MODULE I Introduction to Computer networks and Network models 1. Types of computer networks, Internet, Intranet, Network topologies, Network classifications. 2. Network Architecture Models: Layered architecture approach, OSI Reference Model, TCP/IP 3. Physical Layer: Analog signal, digital signal, Analog to Digital, Digital to Analog, maximum data rate of a channel transmission 4. Transmission media (guided transmission media, wireless transmission, satellite communication). 5. multiplexing (frequency division multiplexing, time division multiplexing, wavelength division multiplexing Introduction to Computer networks A computer network is a system that connects many independent computers to share information (data) and resources. The integration of computers and other different devices allows users to communicate more easily. It is a collection of two or more computer systems that are linked together. A network connection can be established using either cable or wireless media. Hardware and software are used to connect computers and tools in any network. Uses of Computer Networks ● Communicating using email, video, instant messaging, etc. ● Sharing devices such as printers, scanners, etc. ● Sharing files. ● Sharing software and operating programs on remote systems. ● Allowing network users to easily access and maintain information. Advantages of Computer Network ● Central Storage of Data: Files are stored on a central storage database which helps to easily access and available to everyone. ● Connectivity: A single connection can be routed to connect multiple computing devices. ● Sharing of Files: Files and data can be easily shared among multiple devices which helps in easily communicating among the organization. ● Security through Authorization: Computer Networking provides additional security and protection of information in the system. Disadvantages of Computer Network ● Virus and Malware: A virus is a program that can infect other programs by modifying them. Viruses and Malware can corrupt the whole network. ● High Cost of Setup: The initial setup of Computer Networking is expensive because it consists of a lot of wires and cables along with the device. ● loss of Information: In case of a System Failure, might lead to some loss of data. ● Management of Network: Management of a Network is somehow complex for a person, it requires training for its proper use. CATEGORIES OF NETWORKS Computer networks can be characterized by their size as well as their purpose. The size of a network can be expressed by the geographic area they occupy and the number of computers that are part of the network. Some of the different networks based on size are: ● Personal area network, or PAN ● Local area network, or LAN ● Metropolitan area network, or MAN ● Wide area network, or WAN In terms of purpose, many networks can be considered general purpose, which means they are used for everything from sending files to a printer to accessing the Internet. Some types of networks, however, serve a very particular purpose. Some of the different networks based on their main purpose are: ● Storage area network, or SAN ● Enterprise private network, or EPN ● Virtual private network, or VPN Local Area Network (LAN) A local area network consists of a computer network at a single site, typically an individual office building. A LAN is very useful for sharing resources, such as data storage and printers. LANs can be built with relatively inexpensive hardware, such as hubs, network adapters and Ethernet cables. LAN size is limited to a few kilometers. Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range. Today, however, speeds are normally 100 or 1000 Mbps. Wireless LANs are the newest evolution in LAN technology. If a local area network, or LAN, is entirely Wireless, it is referred to as a wireless local area network, or WLAN. Wide Area Network (WAN) A wide area network (WAN) provides long-distance transmission of data, image, audio, and video information over large geographic areas that may comprise a country, a continent, or even the whole world. A WAN can contain multiple smaller networks, such as LANs or MANs. The Internet is the best-known example of a public WAN. We normally refer to the first as a switched WAN and to the second as a point-to-point WAN. The switched WAN connects the end systems, which usually comprise a router that connects to another LAN or WAN. The point-to-point WAN is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an Internet service provider (lSP). This type of WAN is often used to provide Internet access. Metropolitan Area Network (MAN) A metropolitan area network is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high-speed connectivity, normally to the Internet, and have endpoints spread over a city or part of city. A good example is example is the cable TV network. Personal area network A personal area network is a computer network organized around an individual person within a single building. This could be inside a small office or residence. A typical PAN would include one or more computers, telephones, peripheral devices, video game consoles and other personal entertainment devices. Internet and Intranet The Internet is a global, public network accessible