1. Introduction
− Computer networks : definition and advantages
− Classification of computer networks
− Introduction and differences among Local Area Networks (LANs), Metropolitan Area Networks (MANs), Wide Area Networks (WANs)
− Uses of Computer Networks
− Meaning of the basic terms : topology, data rate, modulation rate, spectrum, bandwidth, server, host
Computer networks : definition and advantages
· A computer networks is a collection of computers and other hardware interconnected by communication channels that allow sharing of resources and information.
· Where at least one process in one device is able to send/receive data to/from at least one process residing in a remote device, then the two devices are said to be in a network.
· A network is a group of devices connected to each other.
· Networks may be classified into a wide variety of characteristics, such as the medium used to transport the data, communications protocol used, scale, topology, benefit, and organizational scope.
A computer network consists of two or more computers that are linked in order to share resources such as printers and CD-ROMs, exchange files, or allow electronic communications. The computers on a computer network may be linked through cables, telephone lines, radio waves, satellites, or infrared light beams.
Advantages of Computer Network
The following are the distinct notes in favor of computer network.
· The computers, staff and information can be well managed
· A network provides the means to exchange data among the computers and to make programs and data available to people
· It permits the sharing of the resources of the machine
· Networking also provides the function of back-up.
· Networking provides a flexible networking environment. Employees can work at home by using through networks ties through networks into the computer at office.
Use of Computer Networks:
Computer networks can be used for several purposes:
· Facilitating communications. Using a network, people can communicate efficiently and easily via email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing.
· Sharing hardware. In a networked environment, each computer on a network may access and use hardware resources on the network, such as printing a document on a shared network printer.
· Sharing files, data, and information. In a network environment, authorized user may access data and information stored on other computers on the network. The capability of providing access to data and information on shared storage devices is an important feature of many networks.
· Sharing software. Users connected to a network may run application programs on remote computers.
Classification of computer networks
Computer network can be classified on the basis of following features :
By Scale: Computer networks may be classified according to the scale :
- Local Area Network (LAN)
- Metropolitan Area Network (MAN)
- Wide Area Network (WAN)
By Connection Method:
Computer networks can also be classified according to the hardware technology that is used to connect the individual devices in the network such as Optical fibre, Ethernet, Wireless LAN.
By Functional Relationship (Network Architectures) : Computer networks may be classified according to the functional relationships which exist between the elements of the network. This classification also called computer architecture. There are two type of network architecture :
- Client-Server
- Peer-to-Peer Architecture
By Network Topology: Network Topology signifies the way in which intelligent devices in the network see their logical or physical relations to one another. Computer networks may be classified according to the network topology upon which the network is based, such as :
- Bus Network
- Star Network
- Ring Network
- Mesh Network
- Star-Bus Network
- Tree or Hierarchical Topology Network
Connection method
Computer networks can be classified according to the hardware and software technology that is used to interconnect the individual devices in the network, such as optical fiber, Ethernet, wireless LAN, HomePNA, power line communication or Ethernet uses physical wiring to connect devices. Frequently deployed devices include hubs, switches, bridges, or routers. Wireless LAN technology is designed to connect devices without wiring. These devices use radio waves or infrared signals as a transmission medium. ITU- G.hn technology uses existing home wiring (coaxial cable, phone lines and power lines) to create a high-speed (up to 1Gigabit/s) local area network.
a) Wired technologies
Twisted pair wire is the most widely used medium for telecommunication. Twisted-pair wires are ordinary telephone wires which consist of two insulated copper wires
twisted into pairs and are used for both voice and data transmission. The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The
transmission speed ranges from 2 million bits per second to 100 million bits per second.
Coaxial cable is widely used for cable television systems, office buildings, and other worksites for local area networks. The cables consist of copper or aluminum wire wrapped with insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer. The layers of insulation help minimize interference and distortion. Transmission speed range from 200 million to more than 500 million bits per second.
