Figure 7.8: Unidirectional antennas Outgoing transmissions are broadcast through a horn aimed at the dish. The microwaveshit the dish and are deflected outward in a reversal of the receipt path.A horn antenna looks like a gigantic scoop. Outgoing transmissions are broadcastup a stem (resembling a handle) and deflected outward in a series of narrow parallelbeams by the curved head. Received transmissions are collected by the scooped shape ofthe horn, in a manner similar to the parabolic dish, and are deflected down into the stem. Applications Microwaves, due to their unidirectional properties, are very useful when unicast(one-to-one) communication is needed between the sender and the receiver. They areused in cellular phones, satellite networks and wireless LANs.Microwaves are used for unicast communication such as cellular telephones,satellite networks, and wireless LANs. 7.3 INFRARED TRANSMISSION Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mmto 770 nm), can be used for short-range communication. Infrared waves, having highfrequencies, cannot penetrate walls. This advantageous characteristic prevents interferencebetween one system and another; a short-range communication system in one roomcannot be affected by another system in the next room. When we use our infrared remotecontrol, we do not interfere with the use of the remote by our neighbours. However, thissame characteristic makes infrared signals useless for long-range communication. Inaddition, we cannot use infrared waves outside a building because the sun's rays containinfrared waves that can interfere with the communication. Applications The infrared band, almost 400 THz, has an excellent potential for data transmission.Such a wide bandwidth can be used to transmit digital data with a very high data rate.The Infrared 101 CU IDOL SELF LEARNING MATERIAL (SLM)
Data Association (IrDA), an association for sponsoring the use of infraredwaves, has established standards for using these signals for communication betweendevices such as keyboards, mice, PCs, and printers. For example, some manufacturersprovide a special port called the IrDA port that allows a wireless keyboard to communicatewith a PC. The standard originally defined a data rate of 75 kbps for a distanceup to 8 m. The recent standard defines a data rate of 4 Mbps.Infrared signals defined by IrDA transmit through line of sight; the IrDA port onthe keyboard needs to point to the PC for transmission to occur.Infrared signals can be used for short-range communicationin a closed area using line-of-sight propagation. 7.4 LASER TRANSMISSION Highest most electromagnetic spectrum which can be used for data transmission is light or optical signalling. This is achieved by means of LASER. Because of frequency light uses, it tends to travel strictly in straight line. Hence the sender and receiver must be in the line-of- sight. Because laser transmission is unidirectional, at both ends of communication the laser and the photo-detector need to be installed. Laser beam is generally 1mm wide hence it is a work of precision to align two far receptors each pointing to lasers source. Figure 7.9: Laser transmission Laser works as Tx (transmitter) and photo-detectors works as Rx (receiver).Lasers cannot penetrate obstacles such as walls, rain, and thick fog. Additionally, laser beam is distorted by wind, atmosphere temperature, or variation in temperature in the path. Laser is safe for data transmission as it is very difficult to tap 1mm wide laser without interrupting the communication channel. 102 CU IDOL SELF LEARNING MATERIAL (SLM)
7.5 SUMMARY Electromagnetic waves above 100MHz tend to travel in a straight line and signals over them can be sent by beaming those waves towards one particular station. Because Microwaves travels in straight lines, both sender and receiver must be aligned to be strictly in line-of-sight. Microwaves can have wavelength ranging from 1mm – 1meter and frequency ranging from 300MHz to 300GHz. Microwave antennas concentrate the waves making a beam of it. As shown in picture above, multiple antennas can be aligned to reach farther. Microwaves have higher frequencies and do not penetrate wall like obstacles. Microwave transmission depends highly upon the weather conditions and the frequency it is using. Infrared wave lies in between visible light spectrum and microwaves. It has wavelength of 700nm to 1mm and frequency ranges from 300GHz to 430THz. Infrared wave is used for very short range communication purposes such as television and it’s remote. Infrared travels in a straight line hence it is directional by nature. Because of high frequency range, Infrared cannot cross wall-like obstacles. Wireless data are transmitted through ground propagation, sky propagation, and line of-sight propagation. Wireless waves can be classified as radio waves, microwaves, or infrared waves. Radio waves are omnidirectional; microwaves are unidirectional. Microwaves are used for cellular phone, satellite, and wireless LAN communications. Lasers cannot penetrate obstacles such as walls, rain, and thick fog. Additionally, laser beam is distorted by wind, atmosphere temperature, or variation in temperature in the path. Laser is safe for data transmission as it is very difficult to tap 1mm wide laser without interrupting the communication channel. Infrared waves are used for short-range communications such as those between a PC and a peripheral device. It can also be used for indoor LANs. The Infrared Data Association (IrDA), an association for sponsoring the use of infrared waves, has established standards for using these signals for communication between devices such as keyboards, mice, PCs, and printers. For example, some manufacturers provide a special port called the IrDA port that allows a wireless keyboard to communicate with a PC. The standard originally defined a data rate of 75 kbps for a distance up to 8 m. The recent standard defines a data rate of 4 Mbps. Infrared signals defined by IrDA transmit through line of sight; the IrDA port on the keyboard needs to point to the PC for transmission to occur. Highest most electromagnetic spectrum which can be used for data transmission is light or optical signalling. This is achieved by means of LASER. Because of frequency light uses, it tends to travel strictly in straight line. Hence the sender and 103 CU IDOL SELF LEARNING MATERIAL (SLM)
receiver must be in the line-of-sight. Because laser transmission is unidirectional, at both ends of communication the laser and the photo-detector need to be installed. 7.6 KEYWORDS Attenuation- It is loss of signal’s strength in networking cables or connections.the act or process of attenuating something or the state of being attenuated: such as. a : a lessening in amount, force, magnitude, or value : weakening Sound can travel thousands of kilometres in this planar acoustic waveguide with little attenuation Bandwidth- The maximum amount of data transmitted over an internet connection in a given amount of time.The maximum amount of data transmitted over an internet connection in a given amount of time. Bandwidth is often mistaken for internet speed when it's actually the volume of information that can be sent over a connection in a measured amount of time – calculated in megabits per second. Bit Rate- The rate at which the bits are transferred from one location to another is termed as bit rate.In telecommunications and computing, bit rate is the number of bits that are conveyed or processed per unit of time. The bit rate is expressed in the unit bit per second unit, often in conjunction with an SI prefix such as kilo, mega, giga or tera. Composite Signal – It is a combination of two or more simple sine waves with different frequency, phase and amplitude.Any composite signal is a combination of simple sine waves with different amplitudes and frequencies and phases. Composite signals can be periodic or non-periodic. Distortion - It is used to describe an interruption of transmitting signals that cause an unclear reception. It is the act of twisting or altering something out of its true, natural, or original state: the act of distorting a distortion of the facts. 7.7 LEARNING ACTIVITY 1. Create a survey of unidirectional antenna and its features in your locality. ___________________________________________________________________________ ___________________________________________________________________________ 2. Identify the various infrared transmission methods used in your state. ___________________________________________________________________________ ___________________________________________________________________________ 7.8 UNIT END QUESTIONS A.Descriptive Questions 104 CU IDOL SELF LEARNING MATERIAL (SLM)
Short Questions: 1. What is unidirectional antenna? 2. Write the applications of microwave propagation 3. What is Infrared data association? 4. What is a horn antenna? 5. What is electromagnetic spectrum? Long Questions: 1. Explain infrared transmission. 2. Describe laser transmission in detail. 3. Briefly explain microwave transmission. 4. Explain the characteristics of microwave transmission. 5. Explain the features of electromagnetic spectrum. B. Multiple Choice Questions 1. What are micro waves? a. Omni directional b. Unidirectional c. Bi directional d. None of these 2. Which waves are used for cellular phones, satellite and wireless LAN communications? a. Radio waves b. Micro waves c. Infrared waves d. None of these 3. Which waves are used for short range communications such as those between a PC and a peripheral device? a. Micro waves b. Infrared waves c. Radio waves d. All of these 105 CU IDOL SELF LEARNING MATERIAL (SLM)