by anyone, while an intranet is a private, internal network typically used by an organization. The Internet connects computers worldwide for communication and information exchange, while an intranet enables internal communication and resource sharing within an organization. Internet: ● Public Network: Open to anyone with an internet connection. ● Global: Connects computers and networks across the world. ● External Communication: Used for communicating with customers, stakeholders, and the general public. ● Diverse Uses: Supports various applications like email, social media, web browsing, and streaming. Intranet: ● Private Network: Restricted to authorized users within an organization. ● Internal: Connects computers and resources within a specific organization. ● Internal Communication: Used for sharing information, resources, and tools within a company. ● Security: Often protected by firewalls, encryption, and other security measures to prevent unauthorized access. ● Example: A company might use an intranet for employee communications, document storage, or project collaboration. NETWORK TOPOLOGIES The term physical topology refers to the way in which a network is laid out physically. 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 Star Topology In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. 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. . Star topology is used in local-area networks Advantages • Robustness - Failure of a node does not affect the network • Easy fault identification and fault isolation • Fast performance with few nodes and low network traffic. • Hub can be upgraded easily. • Easy to troubleshoot. • Easy to setup and modify (Add or Remove nodes) Disadvantages • If the hub goes down, the whole system is dead. • Cost of installation is high 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 Advantages • Relatively easy to install and reconfigure • fault isolation is simplified Disadvantages • Failure of a node affects the entire network • Troubleshooting is difficult in ring topology. • Adding or deleting the computers disturbs the network activity Bus Topology A bus topology is a multipoint connection. One long cable acts as a backbone to link all the devices in a network. 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. 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. Advantages • Ease of installation • Bus topology uses less cabling • Failure of a node does not affect the network • It is cost effective Disadvantages • Difficult reconnection and fault isolation • A fault or break in the bus cable stops all transmission Mesh topology In a mesh topology, every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. Mesh has n (n-1)/2 physical channels to link n devices. Advantages • Eliminating the traffic problems because each connection can carry its own data load • Robust • Privacy or security. • Easy fault identification and fault isolation Disadvantages • The amount of cabling and the number of I/O ports required Installation and configuration is difficult. • Expensive Hybrid topology A network can be hybrid. It is a combination of different topologies. For example: a star backbone with three bus networks NETWORK ARCHITECTURE MODELS Network architecture generally refers to the design of a computer network or communications network. It is simply a way in which all network devices and services are organized and managed to connect clients like laptops, tablets, servers, etc., and also how tasks are allocated to computers. It is defined as the physical and logical design of software, hardware, protocols, and media of data transmission. Computer networks can be classified based on architecture into two primary types: ● Peer-to-Peer Architecture ● Client/Server Architecture Peer-to-Peer Architecture In the P2P (Peer-to-Peer) network, “peers” generally represent computer systems. These peers are connected to each other with the help of the Internet. Files might be shared directly without the requirement of a central server among these systems on the network. It can be said that each computer on a P2P network usually becomes a file server even as a client also. In this architecture, the system is generally decomposed into various computational nodes that contain the same and equivalent capabilities, abilities, and responsibilities. In this network, tasks are allocated at each and every device available on the network. This network is very essential and important for small environments, usually up to at least 10 computers. There is also no separate division as clients and servers. Each and every computer in this network is treated the same and equally and might send or even receive messages directly. This P2P network is generally useful in various fields such as business, education, military, etc. Advantages ● P2P networks are cheaper. It is affordable. ● P2P is very simple. This is because all computers that are connected in network communication are efficient and well-mannered with each other. ● It is very easy to set up and manage as installation and setup is less painless and the computer manages itself. Disadvantages ● It is more difficult to manage security policies consistently. ● Each peer demands individual