A computer network is a collection of computers and various electronic devices connected to them

A computer network is a collection of computers and various electronic devices connected to them.Such an exchange allows network users not only to form a common information space in which everyone

1. Unit III

2. Computer Network A computer network is a collection of computers and various electronic devices connected to them, for which, with the help of special communication facilities, it is possible to directly exchange information in real time. Such an exchange allows network users not only to form a common information space in which everyone who has access to this network becomes a potential owner of all the information in it, but also simultaneously work together to transform this information of several users.

3. Continue.. In addition, combining computers and related electronic devices into a computer network allows each user to remotely control the operation of any of these devices. A system of interconnected computers and computerized peripherals such as printers is called computer network. This interconnection among computers facilitates information sharing among them. Computers may connect to each other by either wired or wireless media.

4. Classification of Computer Networks The generally accepted classification of computer networks is based on the following principles. Computer networks are distinguished: 1) by the size of the territory, which covers this network (territorial prevalence). On this basis, the networks are divided into local, regional and global; 2) network owner (by network ownership). On this basis, the networks are divided into departmental (corporate), state and commercial;

5. Continue.. 3) speed of information transfer on the network. On this basis, the networks are divided into high-speed (over 100 Mbps), medium-speed (up to 100 Mbps) and low-speed (up to 10 Mbps); 4) type of physical devices, providing communication (media type). On this basis, the networks are divided into fiber -optic networks, networks using coaxial cable, twisted pair, radio channels, satellite communication, etc.

6. Local Computer Networks Local computer networks are used to combine computers and related devices in a relatively small enclosed space. For example, in a school or college classroom or in several adjacent lecture rooms; for all computer devices of the department, laboratory, faculty or university, which are located in the same building or in adjacent buildings, etc. In small local networks, each computer has a certain degree of independence, i.e. the user working on it has the right to decide what information resources located on this computer, he wants to make publicly available. Such networks are called peer-to-peer networks.

7. Continue.. If the local network contains more than 10 computers, then to increase its bandwidth and reliability, i.e. for greater efficiency of work in this network, as well as for security, the network allocates special computers on which the most popular software applications and files are stored. Such computers are called servers. In a local network, computers usually use special cables or special wireless facilities to connect computers (most recently Wi-Fi technology is used for this purpose).

8. Continue.. If it is necessary to combine computer devices located in geographically remote locations, it is necessary to form a so-called regional network. Such networks are able to connect computers within one region (city, country, continent).

9. Global Network The global network is currently the Internet. It also includes many local, regional and corporate networks; this is the largest computer network that includes hundreds of millions of computers (according to some estimates, by 2013, about 1.5 billion computers have been registered in the global Internet network). The reliability of the global network is provided by high redundancy of communication channels.

10. Analog Transmission An analogue signal (otherwise known as a wave form) is characterised by being continuously variable along both amplitude and frequency. In the case of telephony, when we speak into a handset, our voice is converted into current, or voltage fluctuations. Those fluctuations in current are an analogue transmission of the actual voice pattern.

11. Continue.. To transmit an analogue signal effectively, we need to define the frequency in which is operates. In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz, and so the network provider (phone company) will allocate a bandwidth of around 4,000Hz for voice transmission. Because of the limited bandwidth analogue facilities have, they cannot support high-speed data transmission.

12. Digital Transmission Digital signals are much simpler than analogue signals. Instead of a continuous wave form, analogue signals are made up of a series of pulses that represent either one bit or zero bits. Each computer system uses a coding scheme which defines what combinations of ones and zeros make up all the characters in the character set. The data (ones and zeros) are carried throughout the network depending on whether it is an electrical or optical transmission system.

13. Continue.. Electrical Transmitting digital signals over an electrical system essentially means that the ones are represented by high voltage and zero bits are represented as low voltage (or nothing at all). Optical In optical networks, the ones are represented as the presence of light and zeros as the absence of light.

14. Components of Data Communication System Data Communication is defined as exchange of data between two devices via some form of transmission media such as a cable, wire or it can be air or vacuum also. For occurrence of data communication, communicating devices must be a part of communication system made up of a combination of hardware or software devices and programs.

15. Data Communication System Components : There are mainly five components of a data communication system: 1. Message 2. Sender 3. Receiver 4. Transmission Medium 5. Set of rules (Protocol)

16. Figure – Components of Data Communication System

17. Message : This is most useful asset of a data communication system. The message simply refers to data or piece of information which is to be communicated. A message could be in any form, it may be in form of a text file, an audio file, a video file, etc.

