BCA_Sem IV_Computer Networks_Second Draft

can easily be encapsulated in a packet with all the error control and addressing information.  A permanent telephone connection between two points set up by a telecommunications common carrier. Not a dedicated cable, a leased line is actually a reserved circuit between two points. Leased lines can span short or long distances. They maintain a single open circuit at all times, as opposed to traditional telephone services that reuse the same lines for many different conversations through a process called “switching.”  Leased lines most commonly are rented by businesses to connect branch offices, because these lines guarantee bandwidth for network traffic. So-called T1 leased lines are common and offer the same data rate as symmetric DSL (1.544 Mbps).  The switching equipment provides a temporary communication path between the two user terminals, giving the two users exclusive use of the link. The communication path provided by the switched line may vary each time a connection is established between two users. Switched lines are commonly used for ordinary voice telephone systems where the telephone company reserves the established physical path between a caller and the called number. 3.6 KEYWORDS  Protocols - It is a set of rules that need to be followed by the communicating parties in order to have successful and reliable data communication. You have already come across protocols such as Ethernet and HTTP.  Telephony - Telephony is a technology which allows voice and/or interactive communication between two points through the usage of appropriate equipment. Analogue sound signals are translated into electrical signals after a communication request is initiated. These electrical signals are converted back to analogue sound signals once received at the destination.  Terminal - A terminal is an electronic communication hardware device that handles the input and display of data. A terminal may be a PC or workstation connected to a network, Voice over Internet Protocol (VOIP) network endpoint, mobile data terminal such as a telematics device, or a text terminal, or textual language interface.  Communication Media - It is the path through which the message travels between source and destination. It is also called medium or link which is either wired or wireless. For example, a television cable, telephone cable, Ethernet cable, satellite link, microwaves, etc.  Private Branch Exchange (PBX) – It is a private telephone network used within an enterprise. It is a business-oriented phone system designed to supply efficient voice 51 CU IDOL SELF LEARNING MATERIAL (SLM)

communications between the users of an organization. Acting as the phone company’s central office within an organization, a PBX works as the exchange point and point for routing calls. 3.7 LEARNING ACTIVITY 1. Create a survey regarding the connection of telephone exchange in your locality. ___________________________________________________________________________ ___________________________________________________________________________ 2. Describe how does switch line work in your office. ___________________________________________________________________________ ___________________________________________________________________________ 3.8 UNIT END QUESTIONS A. Descriptive Questions Short Questions 1. What is termed by wired transmission? 2. Write short note on switch lines. 3. Write a short note on point of presence (POP) 4. Write short note on DSL? 5. Explain leased lines of wired transmission. Long Questions 1. Explain the major components of a telephone network. 2. Explain the differences between data transfer network and signalling network with diagram. 3. Explain different methods of wired transmission. 4. Explain signalling with an illustration.Write its applications. 5. Explain signalling system seven (SS7) using diagram. B. Multiple Choice Questions 1. Which does provide the Internet accessfor transmitting digital data over the wires of a local network? a. Leased line 52 CU IDOL SELF LEARNING MATERIAL (SLM)

b. Digital subscriber line c. Digital signal line d. Digital leased line 2. Which is the category to which the modern telephone network belongs to? a. Digital b. Analogue c. Digital as well as analogue d. None of these 3. Which is the system to in which the original telephone network, which is referred to as the plain old telephone systems (POTS), belong to? a. Digital b. Analogue c. Digital as well as analogue d. None of these 4. How many components are there in the telephone network? a. 2 b. 3 c. 4 d. 5 5. Which protocol is used for signalling the telephone network? a. POP b. SSS c. SS7 d. None of these Answers 1-b, 2-c, 3-b, 4-c, 5-c 53 CU IDOL SELF LEARNING MATERIAL (SLM)

3.9 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://ecomputernotes.com/computernetworkingnotes/multiple-access/what-is-wired- transmission-type-of-wired-transmission  https://www.techopedia.com/definition/30527/switched-line  https://www.c-sharpcorner.com/uploadfile/abhikumarvatsa/basics-of-data- communication-part-1/  https://www.techopedia.com/definition/24871/analogue 54 CU IDOL SELF LEARNING MATERIAL (SLM)

UNIT– 4: WIRED TRANSMISSIONS PART 2 STRUCTURE 4.0 Learning Objectives 4.1 Introduction 4.2 Coaxial Cables-Base Band 4.3 Broadband 4.4 Optical Fibre Transmission 4.5 Summary 4.6 Keywords 4.7 Learning Activity 4.8 Unit End Questions 4.9 Reference 4.0 LEARNING OBJECTIVES After studying this unit, you will be able to:  Explain different transmission media.  Describe the features of Coaxial cable.  Illustrate the importance of optical fibre and its applications. 4.1 INTRODUCTION Transmission media can be defined as physical path between transmitter and receiver in a data transmission system. And it may be classified into two types as shown in figure 4.1. Guided - Transmission capacity depends critically on the medium, the length, and whether the medium is point-to-point or multipoint (e.g. LAN). Examples are co-axial cable, twisted pair, and optical fibre. Unguided - It provides a means for transmitting electro-magnetic signals but do not guide them. Example: Wireless transmission. 55 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 4.1: Classification of the transmission media In this section we shall discuss about the most commonly used guided transmission media such as coaxial cable and optical fibre. With “coax'', the medium consists of a copper core surrounded by insulating material and a braided outer conductor as shown in figure.4.2. The term base band indicates digital transmission (as opposed to broadband analogue). Figure 4.2: Co-axial cable Physical connection consists of metal pin touching the copper core. There are two common ways to connect to a coaxial cable. 1. With vampire taps, a metal pin is inserted into the copper core. A special tool drills a hole into the cable, removing a small section of the insulation, and a special connector is screwed into the hole. The tap makes contact with the copper core. 2. With a T-junction, the cable is cut in half, and both halves connect to the T-junction. A T-connector is analogous to the signal splitters used to hook up multiple TVs to the same cable wire. 56 CU IDOL SELF LEARNING MATERIAL (SLM)

4.2 COAXIAL CABLES-BASE BAND Co-axial cable has superior frequency characteristics compared to twisted-pair and can be used for both analogue and digital signalling. In baseband LAN, the data rates lies in the range of 1 KHz to 20 MHz over a distance in the range of 1 Km. Co-axial cables typically have a diameter of 3/8\". Coaxial cables are used both for baseband and broadband communication. For broadband CATV application coaxial cable of 1/2\" diameter and 75Ω impedance is used. This cable offers bandwidths of 300 to 400 MHz facilitating high-speed data communication with low bit-error rate. In broadband signalling, signal propagates only in one direction, in contrast to propagation in both directions in baseband signalling. Broadband cabling uses either dual-cable scheme or single-cable scheme with a headend to facilitate flow of signal in one direction. Because of the shielded, concentric construction, co- axial cable is less susceptible to interference and cross talk than the twisted-pair. For long distance communication, repeaters are needed for every kilometre or so. Data rate depends on physical properties of cable, but 10 Mbps is typical. Use One of the most popular uses of co-axial cable is in cable TV (CATV) for the distribution of TV signals. Another importance use of co-axial cable is in LAN. 4.3 BROADBAND The term broadband refers to analogue transmission over coaxial cable. (Note, however, that the telephone folks use broadband to refer to any channel wider than 4 kHz). The Technology Typically bandwidth of 300 MHz has total data rate of about 150 Mbps.  Operates at distances up to 100 km (metropolitan area).  Uses analoguesignalling.  Technology used in cable television. Thus, it is already available at sites such as universities that may have TV classes.  Total available spectrum typically divided into smaller channels of 6 MHz each. That is, to get more than 6MHz of bandwidth, you have to use two smaller channels and somehow combine the signals.  Requires amplifiers to boost signal strength; because amplifiers are one way, data flows in only one direction. Two types of systems have emerged. 1. Dual cable systems use two cables, one for transmission in each direction. i. One cable is used for receiving data. 57 CU IDOL SELF LEARNING MATERIAL (SLM)

