chapter three handout, Lecture notes of Information Systems

Download chapter three handout and more Lecture notes Information Systems in PDF only on Docsity! Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 1 Chapter-Three 3. Data Communications, Networks and Internet Objectives: To know basic component used in data transmission To know, what a computer network is and different types of networks To know what is Internet and what service Internet has. Information is transmitted in the form of analog or digital. Most communication lines are designed to carry analog signals. Digital transmission a rarely used for telecommunications. Therefore, a technical must be used to represent a digital signal or an analog carrier  Modulation: Converting digital signals into analog signals.  Demodulation: Converting analog signals back into digital signals.  Modem: Short for modulator/demodulator: A communications device that converts one form of a signal to another that is suitable for transmission over communication circuits, typically from digital to analog and then from analog to digital Types of Signals a. Digital signal A digital signal is a physical signal that is a representation of a sequence of discrete values (a quantified discrete-time signal), for example an arbitrary bit stream, or of a digitized (sampled). A signal that is generated by means of a digital modulation method (digital passband transmission), produced by a modem, is in the first case considered as a digital signal, and in the second case as converted to an analog signal. b. Analog signal An analog signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e., analogous/similar to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are meaningful. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 2 Any information may be conveyed by an analog signal; often such a signal is a measured response to changes in physical phenomena, such as sound, light, temperature, or pressure. An analog signal is one where at each point in time the value of the signal is significant, whereas a digital signal is one where at each point in time, the value of the signal must be above or below some discrete threshold. The primary disadvantage of analog signaling is that any system has noise – i.e., random unwanted variation. As the signal is copied and re-copied, or transmitted over long distances, these apparently random variations become dominant. Electrically, these losses can be diminished by shielding, good connections, and several cable types such as coaxial or twisted pair. The effects of noise create signal loss and distortion. This is impossible to recover, since amplifying the signal to recover attenuated parts of the signal amplifies the noise (distortion/interference) as well. Even if the resolution of an analog signal is higher than a comparable digital signal, the difference can be overshadowed by the noise in the signal. 3.1. Data Transmission The need of information has increased from time to time. This leads to the need of sharing of information among different agents (individual), which may be at different places or locations. Data communication is the exchange of information between two agents. For exchange of information the information should be transmitted from one point to another through a transmission media called Channel. The following figure shows the different components of data communication. Agent agent 1 2 3 4 5 6 Source System Destination system Input device Transmi tter Transmiss ion medium Receiver Output device Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 5  Category 2 ― good for voice and low data rates (up to 4Mbps for low-speed token ring networks)  Category 3 ― at least 3 twists per foot, for up to 10 Mbps (common in phone networks in residential buildings)  Category 4 ― up to 16 Mbps (mainly for token rings)  Category 5 (or 5e) ― up to 100 Mbps (common for networks targeted for high-speed data communications)  Category 6 ― more twists than Cat 5, up to 1 Gbps Shielded Twisted Pairs (STP)  Are similar to UTP except the extra shield applied to the cables. The shield is used to reduce external electrical interference and is suitable to be used in large industries where there is a high level of electrical interference b. Coaxial Cable  Most versatile medium used in LANs, Cable TV, VCR-to-TV connections  Noise immunity is better than twisted pair  Less susceptible to interference and cross talk but there still is attenuation and thermal noise problem  Can go up to 185m or 500m without the need for an amplifier/repeater c. Optical Fiber cables  Flexible, thin, made of very pure glass / plastic fiber capable of conducting optical (light) rays  Extremely high bandwidth. They are used for high data transmission rate  Very high noise immunity, resistant to electromagnetic interference  Does not radiate energy/cause interference  Very difficult to tap  Better security but multipoint not easy  Need optical-electrical interface (more expensive than electrical interface) Fiber Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 6  There are two types of fiber optic cables: Multimode Fiber optic cable  Fiber optic cable where the light signal travels dispersed through the core  Core is usually 50-62m in diameter  Maximum distance signal travels without a repeater is 500m Single Mode fiber  Fiber optic cable where the light signal travels in a single mode through the core  Maximum distance signal travels without a repeater is 10km (with the appropriate modulation up to 100km) 2. Unguided Transmission media (Wireless transmission) Data transmission through air /space (i.e wireless system). In unguided media transmission and reception are achieved by means of an antenna. There are two types of configuration for wireless transmission namely, directional and omni directional. Wireless transmission can of the following types  Terrestrial Microwave  Satellite Microwave  Infrared a. Terrestrial Microwave  Typically used where laying a cable is not practical  Parabolic dish shaped antenna for directional and bar-like antenna for omni directional transmission  Transmits/receives electromagnetic waves in the 2-40 GHz range  Travels in a straight line (line-of-sight propagation)  High data rates: 100’s Mbps  Repeaters spaced 10 - 100 km apart  