagnetic spectrum, typically inthe range 1 to 40 GHz, with 4-6GHz and now 11GHz bands the most common. The higher the frequencyused, the higher the potential bandwidth and therefore the higher the potential data rate. As with anytransmission system, a main source of loss is attenuation, related to the square of distance. The effects ofrainfall become especially noticeable above 10 GHz. Another source of impairment is interference.A communication satellite is, in effect, a microwave relay station. It is used to link two or more ground-based microwave transmitter/receivers, known as earth stations, or ground stations. The satellite receivestransmissions on one frequency band (uplink), amplifies or repeats the signal, and transmits it on anotherfrequency (downlink). A single orbiting satellite will operate on a number of frequency bands, calledtransponder channels, or simply transponders. The optimum frequency range for satellite transmission isin the range 1 to 10 GHz. Most satellites providing point-to-point service today use a frequency bandwidthin the range 5.925 to 6.425 GHz for transmission from earth to satellite (uplink) and a bandwidth in therange 3.7 to 4.2 GHz for transmission from satellite to earth (downlink). This combination is referred to asthe 4/6-GHz band, but has become saturated. So the 12/14-GHz band has been developed (uplink: 14 - 14.5GHz; downlink: 11.7 - 12.2 GHz).K. Adisesha, 15Presidency College COPY: Jan 2009
  16. 16. Data Communication & Networking IV Sem BCA For a communication satellite to function effectively, it is generally required that it remain stationarywith respect to its position over the earth to be within the line of sight of its earth stations at all times. Toremain stationary, the satellite must have a period of rotation equal to the earths period of rotation, whichoccurs at a height of 35,863 km at the equator. Two satellites using the same frequency band, if closeenough together, will interfere with each other. To avoid this, current standards require a 4° spacing in the4/6-GHz band and a 3° spacing at 12/14 GHz. Thus the number of possible satellites is quite limited. Among the most important applications for satellites are: Television distribution, Long-distancetelephone transmission, Private business networks, and Global positioning. Satellite Point to Point LinkFigure a, depicts in a general way two common configurations for satellite communication. In the first, thesatellite is being used to provide a point-to-point link between two distant ground-based antennas. Satellite Broadcast LinkFigure b, depicts in a general way two common configurations for satellite communication. In the second,the satellite provides communications between one ground-based transmitter and a number of ground-basedreceivers.Radio is a general term used to encompass frequencies in the range of 3 kHz to 300 GHz. We are using theinformal term broadcast radio to cover the VHF and part of the UHF band: 30 MHz to 1 GHz. This rangecovers FM radio and UHF and VHF television. This range is also used for a number of data networkingapplications. The principal difference between broadcast radio and microwave is that the former isomnidirectional and the latter is directional. Thus broadcast radio does not require dish-shaped antennas,and the antennas need not be rigidly mounted to a precise alignment. The range 30 MHz to 1 GHz is an effective one for broadcast communications. Unlike the case forlower-frequency electromagnetic waves, the ionosphere is transparent to radio waves above 30 MHz. Thustransmission is limited to the line of sight, and distant transmitters will not interfere with each other due toreflection from the atmosphere. Unlike the higher frequencies of the microwave region, broadcast radioK. Adisesha, 16Presidency College COPY: Jan 2009
  17. 17. Data Communication & Networking IV Sem BCAwaves are less sensitive to attenuation from rainfall. A prime source of impairment for broadcast radiowaves is multipath interference. Reflection from land, water, and natural or human-made objects can createmultiple paths between antennas, eg ghosting on TV pictures.Infrared communications is achieved using transmitters/receivers (transceivers) that modulate noncoherentinfrared light. Transceivers must be within the line of sight of each other either directly or via reflectionfrom a light-colored surface such as the ceiling of a room.Wireless PropagationA signal radiated from an antenna travels along one of three routes: ground wave, sky wave, or line of sight(LOS), as shown in Figure.Ground Wave PropagationGround wave propagation more or less follows the contour of the earth and can propagate considerabledistances, well over the visual horizon. This effect is found in frequencies up to about 2 MHz. Severalfactors account for the tendency of electromagnetic wave in this frequency band to follow the earthscurvature. One factor is that the electromagnetic wave induces a current in the earths surface, the result ofwhich is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow theearths curvature. Another factor is diffraction, which is a phenomenon having to do with the behavior ofelectromagnetic waves in the presence of obstacles. Electromagnetic waves in this frequency range arescattered by the atmosphere in such a way that they do not penetrate the upper atmosphere. The best-knownexample of ground wave communication is AM radio.Sky Wave PropagationSky wave propagation is used for amateur radio, CB radio, and international broadcasts such as BBC andVoice of America. With sky wave propagation, a signal from an earth-based antenna is reflected from theionized layer of the upper atmosphere (ionosphere) back down to earth. Although it appears the wave isreflected from the ionosphere as if the ionosphere were a hard reflecting surface, the effect is in fact causedK. Adisesha, 17Presidency College COPY: Jan 2009