ii. Second cable used to communicate with headend. When a node wishes to transmit data, it sends the data to a special node called the headend. The headend then resends the data on the first cable. Thus, the headend acts as a root of the tree, and all data must be sent to the root for redistribution to the other nodes. 2. Mid-split systems divide the raw channel into two smaller channels, with each sub channel having the same purpose as above. Which is better, broadband or base band? There is rarely a simple answer to such questions. Base band is simple to install, interfaces are inexpensive, but doesn't have the same range. Broadband is more complicated, more expensive, and requires regular adjustment by a trained technician, but offers more services (e.g., it carries audio and video too) 4.4 OPTICAL FIBRE TRANSMISSION In fibre optic technology, the medium consists of a hair-width strand of silicon or glass, and the signal consists of pulses of light. For instance, a pulse of light means ``1'', lack of pulse means ``0''. It has a cylindrical shape and consists of three concentric sections: the core, the cladding, and the jacket as shown in figure 4.3. Figure 4.3: Optical fibre The core, innermost section consists of a single solid dielectric cylinder of diameter d1 and of refractive index n1. The core is surrounded by a solid dielectric cladding of refractive index n2 that is less than n1. As a consequence, the light is propagated through multiple total internal reflections. The core material is usually made of ultra-pure fused silica or glass and the cladding is either made of glass or plastic. The cladding is surrounded by a jacket made of plastic. The jacket is used to protect against moisture, abrasion, crushing and other environmental hazards. Three components are required. 1. Fibre medium: Current technology carries light pulses for tremendous distances (e.g., 100s of kilometres) with virtually no signal loss. 58 CU IDOL SELF LEARNING MATERIAL (SLM)

2. Light source: typically a Light Emitting Diode (LED) or laser diode. Running current through the material generates a pulse of light. 3. A photo diode light detector, which converts light pulses into electrical signals. Advantages 1. Very high data rate, low error rate. 1000 Mbps (1Gbps) over distances of kilometres common. Error rates are so low they are almost negligible. 2. Difficult to tap which makes it hard for unauthorized taps as well. This is responsible for higher reliability of this medium. How difficult is it to prevent coax taps? Very difficult indeed, unless one can keep the entire cable in a locked room 3. Much thinner (per logical phone line) than existing copper circuits. Because of its thinness, phone companies can replace thick copper wiring with fibres having much more capacity for same volume. This is important because it means that aggregate phone capacity can be upgraded without the need for finding more physical space to hire the new cables. 4. Not susceptible to electrical interference (lightning) or corrosion (rust). 5. Greater repeater distance than coax. Disadvantages  Difficult to tap - It really is point-to-point technology. In contrast, tapping into coax is trivial. No special training or expensive tools or parts are required.  One-way channel - Two fibres needed to get full duplex (both ways) communication. Optical Fibre works in three different types of modes (or we can say that we have 3 types of communication using Optical fibre). Optical fibres are available in two varieties; Multi-Mode Fibre (MMF) and Single-Mode Fibre (SMF). For multi-mode fibre the core and cladding diameter lies in the range 50-200μm and 125-400μm, respectively. In single-mode fibrethe core and cladding diameters lies in the range 8-12μm and 125μm, respectively. Single-mode fibres are also known as Mono-Mode Fibre. Moreover, both single-mode and multi-mode fibres can have two types; step index and graded index. In the former case the refractive index of the core is uniform throughout and at the core cladding boundary there is an abrupt change in refractive index. In the latter case, the refractive index of the core varies radially from the centre to the core-cladding boundary from n1 to n2 in a linear manner. Figure.4.5 shows the optical fibre transmission modes. 59 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 4.4: Schematics of three optical fibre types, (a) Single-mode step-index, (b) Multi-mode step-index, and (c) Multi-mode graded-index Characteristics Optical fibre acts as a dielectric waveguide that operates at optical frequencies (1014 to 1015 Hz). Three frequency bands cantered around 850, 1300 and 1500 nanometres are used for best results. When light is applied at one end of the optical fibre core, it reaches the other end by means of total internal reflection because of the choice of refractive index of core and cladding material (n1 > n2). The light source can be either light emitting diode (LED) or injection laser diode (ILD). These semiconductor devices emit a beam of light when a voltage is applied across the device. At the receiving end, a photodiode can be used to detect the signal-encoded light. Either PIN detector or APD (Avalanche photodiode) detector can be used as the light detector. In a multi-mode fibre, the quality of signal-encoded light deteriorates more rapidly than single-mode fibre, because of interference of many light rays. As a consequence, single-mode fibre allows longer distances without repeater. For multi-mode fibre, the typical maximum length of the cable without a repeater is 2km, whereas for single-mode fibre it is 20km. FibreUses Because of greater bandwidth (2Gbps), smaller diameter, lighter weight, low attenuation, immunity to electromagnetic interference and longer repeater spacing, optical fibre cables are finding widespread use in long-distance telecommunications. Especially, the single mode fibre is suitable for this purpose. Fibre optic cables are also used in high-speed LAN applications. Multi-mode fibre is commonly used in LAN.  Long-haul trunks-increasingly common in telephone network (Sprint ads).  Metropolitan trunks-without repeaters (average 8 miles in length).  Rural exchange trunks-link towns and villages  Local loops-direct from central exchange to a subscriber (business or home). 60 CU IDOL SELF LEARNING MATERIAL (SLM)

 Local area networks-100Mbps ring networks. 4.5 SUMMARY  A guided medium provides a physical conduit from one device to another. Twisted pair cable, coaxial cable, and optical fibre are the most popular types of guided media.  Twisted-pair cable consists of two insulated copper wires twisted together. Twisted pair cable is used for voice and data communications.  Coaxial cable consists of a central conductor and a shield. Coaxial cable can carry signals of higher frequency ranges than twisted-pair cable. Coaxial cable is used in cable TV networks and traditional Ethernet LANs.  Physical connection consists of metal pin touching the copper core. There are two common ways to connect to a coaxial cable. With vampire taps, a metal pin is inserted into the copper core. A special tool drills a hole into the cable, removing a small section of the insulation, and a special connector is screwed into the hole. The tap makes contact with the copper core.  With a T-junction, the cable is cut in half, and both halves connect to the T-junction. A T-connector is analogous to the signal splitters used to hook up multiple TVs to the same cable wire.  Fibre-optic cables are composed of a glass or plastic inner core surrounded by cladding, all encased in an outside jacket. Fibre-optic cables carry data signals in the form of light. The signal is propagated along the inner core by reflection.  Fiberoptic transmission is becoming increasingly popular due to its noise resistance, low attenuation, and high-bandwidth capabilities. Fibre-optic cable is used in backbone networks, cable TV networks, and Fast Ethernet networks.  Typically bandwidth of 300 MHz has total data rate of about 150 Mbps. It operates at distances up to 100 km (metropolitan area). It uses analoguesignalling. Technology used in cable television. Thus, it is already available at sites such as universities that may have TV classes.  The term broadband refers to analogue transmission over coaxial cable. (Note, however, that the telephone folks use broadband to refer to any channel wider than 4 kHz). Typically bandwidth of 300 MHz has total data rate of about 150 Mbps. Operates at distances up to 100 km (metropolitan area). It uses analoguesignalling.  Technology used in cable television. Thus, it is already available at sites such as universities that may have TV classes. Total available spectrum typically divided into smaller channels of 6 MHz each. That is, to get more than 6MHz of bandwidth, you have to use two smaller channels and somehow combine the signals. 61 CU IDOL SELF LEARNING MATERIAL (SLM)