18. Sender : To transfer message from source to destination, someone must be there who will play role of a source. Sender plays part of a source in data communication system. It is simple a device that sends data message. The device could be in form of a computer, mobile, telephone, laptop, video camera, or a workstation, etc.

19. Receiver : It is destination where finally message sent by source has arrived. It is a device that receives message. Same as sender, receiver can also be in form of a computer, telephone mobile, workstation, etc.

20. Transmission Medium : In entire process of data communication, there must be something which could act as a bridge between sender and receiver, Transmission medium plays that part. It is physical path by which data or message travels from sender to receiver. Transmission medium could be guided (with wires) or unguided (without wires), for example, twisted pair cable, fiber optic cable, radio waves, microwaves, etc.

21. Set of rules (Protocol) : To govern data communications, various sets of rules had been already designed by the designers of the communication systems, which represent a kind of agreement between communicating devices. These are defined as protocol. In simple terms, the protocol is a set of rules that govern data communication. If two different devices are connected but there is no protocol among them, there would not be any kind of communication between those two devices. Thus the protocol is necessary for data communication to take place.

22. Continue.. A typical example of a data communication system is sending an e-mail. The user which send email act as sender, message is data which user wants to send, receiver is one whom user wants to send message, there are many protocols involved in this entire process, one of them is Simple Mail Transfer Protocol (SMTP), both sender and receiver must have an internet connection which uses a wireless medium to send and receive email.

23. Transmission Modes There are 3 types of transmission modes which are given below: Simplex mode, Half duplex mode, and Full duplex mode. These are explained as following below.

24. Simplex mode: In simplex mode, Sender can send the data but that sender can’t receive the data. It is a unidirectional communication.

25. Half-duplex mode: In half duplex mode, Sender can send the data and also can receive the data but one at a time. It is two-way directional communication but one at a time.

26. Full duplex mode: In full duplex mode, Sender can send the data and also can receive the data simultaneously. It is two-way directional communication simultaneously.

27. Difference between Simplex, Half duplex and Full Duplex Transmission Modes: Simplex Half duplex Full duplex Simplex mode is a uni-directional communication. Half duplex mode is a two-way directional communication but one at a time. Full duplex mode is a two-way directional communication simultaneously. In simplex mode, Sender can send the data but that sender can’t receive the data. In half duplex mode, Sender can send the data and also can receive the data but one at a time. In full duplex mode, Sender can send the data and also can receive the data simultaneously. The simplex mode provides less performance than half duplex and full duplex. The half duplex mode provides less performance than full duplex. Full duplex provides better performance than simplex and half duplex mode. Example of simplex mode are:Keyboard and monitor. Example of half duplex mode is: Walkie-Talkies. Example of full duplex mode is:Telephone .

28. Network Topologies The way in which devices are interconnected to form a network is called network topology. Some of the factors that affect choice of topology for a network are − Cost − Installation cost is a very important factor in overall cost of setting up an infrastructure. So cable lengths, distance between nodes, location of servers, etc. have to be considered when designing a network. Flexibility − Topology of a network should be flexible enough to allow reconfiguration of office set up, addition of new nodes and relocation of existing nodes.

29. Continue.. Reliability − Network should be designed in such a way that it has minimum down time. Failure of one node or a segment of cabling should not render the whole network useless. Scalability − Network topology should be scalable, i.e. it can accommodate load of new devices and nodes without perceptible drop in performance. Ease of installation − Network should be easy to install in terms of hardware, software and technical personnel requirements. Ease of maintenance − Troubleshooting and maintenance of network should be easy.

30. Bus Topology Data network with bus topology has a linear transmission cable, usually coaxial, to which many network devices and workstations are attached along the length. Server is at one end of the bus. When a workstation has to send data, it transmits packets with destination address in its header along the bus. The data travels in both the directions along the bus. When the destination terminal sees the data, it copies it to the local disk.

31. Advantages and Disadvantages Advantages of Bus Topology Easy to install and maintain Can be extended easily Very reliable because of single transmission line Disadvantages of Bus Topology Troubleshooting is difficult as there is no single point of control One faulty node can bring the whole network down Dumb terminals cannot be connected to the bus

32. Ring Topology In ring topology each terminal is connected to exactly two nodes, giving the network a circular shape. Data travels in only one pre-determined direction. When a terminal has to send data, it transmits it to the neighboring node which transmits it to the next one. Before further transmission data may be amplified. In this way, data raverses the network and reaches the destination node, which removes it from the network. If the data reaches the sender, it removes the data and resends it later.