  18. 18. Data Communication & Networking IV Sem BCAby refraction. Refraction is described subsequently. A sky wave signal can travel through a number of hops,bouncing back and forth between the ionosphere and the earths surface, as shown in figure b. With thispropagation mode, a signal can be picked up thousands of kilometers from the transmitter.Line of Sight PropagationAbove 30 MHz, neither ground wave nor sky wave propagation modes operate, and communication must beby line of sight. For satellite communication, a signal above 30 MHz is not reflected by the ionosphere andtherefore a signal can be transmitted between an earth station and a satellite overhead that is not beyond thehorizon. For ground-based communication, the transmitting and receiving antennas must be within aneffective line of sight of each other. The term effective is used because microwaves are bent or refracted bythe atmosphere. The amount and even the direction of the bend depends on conditions, but generallymicrowaves are bent with the curvature of the earth and will therefore propagate farther than the optical lineof sight. In this book, we are almost exclusively concerned with LOS communications.K. Adisesha, 18Presidency College COPY: Jan 2009
  19. 19. Data Communication & Networking IV Sem BCADigital Data CommunicationsData CommunicationsThe distance over which data moves within a computer may vary from a few thousandths of an inch, as isthe case within a single IC chip, to as much as several feet along the backplane of the main circuit board.Over such small distances, digital data may be transmitted as direct, two-level electrical signals over simplecopper conductors. Except for the fastest computers, circuit designers are not very concerned about theshape of the conductor or the analog characteristics of signal transmission.Data Communications concerns the transmission of digital messages to devices external to the messagesource. "External" devices are generally thought of as being independently powered circuitry that existsbeyond the chassis of a computer or other digital message source. As a rule, the maximum permissibletransmission rate of a message is directly proportional to signal power, and inversely proportional to channelnoise. It is the aim of any communications system to provide the highest possible transmission rate at thelowest possible power and with the least possible noise.Communications ChannelsA communications channel is a pathway over which information can be conveyed. It may be defined by aphysical wire that connects communicating devices, or by a radio, laser, or other radiated energy source thathas no obvious physical presence. Information sent through a communications channel has a source fromwhich the information originates, and a destination to which the information is delivered. Althoughinformation originates from a single source, there may be more than one destination, depending upon howmany receive stations are linked to the channel and how much energy the transmitted signal possesses.In a digital communications channel, the information is represented by individual data bits, which may beencapsulated into multibit message units. A byte, which consists of eight bits, is an example of a messageunit that may be conveyed through a digital communications channel. A collection of bytes may itself begrouped into a frame or other higher-level message unit. Such multiple levels of encapsulation facilitate thehandling of messages in a complex data communications network.Any communications channel has a direction associated with it:K. Adisesha, 19Presidency College COPY: Jan 2009
  20. 20. Data Communication & Networking IV Sem BCAThe message source is the transmitter, and the destination is the receiver. A channel whose direction oftransmission is unchanging is referred to as a simplex channel. For example, a radio station is a simplexchannel because it always transmits the signal to its listeners and never allows them to transmit back.A half-duplex channel is a single physical channel in which the direction may be reversed. Messages mayflow in two directions, but never at the same time, in a half-duplex system. In a telephone call, one partyspeaks while the other listens. After a pause, the other party speaks and the first party listens. Speakingsimultaneously results in garbled sound that cannot be understood.A full-duplex channel allows simultaneous message exchange in both directions. It really consists of twosimplex channels, a forward channel and a reverse channel, linking the same points. The transmission rateof the reverse channel may be slower if it is used only for flow control of the forward channel.Serial CommunicationsMost digital messages are vastly longer than just a few bits. Because it is neither practical nor economic totransfer all bits of a long message simultaneously, the message is broken into smaller parts and transmittedsequentially. Bit-serial transmission conveys a message one bit at a time through a channel. Each bitrepresents a part of the message. The individual bits are then reassembled at the destination to compose themessage. In general, one channel will pass only one bit at a time. Thus, bit-serial transmission is necessaryin data communications if only a single channel is available. Bit-serial transmission is normally just calledserial transmission and is the chosen communications method in many computer peripherals.Byte-serial transmission conveys eight bits at a time through eight parallel channels. Although the rawtransfer rate is eight times faster than in bit-serial transmission, eight channels are needed, and the cost maybe as much as eight times higher to transmit the message. When distances are short, it may nonetheless beboth feasible and economic to use parallel channels in return for high data rates. This figure illustrates theseideas:The