 Optical fibre acts as a dielectric waveguide that operates at optical frequencies (1014 to 1015 Hz). Three frequency bands cantered around 850, 1300 and 1500 nanometres are used for best results.  When light is applied at one end of the optical fibre core, it reaches the other end by means of total internal reflection because of the choice of refractive index of core and cladding material (n1 > n2).  The light source can be either light emitting diode (LED) or injection laser diode (ILD). These semiconductor devices emit a beam of light when a voltage is applied across the device. At the receiving end, a photodiode can be used to detect the signal- encoded light. Either PIN detector or APD (Avalanche photodiode) detector can be used as the light detector.  In a multi-mode fibre, the quality of signal-encoded light deteriorates more rapidly than single-mode fibre, because of interference of many light rays. As a consequence, single-mode fibre allows longer distances without repeater. For multi-mode fibre, the typical maximum length of the cable without a repeater is 2km, whereas for single- mode fibre it is 20km. 4.6KEYWORDS  Base Band Channel - A base band channel is a communication channel that can transfer frequency that is very near zero.Baseband is a signal that has a near-zero frequency range from close to zero hertz up to a higher cut-off frequency. An example of a baseband signal would be the audio signal output of a microphone, or a single musical note or synthesized tone.  Decibel (dB) - It is the ratio of two values that measure signal strength such as voltage, current or power.The decibel is a relative unit of measurement equal to one tenth of a bell. It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10.  Analogue Signal – It is any signal which is continuous for which the time varying feature (variable) of the signal is a representation of some other time varying quantity.  Cladding – The plastic or glass sheath that is fused to and surrounds the core of an optical fibre. Cladding is the application of one material over another to provide a skin or layer. In construction, cladding is used to provide a degree of thermal insulation and weather resistance, and to improve the appearance of buildings.  Broadband- In telecommunication, broad band is wide bandwidth data transmission which transports multiple signals and traffic types.In telecommunications, broadband 62 CU IDOL SELF LEARNING MATERIAL (SLM)

is wide bandwidth data transmission which transports multiple signals and traffic types. 4.7 LEARNING ACTIVITY 1. Give a popular example where co-axial cables are used for broadband signalling. Explain its features in detail. ________________________________________________________________________ _____________________________________________________________________ 2. Describe the optical fibre connection taken in your locality. ________________________________________________________________________ _____________________________________________________________________ 4.8 UNIT END QUESTIONS A. Descriptive Questions Short Questions: 1. Define transmission media. 2. What are the uses of co-axial cable? 3. Define the term broadband. Briefly explain its technology 4. What are the three types of optical fibre transmission mode using a diagram? 5. What are the uses of optical fibre? Long Questions: 1. Explain transmission media. What are its classifications with an illustration? 2. What is termed as co-axial cable? What are the two common ways of connecting to a co-axial cable? 3. Explain the characteristics of co-axial cable. 4. Explain the advantages and disadvantages of optical fibre. 5. Explain the structure of optical fibre with an illustration. What are its components? B.Multiple Choice Questions 1. Which is the media in which transmission system is widely used as the backbone of a network? a. Fibre optics 63 CU IDOL SELF LEARNING MATERIAL (SLM)

b. Co-axial c. UTP d. STP 2. Which is less affected due to the external magnetic field? a. Optical fibre cable b. Co-axial cable c. STP cable d. UTP cable 3. Identify the media in which EMI get reduced due to shielding? a. Optical fibre cable b. Co-axial cable c. STP cable d. UTP cable 4. Identify the media in which the power loss occurs because of absorption, scattering, dispersion and bending? a. STP cable b. UTP cable c. Optical fibre cable d. None of these 5. Which cable has much lower attenuation and can carry the signal to a longer distance without using amplifiers and repeaters in between? a. Optical fibre b. STP cable c. Co-axial cable d. None of these Answers 1-a, 2-b, 3-b, 4-c, 5-a. 4.9 REFERENCES References 64 CU IDOL SELF LEARNING MATERIAL (SLM)

 Behrouz, A, Forouzan & Coombs, Ann, Catherine & Chung, Sophia. (2001). Data Communication & Networking. Second edition. Mcgraw Hill.  Stamper, D. (1993). Local Area Networks, Addison-Wesley, Reading. MA.  Stamper, D. (1991). Business Data Communications, Third Edition. Addison-Wesley, Reading, MA. 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 65 CU IDOL SELF LEARNING MATERIAL (SLM)

UNIT – 5: TRANSMISSION MEDIA PART 1 STRUCTURE 5.0 Learning Objectives 5.1 Introduction 5.2 Guided and Unguided 5.3 Attenuation 5.4 Distortion 5.5 Noise 5.6 Throughput 5.6 Summary 5.7 Keywords 5.8 Learning Activity 5.9 Unit End Questions 5.10 References 5.0 LEARNING OBJECTIVES After studying this unit, you will be able to:  Distinguish guided and unguided transmission media.  Illustrate the importance of attenuation.  Describe the features of noise.  Explain the features of distortion. 5.1 INTRODUCTION In a data transmission system, the transmission medium is the physical path between transmitter and receiver. For guided media, electromagnetic waves are guided along a solid medium, such as copper twistedpair, copper coaxial cable, and optical fibre. For unguided media, wireless transmissionoccurs through the atmosphere, outer space, or water.The characteristics and quality of a data transmission are determined bothby the characteristics of the medium and the characteristics of the signal. In thecase of guided media, the medium itself is more important in determining thelimitations of transmission.For unguided media, the bandwidth of the signal produced by the transmittingantenna is more important than the medium in determining transmissioncharacteristics. One key property of signals transmitted 66 CU IDOL SELF LEARNING MATERIAL (SLM)

by antenna is directionality.In general, signals at lower frequencies are omnidirectional; that is, the signalpropagates in all directions from the antenna. At higher frequencies, it is possibleto focus the signal into a directional beam.In considering the design of data transmission systems, key concerns aredata rate and distance: the greater the data rate and distance the better. Anumber of design factors relating to the transmission medium and the signaldetermine the data rate and distance: Bandwidth: All other factors remaining constant, the greater the bandwidthof a signal, the higher the data rate that can be achieved. Transmission impairments: Impairments, such as attenuation, limit the distance.For guided media, twisted pair generally suffers more impairmentthan coaxial cable, which in turn suffers more than optical fibre. Interference: Interference from competing signals in overlapping frequencybands can distort or wipe out a signal. Interference is of particularconcern for unguided media, but is also a problem with guided media. Forguided media, interference can be caused by emanations from nearbycables. For example, twisted pairs are often bundled together and conduitsoften carry multiple cables. Interference can also be experienced fromunguided transmissions. Proper shielding of a guided medium can minimizethis problem. Number of receivers: A guided medium can be used to construct a point to-point link or a shared link with multiple attachments. In the latter case,each attachment introduces some attenuation and distortion on the line,limiting distance and/or data rate. It depicts the electromagnetic spectrum and indicates the frequencies at which various guided media and unguided transmission techniques operate. In this chapter we examine these guided and unguided alternatives. In all cases, we describe the systems physically, briefly discuss applications, and summarize key transmission characteristics. 5.2 GUIDED AND UNGUIDED Guided Media Guided media, which are those that provide a conduit from one device to another,include twisted-pair cable, coaxial cable, and fibre-optic cable. A signal travelingalong any of these media is directed and contained by the physical limits of themedium. Twisted-pair and coaxial cable use metallic (copper) conductors that acceptand transport signals in the form of electric current. Optical fibre is a cable that acceptsand transports signals in the form of light. Twisted-Pair Cable A twisted pair consists of two conductors (normally copper), each with its own plasticinsulation, twisted together, as shown in figure 5.1. 67 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 5.1: Twisted pair cable One of the wires is used to carry signals to the receiver, and the other is used onlyas a ground reference. The receiver uses the difference between the two.In addition to the signal sent by the sender on one of the wires, interference (noise)and crosstalk may affect both wires and create unwanted signals.If the two wires are parallel, the effect of these unwanted signals is not the same inboth wires because they are at different locations relative to the noise or crosstalk sources(e.g., one is closer and the other is farther). This results in a difference at the receiver. By twisting the pairs, a balance is maintained. For example, suppose in one twist, one wireis closer to the noise source and the other is farther; in the next twist, the reverse is true.Twisting makes it probable that both wires are equally affected by external influences(noise or crosstalk). This means that the receiver, which calculates the difference betweenthe two, receives no unwanted signals. The unwanted signals are mostly cancelled out.From the above discussion, it is clear that the number of twists per unit of length (e.g., inch) has some effect on the quality of the cable. Unshielded Versus Shielded Twisted-Pair Cable The most common twisted-pair cable used in communications is referred to asunshielded twisted-pair (UTP). IBM has also produced a version of twisted-pair cablefor its use called shielded twisted-pair (STP). STP cable has a metal foil or braidedmeshcovering that encases each pair of insulated conductors. Although metal casingimproves the quality of cable by preventing the penetration of noise or crosstalk, it isbulkier and more expensive. Figure 5.2 shows the difference between UTP and STP.Our discussion focuses primarily on UTP because STP is seldom used outside of IBM. Figure 5.2: UTP and STP cables 68 CU IDOL SELF LEARNING MATERIAL (SLM)