4. Which waves does wireless transmission of signals use? a. Radio waves b. Micro waves c. Infrared d. All of these 5. Which layer does transmission media lie in? a. Physical b. Network c. Transport d. Application Answers 1-b, 2-b, 3-b, 4-d, 5-a 7.9 REFERENCES References Stamper, D. (1993). Local Area Networks, Addison-Wesley, Reading. MA. Stamper, D. (1991). Business Data Communications, Third Edition. Addison-Wesley, Reading, MA. W, Stallings. (n.d). Data and Computer Communications. Eight Editions. Pearson Education. Textbooks Dr. Sidnie, Feit. (n.d). TCP/IP. Second Edition. TMH Behrouz, A, Forouzan. (n.d). Data communications and Networking. Fourth Edition. Mc-Graw Hill Achyut Godbole, ―Data communications and Networks, TMH. Computer Networks – Andrew Tannenbaum. Websites http://fcit.usf.edu/network/chap2/chap2.htm www.pragsoft.com http://pages.cs.wisc.edu/~tvrdik/7/html/Section7.html http://www.garymgordon.com/misc/tutorials/networking/Lesson2.pdf http://networkworld.com/ns/books/ciscopress/samples/0735700745.pdf 106 CU IDOL SELF LEARNING MATERIAL (SLM)
UNIT- 8: WIRELESS TRANSMISSION PART 2 STRUCTURE 8.0 Learning Objectives 8.1 Introduction 8.2 Radio Transmission 8.2.1 Very High Frequency 8.2.2 Ultra High Frequency 8.3 Satellite Transmission 8.4 Summary 8.5 Keywords 8.6 Learning Activity 8.7 Unit End Questions 8.8 References 8.0 LEARNING OBJECTIVES After studying this unit, you will be able to: Explain radio transmission. Describe very high frequency of radio transmission. Illustrate satellite transmission method. Explain ultra-high frequency of radio transmission. 8.1 INTRODUCTION Unguided media transport electromagnetic waves without using a physical conductor.This type of communication is often referred to as wireless communication. Signalsare normally broadcast through free space and thus are available to anyone who has adevice capable of receiving them.Figure 8.1 shows the part of the electromagnetic spectrum, ranging from 3 kHz to900 THz, used for wireless communication. 107 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 8.1: Electromagnetic spectrumfor wireless communication Unguided signals can travel in different ways from the source to destination and they are i. Ground propagation ii. Sky propagation, and iii. Line-of-sight propagation. Radio waves will almost travel through the atmosphere’s low portion which is nearly hugging the earth. These signals which are low frequencyemanate in all directionsfrom the transmitting antenna. It also followsearth’s curvature. Distance dependson the amount of power in the signal: The greater the power, the greater the distance. Insky propagation, higher-frequency radio waves radiate upward into the ionosphere(the layer of atmosphere where particles exist as ions) where they are reflected back toearth. This type of transmission allows for greater distances with lower output power.In line-or-sight propagation, very high- frequency signals are transmitted in straightlines directly from antenna to antenna. Antennas must be directional, facing each other. Although there is no clear-cut demarcation between radio waves and microwaves, electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called radio waves; waves ranging in frequencies between 1 and 300 GHz are called microwaves. However, the behaviour of the waves, rather than the frequencies, is a better criterion for classification. Radio waves, for the most part, are omnidirectional. When an antenna transmits radio waves, they are propagated in all directions. This means that the sending and receiving antennas do not have to be aligned. A sending antenna sends waves that can be received by any receiving antenna. The omnidirectional property has a disadvantage, too. The radio waves transmitted by one antenna are susceptible to interference by another antenna that may send signals using the same frequency or band. Radio waves, particularly those waves that propagate in the sky mode, can travel long distances. This makes radio waves a good candidate for long-distance broadcasting such as AM radio. 8.2 RADIO TRANSMISSION 108 CU IDOL SELF LEARNING MATERIAL (SLM)
Radio waves, particularly those of low and medium frequencies, can penetrate walls.This characteristic can be both an advantage and a disadvantage. It is an advantagebecause, for example, an AM radio can receive signals inside a building. It is a disadvantagebecause we cannot isolate a communication to just inside or outside a building. Theradio wave band is relatively narrow, just under 1 GHz, compared to the microwaveband. When this band is divided into subbands, the subbands are also narrow, leading to alow data rate for digital communications.Almost the entire band is regulated by authorities (e.g., the FCC in the UnitedStates). Using any part of the band requires permission from the authorities. Omnidirectional Antenna Radio waves use omnidirectional antennas that send out signals in all directions.Based on the wavelength, strength, and the purpose of transmission, we can have severaltypes of antennas. Figure 8.2 shows an omnidirectional antenna. Figure 8.2: Omnidirectional antenna Radio waves are used for multicast communications, such as radio and television, and paging systems. Applications The omnidirectional characteristics of radio waves make them useful for multicasting,in which there is one sender but many receivers. AM and FM radio, television, maritimeradio, cordless phones, and paging are examples of multicasting. 8.2.1 Very High Frequency Very high frequency (VHF) is the ITUdesignationfor the range of radio frequencyelectromagnetic waves(radio waves) from 30 to 300 megahertz(MHz), with corresponding wavelengths of ten meters to one meter. Frequencies immediately below VHF are denoted high frequency(HF), and the next higher frequencies are known as ultra-high frequency(UHF). 109 CU IDOL SELF LEARNING MATERIAL (SLM)
VHF radio waves propagate mainly by line-of-sight, so they are blocked by hills and mountains, although due to refraction they can travel somewhat beyond the visual horizon out to about 160 km (100 miles). Common uses for radio waves in the VHF band are Digital Audio Broadcasting (DAB) and FM radio broadcasting, television broadcasting, two- way land mobile radio systems (emergency, business, private use and military), long range data communication up to several tens of kilometres with radio modems, amateur radio, and marine communications. Air traffic control communications and air navigation systems (e.g. VOR & ILS) work at distances of 100 kilometres (62 mi) or more to aircraft at cruising altitude. Propagation Characteristics Radio waves in the VHF band propagate mainly by line-of-sightand ground-bounce paths; unlike in the HFband there is only some reflection at lower frequencies from the ionosphere(sky wavepropagation). They do not follow the contour of the Earth as ground wavesand so are blocked by hills and mountains, although because they are weakly refracted (bent) by the atmosphere they can travel somewhat beyond the visual horizon out to about 160 km (100 miles). They can penetrate building walls and be received indoors, although in urban areas reflections from buildings cause multipath propagation, which can interfere with television reception. Atmospheric radio noise and interference (RFI) from electrical equipment is less of a problem in this and higher frequency bands than at lower frequencies. The VHF band is the first band at which efficient transmitting antennas are small enough that they can be mounted on vehicles and portable devices, so the band is used for two-way land mobile radio systems, such as walkie-talkies, and two way radio communication with aircraft (Air band) and ships (marine radio). Occasionally, when conditions are right, VHF waves can travel long distances by tropospheric ducting due to refraction by temperature gradients in the atmosphere. 8.2.2 Ultra High Frequency Ultra high frequency (UHF) is the ITUdesignation for radio frequencies in the range between 300 megahertz (MHz) and 3 gigahertz (GHz), also known as the decimetre band as the wavelengths range from one meter to one tenth of a meter (one decimetre). Radio waves with frequencies above the UHF band fall into the super-high frequency (SHF) or microwavefrequency range. Lower frequency signals fall into the VHF (very high frequency) or lower bands. UHF radio waves propagate mainly by line of sight; they are blocked by hills and large buildings although the transmission through building walls is strong enough for indoor reception. They are used for television broadcasting, cell phones, satellite communication including GPS, personal radio services including Wi-Fi and Bluetooth, walkie-talkies, cordless phones, and numerous other applications. 110 CU IDOL SELF LEARNING MATERIAL (SLM)