Optical fiber cable consists of one or more filaments of glass fiber wrapped in protective layers. It transmits light which can travel over extended distances. Fiber-optic cables are not affected by electromagnetic radiation. Transmission speed may reach trillions of bits per second. The transmission speed of fiber optics is hundreds of times faster than for coaxial cables and thousands of times faster than a twisted-pair wire.
b) Wireless technologies
Terrestrial microwave – Terrestrial microwaves use Earth- based transmitter and receiver. The equipment looks similar to satellite dishes. Terrestrial microwaves use low-gigahertz range, which limits all communications to line-of-sight. Path between relay stations spaced approx, 30 miles apart. Microwave antennas are usually placed on top of buildings, towers, hills, and mountain peaks.
Communications satellites – The satellites use microwave radio as their telecommunications medium which are not deflected by the Earth's atmosphere. The satellites are stationed in space, typically 22,000 miles (for geosynchronous satellites) above the equator. These Earthorbiting systems are capable of receiving
and relaying voice, data, and TV signals.
Cellular and PCS systems – Use several radio communications technologies. The systems are divided to different geographic areas. Each area has a low-power transmitter or radio relay antenna device to relay calls from one area to the next area.
Wireless LANs – Wireless local area network use a highfrequency radio technology
Similar to digital cellular and a low-frequency radio technology. Wireless LANs use spread spectrum technology to enable communication between multiple devices in a limited area. An example of openstandards wireless radio-wave technology is IEEE.
Infrared communication , which can transmit signals between devices within small distances not more than 10 meters peer to peer or (face to face) without anybody in the line of transmitting.
2. Scale:
Networks are often classified as
• local area network (LAN),
• wide area network (WAN),
• metropolitan area network (MAN),
• personal area network (PAN),
• virtual private network (VPN),
• campus area network (CAN),
• storage area network (SAN), and others, depending on their scale, scope and purpose, e.g., controller area network (CAN) usage, trust level, and access right often differ between these types of networks.
LANs tend to be designed for internal use by an organization's internal systems and employees in individual physical locations, such as a building, while WANs may connect physically separate parts of an organization and may include connections to third parties.
3. Functional relationship (network architecture)
Computer networks may be classified according to the functional relationships which exist among the elements of the network, e.g., active networking, client–server and peer-to-peer (workgroup) architecture.
4. Network topology
Computer networks may be classified according to the network topology upon which the network is based, such as bus network, star network, ring network, mesh network.
Network topology is the coordination by which devices in the network are arranged in their logical relations to one another, independent of physical arrangement. Even if networked computers are physically placed in a linear arrangement and are connected to a hub, the network has a star topology, rather than a bus topology. In this regard the visual and operational characteristics of a network are distinct. Networks may be classified based on the method of data used to convey the data;
these include digital and analog networks.
Local Area Network
A Local Area Network is a computer network covering a small Networks geographical area, like a home, office, or groups of buildings e.g. a school Network. For example, a library will have a wired or wireless LAN Network for users to interconnect local networking devices e.g., printers and servers to connect to the internet. The defining characteristics of LANs Network, in contrast to Wide Area Networks (WANs), includes their much higher data-transfer rates, smaller geographic range, and lack of need for leased telecommunication lines. Although switched Ethernet is now the most common protocol for Networks. Current Ethernet or other IEEE 802.3 LAN technologies operate at speeds up to 10 Gbit/s.IEEE has projects investigating the standardization of 100 Gbit/s, and possibly 40 Gbit/s. Smaller Networks generally consist of a one or more switches linked to each other - often with one connected to a router, cable modem, or DSL modem for Internet access. LANs Network may have connections with other LANs Network via leased lines, leased services.
LAN is a group of computers located in the same room, on the same floor or in the same building that are connected to form a single network as to share resources such as disk drives , printers, data, CPU, fax/modem, application. etc.
LAN is generally limited to specific geographical area less than 2 K.M., supporting high speed networks. A wide variety of LANs have been built and installed, but a few types have more recently become dominant. The most widely used LAN system is the Ethernet system based on the bus topology.