care and control. ● As the network expands in size, it may become inefficient. ● Performance, security, and access can also become major problems with an increase in the number of computers on this network. Client/Server Architecture CSN (Client/Server Network) is a type of computer network in which one of centralized and powerful computers (commonly called a server) is the hub to which many personal computers that are less powerful or workstations (commonly known as clients) are connected. It is a type of system where clients are connected to servers to just share or use resources. These servers are generally considered the heart of the system. This type of network is more stable and scalable as compared to P2P networks. In this architecture, the system is generally decomposed into client and server processors or processes. Advantages ● A special Network Operating System (NOS) is provided by the server to provide resources to many users that request them. ● It is also very simple to set up and manage data updates. This is because data is generally stored in a centralized manner on the server. ● The server usually controls resources and data security.. ● This network also boosts the speed of sharing resources. Disadvantages ● If the server fails, clients may lose access to services. ● It is very expensive as compared to P2P. This is due to the need for servers with more memory as well as the need for many networking devices such as hubs, routers, switches, etc. ● Managing servers requires skilled personnel. ● Cost of NOS being provided is very high. Network Architecture Models Layered Architecture Approach Every network consists of a specific number of functions, layers, and tasks to perform. Layered Architecture in a computer network is defined as a model where a whole network process is divided into various smaller sub-tasks. These divided sub-tasks are then assigned to a specific layer to perform only the dedicated tasks. A single layer performs only a specific type of task. To run the application and provide all types of services to clients a lower layer adds its services to the higher layer present above it. Therefore layered architecture provides interactions between the sub-systems. If any type of modification is done in one layer it does not affect the next layer. As shown in the above diagram, there are five different layers. Therefore, it is a five-layered architecture. Each layer performs a dedicated task. The lower-level data for example from layer 1 data is transferred to layer 2. Below all the layers Physical Medium is present. The physical medium is responsible for the actual communication to take place. For the transfer of data and communication layered architecture provides a simple interface. In computer networks, layered architecture is majorly used for communication. The two network models that makes use of layered architecture are: ● OSI Model ● TCP/IP Model 1. OSI Model OSI stands for Open Systems Interconnection. OSI is a seven layered architecture. All these seven layers work collaboratively to transmit data from one layer to another. Below are the layers of OSI Model. ● Physical Layer: Physical layer is the lowest layer of OSI model and is responsible for the physical connection between all the required devices. The information present in the physical layer is in the form of bits. Physical layer performs various functions such as bit rate control, bit synchronization, transmission mode etc. ● Data Link Layer: Data Link layer provides successful delivery of messages from one node to the another. It checks whether this delivery of the message is error free. Other functions performed by the data link layer are error control, framing, flow control etc. ● Network Layer: Network Layer is responsible for the transmission of data from one host to another host that is connected in a different network. It performs other tasks such as routing and logical addressing. ● Transport Layer: Transport Layer is defined as a layer that takes services from the network layer and provides services to the application layer. Other tasks performed by the transport layer are service point addressing, segmentation and reassembling. ● Session Layer: Session layer is defined as a layer that is responsible for establishing a connection, maintenance of session and to provide security. Other functions of session Layer are to establish session, termination and synchronization. ● Presentation Layer: The data from the application layer is extracted at the presentation layer. This layer is also known as the translation layer. The functions of the presentation layer are encryption, decryption, compression and translation. ● Application Layer: Application layer is the topmost layer of OSI Model. Application layer is also known as the desktop layer. It provides other functions such as directory services, mail services, network virtual terminal etc. 2. TCP/IP Model 1. Network Access Layer It is a group of applications requiring network communications. This layer is responsible for generating the data and requesting connections. It acts on behalf of the sender and the Network Access layer on the behalf of the receiver. 