A computer network, or simply a network, is a collection of computers and other hardware components 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. Simply, more than one computer interconnected through a communication medium for information interchange is called a computer network.
Each of the devices on the network can be thought of as a node; each node has a unique address.Networks may be classified according to a wide variety of characteristics, such as the medium used to transport the data, communications protocol used, scale, topology, and organizational scope.Communications protocols define the rules and data formats for exchanging information in a computer network, and provide the basis for network programming i.e.
for data communication between multiple networks, as well as host-to-host data transfer, and application-specific data transmission formats.Characteristics of computer networks:Facilitate communicationsUsing a network, people can communicate efficiently and easily via email, instant messaging, chat rooms, telephone, video telephone calls, and video conferencing.Permit sharing of files, data, and other types of informationIn a network environment, authorized users 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.
Share network and computing resourcesIn a networked environment, each computer on a network may access and use resources provided by devices on the network, such as printing a document on a shared network printer. Distributed computing uses computing resources across a network to accomplish tasks.Types of NetworksThere are three kinds of networks: Local Area Networks (LANs), Metropolitan area network (MAN) and Wide Area Networks (WANs)Local Area NetworksA local area network (LAN) is a network that connects computers and devices in a limited geographical area such as home, school, computer laboratory, office building, or closely positioned group of buildings. Each computer or device on the network is a node.
Examples of LAN technology: Ethernet, Token Ring, and Fiber Distributed Data Interconnect (FDDI).Ethernet LANs: based on a bus topology and broadcast communication. Token ring LANs: based on ring topology. FDDI LANs: use optical fibers and an improved token ring mechanism based on two rings flowing in opposite directions.Metropolitan area networkA Metropolitan area network (MAN) is a large computer network that usually spans a city or a large campus.
Wide Area NetworksA wide area network (WAN) is a computer network that covers a large geographic area such as a city, country, or spans even intercontinental distances, using a communications channel that combines many types of media such as telephone lines, cables, and air waves. WAN is based on packet switching technology. Examples of WAN technology: Asynchronous Transfer Mode (ATM), Integrated Services Digital Network (ISDN).Network TopologyA network topology is the layout of the interconnections of the nodes of a computer network. There are 4 types of topology:1. Star topology2.
Bus topology3. Ring topology4. Mesh topologyA bus network: all nodes are connected to a common medium along this medium. This was the layout used in the original Ethernet, called 10BASE5 and 10BASE2.A star network: all nodes are connected to a special central node.
This is the typical layout found in a Wireless LAN, where each wireless client connects to the central Wireless access point.A ring network: each node is connected to its left and right neighbour node, such that all nodes are connected and that each node can reach each other node by traversing nodes left- or rightwards. The Fiber Distributed Data Interface (FDDI) made use of such a topology.A mesh network: each node is connected to an arbitrary number of neighbors in such a way that there is at least one traversal from any node to any other.Basic hardware componentsApart from the physical communications media themselves as described above, networks comprise additional basic hardware building blocks interconnecting their terminals, such as network interface cards (NICs), hubs, bridges, switches, and routers.
Network interface cardsA 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.Repeaters and hubsA 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. Today, repeaters and hubs have been made mostly obsolete by switches.
SwitchA network switch is a small hardware device that joins multiple computers together within one local area network (LAN). Network switches appear nearly identical to network hubs, but a switch generally contains more intelligence (and a slightly higher price tag) than a hub. Unlike hubs, network switches are capable of inspecting data packets as they are received, determining the source and destination device of each packet, and forwarding them appropriately. By delivering messages only to the connected device intended, a network switch conserves network bandwidth and offers generally better performance than a hub.
BridgesA network bridge connects multiple network segments at the data link layer 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.
RoutersA router is an internetworking device that forwards packets between networks by processing information found in the datagram or packet.FirewallsA 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.ProtocolsDefine the rules that govern the communications between two computers connected to the network.
Roles: addressing and routing of messages, error detection and recovery, sequence and flow controls etc. A protocol specification consists of the syntax, which defines the kinds and formats of the messages exchanged, and the semantic, which specifies the action taken by each entity when specific events occur. Example: HTTP protocol for communication between web browsers and servers. -Protocols are designed based on a layered architecture such as the OSI reference model.