baud rate refers to the signalling rate at which data is sent through a channel and is measured inelectrical transitions per second. In the EIA232 serial interface standard, one signal transition, at most,occurs per bit, and the baud rate and bit rate are identical. In this case, a rate of 9600 baud corresponds to atransfer of 9,600 data bits per second with a bit period of 104 microseconds (1/9600 sec.). If two electricaltransitions were required for each bit, as is the case in non-return-to-zero coding, then at a rate of 9600 baud,only 4800 bits per second could be conveyed. The channel efficiency is the number of bits of usefulinformation passed through the channel per second. It does not include framing, formatting, and errordetecting bits that may be added to the information bits before a message is transmitted, and will always beless than one.K. Adisesha, 20Presidency College COPY: Jan 2009
  21. 21. Data Communication & Networking IV Sem BCAThe data rate of a channel is often specified by its bit rate (often thought erroneously to be the same as baudrate). However, an equivalent measure channel capacity is bandwidth. In general, the maximum data rate achannel can support is directly proportional to the channels bandwidth and inversely proportional to thechannels noise level.A communications protocol is an agreed-upon convention that defines the order and meaning of bits in aserial transmission. It may also specify a procedure for exchanging messages. A protocol will define howmany data bits compose a message unit, the framing and formatting bits, any error-detecting bits that maybe added, and other information that governs control of the communications hardware. Channel efficiency isdetermined by the protocol design rather than by digital hardware considerations. Note that there is atradeoff between channel efficiency and reliability - protocols that provide greater immunity to noise byadding error-detecting and -correcting codes must necessarily become less efficient.Digital Data TransmissionThe transmission of a stream of bits from one device to another across a transmission link involves a greatdeal of cooperation and agreement between the two sides. One of the most fundamental requirements issynchronization. The receiver must know the rate at which bits are being received so that it can sample theline at appropriate intervals to determine the value of each received bit. Two techniques are in common usefor this purpose are:•Asynchronous transmission.•Synchronous transmission.The reception of digital data involves sampling the incoming signal once per bit time to determine thebinary value. This is compounded by a timing difficulty: In order for the receiver to sample the incomingbits properly, it must know the arrival time and duration of each bit that it receives. Typically, the receiverwill attempt to sample the medium at the center of each bit time, at intervals of one bit time. If the receivertimes its samples based on its own clock, then there will be a problem if the transmitters and receiversclocks are not precisely aligned. If there is a drift in the receivers clock, then after enough samples, thereceiver may be in error because it is sampling in the wrong bit time For smaller timing differences, theerror would occur later, but eventually the receiver will be out of step with the transmitter if the transmittersends a sufficiently long stream of bits and if no steps are taken to synchronize the transmitter and receiver.Asynchronous vs. Synchronous TransmissionSerialized data is not generally sent at a uniform rate through a channel. Instead, there is usually a burst ofregularly spaced binary data bits followed by a pause, after which the data flow resumes. Packets of binarydata are sent in this manner, possibly with variable-length pauses between packets, until the message hasbeen fully transmitted. In order for the receiving end to know the proper moment to read individual binarybits from the channel, it must know exactly when a packet begins and how much time elapses between bits.When this timing information is known, the receiver is said to be synchronized with the transmitter, andaccurate data transfer becomes possible. Failure to remain synchronized throughout a transmission willcause data to be corrupted or lost.In synchronous systems, separate channels are used to transmit data and timing information. The timingchannel transmits clock pulses to the receiver. Upon receipt of a clock pulse, the receiver reads the datachannel and latches the bit value found on the channel at that moment. The data channel is not read againuntil the next clock pulse arrives. Because the transmitter originates both the data and the timing pulses, thereceiver will read the data channel only when told to do so by the transmitter (via the clock pulse), andsynchronization is guaranteed. Techniques exist to merge the timing signal with the data so that only asingle channel is required. This is especially useful when synchronous transmissions are to be sent through amodem. Two methods in which a data signal is self-timed are nonreturn-to-zero and biphase Manchestercoding. These both refer to methods for encoding a data stream into an electrical waveform for transmission.K. Adisesha, 21Presidency College COPY: Jan 2009