Propagation Characteristics Radio waves in the UHF band travel almost entirely by line-of-sight propagation(LOS) and ground reflection; unlike in the HFband there is little to no reflection from the ionosphere (skywavepropagation), or ground wave. UHF radio waves are blocked by hills and cannot travel beyond the horizon, but can penetrate foliage and buildings for indoor reception. Since the wavelengthsof UHF waves are comparable to the size of buildings, trees, vehicles and other common objects, reflection and diffractionfrom these objects can cause fading due to multipath propagation, especially in built-up urban areas. Atmospheric moisture reduces, or attenuates, the strength of UHF signals over long distances, and the attenuation increases with frequency. UHF TV signals are generally more degraded by moisture than lower bands, such as VHFTV signals. Since UHF transmission is limited by the visual horizon to 30–40 miles (48–64 km) and usually to shorter distances by local terrain, it allows the same frequency channels to be reused by other users in neighbouring geographic areas (frequency reuse). Radio repeaters are used to retransmit UHF signals when a distance greater than the line of sight is required. Occasionally when conditions are right, UHF radio waves can travel long distances by tropospheric ductingas the atmosphere warms and cools throughout the day. 8.3 SATELLITE TRANSMISSION A satellite network is a combination of nodes, some of which are satellites, that providescommunication from one point on the Earth to another. A node in the network can be asatellite, an Earth station, or an end-user terminal or telephone. Although a natural satellite,such as the Moon, can be used as a relaying node in the network, the use of artificialsatellites is preferred because we can install electronic equipment on the satellite to regeneratethe signal that has lost its energy during travel. Another restriction on using naturalsatellites is their distances from the Earth, which create a long delay in communication.Satellite networks are like cellular networks in that they divide the planet into cells.Satellites can provide transmission capability to and from any location on Earth, nomatter how remote. This advantage makes high-quality communication available toundeveloped parts of the world without requiring a huge investment in ground- basedinfrastructure. Orbits An artificial satellite needs to have an orbit the path in which it travels around the Earth.The orbit can be equatorial, inclined, or polar, as shown in figure 8.3. 111 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 8.3: Satellite orbits The period of a satellite, the time required for a satellite to make a complete triparound the Earth, is determined by Kepler's law, which defines the period as a functionof the distance of the satellite from the center of the Earth. Footprint Satellites process microwaves with bidirectional antennas (line-of-sight). Therefore, thesignal from a satellite is normally aimed at a specific area called the footprint. The signalpower at the center of the footprint is maximum. The power decreases as we moveout from the footprint center. The boundary of the footprint is the location where thepower level is at a predefined threshold. Three Categories of Satellites Based on the location of the orbit, satellites can be divided into three categories. GeostationaryEarth orbit (GEO), low-Earth-orbit (LEO) and middle-Earth-orbit (MEO). Figure 8.4 shows the representation. Figure 8.4: Satellite categories Figure 8.5 shows the satellite altitudes with respect to the surface of the Earth.There is only one orbit, at an altitude of 35,786 km for the GEO satellite. MEO satellitesare located at altitudes between 5000 and 15,000 km. LEO satellites are normally belowan altitude of 2000 km. 112 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 8.5: Satellite orbit altitudes One reason for having different orbits is due to the existence of two Van Allenbelts. A Van Allen belt is a layer that contains charged particles. A satellite orbiting inone of these two belts would be totally destroyed by the energetic charged particles.The MEO orbits are located between these two belts. Frequency Bands for Satellite Communication The frequencies reserved for satellite microwave communication are in the gigahertz(GHz) range. Each satellite sends and receives over two different bands. Transmissionfrom the Earth to the satellite is called the uplink. Transmission from the satellite to the Earthis called the downlink. Table 8.1 gives the band names and frequencies for each range. Table 8.1: Satellite frequency bands GEO Satellites Line-of-sight propagation requires that the sending and receiving antennas be lockedonto each other's location at all times (one antenna must have the other in sight). Forthis reason, a satellite that moves faster or slower than the Earth's rotation is usefulonly for short periods. To ensure constant communication, the satellite must move atthe same speed as the Earth so 113 CU IDOL SELF LEARNING MATERIAL (SLM)
that it seems to remain fixed above a certain spot. Suchsatellites are called geostationary.Because orbital speed is based on the distance from the planet, only one orbit can begeostationary. This orbit occurs at the equatorial plane and is approximately 22,000 mifrom the surface of the Earth.But one geostationary satellite cannot cover the whole Earth. One satellite in orbithas line-of-sight contact with a vast number of stations, but the curvature of the Earthstill keeps much of the planet out of sight. It takes a minimum of three satellites equidistant from each other in geostationary Earth orbit (OEO) to provide full global transmission.Figure 8.6 shows three satellites, each 120° from another in geosynchronousorbit around the equator. The view is from the North Pole. Figure 8.6: Satellites in geostationary orbit MEO Satellites Medium-Earth-orbit (MEO) satellites are positioned between the two Van Allenbelts. A satellite at this orbit takes approximately 6-8 hours to circle the Earth. Global Positioning System One example of a MEO satellite system is the Global Positioning System (GPS), constructedand operated by the US Department of Defence, orbiting at an altitude about18,000 km (11,000 mi) above the Earth. The system consists of 24 satellites and is used forland, sea, and air navigation to provide time and locations for vehicles and ships. GPSuses 24 satellites in six orbits, as shown in figure 8.7. The orbits and the locations ofthe satellites in each orbit are designed in such a way that, at any time, four satellitesare visible from any point on Earth. A GPS receiver has an almanac that tells the currentposition of each satellite. 114 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 8.7: Orbits for global positioning system (GPS) satellites Trilateration GPS is based on a principle called trilateration.t on a plane, if weknow our distance from three points; we know exactly where we are. Let us say that weare 10 miles away from point A, 12 miles away from point B, and 15 miles away frompoint C. If we draw three circles with the centres at A, B, and C, we must be somewhereon circle A, somewhere on circle B, and somewhere on circle C. These three circles meetat one single point (if our distances are correct), our position. Figure 8.8 shows theconcept. Figure: 8.8 Trilateration on a plane 115 CU IDOL SELF LEARNING MATERIAL (SLM)
In three-dimensional space, the situation is different. Three spheres meet in twopoints as shown in figure 8.8b. We need at least four spheres to find our exact positionin space (longitude, latitude, and altitude). However, if we have additional facts aboutour location (for example, we know that we are not inside the ocean or somewhere inspace), three spheres are enough, because one of the two points, where the spheres meet,is so improbable that the other can be selected without a doubt.Measuring the Distance The trilateration principle can find our location on the earth ifwe know our distance from three satellites and know the position of each satellite. Theposition of each satellite can be calculated by a GPS receiver (using the predetermined pathof the satellites). The GPS receiver, then, needs to find its distance from at least three GPSsatellites (center of the spheres). Measuring the distance is done using a principle calledone-way ranging. For the moment, let us assume that all GPS satellites and the receiver onthe Earth are synchronized. Each of 24 satellites synchronously transmits a complex signaleach having a unique pattern. The computer on the receiver measures the delay between thesignals from the satellites and its copy of signals to determine the distances to the satellites.Synchronization The previous discussion was based on the assumption that the satellites clock is synchronized with each other and with the receiver's clock. Satellitesuse atomic clock that are precise and can function synchronously with each other. Thereceiver's clock however, is a normal quartz clock (an atomic clock costs more than$50,000), and there is no way to synchronize it with the satellite clocks. There is anunknown offset between the satellite clocks and the receiver clock that introduces acorresponding offset in the distance calculation. Because of this offset, the measureddistance is called a pseudorange.GPS uses an elegant solution to the clock offset problem, by recognizing that the offset'svalue is the same for all satellite being used. The calculation of position becomes findingfour unknowns: the xp YP zp coordinates of the receiver, and common clock offset dt.For finding these four unknown values, we need at least four equations. This means thatwe need to measure pseudo ranges from four satellites instead of three. If we call the four measured pseudoranges PRI, PR2, PR3 and PR4 and the coordinates of each satelliteXi, yj, and Zj (for i =1 to 4), we can find the four previously mentioned unknown valuesusing the following four equations.The coordinates used in the above formulas are in an Earth-Centred Earth-Fixed(ECEF) reference frame, which means that the origin of the coordinate space is at thecenter of the Earth and the coordinate space rotate with the Earth. This implies that theECEF coordinates of a fixed point on the surface of the earth do not change. Application GPS is used by military forces. For example, thousands of portable GPSreceivers were used during the Persian Gulf War by foot soldiers, vehicles, and helicopters.Another use of GPS is in navigation. The driver of a car can find the location ofthe car. The driver can then consult a database in the memory of the automobile to bedirected to the destination. In other words, GPS gives the location of the car, and thedatabase uses this information to find a path to the destination. A very interesting applicationis clock synchronization. As we mentioned 116 CU IDOL SELF LEARNING MATERIAL (SLM)