Categories The Electronic Industries Association (EIA) has developed standards to classifyunshielded twisted-pair cable into seven categories. Categories are determined by cablequality, with 1 as the lowest and 7 as the highest. Each EIA category is suitable forspecific uses. Table 5.1 shows these categories. Table 5.1: Categories of unshielded twisted-pair cables Applications Twisted-pair cables are used in telephone lines to provide voice and data channels. The local loop-the line that connects subscribers to the central telephone office---commonly consists of unshielded twisted-pair cables. The DSL lines that are used by the telephone companies to provide high-data-rate connections also use the high-bandwidth capability of unshielded twisted-pair cables. Local-area networks, such as lOBase-T and lOOBase-T, also use twisted- pair cables. Unguided Transmission Unguided transmission is used when running a physical cable (either fibre or copper) between two end points is not possible. For example, running wires between buildings is probably not legal if the building is separated by a public street. Infrared signals typically used for short distances (across the street or within same room). Microwave signals commonly used for longer distances (10's of km). Sender and receiver use some sort of dish antenna as shown in figure 5.1. 69 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 5.3: Communication using terrestrial microwave Difficulties  Weather interferes with signals. For instance, clouds, rain, lightning, etc. may adversely affect communication.  Radio transmissions easy to tap. A big concern for companies worried about competitors stealing plans.  Signals bouncing off of structures may lead to out-of-phase signals that the receiver must filter out. 5.3 ATTENUATION Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise. Figure 5.4: Causes of impairment Attenuation means a loss of energy. When a signal, simple or composite, travelsthrough a medium, it loses some of its energy in overcoming the resistance of themedium. That is why a wire carrying electric signals gets warm, if not hot, after awhile. Some of the electrical 70 CU IDOL SELF LEARNING MATERIAL (SLM)

energy in the signal is converted to heat. To compensatefor this loss, amplifiers are used to amplify the signal. Figure 5.3 shows the effect ofattenuation and amplification. Figure 5.5: Attenuation Decibel To show that a signal has lost or gained strength, engineers use the unit of the decibel.The decibel (dB) measures the relative strengths of two signals or one signal at two differentpoints. Note that the decibel is negative if a signal is attenuated and positive if asignal is amplified. dB = 10log10P2 P1 Variables PI and P2 are the powers of a signal at points 1 and 2, respectively. Note thatsome engineering books define the decibel in terms of voltage instead of power. In thiscase, because power is proportional to the square of the voltage, the formula is dB =20 log 10 (V2IV1). It is good to express dB in terms of power. 5.4 DISTORTION Distortion means that the signal changes its form or shape. Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed (see the next section) through a medium and, therefore, its own delay in arriving at the final destination. Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration. In other words, signal components at the receiver have phases different from what they had at the sender. The shape of the composite signal is therefore not the same. Figure 5.4 shows the effect of distortion on a composite signal. 71 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 5.6: Distortion 5.5 NOISE Noise is another cause of impairment. Several types of noise, such as thermal noise, induced noise, crosstalk, and impulse noise, may corrupt the signal. Thermal noise is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna. Crosstalk is the effect of one wire on the other. One wire acts as a sending antenna and the other as the receiving antenna. Impulse noise is a spike (a signal with high energy in a very short time) that comes from power lines, lightning, and so on. Figure 5.6 shows the effect of noise on a signal. Figure 5.7: Noise Signal-to-Noise Ratio (SNR) To find the theoretical bit rate limit, we need to know the ratio ofthe signal power to the noise power. The signal-to-noise ratio is defined as signal-to-noise ratio. (SNR). SNR = (average signal power) / (average noise power) 72 CU IDOL SELF LEARNING MATERIAL (SLM)

We need to consider the average signal power and the average noise power because these may change with time. Figure 5.7 shows the idea of SNR. Figure 5.8: Two cases ofSNR: a high SNR and a low SNR SNR is actually the ratio of what is wanted (signal) to what is not wanted (noise). A high SNR means the signal is less corrupted by noise; a low SNR means the signal is more corrupted by noise. Because SNR is the ratio of two powers, it is often described in decibel units, SNR (db), defined as SNR (db) = 10log SNR. 5.6 THROUGHPUT The throughput is a measure of how fast we can actually send data through a network. Although, at first glance, bandwidth in bits per second and throughput seem the same, they are different. A link may have a bandwidth of B bps, but we can only send T bps through this link with T always less than B. In other words, the bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data. For example, we may have a link with a bandwidth of 1 Mbps, but the devices connected to the end of the link may handle only 200 kbps. This means that we cannot send more than 200 kbps through this link. Imagine a highway designed to transmit 1000 cars per minute from one point to another. However, if there is congestion on the road, this figure may be reduced to 100 cars per minute. The bandwidth is 1000 cars per minute; the throughput is 100 cars per minute. Example A network with bandwidth of 10 Mbps can pass only an average of 12,000 frames per minute with each frame carrying an average of 10,000 bits. What is the throughput of this network? We can calculate the throughput as 73 CU IDOL SELF LEARNING MATERIAL (SLM)

Throughput= 12,000 x 10,000 =2 Mbps The throughput is almost one-fifth of the bandwidth in this case. 5.6 SUMMARY  Transmission medium - Physical path between transmitter and receiver may be guided (wired) or unguided (wireless). Communication achieved by using EM waves. Characteristics and quality of data transmission is dependent on characteristics of medium and signal.  Guided medium - Medium is more important in setting transmission parameters.  Unguided medium - Bandwidth of the signal produced by transmitting antenna is important in setting transmission parameters. Signal directionality – It has lower frequency signals are omnidirectional. It has higher frequency signals can be focused in a directional beam.  The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise.  When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in the signal is converted to heat. To compensate for this loss, amplifiers are used to amplify the signal.  Differences in delay may create a difference in phase if the delay is not exactly the same as the period duration. In other words, signal components at the receiver have phases different from what they had at the sender. The shape of the composite signal is therefore not the same.  Thermal noise is the random motion of electrons in a wire which creates an extra signal not originally sent by the transmitter. Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna. Crosstalk is the effect of one wire on the other. One wire acts as a sending antenna and the other as the receiving antenna.  SNR means the signal is less corrupted by noise; a low SNR means the signal is more corrupted by noise. Because SNR is the ratio of two powers, it is often described in decibel units, SNR (db). 74 CU IDOL SELF LEARNING MATERIAL (SLM)