  22. 22. Data Communication & Networking IV Sem BCA Synchronous transmissionIn asynchronous systems, a separate timing channel is not used. The transmitter and receiver must be presetin advance to an agreed-upon baud rate. A very accurate local oscillator within the receiver will thengenerate an internal clock signal that is equal to the transmitters within a fraction of a percent. For the mostcommon serial protocol, data is sent in small packets of 10 or 11 bits, eight of which constitute messageinformation. When the channel is idle, the signal voltage corresponds to a continuous logic 1. A data packetalways begins with a logic 0 (the start bit) to signal the receiver that a transmission is starting. The start bittriggers an internal timer in the receiver that generates the needed clock pulses. Following the start bit, eightbits of message data are sent bit by bit at the agreed upon baud rate. The packet is concluded with a paritybit and stop bit. One complete packet is illustrated below:The packet length is short in asynchronous systems to minimize the risk that the local oscillators in thereceiver and transmitter will drift apart. When high-quality crystal oscillators are used, synchronization canbe guaranteed over an 11-bit period. Every time a new packet is sent, the start bit resets the synchronization,so the pause between packets can be arbitrarily long.Parity and Checksums Noise and momentary electrical disturbances may cause data to be changed as it passes through acommunications channel. If the receiver fails to detect this, the received message will be incorrect, resultingin possibly serious consequences. As a first line of defense against data errors, they must be detected. If anerror can be flagged, it might be possible to request that the faulty packet be resent, or to at least prevent theflawed data from being taken as correct. If sufficient redundant information is sent, one- or two-bit errorsmay be corrected by hardware within the receiver before the corrupted data ever reaches its destination.A parity bit is added to a data packet for the purpose of error detection. In the even-parity convention, thevalue of the parity bit is chosen so that the total number of 1 digits in the combined data plus parity packetis an even number. Upon receipt of the packet, the parity needed for the data is recomputed by localhardware and compared to the parity bit received with the data. If any bit has changed state, the parity willnot match, and an error will have been detected. In fact, if an odd number of bits (not just one) have beenaltered, the parity will not match. If an even number of bits have been reversed, the parity will match eventhough an error has occurred. However, a statistical analysis of data communication errors has shown that asingle-bit error is much more probable than a multibit error in the presence of random noise. Thus, parity isa reliable method of error detection.K. Adisesha, 22Presidency College COPY: Jan 2009
  23. 23. Data Communication & Networking IV Sem BCAAnother approach to error detection involves the computation of a checksum. In this case, the packets thatconstitute a message are added arithmetically. A checksum number is appended to the packet sequence sothat the sum of data plus checksum is zero. When received, the packet sequence may be added, along withthe checksum, by a local microprocessor. If the sum is nonzero, an error has occurred. As long as the sum iszero, it is highly unlikely (but not impossible) that any data has been corrupted during transmission.Errors may not only be detected, but also corrected if additional code is added to a packet sequence. If theerror probability is high or if it is not possible to request retransmission, this may be worth doing. However,including error-correcting code in a transmission lowers channel efficiency, and results in a noticeable dropin channel throughput.Data CompressionIf a typical message were statistically analyzed, it would be found that certain characters are used muchmore frequently than others. By analyzing a message before it is transmitted, short binary codes may beassigned to frequently used characters and longer codes to rarely used characters. In doing so, it is possibleto reduce the total number of characters sent without altering the information in the message. Appropriatedecoding at the receiver will restore the message to its original form. This procedure, known as datacompression, may result in a 50 percent or greater savings in the amount of data transmitted. Even thoughtime is necessary to analyze the message before it is transmitted, the savings may be great enough so that thetotal time for compression, transmission, and decompression will still be lower than it would be whensending an uncompressed message.A compression method called Huffman coding is frequently used in data communications, and particularlyin fax transmission. Clearly, most of the image data for a typical business letter represents white paper, andonly about 5 percent of the surface represents black ink. It is possible to send a single code that, forexample, represents a consecutive string of 1000 white pixels rather than a separate code for each whitepixel. Consequently, data compression will significantly reduce the total message length for a faxedbusiness letter. Were the letter made up of randomly distributed black ink covering 50 percent of the whitepaper surface, data compression would hold no advantages.Data EncryptionPrivacy is a great concern in data communications. Faxed business letters can be intercepted at will throughtapped phone lines or intercepted microwave transmissions without the knowledge of the sender or receiver.To increase the security of this and other data communications, including digitized telephone conversations,the binary codes representing data may be scrambled in such a way that unauthorized interception willproduce an indecipherable sequence of characters. Authorized receive stations will be equipped with aK. Adisesha, 23Presidency College COPY: Jan 2009