previously, the IS-95 cellular telephonesystem uses GPS to create time synchronization between the base stations. LEO Satellites Low-Earth-orbit (LEO) satellites have polar orbits. The altitude is between 500 and2000 km, with a rotation period of 90 to 120 min. The satellite has a speed of 20,000 to25,000 km/h. An LEO system usually has a cellular type of access, similar to the cellulartelephone system. The footprint normally has a diameter of 8000 km. Because LEOsatellites are close to Earth, the round-trip time propagation delay is normally less than20 ms, which is acceptable for audio communication.An LEO system is made of a constellation of satellites that work together as a network;each satellite acts as a switch. Satellites that are close to each other are connected throughintersatellite links (ISLs). A mobile system communicates with the satellite through a usermobile link (UML). A satellite can also communicate with an Earth station (gateway)through a gateway link (GWL). Figure 8.9 shows a typical LEO satellite network. Figure 8.9: LEO satellite system LEO satellites can be divided into three categories: little LEOs, big LEOs, and broadbandLEOs. The little LEOs operate less than 1 GHz. They are mostly used for low- data-ratemessaging. The big LEOs operate between 1 and 3 GHz. Globalstar and Iridium systemsare examples of big LEOs. The broadband LEOs provide communication similar to fibreopticnetworks. The first broadband LEO system was Teledesic. Global star Global star is another LEO satellite system. The system uses 48 satellites in six polarorbits with each orbit hosting eight satellites. The orbits are located at an altitude ofalmost 1400 LAN.The Global star system is similar to the Iridium system; the main difference is the relayingmechanism. Communication between two distant users in the Iridium system requiresrelaying between several satellites; Global star communication requires both satellites andEarth stations, which means that ground stations can create more powerful signals. 117 CU IDOL SELF LEARNING MATERIAL (SLM)
Teledesic Teledesic is a system of satellites that provides fibre-optic-like (broadband channels,low error rate, and low delay) communication. Its main purpose is to provide broadbandInternet access for users allover the world. It is sometimes called \"Internet in the sky.\"The project was started in 1990 by Craig McCaw and Bill Gates; later, other investorsjoined the consortium. The project is scheduled to be fully functional in the near future.Constellation Teledesic provides 288 satellites in 12 polar orbits with each orbithosting 24 satellites. The orbits are at an altitude of 1350 LAN, as shown in figure 8.10. Figure 8.10: Teledesic Communication The system provides three types of communication. Inter-satellite communicationallows eight neighbouring satellites to communicate with one another. Communicationis also possible between a' satellite and an Earth gateway station. Users cancommunicate directly with the network using terminals. Earth is divided into tens of thousandsof cells. Each cell is assigned a time slot, and the satellite focuses its beam to the cellat the corresponding time slot. The terminal can send data during its time slot. A terminalreceives all packets intended for the cell, but selects only those intended for its address. Bands Transmission occurs in the Ka bands. Data Rate The data rate is up to 155 Mbps for the uplink and up to 1.2 Gbps for thedownlink. 8.4 SUMMARY A satellite network uses satellites to provide communication between any points on Earth. A geostationary Earth orbit CGEO) is at the equatorial plane and revolves in phase with Earth. 118 CU IDOL SELF LEARNING MATERIAL (SLM)
Global Positioning System CGPS) satellites are medium-Earth-orbit (MEO) satellites that provide time and location information for vehicles and ships. Iridium satellites are low-Earth-orbit (LEO) satellites that provide direct universal voice and data communications for handheld terminals. Teledesic satellites are low-Earth-orbit satellites that will provide universal broadband Internet access Radio waves will almost travel through the atmosphere’s low portion which is nearly hugging the earth. These signals which are low frequency emanate in all directions from the transmitting antenna. It also follows earth’s curvature. Distance depends on the amount of power in the signal: The greater the power, the greater the distance. In sky propagation, higher-frequency radio waves radiate upward into the ionosphere (the layer of atmosphere where particles exist as ions) where they are reflected back to earth. This type of transmission allows for greater distances with lower output power. In line-or-sight propagation, very high-frequency signals are transmitted in straight lines directly from antenna to antenna. Antennas must be directional, facing each other. Radio waves use omnidirectional antennas that send out signals in all directions. Based on the wavelength, strength, and the purpose of transmission, we can have several types of antennas. Radio waves are used for multicast communications, such as radio and television, and paging systems. VHF radio waves propagate mainly by line-of-sight, so they are blocked by hills and mountains, although due to refraction they can travel somewhat beyond the visual horizon out to about 160 km (100 miles). Common uses for radio waves in the VHF band are Digital Audio Broadcasting (DAB) and FM radio broadcasting, television broadcasting, two-way land mobile radio systems (emergency, business, private use and military), long range data communication up to several tens of kilometres with radio modems, amateur radio, and marine communications. Air traffic control communications and air navigation systems (e.g. VOR & ILS) work at distances of 100 kilometres (62 mi) or more to aircraft at cruising altitude. UHF radio waves are blocked by hills and cannot travel beyond the horizon, but can penetrate foliage and buildings for indoor reception. Since the wavelengths of UHF waves are comparable to the size of buildings, trees, vehicles and other common objects, reflection and diffractionfrom these objects can cause fading due to multipath propagation, especially in built-up urban areas. Another restriction on using natural satellites is their distances from the Earth, which create a long delay in communication. Satellite networks are like cellular networks in that they divide the planet into cells. Satellites can provide transmission capability to and from any location on Earth, no matter how remote. This advantage makes high- 119 CU IDOL SELF LEARNING MATERIAL (SLM)
quality communication available to undeveloped parts of the world without requiring a huge investment in ground-based infrastructure. 8.5 KEYWORDS Digital Signal- It is a signal that represents data as a sequence of discrete values.A digital signal is a signal that is being used to represent data as a sequence of discrete values; at any given time it can only take on, at most, one of a finite number of values Periodic Signal – Signal which repeats itself after a fixed time period are called periodic signal. Sine wave is an example.A periodic signal is one that repeats the sequence of values exactly after a fixed length of time, known as the period. ... Examples of periodic signals include the sinusoidal signals and periodically repeated non-sinusoidal signals, such as the rectangular pulse sequences used in radar. Non-periodic Signal – A signal which does not repeat itself after a fixed time period are called periodic signal.Non-periodic signals (also known as aperiodic signals), unlike periodic signals, do not have just one particular frequency. Instead, they are spread out over a continuous range of frequencies. Low-Pass Channel - A low pass channel is a communication channel that can transfer frequency that is very near zero.A baseband channel or low pass channel (or system, or network) is a communication channel that can transfer frequencies that are very near zero. Frequency division multiplexing (FDM) allows an analogue telephone wire to carry a baseband telephone call, concurrently as one or several carrier- modulated telephone calls. Nyquist Bit Rate- Its formula gives the upper bound for the data rate of transmission system by calculating the bit rate directly from the number of signal levels and the bandwidth of the system. 8.6 LEARNING ACTIVITY 1. Use Kepler's formula to check the accuracy of a given period and altitude for aGPS satellite. ___________________________________________________________________________ ___________________________________________________________________________ 2. Use Kepler's formula to check the accuracy of a given period and altitude for a Global star satellite. ___________________________________________________________________________ ___________________________________________________________________________ 120 CU IDOL SELF LEARNING MATERIAL (SLM)
8.7 UNIT END QUESTIONS A. Descriptive Questions Short Questions: 1. Define foot prints. 2. What are the three categories of satellite? 3. Which type of orbit does a GEO satellite have? Explain your answer. 4. What is very high frequency radio transmission? 5. What are the benefits of ultra-high frequency radio transmission? Long Questions: 1. What is the purpose of GPS? 2. What is the relationship between the Van Allen belts and satellites? 3. What is the main difference between Iridium and Global star? 4. Explain satellite transmission. 5. Explain radio transmission. B. Multiple choice Questions 1. What are radio waves? a. Omni directional b. Uni directional c. Bi directional d. None of these 2. Which channel is required for a base band transmission of a digital signal to be possible? a. Low pass b. Band pass c. Low rate d. High rate 3. When does available channel cannot send a digital signal directly? 121 a. Low pass b. Bandpass CU IDOL SELF LEARNING MATERIAL (SLM)