 A link may have a bandwidth of B bps, but we can only send T bps through this link with T always less than B. In other words, the bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data. For example, we may have a link with a bandwidth of 1 Mbps, but the devices connected to the end of the link may handle only 200 kbps. 5.7 KEYWORDS  Sender - A sender is a computer or any such device which is capable of sending data over a network. It can be a computer, mobile phone, smartwatch, walkie-talkie, video recording device, etc.  Receiver - A receiver is a computer or any such device which is capable of receiving data from the network. It can be any computer, printer, laptop, mobile phone, television, etc. In computer communication, the sender and receiver are known as nodes in a network.  Decibel (dB) - The decibel is a relative unit of measurement equal to one tenth of a bell. It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10.  Period – It refers to the amount of time in seconds,a signal needs to complete one cycle or the period of a wave is the time for a particle on a medium to make one complete vibrational cycle.  Guided Media- Are those that provide a conduit from one device to another.Guided media, which are those that provide a conduit from one device to another, include Twisted-Pair Cable, Coaxial Cable, and Fibre-Optic Cable. A signal travelling along any of these media is directed and contained by the physical limits of the medium. 5.8 LEARNING ACTIVITY 1. The loss in a cable is usually defined in decibels per kilometres (dB/km). If the signal at the beginning of a cable with 0.3dBlkm has a power of 2 mW, what is the power of the signal at 5 km? ___________________________________________________________________________ ___________________________________________________________________________ 2. Sometimes the decibel is used to measure signal power in milliwatts. In this case, it is referred to as dBm and is calculated as dBm = 10 loglO Pm' where Pm is the power in milliwatts. Calculate the power of a signal if its dBm =-30. 75 CU IDOL SELF LEARNING MATERIAL (SLM)

___________________________________________________________________________ __________________________________________________________________________ 5.9 UNIT END QUESTIONS A. Descriptive Questions Short Questions: 1. What are the values of SNR and SNRdBin the following case: The power of a signal is 10 mW and the power of the noise is 1 /lW.? 2. Write short note on guided transmission media. 3. Write a short note on twisted pair cable with an illustration. 4. Explain the difference between unshielded and shielded twisted pair cable. 5. What are the applications of twisted pair cable? Long Questions: 1. Write a short note on unguided transmission. 2. Explain attenuation with an illustration 3. Explain distortion using suitable diagram. 4. Define noise. Explain signal-to-Noise ratio. 5. Explain throughput. B.Multiple Choice Questions 1. Which can impair a signal? a. Noise b. Attenuation c. Distortion d. All of these 2. Which is the type of transmission impairment in which the signal loses strength due to the resistance of the transmission medium? a. Distortion b. Attenuation c. Noise d. Decibel 76 CU IDOL SELF LEARNING MATERIAL (SLM)

3. Which is a type of transmission impairment in which the signal losses the strength due to the different propagation speed of each frequency that makes up the signal? a. Noise b. Distortion c. Decibel d. Attenuation 4. Which is a type of transmission impairment in which an outside source such as cross talk corrupts a signal? a. Noise b. Distortion c. Decibel d. Attenuation 5. What is the term which refers to the loss of strength of a signal? a. Attenuation b. Distortion c. Noise d. Impairment Answers 1-d, 2-b, 3- b, 4s-a, 5-a 5.10 REFERENCES References  Madhow, Upamanya. (2014). Introduction to communication system. United Kingdom. Cambridge University Press.  Hildebrant, Michael, Benjamin. (1950). Distortion in audio systems. Cornell University.  Hansler, Eberhard & Schmidt, Gerhard. (2008). Speech and audio processing in adverse environment. Springer publications. Textbooks  Behrouz, A, Forouzan & Coombs, Ann, Catherine & Chung, Sophia. (2001). Data Communication & Networking. Second edition. Mcgraw Hill. 77 CU IDOL SELF LEARNING MATERIAL (SLM)

 Van, Duuren, J. Schoute, F & Kastelein, P. (1992) Telecommunication.  Networks and Services, Addison-Wesley, Reading, MA. 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 78 CU IDOL SELF LEARNING MATERIAL (SLM)

UNIT – 6: TRANSMISSION MEDIA PART 2 STRUCTURE 6.0 Learning Objectives 6.1 Introduction 6.2 Propagation Speed and Time 6.3 Wavelength 6.3.1 Time and Frequency Domains 6.4 Shannon Capacity 6.5 Comparison of Media 6.6 Summary 6.7 Keywords 6.8 Learning Activity 6.9 Unit End Questions 6.10 References 6.0 LEARNING OBJECTIVES After studying this unit, you will be able to:  Identify the characteristics of transmission media.  Describe the propagation speed and time.  Explain the wavelength characteristics of media.  Explain shannon capacity in detail. 6.1 INTRODUCTION We start the discussion of the Internet model with the bottom-most layer, the physical layer. It is the layer that actually interacts with the transmission media, the physical part of the network that connects network components together. This layer is involved in physically carrying information from one node in the network to the next. The physical layer has complex tasks to perform. One major task is to provide services for the data link layer. The data in the data link layer consists of 0s and 1s organized into frames that are ready to be sent across the transmission medium. This stream of 0s and 1 s must first be converted into another entity: signals. One of the services provided by the physical layer is to create a signal that represents this stream of bits. 79 CU IDOL SELF LEARNING MATERIAL (SLM)

The physical layer must also take care of the physical network, the transmission medium. The transmission medium is a passive entity; it has no internal program or logic for control like other layers. The transmission medium must be controlled by the physical layer. The physical layer decides on the directions of data flow. The physical layer decides on the number of logical channels for transporting data coming from different sources. We discuss issues related to the physical layer and the transmission medium that is controlled by the physical layer. Also we discuss the structure and the physical layers of the telephone network and the cable network. 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 devicesand interfaces have to perform for transmission to occur. Figure 2.5 shows the position ofthe physical layer with respect to the transmission medium and the data link layer.The physical layer is also concerned with the following. The sender and receiver not only must use the same bit rate but also must be synchronized at the bit level. In other words, the sender and the receiver clocks must be synchronized.The physical layer is concerned with the connection of devices to the media. In a point-to-point configuration, two devices are connected through a dedicated link. In a multipoint configuration, a link is shared among several devices. The physical topology defines how devices are connected to make a network. Devices can be connected by using a mesh topology (every device is connected to every other device), a star topology (devices are connected through a central device), a ring topology (each device is connected to the next, forming a ring), a bus topology (every device is on a common link), or a hybrid topology (this is a combination of two or more topologies). 6.2 PROPAGATION SPEED AND TIME The latency or delay defines how long it takes for an entire message to completelyarrive at the destination from the time the first bit is sent out from the source. We cansay that latency is made of four components: propagation time, transmission time,queuing time and processing delay. Latency = propagation time +transmission time +queuing time + processing delay Propagation Time Propagation time measures the time required for a bit to travel from the source to the destination. The propagation time is calculated by dividing the distance by the propagation speed. Propagation time = (Distance) / (Propagation speed) 80 CU IDOL SELF LEARNING MATERIAL (SLM)

The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal For example, in a vacuum, light is propagated with a speed of 3 x 108 mfs. It is lower in air; it is much lower in cable. Transmission Time In data communications we don't send just 1 bit, we send a message. The first bit may take a time equal to the propagation time to reach its destination; the last bit also may take the same amount of time. However, there is a time between the first bit leaving the sender and the last bit arriving at the receiver. The first bit leaves earlier and arrives earlier; the last bit leaves later and arrives later. The time required for transmission of a message depends on the size of the message and the bandwidth of the channel. Transmission time = (Message size) / (Bandwidth) Queuing Time The third component in latency is the queuing time, the time needed for each intermediate or end device to hold the message before it can be processed. The queuing time is not a fixed factor; it changes with the load imposed on the network. When there is heavy traffic on the network, the queuing time increases. An intermediate device, such as a router, queues the arrived messages and processes them one by one. If there are many messages, each message will have to wait. Bandwidth-Delay Product Bandwidth and delay are two performance metrics of a link. However, as we will see inthis chapter and future chapters, what is very important in data communications is theproduct of the two, the bandwidth-delay product. Let us elaborate on this issue, usingtwo hypothetical cases as examples. Figure 6.1: Case-1 81 CU IDOL SELF LEARNING MATERIAL (SLM)