2. Internet Layer (Network layer) This layer parallels the functions of OSI’s Network layer. It defines the protocols which are responsible for the logical transmission of data over the entire network. The main protocols residing at this layer are as follows: ● IP: IP stands for Internet Protocol and it is responsible for delivering packets from the source host to the destination host by looking at the IP addresses in the packet headers. IP has 2 versions: IPv4 and IPv6. IPv4 is the one that most websites are using currently. But IPv6 is growing as the number of IPv4 addresses is limited in number when compared to the number of users. ● ICMP: ICMP stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is responsible for providing hosts with information about network problems. ● ARP: ARP stands for Address Resolution Protocol. Its job is to find the hardware address of a host from a known IP address. ARP has several types: Reverse ARP, Proxy ARP, Gratuitous ARP, and Inverse ARP. 3. Transport Layer The TCP/IP transport layer protocols exchange data receipt acknowledgments and retransmit missing packets to ensure that packets arrive in order and without error. End-to-end communication is referred to as such. Transmission Control Protocol (TCP) and User Datagram Protocol are transport layer protocols at this level (UDP). ● TCP: Applications can interact with one another using TCP as though they were physically connected by a circuit. TCP transmits data in a way that resembles character-by-character transmission rather than separate packets. A starting point that establishes the connection, the whole transmission in byte order, and an ending point that closes the connection make up this transmission. ● UDP: The datagram delivery service is provided by UDP, the other transport layer protocol. Connections between receiving and sending hosts are not verified by UDP. Applications that transport little amounts of data use UDP rather than TCP because it eliminates the processes of establishing and validating connections. 4. Application Layer This layer is analogous to the transport layer of the OSI model. It is responsible for end-to-end communication and error-free delivery of data. It shields the upper-layer applications from the complexities of data. The three main protocols present in this layer are: ● HTTP and HTTPS: HTTP stands for Hypertext transfer protocol. It is used by the World Wide Web to manage communications between web browsers and servers. HTTPS stands for HTTP-Secure. It is a combination of HTTP with SSL(Secure Socket Layer). It is efficient in cases where the browser needs to fill out forms, sign in, authenticate, and carry out bank transactions. ● SSH: SSH stands for Secure Shell. It is a terminal emulation software similar to Telnet. The reason SSH is preferred is because of its ability to maintain the encrypted connection. It sets up a secure session over a TCP/IP connection. ● NTP: NTP stands for Network Time Protocol. It is used to synchronize the clocks on our computer to one standard time source. It is very useful in situations like bank transactions. Assume the following situation without the presence of NTP. Suppose you carry out a transaction, where your computer reads the time at 2:30 PM while the server records it at 2:28 PM. The server can crash very badly if it’s out of sync. Physical Layer: Data and Signals One of the major functions of the physical layer is to move data in the form of electromagnetic signals across a transmission medium. Analog and Digital Both data and the signals that represent them can be either analog or digital in form. Analog and Digital Data Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states. For example, an analog clock that has hour, minute, and second hands gives information in a continuous form; the movements of the hands are continuous. On the other hand, a digital clock that reports the hours and the minutes will change suddenly from 8:05 to 8:06. Analog and Digital Signal Signals can be analog or digital. Analog signals can have an infinite number of values in a range. Digital signals can have only a limited number of values. Both analog and digital signals can take one of two forms: periodic or nonperiodic (aperiodic). A periodic signal completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods. The completion of one full pattern is called a cycle. A nonperiodic signal changes without exhibiting a pattern or cycle that repeats over time. In data communications, we commonly use periodic analog signals and nonperiodic digital signals. DIGITAL SIGNALS ● Digital data is discrete, discontinuous representation of any data or information ● Digital signal is discrete and have only limited set of values with transition from one value to another being instantaneous ● Bit interval refers to the time taken to send a single bit ● Bit rate refers to the number of bit intervals per second or number of bits sent in one second ● Computers, CDs, DVDs are examples of digital devices ANALOG TO DIGITAL CONVERSION Two techniques are used for changing an analog signal to digital data. ● Pulse code modulation ● Delta modulation Pulse Code Modulation (PCM) Pulse Code Modulation (PCM) is a digital encoding technique that converts analog signals into a digital format for transmission. It involves sampling, quantization, and encoding the analog signal into a series of binary digits (0s and 1s). This digital representation allows for easier processing, storage, and transmission of the signal, especially over digital communication networks. 