c. Low rate d. High rate 4. What is the lowest range of frequency of radio communication where it’s upper frequency being300 GHz? a. 3KHz b. 3MHz c. 3GHz d. None of these 5. In which range does FM radio uses its frequencies? a. LF b. MF c. HF d. VHF Answers 1-a, 2-a, 3- b, 4-a, 5-d 8.8 REFERENCES References Zitsen, W. (1990) ‗Metropolitan Area Networks: Taking LANs into the Public, Network, ‘Telecommunications, pp. 53-60. Black, U. (1989), Data Networks: Concepts, Theory, and Practice. NJ. Prentice Hall, Englewood Cliffs. Gitlin, R. D. Hayes, J. F & Weinstein, S. B. (1992). Data Communication Principles, Plenum, New York, NY. Textbooks Hughes, L. (1992) Data Communications, McGraw-Hill, NY. Kessler, G. and Train, D. (1992) Metropolitan Area Networks: Concepts. Standards, and Service, McGraw-Hill, NY. Websites https://whatis.techtarget.com/definition/Open-Data-Link-Interface-ODI https://ecomputernotes.com/computernetworkingnotes/multiple-access/what-is-wired- transmission-type-of-wired-transmission 122 CU IDOL SELF LEARNING MATERIAL (SLM)
https://www.techopedia.com/definition/30527/switched-line 123 CU IDOL SELF LEARNING MATERIAL (SLM)
UNIT – 9: WIRELESS TRANSMISSION PART 3 STRUCTURE 9.0 Learning Objectives 9.1 Introduction 9.2 Communication Switching Techniques 9.3 Circuit Switching 9.3.1 Networks 9.4 Message Switching 9.4.1 Concepts 9.5 Packet Switching 9.5.1 Principles 9.6 Summary 9.7 Keywords 9.8 Learning Activity 9.9 Unit End Questions 9.10 References 9.0 LEARNING OBJECTIVES After studying this unit, you will be able to: Explain the different communication switching techniques. Describe circuit switching method. Illustrate message switching techniques. Explain packet switching and its principles. 9.1 INTRODUCTION A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM. It shows a trivial circuit-switched network with four switches and four links. Each link is divided into n (n is 3 in the figure) channels by using FDM or TDM. We have explicitly shown the multiplexing symbols to emphasize the division 124 CU IDOL SELF LEARNING MATERIAL (SLM)
of the link into channels even though multiplexing can be implicitly included in the switch fabric. The end systems, such as computers or telephones, are directly connected to a switch. We have shown only two end systems for simplicity. When end system A needs to communicate with end system M, system A needs to request a connection to M that must be accepted by all switches as well as by M itself. This is called the setup phase; a circuit (channel) is reserved on each link, and the combination of circuits or channels defines the dedicated path. After the dedicated path made of connected circuits (channels) is established, data transfer can take place. After all data have been transferred, the circuits are tom down. We need to emphasize several points here. Circuit switching takes place at the physical layer. Before starting communication, the stations must make a reservation for the resources to be used during the communication. These resources, such as channels (bandwidth in FDM and time slots in TDM), switch buffers, switch processing time, and switch input/output ports, must remain dedicated during the entire duration of data transfer until the teardown phase. Data transferred between the two stations are not packetized (physical layer transfer of the signal). The data are a continuous flow sent by the source station and received by the destination station, although there may be periods of silence. There is no addressing involved during data transfer. The switches route the data based on their occupied band (FDM) or time slot (TDM). Of course, there is end-to-end addressing used during the setup phase, as we will see shortly. In circuit switching, the resources need to be reserved during the setup phase; the resources remain dedicated for the entire durationof data transfer until the teardown phase The actual communication in a circuit-switched network requires three phases: connection setup, data transfer, and connection teardown.Before the two parties (or multiple parties in a conference call) can communicate, a dedicated circuit (combination of channels in links) needs to be established. The end systems are normally connected through dedicated lines to the switches, so connection setup means creating dedicated channels between the switches. When system a needs to connect to system M, it sends a setup request that includes the address of system M, to switch I. Switch I finds a channel between itself and switch IV that can be dedicated for this purpose. Switch I then sends the request to switch IV, which finds a dedicated channel between itself and switch III. Switch III informs system M of system A's intention at this time. In the next step to making a connection, an acknowledgment from system M needs to be sent in the opposite direction to system A. Only after system A receives this acknowledgment is the connection established. Note that end-to-end addressing is required for creating a connection between thetwo end systems. These can be, for example, the addresses of the computers assigned by the administrator in a TDM network, or telephone numbers in an FDM network. 125 CU IDOL SELF LEARNING MATERIAL (SLM)
9.2 COMMUNICATION SWITCHING TECHNIQUES Switching is process to forward packets coming in from one port to a port leading towards the destination. When data comes on a port it is called ingress, and when data leaves a port or goes out it is called egress. A communication system may include number of switches and nodes. At broad level, switching can be divided into two major categories. Connectionless: The data is forwarded on behalf of forwarding tables. No previous handshaking is required and acknowledgements are optional. Connection Oriented: Before switching data to be forwarded to destination, there is a need to pre-establish circuit along the path between both endpoints. Data is then forwarded on that circuit. After the transfer is completed, circuits can be kept for future use or can be turned down immediately. 9.3 CIRCUIT SWITCHING When two nodes communicate with each other over a dedicated communication path, it is called circuit switching. There is a need of pre-specified route from which data travels and no other data is permitted. In circuit switching to transfer the data, circuit must be established so that the data transfer can take place. Circuits can be permanent or temporary. Applications which use circuit switching may have to go through three phases: Establish a circuit Transfer the data Disconnect the circuit 126 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 9.1: Circuit Switching Circuit switching was designed for voice applications. Telephone is the best suitable example of circuit switching. Before a user can make a call, a virtual path between caller and callee is established over the network. 9.3.1 Networks A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM. Figure 9.2 shows a trivial circuit-switched network with four switches and four links. Each link is divided into n (n is 3 in the figure) channels by using FDM or TDM. 127 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 9.2: A trivial circuit-switched network We have explicitly shown the multiplexing symbols to emphasize the division of the link into channels even though multiplexing can be implicitly included in the switch fabric. The end systems, such as computers or telephones, are directly connected to a switch. We have shown only two end systems for simplicity. When end system A needs to communicate with end system M, system A needs to request a connection to M that must be accepted by all switches as well as by M itself. This is called the setup phase; a circuit (channel) is reserved on each link, and the combination of circuits or channels defines the dedicated path. After the dedicated path made of connected circuits (channels) is established, data transfer can take place. After all data have been transferred, the circuits are tom down. We need to emphasize several points here. Circuit switching takes place at the physical layer. Before starting communication, the stations must make a reservation for the resources to be used during the communication. These resources, such as channels (bandwidth in FDM and time slots in TDM), switch buffers, switch processing time, and switch input/output ports, must remain dedicated during the entire duration of data transfer until the teardown phase. Data transferred between the two stations are not packetized (physical layer transfer of the signal). The data are a continuous flow sent by the source station and received by the destination station, although there may be periods of silence. There is no addressing involved during data transfer. The switches route the data based on their occupied band (FDM) or time slot (TDM). Of course, there is end-to end addressing used during the setup phase. 9.4 MESSAGE SWITCHING 128 CU IDOL SELF LEARNING MATERIAL (SLM)