  24. 24. Data Communication & Networking IV Sem BCAdecoder that enables the message to be restored. The process of scrambling, transmitting, and descramblingis known as encryption.Data Storage TechnologyNormally, we think of communications science as dealing with the contemporaneous exchange ofinformation between distant parties. However, many of the same techniques employed in datacommunications are also applied to data storage to ensure that the retrieval of information from a storagemedium is accurate. We find, for example, that similar kinds of error-correcting codes used to protect digitaltelephone transmissions from noise are also used to guarantee correct readback of digital data from compactaudio disks, CD-ROMs, and tape backup systems.Data Transfer in Digital Circuits Data is typically grouped into packets that are either 8, 16, or 32 bits long, and passed between temporaryholding units called registers. Data within a register is available in parallel because each bit exits the registeron a separate conductor. To transfer data from one register to another, the output conductors of one registerare switched onto a channel of parallel wires referred to as a bus. The input conductors of another register,which is also connected to the bus, capture the information:Following a data transaction, the content of the source register is reproduced in the destination register. It isimportant to note that after any digital data transfer, the source and destination registers are equal; the sourceregister is not erased when the data is sent.The transmit and receive switches shown above are electronic and operate in response to commands from acentral control unit. It is possible that two or more destination registers will be switched on to receive datafrom a single source. However, only one source may transmit data onto the bus at any time. If multiplesources were to attempt transmission simultaneously, an electrical conflict would occur when bits ofopposite value are driven onto a single bus conductor. Such a condition is referred to as a bus contention.Not only will a bus contention result in the loss of information, but it also may damage the electroniccircuitry. As long as all registers in a system are linked to one central control unit, bus contentions shouldnever occur if the circuit has been designed properly. Note that the data buses within a typicalmicroprocessor are funda-mentally half-duplex channels.Transmission over Short Distances (< 2 feet)When the source and destination registers are part of an integrated circuit (within a microprocessor chip, forexample), they are extremely close (thousandths of an inch). Consequently, the bus signals are at very lowpower levels, may traverse a distance in very little time, and are not very susceptible to external noise anddistortion. This is the ideal environment for digital communications. However, it is not yet possible tointegrate all the necessary circuitry for a computer (i.e., CPU, memory, disk control, video and displaydrivers, etc.) on a single chip. When data is sent off-chip to another integrated circuit, the bus signals mustbe amplified and conductors extended out of the chip through external pins. Amplifiers may be added to thesource register:K. Adisesha, 24Presidency College COPY: Jan 2009
  25. 25. Data Communication & Networking IV Sem BCABus signals that exit microprocessor chips and other VLSI circuitry are electrically capable of traversingabout one foot of conductor on a printed circuit board, or less if many devices are connected to it. Specialbuffer circuits may be added to boost the bus signals sufficiently for transmission over several additionalfeet of conductor length, or for distribution to many other chips (such as memory chips).Noise and Electrical DistortionBecause of the very high switching rate and relatively low signal strength found on data, address, and otherbuses within a computer, direct extension of the buses beyond the confines of the main circuit board orplug-in boards would pose serious problems. First, long runs of electrical conductors, either on printedcircuit boards or through cables, act like receiving antennas for electrical noise radiated by motors, switches,and electronic circuits:Such noise becomes progressively worse as the length increases, and may eventually impose anunacceptable error rate on the bus signals. Just a single bit error in transferring an instruction code frommemory to a microprocessor chip may cause an invalid instruction to be introduced into the instructionstream, in turn causing the computer to totally cease operation.A second problem involves the distortion of electrical signals as they pass through metallic conductors.Signals that start at the source as clean, rectangular pulses may be received as rounded pulses with ringing atthe rising and falling edges:These effects are properties of transmission through metallic conductors, and become more pronounced asthe conductor length increases. To compensate for distortion, signal power must be increased or thetransmission rate decreased.K. Adisesha, 25Presidency College COPY: Jan 2009
  26. 26. Data Communication & Networking IV Sem BCATransmission over Medium Distances (< 20 feet)Computer peripherals such as a printer or scanner generally include mechanisms that cannot be situatedwithin the computer itself. Our first thought might be just to extend the computers internal buses with acable of sufficient length to reach the peripheral. Doing so, however, would expose all bus transactions toexternal noise and distortion even though only a very small percentage of these transactions concern thedistant peripheral to which the bus is connected.If a peripheral can be located within 20 feet of the computer, however, relatively simple electronics may beadded to make data transfer through a cable efficient and reliable. To accomplish this, a bus interface circuitis installed in the computer:It consists of a holding register for peripheral data, timing and formatting circuitry for external datatransmission, and signal amplifiers to boost the signal sufficiently for transmission through a cable. Whencommunication with the peripheral is necessary, data is first deposited in the holding register by themicroprocessor. This data will then be reformatted, sent with error-detecting