1. Sampling: The analog signal is sampled at regular intervals, converting a continuous waveform into a series of discrete values. 2. Quantization: The sampled values are then rounded off to the nearest predefined level, representing them with a finite number of discrete levels. 3. Encoding: These quantized values are then converted into binary code, representing them as a series of 0s and 1s. Delta modulation Delta Modulation (DM) is a method of analog-to-digital signal conversion used in signal processing and communications. It simplifies the process of converting analog signals to digital by encoding the difference between successive samples, rather than the absolute sample values as in Pulse Code Modulation (PCM). 1. Sampling: The analog signal is sampled at a regular interval. 2. Quantization: Instead of encoding the absolute value of the sample, DM encodes whether the signal is increasing or decreasing compared to the previous sample. 3. 1-bit output: The result is a stream of 1-bit values: ○ 1 if the signal is going up ○ 0 if the signal is going down DIGITAL-TO-ANALOG CONVERSION Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. A sine wave is defined by three characteristics: amplitude, frequency, and phase. By changing one characteristic of a simple electric signal, we can use it to represent digital data. Any of the three characteristics can be altered in this way, giving us at least three mechanisms for modulating digital data into an analog signal: amplitude shift keying (ASK), frequency shift keying (FSK), and phase shift keying (PSK). In addition, there is a fourth (and better) mechanism that combines changing both the amplitude and phase, called quadrature amplitude modulation (QAM). QAM is the most efficient of these options and is the mechanism commonly used today. Bit rate is the number of bits per second. Baud rate is the number of signal elements per second. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate.In analog transmission, the sending device produces a high-frequency signal that acts as a base for the information signal. This base signal is called the carrier signal or carrier frequency. The receiving device is tuned to the frequency of the carrier signal that it expects from the sender. Digital information then changes the carrier signal by modifying one or more of its characteristics (amplitude, frequency, or phase). This kind of modification is called modulation (shift keying). Amplitude Shift Keying In amplitude shift keying, the amplitude of the carrier signal is varied to create signal elements. Both frequency and phase remain constant while the amplitude changes. ASK is normally implemented using only two levels. This is referred to as binary amplitude shift keying (BASK) or on-off keying (OOK). The peak amplitude of one signal level is 0; the other is the same as the amplitude of the carrier frequency. Frequency Shift Keying In frequency shift keying, the frequency of the carrier signal is varied to represent data. The frequency of the modulated signal is constant for the duration of one signal element, but changes for the next signal element if the data element changes. Both peak amplitude and phase remain constant for all signal elements. Phase Shift Keying In phase shift keying, the phase of the carrier is varied to represent two or more different signal elements. Both peak amplitude and frequency remain constant as the phase changes. Today, PSK is more common than ASK or FSK. Binary PSK (BPSK) The simplest PSK is binary PSK, in which we have only two signal elements, one with a phase of 0°, and the other with a phase of 180°. Binary PSK is as simple as binary ASK with one big advantage-it is less susceptible to noise. Noise can change the amplitude easier than it can change the phase. PSK is superior to FSK because we do not need two carrier signals. The bandwidth is the same as that for binary ASK, but less than that for BFSK. No bandwidth is wasted for separating two carrier signals. Quadrature PSK The scheme is called quadrature PSK or QPSK because it uses two separate BPSK modulations; one is in-phase, the other quadrature (out-of-phase). Maximum data rate of a channel transmission The maximum data rate of a channel, often referred to as its capacity, is the highest amount of data that can be transmitted over that channel in a given time. It is fundamentally limited by the channel's bandwidth and the signal-to-noise ratio, according to the Shannon-Hartley theorem. In essence, the maximum data rate depends on how effectively the channel can carry information without significant interference. TRANSMISSION MEDIA Transmission media are actually located below the physical layer and are directly controlled by the physical layer. A transmission medium can be broadly defined as anything that can carry information from a source to a destination. For example, the transmission medium for two people having a dinner conversation is the air. The transmission medium is usually free space, metallic cable, or fiber-optic cable. The information is usually a signal that is the result of a conversion of data from another form. In telecommunications, transmission media can be divided into two broad categories: guided and unguided. Guided media include twisted-pair cable, coaxial cable, and fiber-optic cable. Unguided medium is free space. GUIDED MEDIA Guided media, which are those that provide a channel from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable. A signal traveling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a cable that accepts and transports signals in the form of light. 