Let us assume that we have a link with a bandwidth of 1 bps (unrealistic, but goodfor demonstration purposes). We also assume that the delay of the link is 5 s (alsounrealistic). We want to see what the bandwidth-delay product means in this case.Looking at figure, we can say that this product 1 x 5 is the maximum number ofbits that can fill the link. There can be no more than 5 bits at any time on the link.Case 2. Now assume we have a bandwidth of 4 bps. Figure 6.2 shows that therecan be maximum 4 x 5 = 20 bits on the line. The reason is that, at each second,there are 4 bits on the line; the duration of each bit is 0.25 s. Figure 6.2: Filling the link with bits for case 1 The above two cases show that the product of bandwidth and delay is the number ofbits that can fill the link. This measurement is important if we need to send data in burstsand wait for the acknowledgment of each burst before sending the next one. To use themaximum capability of the link, we need to make the size of our burst 2 times the productof bandwidth and delay; we need to fill up the full-duplex channel (two directions). Thesender should send a burst of data of (2 x bandwidth x delay) bits. The sender then waitsfor receiver acknowledgment for part of the burst before sending another burst. Theamount 2 x bandwidth x delay is the number of bits that can be in transition at any time.The bandwidth delayproduct defines the number of bits that can rdl the link. Example We can think about the link between two points as a pipe. The cross section of the pipe representsthe bandwidth, and the length of the pipe represents the delay. We can say the volume of the pipedefines the bandwidth-delay product, as shown in figure 6.3. 82 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 6.3: Band width delay product Jitter Another performance issue that is related to delay is jitter. We can roughly say that jitteris a problem if different packets of data encounter different delays and the applicationusing the data at the receiver site is time-sensitive (audio and video data, for example).If the delay for the first packet is 20 ms, for the second is 45 ms, and for the third is40 ms, then the real-time application that uses the packets endures jitter. 6.3 WAVE LENGTH Wavelength is another characteristic of a signal traveling through a transmission medium. Wavelength binds the period or the frequency of a simple sine wave to the propagation speed of the medium (see figure 6.4). Figure 6.4: Wavelength and period While the frequency of a signal is independent of the medium, the wavelengthdepends on both the frequency and the medium. Wavelength is a property of any typeof signal. In data communications, we often use wavelength to describe the transmissionof light in an optical fibre. The wavelength is the distance a simple signal can travelin one period.Wavelength can be calculated if one is given the propagation speed (the speed oflight) and the period of the signal. However, since period and frequency are related toeach other, if we represent wavelength by A, propagation speed by c (speed of light), andfrequency by1, we getWavelength = propagation speed x period = (propagation speed) / Frequency. 83 CU IDOL SELF LEARNING MATERIAL (SLM)

The propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal. For example, in a vacuum, light is propagated with a speed of 3 x 108 mls. That speed is lower in air and even lower in cable. The wavelength is normally measured in micrometres (microns) instead of meters. For example, the wavelength of red light (frequency =4 x 1014) in air is Wavelength = (C/F) = (3 x108) / (4 x 1014) = 0.75 micro-metres. In a coaxial or fibre-optic cable, however, the wavelength is shorter because thepropagation speed in the cable is decreased. 6.3.1 Time and Frequency Domains A sine wave is comprehensively defined by its amplitude, frequency, and phase. Wehave been showing a sine wave by using what is called a time-domain plot. Thetime-domain plot shows changes in signal amplitude with respect to time (it is anamplitude-versus-time plot). Phase is not explicitly shown on a time-domain plot.To show the relationship between amplitude and frequency, we can use what iscalled a frequency-domain plot. A frequency- domain plot is concerned with only thepeak value and the frequency. Changes of amplitude during one period are not shown.Figure 6.6 shows a signal in both the time and frequency domains. Figure 6.5: The time-domain and frequency-domain plots of sine wave 84 CU IDOL SELF LEARNING MATERIAL (SLM)

It is obvious that the frequency domain is easy to plot and conveys the informationthat one can find in a time domain plot. The advantage of the frequency domain is thatwe can immediately see the values of the frequency and peak amplitude. A completesine wave is represented by one spike. The position of the spike shows the frequency;its height shows the peak amplitude. A complete sine wave in the time domain can be represented by one single spike in the frequency domain. Example The frequency domain is more compact and useful when we are dealing with more than one sinewave. For example, Figure 6.7 shows three sine waves, each with different amplitude and frequency.All can be represented by three spikes in the frequency domain. Figure 6.6: The time domain and frequency domain of three sine waves 6.4 SHANNON CAPACITY In reality, we cannot have a noiseless channel; the channel is always noisy. In 1944, Claude Shannon introduced a formula, called the Shannon capacity, to determine the theoretical highest data rate for a noisy channel: Capacity =bandwidth X log2 (1 +SNR) In this formula, bandwidth is the bandwidth of the channel, SNR is the signal-to-noise ratio, and capacity is the capacity of the channel in bits per second. Note that in the Shannon formula there is no indication of the signal level, which means that no matter how many levels we have, we cannot achieve a data rate higher than the capacity of the channel. In other words, the formula defines a characteristic of the channel, not the method of transmission. 85 CU IDOL SELF LEARNING MATERIAL (SLM)

Example Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero. In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as C=B log2 (1 + SNR) =B 10gz (l + 0) =B log2 1. = B x 0 = 0 This means that the capacity of this channel is zero regardless of the bandwidth. In other words, we cannot receive any data through this channel. Example We can calculate the theoretical highest bit rate of a regular telephone line. A telephone line normally has a bandwidth of 3000 Hz (300 to 3300 Hz) assigned for data communications. The signal-to-noise ratio is usually 3162. For this channel the capacity is calculated as C =B log2 (1 + SNR) =3000 log2 (l + 3162) = 3000 log2 3163 = 3000 x 11.62 = 34,860 bps. This means that the highest bit rate for a telephone line is 34.860 kbps. If we want to send data faster than this, we can either increase the bandwidth of the line or improve the signal-to noise ratio. Example The signal-to-noise ratio is often given in decibels. Assume that SN~B = 36 and the channel bandwidth is 2 MHzthe theoretical channel capacity can be calculated as SNRdB = 10 loglO SNR which implies SNR = lOSNRoB/10 ... SNR; 10 3.6 =3981 C =B log2 (1+ SNR) = 2 X 106 X log2 3982 = 24 Mbps Example 3.40 For practical purposes, when the SNR is very high, we can assume that SNR + I is almost the Same as SNR. In these cases, the theoretical channel capacity can be simplified to C = B x (SNRdB)/ 3 For example, we can calculate the theoretical capacity of the previous example as C= 2 MHz X (36/3) =24 Mbps Using Both Limits In practice, we need to use both methods to find the limits and signal levels. Let us showthis with an example. Example 86 CU IDOL SELF LEARNING MATERIAL (SLM)

We have a channel with an I-MHz bandwidth. The SNR for this channel is 63. What are the appropriate bit rate and signal level? Solution First, we use the Shannon formula to find the upper limit. C =B log2 (l + SNR) =106 log2 (1 + 63) =106 10g2 64 =6 Mbps The Shannon formula gives us 6 Mbps, the upper limit. For better performance we choose something lower, 4 Mbps, for example. Then we use the Nyquist formula to find the number of signal levels. 4Mbps=2x 1 MHz x log2 L which implies L=4 The Shannon capacity gives us the upper limit; the Nyquist formula tells us how many signal levels we need. 6.5 COMPARISON OF MEDIA Guided Transmission Media  Transmission capacity (bandwidth and data rate) depends on distance and type of network (point-to-point or multipoint)  Twisted pair 1. Least expensive and most widely used 2. Physical description i. Two insulated copper wires arranged in regular spiral pattern ii. Number of pairs are bundled together in a cable iii. Twisting decreases the crosstalk interference between adjacent pairs in the cable, by using different twist length for neighbouring pairs  Applications 1. Most common transmission media for both digital and analogue signals 2. Less expensive compared to coaxial cable or optical fibre 3. Limited in terms of data rate and distance 4. Telephone network i. Individual units (residence lines) to local exchange ii. Subscriber loops iii. Supports voice traffic using analogue signalling 87 CU IDOL SELF LEARNING MATERIAL (SLM)