Applications: telephone and data transmission- wireless LANs b. Satellite Microwave  Uses satellite in geostationary (geosynchronous) 36,000 km)  Source transmits signal to satellite which amplifies or repeats it, and retransmits down to destinations Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 7  Optimum transmission in 1 - 10 GHz range  Bandwidth of 100’s MHz  270msSignificant propagation delay about  VSAT ( Very small Aperture Terminal) :- High speed data transmission using satellite c. Infrared Transmission  For short-range communication o Remote controls for TVs, VCRs, and stereos o Indoor wireless LANs  Do not pass through solid walls o Better security and no interference (with a similar system in adjacent rooms)  No government license is needed  Cannot be used outdoors (due to the sunshine) 3.3.Computer Networks When a computer system is processing data all by itself, without any interaction or interconnection with any other computer system, it is called a stand-alone computer system. A Network System is a system of two or more computers that are connected to each other for the purpose of data communication and sharing of resources. A network also consists of communication channels that are responsible for connecting devices with each other and transmitting information... Some of the most important advantages of a computer network system are:  Data sharing between different users of computers with certain amount of data security and access control.  Sharing of software application systems.  Distribution of computing load on computers at distributed locations to reduce peak load.  Sharing or high power computing resources situated at a central location on a need basis.  Sharing of costly special devices by several computer users. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 10  Less disk storage space is required because the program is stored only once on the server, instead of being stored on the hard disks of multiple standalone computers  When a new version of the software is released, it is easier to update on copy of the program on the server than to update many copies stored on standalone computer.  Purchasing a software license for a network can be less expensive than purchasing a single-user license for every workstation on the network In short sharing programs on a network  Saves disk space  Reduces maintenance  Reduce licensing cost b. Sharing Hardware Computer networks enable us to share expensive hardware resource among several computers. A typical example of shared resource is printer. For example, a company may prefer to acquire one expensive printer and connect it to the network to provide high quality printing to users. This avoids the need to have separate printer for each computer in the office. 3.4. 2. Centralizing Administration and Support Networking computers can simplify support tasks as well. It is far more efficient for technical personnel to support one version of one operating system or application and to set up all computers in the same manner than to support many individual and unique systems and setups. Even if your computer is physically connected to a network, you cannot typically use network resources until you log into the network. When you log in, you formally identify yourself to the network by providing your ID and password. 3.4. 3. Sharing Information/data Users in a certain network environment have the liberty of sharing data and information across the network. Data sharing enables different users to work on a certain file concurrently. Few examples of data sharing are: o Database: - databases are often managed centrally and several users can have access to the database at same time. For example, in a networked banking system, different bank branches can have access to the central account database. This enables bank clients to carry their transactions on any branch bank office. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 11 o E-mail: email communication can be achieved over the network enabling networked users in the company to communicate messages across the network using email. o Intranet: Intranets are similar to World Wide Web (WWW) where centrally stored hypertext documents can be accessed using the web. Unlike the WWW, intranets are available only to user within the company network system. Intranet is a very common service in large networked organizations. o Extranet: Although similar to intranet, extranet provides selected users from outside the organization to access data from the internal network. Extranets are commonly used by suppliers to provide data to company clients. 3.5. Types of computer network Computer Network is a collection of computers and terminal devices connected by a communication system. The use of computer network is to share resources like file, device, printer, scanner, and program. Topology refers to the way in which multiple devices are interconnected via communication links. There are different types of network based on the geographical area covered by a network: -  Local area Network (LAN)  Metropolitan Area Network (MAN)  wide Area Network (WAN) and  Personal Area Network (PAN) A local area network is a network confined to a small area like a building. Most LAN has communication stations that are physically linked by a cable. Advantages of LAN  Speed  Cost  Security Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 12  E-mail  Resource Sharing Disadvantages of LAN o Expensive to Install o Requires Administrative Time o File Server May Fail o Cables May Break Metropolitan Area Network (MAN):-is a communication network covering a geographical area such as the size of city or sub-urban. A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations. A MAN often acts as a high speed network to allow sharing of regional resources. A MAN typically covers an area of between 5 and 50 km diameter. Examples of MAN: Telephone company network that provides a high speed DSL to customers and cable TV network. WAN are used to connect computers not located in the same building (or even in the same city. High-speed dedicated data lines or satellite connections may be used to link computers separated by great distance. Multiple LANs can be connected together using devices such as bridges, routers, or gateways, which enable them to share data. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 15 A Peer-to-Peer network is a distributed network in which many Peer-to-Peer In a peer-to-peer network, workstations are connected to each other and do not rely on a server for global software and data, data processing tasks, or communication within the network. Basically, a peer-to-peer network consists of many workstations connected together that can share resources and communicate with each other. Each workstation can be a client at one time and a server at another time. Network Topologies The term topology, or more specifically, network topology, refers to the arrangement or physical layout of computers, cables, and other components on the network. "Topology" is the standard term that most network professionals use when they refer to the network's basic design. In addition to the term "topology," you will find several other terms that are used to define a network's design:  Physical layout  Design  Diagram  Map A network's topology affects its capabilities. The choice of one topology over another will have an impact on the:  Type of equipment that the network needs Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 16  Capabilities of the network  Growth of the network  Way the network is managed Developing a sense of how to use the different topologies is a key to understanding the capabilities of the different types of networks. Before computers can share resources or perform other communication tasks they must be connected. Most networks use cable to connect one computer to another. However, it is not as simple as just plugging a computer into a cable connecting to other computers. Different types of cable—combined with different network cards, network operating systems, and other components—require different types of arrangements. To work well, a network topology takes planning. For example, a particular topology can determine not only the type of cable used but also how the cabling runs through floors, ceilings, and walls. Topology can also determine how computers communicate on the network. Different topologies require different communication methods, and these methods have a great influence on the network. There are five basic types of computer topology: bus, star, ring and mesh Bus topology: - In bus topology, all stations attach through appropriate interfacing hardware, directly to a line as transmission medium, or bus. A transmission from any station propagates the length of the medium in both directions and can be received by all other stations. The message that is transmitted contains the address of the station and the data and each station monitors the medium and copies packets addressed to itself. Because all stations share a common transmission link, only one station can successfully transmit at a time, duo some form of medium access control technique is needed to regulate access. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 17 Ring topology: - In the ring topology the network consists of a set repeaters joined by point –to point links in a closed loop. The links are unidirectional, that is, data are transmitted in one direction only, and all oriented in the same way. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 20  Repeaters and Hubs  Bridges  Switches  Routers  Gateways a) Modem A modem is a device that makes it possible for computers to communicate over a telephone line. When computers are too far apart to be joined by a standard computer cable, a modem can enable communication between them. In a network environment, modems serve as a means of communication between networks and as a way to connect to the world beyond the local network. Computers cannot simply be connected to each other over a telephone line, because computers communicate by sending digital electronic pulses (electronic signals), and a telephone line can send only analog waves (sound). The modem at the sending end converts the computer's digital signals into analog waves and transmits the analog waves onto the telephone line. A modem at the receiving end converts the incoming analog signals back into digital signals for the receiving computer. In other words, sending modem MOdulates digital signals into analog signals, and receiving modem DEModulates analog signals back into digital signals. Modems convert digital signals to analog waves, and convert analog waves to digital signals b) Repeaters Repeaters receive signals and retransmit them at their original strength and definition. This increases the practical length of a cable. (If a cable is very long, the signal weakens and become unrecognizable.) Installing a repeater between cable segments enables to travel farther. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 21 or Repeaters regenerate weakened signals Use a repeater to:  Connect two segments of similar or dissimilar cabling.  Regenerate the signal to increase the distance transmitted.  Transmit all traffic in both directions.  Connect two segments in the most cost-effective manner. Do not use a repeater when:  There is heavy network traffic.  Segments are using different access methods.  Data filtering is needed. c) Hubs Hubs are connectivity devices that connect computers in a star topology. Hubs contain multiple ports for connecting to network components. If you use a hub, a break in the network does not affect the entire network; only the segment and the computer attached to that segment fail. A single data packet sent through a hub goes to all connected computers. Figure 4.5 shows method of hub communication. Chapter-Three Introduction to Computer and Information System Compiled by: Isayas W. 22 Hub communication Use a hub to:  Easily change and expand wiring systems  Enable central monitoring of network activity and traffic d) Bridges A bridge is a device that passes data packets between multiple network segments that use the same communications protocol. A bridge passes one signal at a time. If a packet is destined for a computer within the sender's own network segment, the bridge retains the packet within that segment. If the packet is destined for another segment, it passes the packet to that segment. Figure 4.7 shows a bridge connecting two network segments. A bridge connecting two network segments