1. Twisted-Pair Cable A twisted pair consists of two conductors (normally copper), each with its own plastic insulation, twisted together. One of the wires is used to carry signals to the receiver, and the other is used only as 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. 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 between the two, receives no unwanted signals. The unwanted signals are mostly canceled out. The number of twists per unit of length has some effect on the quality of the cable. The most common twisted-pair cable used in communications is referred to as unshielded twisted-pair (UTP). IBM has also produced a version of twisted-pair cable for its use called shielded twisted-pair (STP). STP cable has a metal foil or braided mesh covering that encases each pair of insulated conductors. Although metal casing improves the quality of cable by preventing the penetration of noise or crosstalk, it is bulkier and more expensive. The most common UTP connector is RJ45 (RJ stands for registered jack). 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 10Base-T and 100Base-T, also use twisted-pair cables. 2. Coaxial Cable Coaxial cable (or coax) carries signals of higher frequency ranges than those in twisted pair cable, in part because the two media are constructed quite differently. Instead of having two wires, coax has a central core conductor of solid or stranded wire (usually copper) enclosed in an insulating sheath, which is, in turn, encased in an outer conductor of metal foil, braid, or a combination of the two. The outer metallic wrapping serves both as a shield against noise and the second conductor, which completes the circuit. This outer conductor is also enclosed in an insulating sheath, and the whole cable is protected by a plastic cover. To connect coaxial cable to devices, we need coaxial connectors. The most common type of connector used today is the Bayone-Neill-Concelman (BNC) connector. Three popular types of these connectors: the BNC connector, the BNC T connector, and the BNC terminator. Coaxial cable has a much higher bandwidth; the signal weakens rapidly and requires the frequent use of repeaters. The attenuation is much higher in coaxial cables than in twisted-pair cable 3. Fiber-Optic Cable A fiber-optic cable is made of glass or plastic and transmits signals in the form of light. Light travels in a straight line as long as it is moving through a single uniform substance. If a ray of light traveling through one substance suddenly enters another substance of a different density, the ray changes direction. If the angle of incidence I is less than the critical angle, the ray refracts and moves closer to the surface. If the angle of incidence is equal to the critical angle, the light bends along the interface. If the angle is greater than the critical angle, the ray reflects and travels again in the denser substance. The critical angle is a property of the substance, and its value differs from one substance to another. Optical fibers use reflection to guide light through a channel. A glass or plastic core is surrounded by a cladding of less dense glass or plastic. The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. The outer jacket is made of either PVC or Teflon. Inside the jacket are Kevlar strands to strengthen the cable. Kevlar is a strong material used in the fabrication of bulletproof vests. Below the Kevlar is another plastic coating to cushion the fiber. The fiber is at the center of the cable, and it consists of cladding and core. There are three types of connectors for fiber-optic cables. The subscriber channel (SC) connector is used for cable TV. It uses a push/pull locking system. The straight-tip (ST) connector is used for connecting cable to networking devices. It uses a bayonet locking system and is more reliable than SC. MT-RJ is a connector that is the same size as RJ45. Advantages of Optical Fiber Higher bandwidth: Fiber-optic cable can support dramatically higher bandwidths (and hence data rates) than either twisted-pair or coaxial cable. Less signal attenuation: Fiber-optic transmission distance is significantly greater than that of other guided media . Immunity to electromagnetic interference: Electromagnetic noise cannot affect fiber- optic cables. Resistance to corrosive materials: Glass is more resistant to corrosive materials than copper. Light weight: Fiber-optic cables are much lighter than copper cables. Greater immunity to tapping: Fiber-optic cables are more immune to tapping than copper cables. Disadvantages of Optical Fiber Installation and maintenance: Fiber-optic