iv. May handle digital data at modest rates using modems 5. Communications within buildings i. Connection to digital data switch or digital pbx within a building ii. Allows data rate of 64 kbps  Transmission characteristics 1. Requires amplifiers every 5-6 km for analogue signals 2. Requires repeaters every 2-3 km for digital signals 3. Attenuation is a strong function of frequency i. Higher frequency implies higher attenuation 4. Susceptible to interference and noise 5. Improvement possibilities i. Shielding with metallic braids or sheathing reduces interference ii. Twisting reduces low frequency interference iii. Different twist length in adjacent pairs reduces crosstalk  Unshielded and shielded twisted pairs 1. Unshielded twisted pair (utp) i. Ordinary telephone wire ii. Subject to external electromagnetic interference 2. Shielded twisted pair (stp) i. Shielded with a metallic braid or sheath ii. Reduces interference iii. Better performance at higher data rates iv. More expensive and difficult to work compared to utp  Coaxial cable 1. Physical description i. Consists of two conductors with construction that allows it to operate over a wider range of frequencies ii. Compared to twisted pair iii. Hollow outer cylindrical conductor surrounding a single inner wire conductor 88 CU IDOL SELF LEARNING MATERIAL (SLM)

iv. Inner conductor held in place by regularly spaced insulating rings or solid di- electrical material v. Outer conductor covered with a jacket or shield vi. Diameter from 1 to 2.5 cm vii. Shielded concentric construction reduces interference and crosstalk viii. Can be used over longer distances and support more stations on a shared line than twisted pair 2. Applications i. Most common use is in cable TV ii. Traditionally part of long distance telephone network iii. Can carry more than 10,000 voice channels simultaneously using frequency- division multiplexing iv. Short range connections between devices 3. Transmission characteristics i. Used to transmit both analogue and digital signals ii. Superior frequency characteristics compared to twisted pair iii. Can support higher frequencies and data rates iv. Shielded concentric construction makes it less susceptible to interference and crosstalk than twisted v. Pair vi. Constraints on performance are attenuation, thermal noise, and intermodulation noise vii. Requires amplifiers every few kilometres for long distance transmission viii. Usable spectrum for analoguesignalling up to 500 MHz ix. Requires repeaters every few kilometres for digital transmission x. For both analogue and digital transmission, closer spacing is necessary for higher frequencies/data rates  Optical fibre 1. Thin, flexible material to guide optical rays 2. Cylindrical cross-section with three concentric links i. Core 89 CU IDOL SELF LEARNING MATERIAL (SLM)

a. Innermost section of the fibre b. One or more very thin (dia. 8-100m) strands or fibres ii. Cladding a. Surrounds each strand b. Plastic or glass coating with optical properties different from core c. Interface between core and cladding prevents light from escaping the core iii. Jacket a. Outermost layer, surrounding one or more claddings b. Made of plastic and other materials c. Protects from environmental elements like moisture, abrasions, and crushing 3. Comparison with twisted pair and coaxial cable i. Capacity a. Much higher bandwidth b. Can carry hundreds of Gbps over tens of kms ii. Smaller size and light weight a. Very thin for similar data capacity b. Much lighter and easy to support in terms of weight (structural properties) iii. Significantly lower attenuation iv. EM isolation v. Not affected by external fields a. Not vulnerable to interference, impulse noise, or crosstalk b. No energy radiation; little interference with other devices; security from eavesdropping vi. Greater repeater spacing a. Lower cost and fewer error sources 4. Applications i. Long haul trunks a. Increasingly common in telephone networks b. About 1500km in length with high capacity (20000 to 60000 voice channels) ii. Metropolitan trunks a. Average length of about 12 km with a capacity of 100,000 voice channels b. Mostly repeater less to join phone exchanges in metro areas iii. Rural exchange trunks 90 CU IDOL SELF LEARNING MATERIAL (SLM)

a. Circuit lengths from 40 to 160 km b. Fewer than 5000 voice channels c. Connect exchanges of different phone companies iv. Subscriber loops a. Central exchange to subscriber b. May be able to handle image and video in addition to voice and data 6.6 SUMMARY  Data must be transformed to electromagnetic signals to be transmitted. Data can be analogue or digital. Analogue data are continuous and take continuous values. Digital data have discrete states and take discrete values. Signals can be analogue or digital. Analogue signals can have an infinite number of values in a range; digital, signals can have only a limited number of values.  In data communications, we commonly use periodic analogue signals and nonperiodic digital signals.Frequency and period are the inverse of each other. Frequency is the rate of change with respect to time. Phase describes the position of the waveform relative to time. A complete sine wave in the time domain can be represented by one single spike in the frequency domain.  A single-frequency sine wave is not useful in data communications; we need to send a composite signal, a signal made of many simple sine waves.According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases.  The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. A digital signal is a composite analogue signal with an infinite bandwidth.  Baseband transmission of a digital signal that preserves the shape of the digital signal is possible only if we have a low-pass channel with an infinite or very wide bandwidth. If the available channel is a band pass channel, we cannot send a digital signal directly to the channel; we need to convert the digital signal to an analogue signal before transmission.  Propagation time measures the time required for a bit to travel from the source to the destination. The propagation time is calculated by dividing the distance by the propagation speed.  The first bit may take a time equal to the propagation time to reach its destination; the last bit also may take the same amount of time. However, there is a time between the first bit leaving the sender and the last bit arriving at the receiver. The first bit leaves earlier and arrives earlier; the last bit leaves later and arrives later. 91 CU IDOL SELF LEARNING MATERIAL (SLM)

 For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate. For a noisy channel, we need to use the Shannon capacity to find the maximum bit rate. Attenuation, distortion, and noise can impair a signal.Attenuation is the loss of a signal's energy due to the resistance of the medium.  We want to see what the bandwidth-delay product means in this case. Looking at figure, we can say that this product 1 x 5 is the maximum number of bits that can fill the link. There can be no more than 5 bits at any time on the link. Case 2. Now assume we have a bandwidth of 4 bps. Figure 6.2 shows that there can be maximum 4 x 5 = 20 bits on the line.  To use the maximum capability of the link, we need to make the size of our burst 2 times the product of bandwidth and delay; we need to fill up the full-duplex channel (two directions). The sender should send a burst of data of (2 x bandwidth x delay) bits. The sender then waits for receiver acknowledgment for part of the burst before sending another burst. The amount 2 x bandwidth x delay is the number of bits that can be in transition at any time.  Distortion is the alteration of a signal due to the differing propagation speeds of each of the frequencies that make up a signal. Noise is the external energy that corrupts a signal. The bandwidth-delay product defines the number of bits that can fill the link.  The wavelength is the distance a simple signal can travel in one period. Wavelength can be calculated if one is given the propagation speed (the speed of light) and the period of the signal. However, since period and frequency are related to each other, if we represent wavelength by A, propagation speed by c (speed of light), and frequency by1, we get Wavelength = propagation speed x period = (propagation speed) / Frequency.  The time-domain plot shows changes in signal amplitude with respect to time (it is an amplitude-versus-time plot). Phase is not explicitly shown on a time-domain plot. To show the relationship between amplitude and frequency, we can use what is called a frequency-domain plot.  Note that in the Shannon formula there is no indication of the signal level, which means that no matter how many levels we have, we cannot achieve a data rate higher than the capacity of the channel. In other words, the formula defines a characteristic of the channel, not the method of transmission. 6.7 KEYWORDS  Electromagnetic Spectrum – The frequencies we use for wireless are only a portion of what is called the electromagnetic spectrum. 92 CU IDOL SELF LEARNING MATERIAL (SLM)