This technique was somewhere in middle of circuit switching and packet switching. In message switching, the whole message is treated as a data unit and is switching / transferred in its entirety. 9.4.1 Concepts A switch working on message switching, first receives the whole message and buffers it until there are resources available to transfer it to the next hop. If the next hop is not having enough resource to accommodate large size message, the message is stored and switch waits. Figure 9.3: Messageswitching technique This technique was considered substitute to circuit switching. As in circuit switching the whole path is blocked for two entities only. Message switching is replaced by packet switching. Message switching has the following drawbacks. Every switch in transit path needs enough storage to accommodate entire message. Because of store-and-forward technique and waits included until resources are available, message switching is very slow. Message switching was not a solution for streaming media and real-time applications. 9.5 PACKET SWITCHING Shortcomings of message switching gave birth to an idea of packet switching. The entire message is broken down into smaller chunks called packets. The switching information is added in the header of each packet and transmitted independently. It is easier for intermediate networking devices to store small size packets and they do not takeresources either on carrier path or in the internal memory of switches. 129 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 9.4: Packet switching Packet switching enhances line efficiency as packets from multiple applications can be multiplexed over the carrier. The internet uses packet switching technique. Packet switching enables the user to differentiate data streams based on priorities. Packets are stored and forwarded according to their priority to provide quality of service. 9.5.1 Principles Packet switching is a connectionless network switching technique. Here, the message is divided and grouped into a number of units called packets that are individually routed from the source to the destination. There is no need to establish a dedicated circuit for communication. Process Each packet in a packet switching technique has two parts: a header and a payload. The header contains the addressing information of the packet and is used by the intermediate routers to direct it towards its destination. The payload carries the actual data. A packet is transmitted as soon as it is available in a node, based upon its header information. The packets of a message are not routed via the same path. So, the packets in the message arrive in the destination out of order. It is the responsibility of the destination to reorder the packets in order to retrieve the original message. 130 CU IDOL SELF LEARNING MATERIAL (SLM)
The process is diagrammatically represented in the following figure. Here the message comprises of four packets, A, B, C and D, which may follow different routes from the sender to the receiver. Figure 9.5: Packet switching 9.6 SUMMARY Switching is process to forward packets coming in from one port to a port leading towards the destination. When data comes on a port it is called ingress, and when data leaves a port or goes out it is called egress. A communication system may include number of switches and nodes. At broad level, switching can be divided into two major categories. When two nodes communicate with each other over a dedicated communication path, it is called circuit switching. There is a need of pre-specified route from which data travels and no other data is permitted. In circuit switching to transfer the data, circuit must be established so that the data transfer can take place. A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM. 131 CU IDOL SELF LEARNING MATERIAL (SLM)
Before starting communication, the stations must make a reservation for the resources to be used during the communication. These resources, such as channels (bandwidth in FDM and time slots in TDM), switch buffers, switch processing time, and switch input/output ports, must remain dedicated during the entire duration of data transfer until the teardown phase. Data transferred between the two stations are not packetized (physical layer transfer of the signal). The data are a continuous flow sent by the source station and received by the destination station, although there may be periods of silence. A switch working on message switching, first receives the whole message and buffers it until there are resources available to transfer it to the next hop. If the next hop is not having enough resource to accommodate large size message, the message is stored and switch waits. This technique was considered substitute to circuit switching. As in circuit switching the whole path is blocked for two entities only. Message switching is replaced by packet switching. Message switching has the following drawbacks. Every switch in transit path needs enough storage to accommodate entire message. Because of store- and-forward technique and waits included until resources are available, message switching is very slow. Message switching was not a solution for streaming media and real-time applications. Shortcomings of message switching gave birth to an idea of packet switching. The entire message is broken down into smaller chunks called packets. The switching information is added in the header of each packet and transmitted independently. 9.7 KEYWORDS Switching- Switched communication networks are those in which the data transferred from source to destination is routed between various intermediate nodes. Peak Amplitude – The peak amplitude of a signal is the absolute value of is highest intensity proportional to the energy it carries. Phase – It is not a property of just one RF signal but instead involves the relationship between two or more signals that share the same frequency. Processing Delay- Time taken to process the data packet by processor. Ie, the time required by intermediate routers to decide whether to forward the packet, update TTL perform header check some calculations. Propagation Speed – In computer network, it is the amount of time it takes for the head of the signal to travel from the centre to the receiver. 132 CU IDOL SELF LEARNING MATERIAL (SLM)
9.8 LEARNING ACTIVITY 1. Create a survey and compare the features of different communication switching techniques. ___________________________________________________________________________ ___________________________________________________________________________ 2. Prepare the advantages of message switching and packet switching and compare the two. ___________________________________________________________________________ ___________________________________________________________________________ 9.9 UNIT END QUESTIONS A. Descriptive Questions Short Questions 1. What are the three phases do the application use the circuit switching? 2. What are the drawbacks of message switching? 3. What are the uses of packet switching? 4. What is a network? 5. What are the two categories of switching? Long Questions 1. Explain the different communication switching techniques. 2. Describe circuit switching method. 3. Illustrate message switching techniques. 4. Explain packet switching and its principles. 5. Explain the applications of packet switching. B. Multiple Choice Questions 1. What is termed as the difference between the highest and the lowest frequencies contained in that signal for a composite signal? a. Frequency b. Period c. Bandwidth 133 CU IDOL SELF LEARNING MATERIAL (SLM)
d. Amplitude 2. How is a switched WANnormally implemented? a. Virtual circuit b. Datagram c. Circuit switched d. None of these 3. Identify the address on which a switch in a datagram network is based on which uses a routing table. a. Source b. Destination c. Local d. None of these 4. Which category does a local telephone network belong to? a. Packet switched b. Circuit switched c. Message switched d. All of these 5. Which is the type of switching that uses the entire capacity of a dedicated link? a. Circuit switching b. Datagram packet switching c. Virtual circuit packet switching d. Message switching Answers 1-c, 2-a, 3-b, 4—b, 5-a 9.10 REFERENCES References Stone, H. (1982). Microcomputer Interfacing, Addison-Wesley, Reading, MA. 134 CU IDOL SELF LEARNING MATERIAL (SLM)
Tanenbaum, A. (1989), Computer Networks, Second Edition, NJ. Prentice Hall, Englewood Cliffs. Viniotis Y. and Onvural R. (editors) (1993) Asynchronous Transfer Mode, Networks, Plenum, New York, NY. Textbooks Van, Duuren, J. Schoute, F & Kastelein, P. (1992) Telecommunication. Networks and Services, Addison-Wesley, Reading, MA. White, G. (1992) Internetworking and Addressing, McGraw-Hill, NY. Websites www.cisco.com/go/ipv6 http://www.cisco.com/en/US/technologies/tk648/tk872/technologies_white_paper090 0aecd80260042.pdf http://www.cisco.com/en/US/products/ps6350/prod_command_reference_list.html http://www.cisco.com/en/US/docs/ios/ipv6/command/reference/ipv6_book.html http://www.cisco.com/en/US/docs/ios/ipv6/configuration/guide/12_4/ipv6_12_4_boo k.html http://www.cisco.com/en/US/tech/tk828/technologies_white_paper09186a0080203e9 0.s 135 CU IDOL SELF LEARNING MATERIAL (SLM)
UNIT 10 – NETWORK REFERENCE MODELS PART 1 STRUCTURE 10.0 Learning Objectives 10.1 Introduction 10.2 Network Topologies 10.3 OSI References Model 10.3.1 Layers of OSI Model 10.4 Summary 10.5 Keywords 10.6 Learning Activity 10.7 Unit End Questions 10.8 References 10.0 LEARNING OBJECTIVES After studying this unit, you will be able to: Describe different network topologies. Explain about OSI reference model. Explain about different layers of OSI layers. Illustrate network layer. 10.1 INTRODUCTION Network Reference Models A computer network connects two or more devices together to share information and services. Multiple networks connected together form an internetwork. Internetworking present challenges - interoperating between products from different manufacturers requires consistent standards. Network reference models were developed to address these challenges. A network reference model serves as a blueprint, detailing how communication between network devices should occur. The two most recognized network reference models are The Open Systems Interconnection (OSI) model The Department of Defence (DoD) model 136 CU IDOL SELF LEARNING MATERIAL (SLM)