codes, and transmitted at arelatively slow rate by digital hardware in the bus interface circuit. In addition, the signal power is greatlyboosted before transmission through the cable. These steps ensure that the data will not be corrupted bynoise or distortion during its passage through the cable. In addition, because only data destined for theperipheral is sent, the party-line transactions taking place on the computers buses are not unnecessarilyexposed to noise.Data sent in this manner may be transmitted in byte-serial format if the cable has eight parallel channels (atleast 10 conductors for half-duplex operation), or in bit-serial format if only a single channel is available.Transmission over Long Distances (< 4000 feet)When relatively long distances are involved in reaching a peripheral device, driver circuits must be insertedafter the bus interface unit to compensate for the electrical effects of long cables:K. Adisesha, 26Presidency College COPY: Jan 2009
  27. 27. Data Communication & Networking IV Sem BCAThis is the only change needed if a single peripheral is used. However, if many peripherals are connected, orif other computer stations are to be linked, a local area network (LAN) is required, and it becomes necessaryto drastically change both the electrical drivers and the protocol to send messages through the cable.Because multiconductor cable is expensive, bit-serial transmission is almost always used when the distanceexceeds 20 feet.A great deal of technology has been developed for LAN systems to minimize the amount of cable requiredand maximize the throughput. The costs of a LAN have been concentrated in the electrical interface cardthat would be installed in PCs or peripherals to drive the cable, and in the communications software, not inthe cable itself (whose cost has been minimized). Thus, the cost and complexity of a LAN are notparticularly affected by the distance between stations.Transmission over Very Long Distances (greater than 4000 feet)Data communications through the telephone network can reach any point in the world. The volume ofoverseas fax transmissions is increasing constantly, and computer networks that link thousands ofbusinesses, governments, and universities are pervasive. Transmissions over such distances are notgenerally accomplished with a direct-wire digital link, but rather with digitally-modulated analog carriersignals. This technique makes it possible to use existing analog telephone voice channels for digital data,although at considerably reduced data rates compared to a direct digital link.Transmission of data from your personal computer to a timesharing service over phone lines requires thatdata signals be converted to audible tones by a modem. An audio sine wave carrier is used, and, dependingon the baud rate and protocol, will encode data by varying the frequency, phase, or amplitude of the carrier.The receivers modem accepts the modulated sine wave and extracts the digital data from it.Signal Encoding TechniquesAnalog and Digital information can be encoded as either analog or digital signals: ♦ Digital data, digital signals: simplest form of digital encoding of digital data ♦ Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analogK. Adisesha, 27Presidency College COPY: Jan 2009
  28. 28. Data Communication & Networking IV Sem BCA ♦ Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities ♦ Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission systemFor digital signaling, a data source g(t), which may be either digital or analog, is encoded into a digitalsignal x(t). The basis for analog signaling is a continuous constant-frequency fc signal known as the carriersignal. Data may be transmitted using a carrier signal by modulation, which is the process of encodingsource data onto the carrier signal. All modulation techniques involve operation on one or more of the threefundamental frequency domain parameters: amplitude, frequency, and phase. The input signal m(t) may beanalog or digital and is called the modulating signal, and the result of modulating the carrier signal is calledthe modulated signal s(t).Encoding - Digital data to digital signals: A digital signal is a sequence of discrete, discontinuous voltagepulses. Each pulse is a signal element. Binary data are transmitted by encoding each data bit into signalelements. In the simplest case, there is a one-to-one correspondence between bits and signal elements. Morecomplex encoding schemes are used to improve performance, by altering the spectrum of the signal andproviding synchronization capability. In general, the equipment for encoding digital data into a digital signalis less complex and less expensive than digital-to-analog modulation equipment.Various Encoding techniques include: • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar -AMI • Pseudoternary • Manchester • Differential Manchester • B8ZS • HDB3 Encoding techniquesThe most common, and easiest, way to transmit digital signals is to use two different voltage levels for thetwo binary digits. Codes that follow this strategy share the property that the voltage level is constant duringa bit interval; there is no transition (no return to a zero voltage level). Can have absence of voltage used toK. Adisesha, 28Presidency College COPY: Jan 2009