cable is a relatively new technology. Its installation and maintenance require expertise that is not yet available everywhere. Unidirectional light propagation: Propagation of light is unidirectional. If we need bidirectional communication, two fibers are needed. Cost: The cable and the interfaces are relatively more expensive than those of other guided media. UNGUIDED MEDIA: WIRELESS Unguided media transport electromagnetic waves without using a physical conductor. This type of communication is often referred to as wireless communication. Signals are normally broadcast through free space and thus are available to anyone who has a device capable of receiving them. We can divide wireless transmission into three broad groups: radio waves, microwaves, and infrared waves. Radio Waves Electromagnetic waves ranging in frequencies between 3 kHz and 1 GHz are normally called radio waves.Radio waves 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. 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 advantage because, for example, an AM radio can receive signals inside a building. It is a disadvantage because we cannot isolate communication to just inside or outside a building. 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, maritime radio, cordless phones, and paging are examples of multicasting. Microwaves Electromagnetic waves having frequencies between 1 and 300 GHz are called microwaves.Microwaves are unidirectional. When an antenna transmits microwave waves, they can be narrowly focused. This means that the sending and receiving antennas need to be aligned. Microwave propagation is line-of-sight. Repeaters are often needed for long distance communication. Very high-frequency microwaves cannot penetrate walls. Microwave band is relatively wide, almost 299 GHz. Therefore wider sub bands can be assigned, and a high data rate is possible. Use of certain portions of the band requires permission from authorities.Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs. Infrared Infrared waves, with frequencies from 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm), can be used for short-range communication. Infrared waves, having high frequencies, cannot penetrate walls. We cannot use infrared waves outside a building because the sun's rays contain infrared waves that can interfere with the communication. Infrared signals can be used in a closed area using line-of-sight propagation. Satellite Communication Satellite communication is transporting information from one place to another using a communication satellite in orbit around the Earth. A communication satellite is an artificial satellite that transmits the signal via a transponder by creating a channel between the transmitter and the receiver at different Earth locations.Telephone, radio, television, internet, and military applications use satellite communications. Believe it or not, more than 2000 artificial satellites are hurtling around in space above your heads. Working The communication satellites are similar to the space mirrors that help us bounce signals such as radio, internet data, and television from one side of the earth to another. Three stages are involved, which explain the working of satellite communications. These are: ● Uplink ● Transponders ● Downlink In the first stage, the signal from the television broadcast on the other side of the earth is first beamed up to the satellite from the ground station on the earth. This process is known as uplink. The second stage involves transponders such as radio receivers, amplifiers, and transmitters. These transponders boost the incoming signal and change its frequency so that the outgoing signals are not altered. Depending on the incoming signal sources, the transponders vary. The final stage involves a downlink in which the data is sent to the other end of the receiver on the earth. It is important to understand that usually, there is one uplink and multiple downlinks. MULTIPLEXING Multiplexing is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. In a multiplexed system, n lines share the bandwidth of one link. Figure shows the basic format of a multiplexed system. The lines on the left direct their transmission streams to a multiplexer (MUX),which combines them into a single stream (many-to-one). At the receiving end, that stream is fed into a demultiplexer (DEMUX), which separates the stream back into its component transmissions (one-to-many) and directs them to their corresponding lines. There are three basic multiplexing techniques: frequency-division multiplexing, wavelength-division multiplexing, and time-division multiplexing. The first two are techniques designed for analog signals, the third, for digital signals. Frequency-division multiplexing Frequency-division multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link. Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. These bandwidth ranges are the channels through which the various signals travel. Channels can be separated by 163 strips of unused bandwidth-guard bands-to prevent signals from overlapping. In addition, carrier frequencies must not interfere with the original data frequencies. Figure gives a conceptual view of FDM. In this illustration, the transmission path is divided into three parts, each representing a channel that carries one transmission. FDM cannot be used to combine sources sending digital signals. A digital signal can be converted to an analog signal before FDM is used to multiplex them. Multiplexing Process Each source generates a signal of a similar frequency range. Inside the multiplexer, these similar signals modulate different carrier frequencies (f1, f2 and f3). The resulting modulated signals are then combined into a single composite signal that is sent out over a media link that has enough bandwidth to accommodate it. Demultiplexing Process The demultiplexer uses a series of filters to decompose the multiplexed signal into its constituent component signals. The individual signals are then passed to a demodulator that separates them from their carriers and passes them to the output lines. Wavelength-Division Multiplexing WDM is conceptually the same as FDM, except that the multiplexing and demultiplexing involve optical signals transmitted through fiber-optic channels. We are combining different signals of different frequencies. The difference is that the frequencies are very high. Very narrow bands of light from different sources are combined to make a wider band of light. At the receiver, the signals are separated by the demultiplexer. Wavelength-division multiplexing (WDM) is designed to use the high-data-rate capability of fiber-optic cable. The optical fiber data rate is higher than the data rate of metallic transmission cable. Using a fiber-optic cable for one single line wastes the available bandwidth. Multiplexing allows us to combine several lines into one. The combining and splitting of light sources are easily handled by a prism. A prism bends a beam of light based on the angle of incidence and the frequency. Using this technique, a multiplexer can be made to combine several input beams of light, each containing a narrow band of frequencies, into one output beam of a wider band of frequencies. A demultiplexer can also be made to reverse the process. Time-division multiplexing Time-division multiplexing (TDM) is a digital process that allows several connections to share the high bandwidth of a link. Instead of sharing a portion of the bandwidth as in FDM, time is shared. Each connection occupies a portion of time in the link. In the figure, portions of signals 1, 2, 3, and 4 occupy the link sequentially. TDM is a digital multiplexing technique for combining several low-rate channels into one high-rate one. We can divide TDM into two different schemes: synchronous and statistical. Synchronous TDM In synchronous TDM, the data flow of each input connection is divided into units, where each input occupies one input time slot. A unit can be 1 bit, one character, or one block of data. Each input unit becomes one output unit and occupies one output time slot. However, the duration of an output time slot is n times shorter than the duration of an input time slot. If an input time slot is T s, the output time slot is T/n s, where n is the number of connections. Figure shows an example of synchronous TDM where n is 3. In synchronous TDM, a round of data units from each input connection is collected into a frame. If we have n connections, a frame is divided into n time slots and one slot is allocated for each unit, one for each input line. If the duration of the input unit is T, the duration of each slot is T/n and the duration of each frame is T. The data rate of the link is n times faster, and the unit duration is n times shorter. Statistical Time-Division Multiplexing In synchronous TDM, each input has a reserved slot in the output frame. This can be inefficient if some input lines have no data to send. In statistical time-division multiplexing, slots are dynamically allocated to improve bandwidth efficiency. Only when an input line has a slot's worth of data to send is it given a slot in the output frame. In statistical multiplexing, the number of slots in each frame is less than the number of input lines. The multiplexer checks each input line in round robin fashion; it allocates a slot for an input line if the line has data to send; otherwise, it skips the line and checks the next line. An output slot in synchronous TDM is totally occupied by data; in statistical TDM, a slot needs to carry data as well as the address of the destination. In synchronous TDM, there is no need for addressing; synchronization and preassigned relationships between the inputs and outputs serve as an address. In statistical multiplexing, there is no fixed relationship between the inputs and outputs because there are no preassigned or reserved slots. We need to include the address of the receiver inside each slot to show where it is to be delivered. The addressing in its simplest form can be n bits to define N different output lines with n =log2 N. For example, for eight different output lines, we need a 3- bit address. In statistical TDM, a block of data is usually many bytes while the address is just a few bytes. There is another difference between synchronous and statistical TDM, but this time it is at the frame level. The frames in statistical TDM need not be synchronized, so we do not need synchronization bits.