 Frequency – The number of times something happens in a particular period.Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as temporal frequency to emphasize the contrast to spatial frequency, and ordinary frequency to emphasize the contrast to angular frequency.  Harmonic It is a signal or wave whose frequency is an integral (whole number) multiple of the frequency of some reference signal or wave.A harmonic is any member of the harmonic series. The term is employed in various disciplines, including music, physics, acoustics, electronic power transmission, radio technology, and other fields. It is typically applied to repeating signals, such as sinusoidal waves.  Jitter - It is when there is a time delay in sending these data packets over your network connection.In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter.  Noise – It is an unwanted signal which interferes with the original message signal and corrects the parameters of the message signal.In communication systems, noise is an error or undesired random disturbance of a useful information signal. Noise is, however, typically distinguished from interference, for example in the signal-to-noise ratio (SNR), signal-to-interference ratio (SIR) and signal-to-noise plus interference ratio (SNIR) measures. 6.8 LEARNING ACTIVITY 1. Identify the propagation time if the distance between the two points is 12,000 km? Assume the propagation speed to be 2.4 x 108 m/s in cable. ___________________________________________________________________________ __________________________________________________________________________ 2. Prepare and identify the propagation time and the transmission time for a 2.5-kbyte message (an e-mail) if the bandwidth of the network is 1 Gbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 x 108 m/sec. ___________________________________________________________________________ ___________________________________________________________________________ 6.9 UNIT END QUESTIONS A. Descriptive Questions 93 Short Questions: CU IDOL SELF LEARNING MATERIAL (SLM)

1. Explain the concept of jitter. 2. What is propagation speed? 3. Define propagation time. 4. What do you mean by wavelength? 5. Define transmission time. Long Questions: 1. What are the propagation time and the transmission time for a 5-Mbyte message (an image) if thebandwidth of the network is 1 Mbps? Assume that the distance between the sender and thereceiver is 12,000 km and that light travels at 2.4 x 108 mls. 2. Explain Shannon capacity in detail. 3. How can we able to compare different media? 4. Explain bandwidth delay product. 5. Explain time and frequency domains. B. Multiple Choice Questions 1. What will happen to period when frequency increases? a. Decreases b. Increases c. Remains the same d. Double 2. What kind of a wave is a sine wave? a. Periodic and discrete b. Periodic and continuous c. Non periodic and continuous d. Non periodic and discrete 3. How does frequency and period relate to each other? a. Inverse of each other b. Proportional to each other c. The same d. None of these 94 CU IDOL SELF LEARNING MATERIAL (SLM)

4. What is termed as the rate of change with respect to time? a. Amplitude b. Time c. Frequency d. Voltage 5. Identify the domain in which a sine wave be represented as one single spike? a. Time b. Frequency c. Phase d. None of these Answers 1-a, 2-b, 3-a, 4-c, 5-a 6.10 REFERENCES References  Martin, J. and Leben, J. (1988), Principles of Data Communication, Prentice Hall, Englewood Cliffs, NJ.  Spohn, D. (1993) Data Network Design, McGraw-Hill, NY.  Stallings, W. (1990), Handbook of Computer Communications Standards, Volumes I and II, Howard Sams and Company, Carmel. Textbooks  Stallings, W. (1992), ISDN and Broadband ISDN, Second Edition, Macmillan, NY.  Stallings, W. (1994), Data and Computer Communications, Fourth Edition, Macmillan, NY.  Davies, J. Understanding IPv6. Redmond, WA: Microsoft Press, 2002. Websites  https://ecomputernotes.com/computernetworkingnotes/multiple-access/what-is-wired- transmission-type-of-wired-transmission  https://www.techopedia.com/definition/30527/switched-line  https://www.c-sharpcorner.com/uploadfile/abhikumarvatsa/basics-of-data- communication-part-1/ 95 CU IDOL SELF LEARNING MATERIAL (SLM)

UNIT – 7: WIRELESS TRANSMISSION PART 1 STRUCTURE 7.0 Learning Objectives 7.1 Introduction 7.2 Microwave Transmission 7.3 Infrared Transmission 7.4 Laser Transmission 7.5 Summary 7.6 Keywords 7.7 Learning Activity 7.8 Unit End Questions 7.9 References 7.0 LEARNING OBJECTIVES After studying this unit, you will be able to:  Explain microwave transmission in detail.  Describe infrared wireless transmission methods.  Illustrate the features of laser transmission.  Illustrate the characteristics of wireless transmission methods. 7.1 INTRODUCTION Wireless transmission is a form of unguided media. Wireless communication involves no physical link established between two or more devices, communicating wirelessly. Wireless signals are spread over in the air and are received and interpreted by appropriate antennas. When an antenna is attached to electrical circuit of a computer or wireless device, it converts the digital data into wireless signals and spread all over within its frequency range. The receptor on the other end receives these signals and converts them back to digital data. A little part of electromagnetic spectrum can be used for wireless transmission. 96 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 7.1: Electromagnetic spectrum 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 7.2 shows the part of the electromagnetic spectrum, ranging from 3 kHz to900 THz, used for wireless communication. Figure 7.2: Electromagnetic spectrum for wireless communication Unguided signals can travel from the source to destination in several ways: groundpropagation, sky propagation, and line-of-sight propagation, as shown in figure 7.3, figure 7.4, figure 7.5 respectively.In ground propagation, radio waves travel through the lowest portion of theatmosphere, hugging the earth. These low-frequency signals emanate in all directionsfrom the transmitting antenna and follow the curvature of the planet. 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 otherand either tall enough or close enough together not to be affected by the curvature ofthe earth. Line-of-sight propagation is tricky because radio transmissions cannot becompletely focused. 97 CU IDOL SELF LEARNING MATERIAL (SLM)

Figure 7.3: Ground wave propagation Figure 7.4: Sky wave propagation Figure 7.5: Light of sight propagation The section of the electromagnetic spectrum defined as radio waves and microwavesis divided into eight ranges, called bands, each regulated by government authorities.These bands are rated from very low frequency (VLF) to extremely highfrequency (EHF).Table 7.1 lists these bands, their ranges, propagation methods, and some applications. 98 CU IDOL SELF LEARNING MATERIAL (SLM)

Table 7.1: Bands We can divide wireless transmission into three broad groups: radio waves, microwaves,and infrared waves as shown in figure 7.6. Figure 7.6: Wireless transmission waves 7.2 MICROWAVE TRANSMISSION Electromagnetic waves having frequencies between me and 300 GHz are called microwaves.Microwaves are unidirectional. When an antenna transmits microwave waves, theycan be narrowly focused. This means that the sending and receiving antennas need tobe aligned. The unidirectional property has an obvious advantage. A pair of antennascan be aligned without interfering with another pair of aligned antennas. The followingdescribes some characteristics of microwave propagation. 99 CU IDOL SELF LEARNING MATERIAL (SLM)

 Microwave propagation is line-of-sight. Since the towers with the mounted antennas need to be in direct sight of each other, towers that are far apart need to be very tall. The curvature of the earth as well as other blocking obstacles does not allow two short towers to communicate by using microwaves. Repeaters are often needed for longdistance communication.  Very high-frequency microwaves cannot penetrate walls. This characteristic can be a disadvantage if receivers are inside buildings.  The microwave band is relatively wide, almost 299 GHz. Therefore wider subbands can be assigned, and a high data rate is possible.  Use of certain portions of the band requires permission from authorities. Figure 7.7: Micro wave transmission Unidirectional Antenna Microwaves need unidirectional antennas that send out signals in one direction. Twotypes of antennas are used for microwave communications: the parabolic dish and thehome (see figure .7.8A parabolic dish antenna is based on the geometry of a parabola: Every lineparallel to the line of symmetry (line of sight) reflects off the curve at angles such thatall the lines intersect in a common point called the focus. The parabolic dish works as afunnel, catching a wide range of waves and directing them to a common point. Inthis way, more of the signal is recovered than would be possible with a single-pointreceiver. 100 CU IDOL SELF LEARNING MATERIAL (SLM)