  29. 29. Data Communication & Networking IV Sem BCArepresent binary 0, with a constant positive voltage used to represent binary 1. More commonly a negativevoltage represents one binary value and a positive voltage represents the other. This is known as Nonreturnto Zero-Level (NRZ-L). NRZ-L is typically the code used to generate or interpret digital data by terminalsand other devices.A variation of NRZ is known as NRZI (Nonreturn to Zero, invert on ones). As with NRZ-L, NRZImaintains a constant voltage pulse for the duration of a bit time. The data bits are encoded as the presence orabsence of a signal transition at the beginning of the bit time. A transition (low to high or high to low) at thebeginning of a bit time denotes a binary 1 for that bit time; no transition indicates a binary 0. NRZI is an example of differential encoding. In differential encoding, the information to betransmitted is represented in terms of the changes between successive signal elements rather than the signalelements themselves. The encoding of the current bit is determined as follows: if the current bit is a binary0, then the current bit is encoded with the same signal as the preceding bit; if the current bit is a binary 1,then the current bit is encoded with a different signal than the preceding bit. One benefit of differentialencoding is that it may be more reliable to detect a transition in the presence of noise than to compare avalue to a threshold. Another benefit is that with a complex transmission layout, it is easy to lose the senseof the polarity of the signal.A category of encoding techniques known as multilevel binary addresses some of the deficiencies of theNRZ codes. These codes use more than two signal levels. In the bipolar-AMI scheme, a binary 0 isrepresented by no line signal, and a binary 1 is represented by a positive or negative pulse. The binary 1pulses must alternate in polarity. There are several advantages to this approach. First, there will be no loss ofsynchronization if a long string of 1s occurs. Each 1 introduces a transition, and the receiver canresynchronize on that transition. A long string of 0s would still be a problem. Second, because the 1 signalsalternate in voltage from positive to negative, there is no net dc component. Also, the bandwidth of theresulting signal is considerably less than the bandwidth for NRZ. Finally, the pulse alternation propertyprovides a simple means of error detection. Any isolated error, whether it deletes a pulse or adds a pulse,causes a violation of this property.The comments on bipolar-AMI also apply to pseudoternary. In this case, it is the binary 1 that isrepresented by the absence of a line signal, and the binary 0 by alternating positive and negative pulses.There is no particular advantage of one technique versus the other, and each is the basis of someapplications.There is another set of coding techniques, grouped under the term biphase, that overcomes the limitations ofNRZ codes. Two of these techniques, Manchester and differential Manchester, are in common use. In the Manchester code, there is a transition at the middle of each bit period. The midbit transitionserves as a clocking mechanism and also as data: a low-to-high transition represents a 1, and a high-to-lowtransition represents a 0. Biphase codes are popular techniques for data transmission. The more commonManchester code has been specified for the IEEE 802.3 (Ethernet) standard for baseband coaxial cable andtwisted-pair bus LANs.In differential Manchester, the midbit transition is used only to provide clocking. The encoding of a 0 isrepresented by the presence of a transition at the beginning of a bit period, and a 1 is represented by theabsence of a transition at the beginning of a bit period. Differential Manchester has the added advantage ofemploying differential encoding.Differential Manchester has been specified for the IEEE 802.5 token ring LAN, using shielded twisted pair.Digital Data, Analog SignalThe most familiar use of transmitting digital data using analog signals transformation is for transmittingdigital data through the public telephone network. The telephone network was designed to receive, switch,and transmit analog signals in the voice-frequency range of about 300 to 3400 Hz. It is not at presentsuitable for handling digital signals from the subscriber locations (although this is beginning to change).K. Adisesha, 29Presidency College COPY: Jan 2009
  30. 30. Data Communication & Networking IV Sem BCAThus digital devices are attached to the network via a modem (modulator-demodulator), which convertsdigital data to analog signals, and vice versa. Having stated that modulation involves operation on one or more of the three characteristics of acarrier signal: amplitude, frequency, and phase. Accordingly, there are three basic encoding or modulationtechniques for transforming digital data into analog signals, as illustrated Figure: amplitude shift keying(ASK), frequency shift keying (FSK), and phase shift keying (PSK). In all these cases, the resulting signaloccupies a bandwidth centered on the carrier frequency. Modulation TechniquesIn ASK, the two binary values are represented by two different amplitudes of the carrier frequency.Commonly, one of the amplitudes is zero; that is, one binary digit is represented by the presence, at constantamplitude, of the carrier, the other by the absence of the carrier, ASK is susceptible to sudden gain changesand is a rather inefficient modulation technique. On voice-grade lines, it is typically used only up to 1200bps. The ASK technique is used to transmit digital data over optical fiber, where one signal element isrepresented by a light pulse while the other signal element is represented by the absence of light.The most common form of FSK is binary FSK (BFSK), in which the two binary values are represented bytwo different frequencies near the carrier frequency, as shown in Figure. BFSK is less susceptible to error than ASK. On voice-grade lines, it is typically used up to 1200 bps.It is also commonly used for high-frequency (3 to 30 MHz) radio transmission. It can also be used at evenhigher frequencies on local area networks that use coaxial cable.In PSK, the phase of the carrier signal is shifted to represent data. The simplest scheme uses two phases torepresent the two binary digits (Figure) and is known as binary phase shift keying. An alternative form of two-level PSK is differential PSK (DPSK). In this scheme, a