Intermediate nodes (i.e., repeaters, bridges and switches) allow to be connected together to from larger LANs. A LAN may also be connected to another LAN or to WANs and MANs using a “router” device.
There are essentially five components of a LAN:
· Network devices such as Workstations, printers, file servers which are normally accessed by all other computers.
· Network Communication Devices i.e., devices such as hubs, routers, switches etc. used for network connectivity.
· Network Interface Cards (NICs) for each network device required to access the network. It is the interface between the machine and the physical network.
· Cable as a physical transmission medium.
· Network Operating System –software applications required to control the use of network operation and administration.
Characteristics of LAN
· It connects computers in a single building, block or campus, i.e. they work in a restricted geographical area.
· LAN’s are private networks, not subject to tariffs or other regulatory controls.
· LAN’s operate at relatively high speed when compared to the typical WAN (.2 to 100 MB /sec).
· There are different types of Media Access Control methods in a LAN, the prominent ones are Ethernet, Token ring, Arcnet.
· The networking is done through Buses or Rings.
ADVANTAGES OF LAN
· It allows sharing of expensive resources such as Laser printers, software and mass storage devices among a number of computers.
· LAN allows for highspeed exchange of essential information.
· It contributes to increased productivity. A LAN installation should be studied closely in the context of its proposed contribution to the long range interest of the organization.
DISADVANTAGES OF LAN
· The financial cost of LAN is still high in comparison with many other alternatives.
· It requires memory space in each of the computers used on the network. This reduces the memory space available for the user’s programs.
· Some type of security system must be implemented if it is important to protect confidential data.
· Some control on the part of the user is lost. You may have to share a printer with other users. You may face a situation like, for example, the entire network suddenly locking up because one user has made a mistake.
ARCNET, Token Ring and other technology standards have been used in the past, but Ethernet over twisted pair cabling, and Wi-Fi are the two most common technologies currently used to build LANs.
ARCNET (Attached Resource Computer NETwork)
ARCNET is one of the oldest, simplest, and least expensive types of Local-Area Network (LAN) protocol, similar in purpose to Ethernet or Token Ring. ARCNET was the first widely available networking system for microcomputers and became popular in the 1980s for office automation tasks. ARCnet was introduced by Datapoint Corporation in 1977. A special advantage of ARCNET (Attached Resource Computer NETwork ) is that it permits various types of transmission media - twisted-pair wire, coaxial cable, and fiber optic cable - to be mixed on the same network.The specification is ANSI 878.1. It can have up to 255 nodes per network.
A new specification, called ARCnet Plus, will support data rates of 20 Mbps
Ethernet is a family of computer networking technologies for local area networks (LANs) commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired local area network (LAN) technologies.Ethernet uses a bus or star topology Network and supports data transfer rates of 10 Mbps.Ethernet Network uses the CSMA/CD access method to handle simultaneous demands. It is one of the most widely implemented LAN standards.A newer version of Ethernet Network, called 100Base-T (or Fast Ethernet), supports data transfer rates of 100 Mbps. And the newest version, Gigabit Ethernet supports data rates of 1 gigabit (1,000 megabits) per second. Ethernet is a physical and data link layer technology for local area networks (LANs). Ethernet Network was invented by engineer Robert Metcalfe.
Metropolitan Area Network (MAN)
Metropolitan area networks, or MANs, are large computer network usually spanning a city. They typically use wireless infrastructure or Optical fiber connections to link their sites.
A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. MANs can also depend on communications channels of moderatetohigh data rates. A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations. MANs might also be owned and operated as public utilities or privately owned. They will often provide means for internetworking of local networks. Metropolitan area networks can span up to 50km, devices used are modem and wire/cable.
A Metropolitan Area Network is a large computer network that spans a metropolitan area or campus. Its geographic scope falls between a WAN and LAN. MANs provide Internet connectivity for LANs in a metropolitan region, and connect them to wider area networks like the Internet.
1) The network size falls intermediate between LAN and WAN. A MAN typically covers an area of between 5 and 50 km diameter. Many MANs cover an area the size of a city, although in some cases MANs may be as small as a group of buildings or as large as the North of Scotland.