Without the framework that network models provide, all network hardware and software would have been proprietary. Organizations would have been locked into a single vendor’s equipment, and global networks like the Internet would have been impractical, if not impossible. Network models are organized into layers, with each layer representing a specific networking function. These functions are controlled by protocols, which are rules that govern end-to-end communication between devices. Protocols on one layer will interact with protocols on the layer above and below it, forming a protocol suite or stack. The TCP/IP suite is the most prevalent protocol suite, and is the foundation of the Internet. A network model is not a physical entity – there is no OSI device. Manufacturers do not always strictly adhere to a reference model’s blueprint, and thus not every protocol fits perfectly within a single layer. Some protocols can function across multiple layers. Physical Structures Before discussing networks, we need to define some network attributes. Type of Connection A network is two or more devices connected through links. A link is a communications pathway that transfers data from one device to another. For visualization purposes, it is simplest to imagine any link as a line drawn between two points. For communication to occur, two devices must be connected in some way to the same link at the same time. There are two possible types of connections: point-to-point and multipoint. Point-to-Point a point- to-point connection provides a dedicated link between two devices. The entire capacity of the link is reserved for transmission between those two devices. Most point-to-point connections use an actual length of wire or cable to connect the two ends, but other options, such as microwave or satellite links, are also possible (see figure 10.1a). When you change television channels by infrared remote control, you are establishing a point-to-point connection between the remote control and the television's control system. Multipoint A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link (see figure 10.1b). In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection. 137 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 10.1: Types of connections: point-to-point and multipoint 10.2 NETWORK TOPOLOGIES Physical Topology The term physical topology refers to the way in which a network is laid out physically. Two or more devices connect to a link; two or more links form a topology. The topology of a network is the geometric representation of the relationship of all the links and linking devices (usually called nodes) to one another. There are four basic topologies possible: mesh, star, bus, and ring (see figure 10.2). Figure 10.2: Categories of topology Mesh 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 138 CU IDOL SELF LEARNING MATERIAL (SLM)
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. To accommodate that many links, every device on the network must have n – 1 input/output (VO) ports (see figure 10.3) to be connected to the other n - 1 stations. Figure 10.3: A fully connected mesh topology (five devices) 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 must be shared by multiple devices. Second, a mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system. 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 network 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 139 CU IDOL SELF LEARNING MATERIAL (SLM)
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 (see figure 10.4) Figure 10.4: A star topology connecting four stations 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. One big 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 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). High-speed LANs often use a star topology with a central hub. Bus Topology The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network (see figure 10.5). Figure 10.5: A bus topology connecting three stations 140 CU IDOL SELF LEARNING MATERIAL (SLM)
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 paththen 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 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 include difficult 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. In addition, 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 now for reasons. 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 (see figure 10.6). Figure 10.6: A ring topology connecting six stations 141 CU IDOL SELF LEARNING MATERIAL (SLM)
A ring is relatively easy to install and reconfigure. Each device is linked to only itsimmediate neighbours (either physically or logically). To add or delete a device requireschanging 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 notreceive a signal within a specified period, it can issue an alarm. The alarm alerts thenetwork operator to the problem and its location.However, unidirectional traffic can be a disadvantage. In a simple ring, a break inthe ring (such as a disabled station) can disable the entire network. This weakness canbe solved by using a dual ring or a switch capable of closing off the break. Ring Topology It was prevalent when IBM introduced its local-area network Token Ring. Today, the need for higher-speed LANs has made this topology less popular. Hybrid Topology A network can be hybrid. For example, we can have a main star topology with each branch connecting several stations in a bus topology as shown in figure 10.7 Figure 10.7: A hybrid topology: a star backbone with three bus networks 10.3 OSI REFERENCES MODEL The Open Systems Interconnection (OSI) model was developed by the International Organization for Standardization (ISO), and formalized in 1984. It provided the first framework governing how information should be sent across a network.The OSI model consists of seven layers, each corresponding to a specific network function: Note that the bottom layer is Layer 1. Various mnemonics make it easier to remember the order of the OSI model’s layers. Established in 1947, the International Standards Organization (ISO) is a multinationalbody dedicated to worldwide agreement on international standards. An ISO standardthat covers all aspects of network communications is the Open 142 CU IDOL SELF LEARNING MATERIAL (SLM)
Systems Interconnectionmodel. It was first introduced in the late 1970s. An open system is a set of protocols thatallows any two different systems to communicate regardless of their underlying architecture.The purpose of the OSI model is to show how to facilitate communicationbetween different systems without requiring changes to the logic of the underlying hardwareand software. The OSI model is not a protocol; it is a model for understanding anddesigning a network architecture that is flexible, robust, and interoperable. Figure 10.8: OSI model layers The OSI model is a layered framework for the design of network systems thatallows communication between all types of computer systems. It consists of seven separatebut related layers, each of which defines a part of the process of moving informationacross a network (see figure 10.8). An understanding of the fundamentals of the OSImodel provides a solid basis for exploring data communications. Layered Architecture The OSI model is composed of seven ordered layers: physical (layer 1), data link (layer 2),network (layer 3), transport (layer 4), and session (layer 5)presentation (layer 6), andapplication (layer 7). Figure 10.9 shows the layers involved when a message is sent fromdevice A to device B. As the message travels from A to B, it may pass through manyintermediate nodes. These intermediate nodes usually involve only the first three layersof the OSI model. 143 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 10.9: The interaction between layers in the OSI model In developing the model, the designers distilled the process of transmitting data toits most fundamental elements. They identified which networking functions had relateduses and collected those functions into discrete groups that became the layers. Eachlayer defines a family of functions distinct from those of the other layers. By definingand localizing functionality in this fashion, the designers created an architecture that isboth comprehensive and flexible. Most importantly, the OSI model allows completeinteroperability between otherwise incompatible systems.Within a single machine, each layer calls upon the services of the layer just belowit. Layer 3, for example, uses the services provided by layer 2 and provides services forlayer 4. Between machines, layer x on one machine communicates with layer x onanother machine. This communication is governed by an agreed-upon series of rulesand conventions called protocols. The processes on each machine that communicate ata given layer are called peer-to-peer processes. Communication between machines istherefore a peer-to-peer process using the protocols appropriate to a given layer. Peer-to-Peer Processes At the physical layer, communication is direct: In figure 10.9, device a sends a streamof bits to device B (through intermediate nodes). At the higher layers, however, communicationmust move down through the layers on device A, over to device B, and then back up through the 144 CU IDOL SELF LEARNING MATERIAL (SLM)