33. Advantages and Disadvantages Advantages of Ring Topology Small cable segments are needed to connect two nodes Ideal for optical fibres as data travels in only one direction Very high transmission speeds possible Disadvantages of Ring Topology Failure of single node brings down the whole network Troubleshooting is difficult as many nodes may have to be inspected before faulty one is identified Difficult to remove one or more nodes while keeping the rest of the network intact

34. Star Topology In star topology, server is connected to each node individually. Server is also called the central node. Any exchange of data between two nodes must take place through the server. It is the most popular topology for information and voice networks as central node can process data received from source node before sending it to the destination node.

35. Advantages and Disadvantages Advantages of Star Topology Failure of one node does not affect the network Troubleshooting is easy as faulty node can be detected from central node immediately Simple access protocols required as one of the communicating nodes is always the central node Disadvantages of Star Topology Long cables may be required to connect each node to the server Failure of central node brings down the whole network

36. Tree Topology Tree topology has a group of star networks connected to a linear bus backbone cable. It incorporates features of both star and bus topologies. Tree topology is also called hierarchical topology.

37. Advantages and Disadvantages Advantages of Tree Topology Existing network can be easily expanded Point-to-point wiring for individual segments means easier installation and maintenance Well suited for temporary networks Disadvantages of Tree Topology Technical expertise required to configure and wire tree topology Failure of backbone cable brings down entire network Insecure network Maintenance difficult for large networks

38. Mesh Topology In this type of topology, a host is connected to one or multiple hosts. This topology has hosts in point-to-point connection with every other host or may also have hosts which are in point-to-point connection to few hosts only. Hosts in Mesh topology also work as relay for other hosts which do not have direct point-to-point links. Mesh technology comes into two types:

39. Continue.. Full Mesh: All hosts have a point-to-point connection to every other host in the network. Thus for every new host n(n-1)/2 connections are required. It provides the most reliable network structure among all network topologies. Partially Mesh: Not all hosts have point-to-point connection to every other host. Hosts connect to each other in some arbitrarily fashion. This topology exists where we need to provide reliability to some hosts out of all.

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41. Hybrid Topology A network structure whose design contains more than one topology is said to be hybrid topology. Hybrid topology inherits merits and demerits of all the incorporating topologies. The above picture represents an arbitrarily hybrid topology. The combining topologies may contain attributes of Star, Ring, Bus, and Daisy-chain topologies. Most WANs are connected by means of Dual-Ring topology and networks connected to them are mostly Star topology networks. Internet is the best example of largest Hybrid topology.

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43. Types of Computer Networks? There are various types of computer networks available. We can categorize them according to their size as well as their purpose. The size of a network should be expressed by the geographic area and number of computers, which are a part of their networks. It includes devices housed in a single room to millions of devices spread across the world.

44. What is PAN (Personal Area Network)? PAN is a computer network formed around a person. It generally consists of a computer, mobile, or personal digital assistant. PAN can be used for establishing communication among these personal devices for connecting to a digital network and the internet.

45. Characteristics of PAN It is mostly personal devices network equipped within a limited area. Allows you to handle the interconnection of IT devices at the surrounding of a single user. PAN includes mobile devices, tablet, and laptop. It can be wirelessly connected to the internet called WPAN. Appliances use for PAN: cordless mice, keyboards, and Bluetooth systems.

46. Advantages & Disadvantages Advantages of PAN PAN networks are relatively secure and safe It offers only short-range solution up to ten meters Strictly restricted to a small area Disadvantages of PAN It may establish a bad connection to other networks at the same radio bands. Distance limits.

47. Local Area Network (LAN) A Local Area Network (LAN) is a group of computer and peripheral devices which are connected in a limited area such as school, laboratory, home, and office building. It is a widely useful network for sharing resources like files, printers, games, and other application. The simplest type of LAN network is to connect computers and a printer in someone's home or office. In general, LAN will be used as one type of transmission medium. It is a network which consists of less than 5000 interconnected devices across several buildings.

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49. Characteristics of LAN It is a private network, so an outside regulatory body never controls it. LAN operates at a relatively higher speed compared to other WAN systems. There are various kinds of media access control methods like token ring and ethernet.

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51. Advantages of LAN Computer resources like hard-disks, DVD-ROM, and printers can share local area networks. This significantly reduces the cost of hardware purchases. You can use the same software over the network instead of purchasing the licensed software for each client in the network. Data of all network users can be stored on a single hard disk of the server computer. You can easily transfer data and messages over networked computers. It will be easy to manage data at only one place, which makes data more secure. Local Area Network offers the facility to share a single internet connection among all the LAN users.