2) A MAN often acts as a high speed network to allow sharing of regional resources. It is also frequently used to provide a shared connection to other networks using a link to a WAN.
Characteristics of MAN
1) It generally covers towns and cities (50 kms)
2) It is developed in 1980s.
3) Communication medium used for MAN are optical fibers, cables etc.
4) Data rates adequate for distributed computing applications.
Wide Area Network (WAN)
Wide Area Network (Figure 8) is a network system connecting cities, countries or continents, a network that uses routers and public communications links. The largest and most wellknown example of a WAN is the Internet.
WANs are used to connect LANs and other types of networks together, so that users and computers in one location can communicate with users and computers in other locations. Many WANs are built for one particular organization and are private. Others, built by Internet service providers, provide connections from an organization's LAN to the Internet. WANs are often built using leased lines. At each end of the leased line, a router connects to the LAN on one side and a hub within the WAN on the other. Leased lines can be very expensive. Instead of using leased lines, WANs can also be built around public network or Internet.
Characterstics of WAN
1) It generally covers large distances (states, countries, continents).
2) Communication medium used are satellite, public telephone networks which are
connected by routers.
3) Routers forward packets from one to another (Table 1) a route from the sender to
the receiver.
Difference between LAN, Man and Wan
Uses of Computer Networks
Uses of Networks for Companies
1.Resource Sharing:
Many organizations have large number of substantial computers in operation often located far apart. Let us consider an example, a company having many factories situated at different locations. A computer at each location (that is in each factory) keeps the track of inventories, monitor productivity and do the local pay roll. Initially each of these computers may have worked in isolation from each other, but at some point, the management may have decided to connect these computers to be able to extract and correlate the information of the entire company.
The issue here is resource sharing. Its goal is to make all the programs, equipments, especially data available to anyone on the network irrespective of the location of the resource and the user.
2.High Reliability:
The second goal or use of networking in companies is to high reliability by having alternative sources of supply. For example all the files can be replicated on two or more machines, so that in case one of them is not available (due to hardware failure), other copies can be used. This feature is used in financial institutions.
3.Saving Money:
The third goal is to save money. Small computers often have better price/performance ratio than the larger ones. Mainframe (room-size) computers are roughly ten faster than the personal computers, but are a thousand times costly. This imbalance caused the system designers to design a system consisting of personal computers, one per user, with data kept on one or more shared file server machines. In this model the user are called the clients and this whole arrangement is known as the client-server model.
In client-server model, the communication generally takes the form of a request message from a client to the receiver asking for some work to be done. Server does the work and sends back the reply.
4.Scalability:
Another goal is scalability. Scalability is the ability to increase the system performance gradually as the workload grows, by just adding more processors.
Uses of Networks to People
1.Access to Remote Information:
Access to remote information occurs in many forms. One of the areas where it is happening is access to the financial institutions. Many people pay their bills, manage bank accounts and handle investments electronically. Home shopping is also becoming popular these days.
Another application that falls under this category is the access to information systems like World Wide Web which contains information about art, business, history, government, geography, economics and several other topics.
All the above applications involve the interaction between the user and a remote database.
2.Person to Person Communication:
Electronic Mail popularly known as email is widely used by millions of people to send text messages, photographs audio as well as video to other people or group of people. This application belongs to person to person communication category.
Videoconferencing is also becoming popular these days. This technology makes it possible to have virtual meetings among far flung people. It is also a type of person to person communication.
3.Interactive Entertainment:
These days we can see many live programmes and shows. The best thing is that we can interact with them by participating in the quizzes and the contests organized by them.
Meaning of the basic terms
Network interface cards
A network card, network adapter, or NIC (network interface card) is a piece of computer hardware designed to allow computers to physically access a networking medium. It provides a low-level addressing system through the use of MAC addresses.Each Ethernet network interface has a unique MAC address which is usually stored in a small memory device on the card, allowing any device to connect to the network without creating an address conflict. Ethernet MAC addresses are composed of six octets. Uniqueness is maintained by the IEEE, which manages the Ethernet address space by assigning 3-octet prefixes to equipment manufacturers. The list of prefixes is publicly available. Each manufacturer is then obliged to both use only their assigned prefix(es) and to uniquely set the 3-octet suffix of every Ethernet interface they produce.