layers. Each layer in the sending device adds its own informationto the message it receives from the layer just above it and passes the whole package tothe layer just below it. At layerthe entire package is converted to a form that can be transmitted to thereceiving device. At the receiving machine, the message is unwrapped layer by layer,with each process receiving and removing the data meant for it. For example, layer 2removes the data meant for it, and then passes the rest to layer 3. Layer 3 then removes thedata meant for it and passes the rest to layer 4, and so on. Interfaces between Layers The passing of the data and network information down through the layers of the sendingdevice and back up through the layers of the receiving device is made possible byan interface between each pair of adjacent layers. Each interface defines the informationand services a layer must provide for the layer above it. Well-defined interfaces andlayer functions provide modularity to a network. As long as a layer provides theexpected services to the layer above it, the specific implementation of its functions canbe modified or replaced without requiring changes to the surrounding layers. Organization ofthe Layers The seven layers can be thought of as belonging to three subgroups. Layers 1, 2, and3- physical, data link, and network-are the network support layers; they deal with the physical aspects of moving data from one device to another (such as electricalspecifications, physical connections, physical addressing, and transport timing andreliability). Layers 5, 6, and 7- session, presentation, and application-can bethought of as the user support layers; they allow interoperability among unrelatedsoftware systems. Layer 4, the transport layer, links the two subgroups and ensuresthat what the lower layers have transmitted is in a form that the upper layers can use.The upper OSI layers are almost always implemented in software; lower layers are acombination of hardware and software, except for the physical layer, which is mostlyhardware.In figure 10.10, which gives an overall view of the OSI layers, D7 means the dataunit at layer 7, D6 means the data unit at layer 6, and so on. The process starts at layer7 (the application layer), then moves from layer to layer in descending, sequentialorder. At each layer, a header, or possibly a trailer, can be added to the data unit.Commonly, the trailer is added only at layer 2. When the formatted data unit passesthrough the physical layer (layer 1), it is changed into an electromagnetic signal andtransported along a physical link. 145 CU IDOL SELF LEARNING MATERIAL (SLM)
Figure 10.10: An exchange using the OSI model Upon reaching its destination, the signal passes into layer 1 and is transformedback into digital form. The data units then move back up through the OSI layers. Aseach block of data reaches the next higher layer, the headers and trailers attached to it atthe corresponding sending layer are removed, and actions appropriate to that layer aretaken. By the time it reaches layer 7, the message is again in a form appropriate to theapplication and is made available to the recipient. Encapsulation Figure 10.10 reveals another aspect of data communications in the OSI model: encapsulation.A packet (header and data) at level 7 is encapsulated in a packet at level 6. Thewhole packet at level 6 is encapsulated in a packet at level 5, and so on.In other words, the data portion of a packet at level N - 1 carries the whole packet(data and header and maybe trailer) from level N. The concept is called encapsulation;level N - 1 is not aware of which part of the encapsulated packet is data and which partis the header or trailer. For level N - 1, the whole packet coming from level N is treatedas one integral unit. 10.3.1 Layers of OSI Model In this section we briefly describe the functions of each layer in the OSI model. Physical Layer The physical layer coordinates the functions required to carry a bit stream over a physicalmedium. It deals with the mechanical and electrical specifications of the interface andtransmission medium. It also defines the procedures and functions that physical 146 CU IDOL SELF LEARNING MATERIAL (SLM)
devicesand interfaces have to perform for transmission to occur. Figure 10.11 shows the position ofthe physical layer with respect to the transmission medium and the data link layer. Figure 10.11: Physical layer The physical layer is also concerned with the following: Physical characteristics of interfaces and medium. The physical layer definesthe characteristics of the interface between the devices and the transmission medium. It also defines the type of transmission medium. Representation of bits. The physical layer data consists of a stream of bits(sequence of Os or 1s) with no interpretation. To be transmitted, bits must be encoded into signals-- electrical or optical. The physical layer defines the type ofencoding (how Os and I s are changed to signals). Data rate. The transmission rate-the number of bits sent each second-is also defined by the physical layer. In other words, the physical layer defines the durationof a bit, which is how long it lasts. Synchronization of bits. The sender and receiver not only must use the same bitrate but also must be synchronized at the bit level. In other words, the sender andthe receiver clocks must be synchronized. Line configuration. The physical layer is concerned with the connection ofdevices to the media. In a point-to-point configuration, two devices are connectedthrough a dedicated link. In a multipoint configuration, a link is shared amongseveral devices. Physical topology. The physical topology defines how devices are connected tomake a network. Devices can be connected by using a mesh topology (every deviceis connected to every other device), a star topology (devices are connected througha central device), a ring topology (each device is connected to the next, forming aring), a bus topology (every device is on a common link), or a hybrid topology (thisis a combination of two or more topologies). 147 CU IDOL SELF LEARNING MATERIAL (SLM)
Transmission mode. The physical layer also defines the direction of transmissionbetween two devices: simplex, half-duplex, or full-duplex. In simplex mode, onlyone device can send; the other can only receive. The simplex mode is a one-waycommunication. In the half-duplex mode, two devices can send and receive, butnot at the same time. In a full-duplex (or simply duplex) mode, two devices cansend and receive at the same time. Data Link Layer The data link layer transforms the physical layer, a raw transmission facility, to a reliablelink. It makes the physical layer appear error-free to the upper layer (networklayer). Figure 10.12 shows the relationship of the data link layer to the network and physicallayers. Figure 10.12: Data link layer Other responsibilities of the data link layer include the following. Framing The data link layer divides the stream of bits received from the networklayer into manageable data units called frames. Physical addressing. If frames are to be distributed to different systems on thenetwork, the data link layer adds a header to the frame to define the sender and/orreceiver of the frame. If the frame is intended for a system outside the sender'snetwork, the receiver address is the address of the device that connects the networkto the next one. Flow control. If the rate at which the data are absorbed by the receiver is less thanthe rate at which data are produced in the sender, the data link layer imposes a flowcontrol mechanism to avoid overwhelming the receiver. Error control. The data link layer adds reliability to the physical layer by addingmechanisms to detect and retransmit damaged or lost frames. It also uses a mechanismto recognize duplicate frames. Error control is normally achieved through atrailer added to the end of the frame. 148 CU IDOL SELF LEARNING MATERIAL (SLM)
Access control. When two or more devices are connected to the same link, datalink layer protocols are necessary to determine which device has control over thelink at any given time. Figure 10.13 illustrates hop-to-hop (node-to-node) delivery by the data link layer. Figure 10.13: Hop-fa-hop delivery As the figure shows, communication at the data link layer occurs between twoadjacent nodes. To send data from A to F, three partial deliveries are made. First, thedata link layer at A sends a frame to the data link layer at B (a router). Second, the data link layer at B sends a new frame to the data link layer at E. Finally, the data link layerat E sends a new frame to the data link layer at F. Note that the frames that areexchanged between the three nodes have different values in the headers. The frame fromA to B has B as the destination address and A as the source address. The frame from B toE has E as the destination address and B as the source address. The frame from E to Fhas F as the destination address and E as the source address. The values of the trailerscan also be different if error checking includes the header of the frame. Network Layer The network layer is responsible for the source-to-destination delivery of a packet,possibly across multiple networks (links). Whereas the data link layer oversees thedelivery of the packet between two systems on the same network (links), the networklayer ensures that each packet gets from its point of origin to its final destination.If two systems are connected to the same link, there is usually no need for a networklayer. However, if the two systems are attached to different networks (links) withconnecting devices between the networks (links), 149 CU IDOL SELF LEARNING MATERIAL (SLM)
there is often a need for the networklayer to accomplish source-to-destination delivery. Figure 10.14 shows the relationship ofthe network layer to the data link and transport layers. Figure 10.14: Network layer Other responsibilities of the network layer include the following. Logical addressing. The physical addressing implemented by the data link layerhandles the addressing problem locally. If a packet passes the network boundary,we need another addressing system to help distinguish the source and destinationsystems. The network layer adds a header to the packet coming from the upperlayer that, among other things, includes the logical addresses of the sender andreceiver. We discuss logical addresses later in this chapter. Routing. When independent networks or links are connected to create intemetworks(network of networks) or a large network, the connecting devices (called routers or switches) route or switch the packets to their final destination. One of the functionsof the network layer is to provide this mechanism. 150 CU IDOL SELF LEARNING MATERIAL (SLM)
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