52. Disadvantages of LAN LAN will indeed save cost because of shared computer resources, but the initial cost of installing Local Area Networks is quite high. The LAN admin can check personal data files of every LAN user, so it does not offer good privacy. Unauthorized users can access critical data of an organization in case LAN admin is not able to secure centralized data repository. Local Area Network requires a constant LAN administration as there are issues related to software setup and hardware failures

53. What is WAN? WAN (Wide Area Network) is another important computer network that which is spread across a large geographical area. WAN network system could be a connection of a LAN which connects with other LAN's using telephone lines and radio waves. It is mostly limited to an enterprise or an organization.

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55. Characteristics of LAN: The software files will be shared among all the users; therefore, all can access to the latest files. Any organization can form its global integrated network using WAN.

56. Advantages of WAN WAN helps you to cover a larger geographical area. Therefore business offices situated at longer distances can easily communicate. Contains devices like mobile phones, laptop, tablet, computers, gaming consoles, etc. WLAN connections work using radio transmitters and receivers built into client devices.

57. Disadvantage of WAN The initial setup cost of investment is very high. It is difficult to maintain the WAN network. You need skilled technicians and network administrators. There are more errors and issues because of the wide coverage and the use of different technologies. It requires more time to resolve issues because of the involvement of multiple wired and wireless technologies. Offers lower security compared to other types of networks.

58. What is MAN?

59. Continue.. A Metropolitan Area Network or MAN is consisting of a computer network across an entire city, college campus, or a small region. This type of network is large than a LAN, which is mostly limited to a site . Depending upon the type of configuration, this type of network allows you to cover an area from several miles to tens of miles.

60. Characteristics of MAN It mostly covers towns and cities in a maximum 50 km range Mostly used medium is optical fibers , cables Data rates adequate for distributed computing applications.

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62. Advantages of MAN It offers fast communication using high-speed carriers, like fiber optic cables. It provides excellent support for an extensive size network and greater access to WANs. The dual bus in MAN network provides support to transmit data in both directions concurrently. A MAN network mostly includes some areas of a city or an entire city.

63. Disadvantages of MAN You need more cable to establish MAN connection from one place to another. In MAN network it is tough to make the system secure from hackers

64. Other Types of Networks WLAN (Wireless Local Area Network) Storage Area Network System Area Network Home Area Network POLAN- Passive Optical LAN Enterprise private network Campus Area Network Virtual Area Network

65. Wireless Networks Computer networks that are not connected by cables are called wireless networks. They generally use radio waves for communication between the network nodes. They allow devices to be connected to the network while roaming around within the network coverage.

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67. 1. Wireless LAN Wireless LAN (WLAN) technology provides internet access within a building or a limited outdoor area. First used within offices and homes, WLAN technology is now also used in stores and restaurants. The use of home networks has greatly increased as the COVID-19 pandemic has forced office workers, students, teachers and others to work and study from home.

68. 2. Wireless MAN Wireless metropolitan area networks have been installed in cities worldwide to provide access for people outside an office or home network. These networks cover a wider area than office or home networks, but the principles are the same. APs are located on the sides of buildings or on telephone poles throughout the coverage area. APs are connected to the internet via a wired network and broadcast a wireless signal throughout the area. Users connect to their desired destination by connecting to the nearest AP, which forwards the connection through its internet connection.

69. 3. Wireless PAN Wireless personal area networks cover a very limited area -- typically a maximum of 100 meters for most applications -- using protocols like Bluetooth and Zigbee. Bluetooth enables hands-free phone calls, connects a phone to earpieces or transmits signals between smart devices. Zigbee connects stations along an IoT network. Infrared technology is limited to line of sight, such as connecting TV remotes to televisions.

70. 4. Wireless WAN Wireless WANs use cellular technology to provide access outside the range of a wireless LAN or metropolitan network. These networks enable users to make phone calls to others connecting either through a wireless WAN or a wired telephone system. Users can also connect to the internet to access websites or server-based applications. Cell towers are located nearly everywhere within the U.S. and most other countries. A user connection is routed to the nearest cell tower which, in turn, is connected either to the wired internet or to another tower connected to wired internet.

71. Advantages of Wireless Networks It provides clutter-free desks due to the absence of wires and cables. It increases the mobility of network devices connected to the system since the devices need not be connected to each other. Accessing network devices from any location within the network coverage or Wi-Fi hotspot becomes convenient since laying out cables is not needed. Installation and setup of wireless networks are easier.

72. Continue.. New devices can be easily connected to the existing setup since they needn’t be wired to the present equipment. Also, the number of equipment that can be added or removed to the system can vary considerably since they are not limited by the cable capacity. This makes wireless networks very scalable. Wireless networks require very limited or no wires. Thus, it reduces the equipment and setup costs.

73. Examples of wireless networks Mobile phone networks Wireless sensor networks Satellite communication networks Terrestrial microwave networks