binary 0 isrepresented by sending a signal burst of the same phase as the previous signal burst sent. A binary 1 isrepresented by sending a signal burst of opposite phase to the preceding one. This term differential refers tothe fact that the phase shift is with reference to the previous bit transmitted rather than to some constantreference signal. In differential encoding, the information to be transmitted is represented in terms of thechanges between successive data symbols rather than the signal elements themselves. DPSK avoids therequirement for an accurate local oscillator phase at the receiver that is matched with the transmitter. Aslong as the preceding phase is received correctly, the phase reference is accurate.More efficient use of bandwidth can be achieved if each signaling element represents more than one bit. Forexample, instead of a phase shift of 180˚, as allowed in BPSK, a common encoding technique, known asquadrature phase shift keying (QPSK), uses phase shifts separated by multiples of π/2 (90˚). Thus eachsignal element represents two bits rather than one. The input is a stream of binary digits with a data rate ofK. Adisesha, 30Presidency College COPY: Jan 2009
  31. 31. Data Communication & Networking IV Sem BCAR = 1/Tb, where Tb is the width of each bit. This stream is converted into two separate bit streams of R/2 bpseach, by taking alternate bits for the two streams. The two data streams are referred to as the I (in-phase) andQ (quadrature phase) streams. The streams are modulated on a carrier of frequency fc by multiplying the bitstream by the carrier, and the carrier shifted by 90˚. The two modulated signals are then added together andtransmitted. Thus, the combined signals have a symbol rate that is half the input bit rate. The use of multiple levels can be extended beyond taking bits two at a time. It is possible to transmitbits three at a time using eight different phase angles. Further, each angle can have more than oneamplitude. For example, a standard 9600 bps modem uses 12 phase angles, four of which have twoamplitude values, for a total of 16 different signal elements.Analog data, digital signalsIn this section we examine the process of transforming analog data into digital signals. Analog data, such asvoice and video, is often digitized to be able to use digital transmission facilities. Strictly speaking, it mightbe more correct to refer to this as a process of converting analog data into digital data; this process is knownas digitization. Once analog data have been converted into digital data, a number of things can happen. Thethree most common are:1. The digital data can be transmitted using NRZ-L. In this case, we have in fact gone directly fromanalog data to a digital signal.2. The digital data can be encoded as a digital signal using a code other than NRZ-L. Thus an extra stepis required.3. The digital data can be converted into an analog signal, using one of the modulation techniques. The device used for converting analog data into digital form for transmission, and subsequentlyrecovering the original analog data from the digital, is known as a codec (coder-decoder). In this section weexamine the two principal techniques used in codecs, pulse code modulation and delta modulation. Digitizing Analog DataThe simplest technique for transforming analog data into digital signals is pulse code modulation (PCM),which involves sampling the analog data periodically and quantizing the samples. Pulse code modulation(PCM) is based on the sampling theorem (quoted above). Hence if voice data is limited to frequencies below4000 Hz (a conservative procedure for intelligibility), 8000 samples per second would be sufficient tocharacterize the voice signal completely. Note, however, that these are analog samples, called pulseamplitude modulation (PAM) samples. To convert to digital, each of these analog samples must beassigned a binary code.K. Adisesha, 31Presidency College COPY: Jan 2009
  32. 32. Data Communication & Networking IV Sem BCA PCM ExampleFigure shows an example in which the original signal is assumed to be bandlimited with a bandwidth of B.PAM samples are taken at a rate of 2B, or once every Ts = 1/2B seconds. Each PAM sample isapproximated by being quantized into one of 16 different levels. Each sample can then be represented by 4bits. But because the quantized values are only approximations, it is impossible to recover the original signalexactly. By using an 8-bit sample, which allows 256 quantizing levels, the quality of the recovered voicesignal is comparable with that achieved via analog transmission. Note that this implies that a data rate of8000 samples per second × 8 bits per sample = 64 kbps is needed for a single voice signal. PCM Block DiagramThus, PCM starts with a continuous-time, continuous-amplitude (analog) signal, from which a digital signalis produced, as shown in Figure. The digital signal consists of blocks of n bits, where each n-bit number isthe amplitude of a PCM pulse. On reception, the process is reversed to reproduce the analog signal. Notice,however, that this process violates the terms of the sampling theorem. By quantizing the PAM pulse, theoriginal signal is now only approximated and cannot be recovered exactly. This effect is known asquantizing error or quantizing noise. Each additional bit used for quantizing increases SNR by about 6dB, which is a factor of 4. Non-Linear CodingTypically, the PCM scheme is refined using a technique known as nonlinear encoding, which means, ineffect, that the quantization levels are not equally spaced. The problem with equal spacing is that the meanabsolute error for each sample is the same, regardless of signal level. Consequently, lower amplitude valuesare relatively more distorted. By using a greater number of quantizing steps for signals of low amplitude,K. Adisesha, 32Presidency College COPY: Jan 2009