Repeaters and hubs
A repeater is an electronic device that receives a signal, cleans it of unnecessary noise, regenerates it, and retransmits it at a higher power level, or to the other side of an obstruction, so that the signal can cover longer distances without degradation. In most twisted pair Ethernet configurations, repeaters are required for cable that runs longer than 100 meters. A repeater with multiple ports is known as a hub. Repeaters work on the Physical Layer of the OSI model. Repeaters require a small amount of time to regenerate the signal. This can cause a propagation delay which can affect network communication when there are several repeaters in a row. Many network architectures limit the number of repeaters that can be used in a row (e.g. Ethernet's 5-4-3 rule).Today, repeaters and hubs have been made mostly obsolete by switches (see below).
Bridges
A network bridge connects multiple network segments at the data link layer (layer 2) of the OSI model. Bridges broadcast to all ports except the port on which the broadcast was received. However, bridges do not promiscuously copy traffic to all ports, as hubs do, but learn which MAC addresses are reachable through specific ports. Once the bridge associates a port and an address, it will send traffic for that address to that port only.Bridges learn the association of ports and addresses by examining the source address of frames that it sees on various ports. Once a frame arrives through a port, its source address is stored and the bridge assumes that MAC address is associated with that port. The first time that a previously unknown destination address is seen, the bridge will forward the frame to all ports other than the one on which the frame arrived.
Bridges come in three basic types:
- Local bridges: Directly connect LANs
- Remote bridges: Can be used to create a wide area network (WAN) link between LANs. Remote bridges, where the connecting link is slower than the end networks, largely have been replaced with routers.
- Wireless bridges: Can be used to join LANs or connect remote stations to LANs.
Switches
A network switch is a device that forwards and filters OSI layer 2 datagrams (chunks of data communication) between ports (connected cables) based on the MAC addresses in the packets.[12] A switch is distinct from a hub in that it only forwards the frames to the ports involved in the communication rather than all ports connected. A switch breaks the collision domain but represents itself as a broadcast domain. Switches make forwarding decisions of frames on the basis of MAC addresses. A switch normally has numerous ports, facilitating a star topology for devices, and cascading additional switches. Some switches are capable of routing based on Layer 3 addressing or additional logical levels; these are called multi-layer switches. The term switch is used loosely in marketing to encompass devices including routers and bridges, as well as devices that may distribute traffic on load or by application content (e.g., a Web URL identifier).Routers
A router is an internetworking device that forwards packets between networks by processing information found in the datagram or packet (Internet protocol information from Layer 3 of the OSI Model). In many situations, this information is processed in conjunction with the routing table (also known as forwarding table). Routers use routing tables to determine what interface to forward packets (this can include the "null" also known as the "black hole" interface because data can go into it, however, no further processing is done for said data).Firewalls
A firewall is an important aspect of a network with respect to security. It typically rejects access requests from unsafe sources while allowing actions from recognized ones. The vital role firewalls play in network security grows in parallel with the constant increase in 'cyber' attacks for the purpose of stealing/corrupting data, planting viruses, etc.Network topology
Network topology is the arrangement of the various elements (links, nodes, etc.) of a computer. Essentially, it is the topological structure of a network, and may be depicted physically or logically. Physical topology refers to the placement of the network's various components, including device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ between two networks, yet their topologies may be identical.
The term Computer Network topology defines the geographic arrangement of computer networking devices. It describes the actual layout of the computer network hardware. Given the location of workstations and peripherals, the goal of topology is to find the most economical and efficient way to connect all the users to the computer network resources while providing adequate capacity to handle user demands, maintain system reliability and minimize delay.
The selection of a Network Topology for a network can not be done in isolation as it affects the choice of media and the access method used. Because it determines the strategy used in wiring a building for a network and deserves some careful study.
Data Rate
Data Rate defines the number of data bits sent in 1 second. The unit is bits per second(bps).
The Data rate is quantified using the bits per second (bit/s) unit, often in conjunction such as kilobits- (kbit/s), megabits- (Mbit/s), gigabits- (Gbit/s) or terabits- (Tbit/s).
Bandwidth
The bit rate is quantified using the bits per second (bit/s) unit, often in conjunction with an SI prefix such as kilo- (kbit/s), mega- (Mbit/s), giga- (Gbit/s) or tera- (Tbit/s).
1) In computer networks, bandwidth is often used as a synonym for data transfer rate - the amount of data that can be carried from one point to another in a given time period (usually a second). This kind of bandwidth is usually expressed in bits (of data) per second (bps). Occasionally, it's expressed as bytes per second (Bps). A modem that works at 57,600 bps hastwice the bandwidth of a modem that works at 28,800 bps. In general, a link with a high bandwidth is one that may be able to carry enough information to sustain the succession of images in a video presentation.
It should be remembered that a real communications path usually consists of a succession of links, each with its own bandwidth. If one of these is much slower than the rest, it is said to be a bandwidth bottleneck.
2) In electronic communication, bandwidth is the width of the range (or band) of frequencies that an electronic signal uses on a given transmission medium. In this usage, bandwidth is expressed in terms of the difference between the highest-frequency signal component and the lowest-frequency signal component. Since the frequency of a signal is measured in hertz (the number of cycles of change per second), a given bandwidth is the difference in hertz between the highest frequency the signal uses and the lowest frequency it uses. A typical voice signal has a bandwidth of approximately three kilohertz (3 kHz); an analog television (TV) broadcast video signal has a bandwidth of six megahertz (6 MHz) -- some 2,000 times as wide as the voice signal.
Modulation
· The Process of converting analog data to analog signal is called Modulation.
· Modulation is used to send an information bearing signal over long distances.
· Modulation is the process of varying some characteristic of a periodic wave with an external signal called carrier signal.
· These carrier signals are high frequency signals and can be transmitted over the air easily and are capable of traveling long distances.
· The characteristics (amplitude, frequency, or phase) of the carrier signal are varied in accordance with the information bearing signal(analog data).
· The information bearing signal is also known as the modulating signal.
· The modulating signal is a slowly varying – as opposed to the rapidly varying carrier frequency.
Spectrum
The word spectrum refers to a collection of various types of electromagnetic radiations of different wavelengths. Spectrum or airwaves are the radio frequencies on which all communication singnals travel. the radio frequencies are being used for different types of services like space communication, mobile communication, broadcasting, radio navigation, mobile satellite service, aeronautical satellite services, defence communication etc. Radio frequency is a natural resource but unlike other resources it will deplete when used. But it will be wasted if not used efficiently.
Server
A Server is a physical computer (a computer hardware system) dedicated to run one or more services, to serve the needs of the users of other computers on a network. Depending on the computing service that it offers it could be a database server, file server, mail server, print server, web server, gaming server, or some other kind of server.
A server is a computer in network that provides services to the client computers such as logon requests processing, files access and storage, internet access, printing access and many other types of services. Servers are mostly equipped with extra hardware such as plenty of external memory (RAM), more data store capacity (hard disks), high processing speed and other features.
Network Server is a computer in Network that is designated to provide one or more network service. For example file server, database server etc.
A network server designated to provide one particular service is known as dedicated server. For example if a computer is assigned for database service only then that computer is known as dedicated database server.
Modem
Modem is abbreviation for Modulator – Demodulator. Modems are used for data transfer from one computer network to another computer network through telephone lines. The computer network works in digital mode, while analog technology is used for carrying massages across phone lines. Modem converts information from digital mode to analog mode at the transmitting end and converts the same from analog to digital at receiving end. The process of converting analog signals of one computer network into digital signals of another computer network so they can be processed by a receiving computer is referred to as digitising.
