Lecture 13

INTRODUCTION TO COMPUTER LECTURE NO 13

THE NETWORK A network is a way to connect computers together so that they can communicate, exchange information, and pool resources.

Uses of Network A network provide tremendous benefits. Four of the most compelling benefits are:  Allowing simultaneous access to critical programs and data  Allowing people to share peripheral deices, such as printers and scanners  Streamlining personal communication with e-mail  Making the backup process easier Simultaneous Access It is a fact business computing that multiple employees, using a computer network, often need access to the same data at the same time. Without a network that enables file-sharing employees keep separate copies of data on different hard disks, and universally updating the data becomes very difficult. It is also true that most office workers use the same programs. One solution t purchasing separate copies of applications for every wo4rker is to use network versions of programs. A network version of a software application is also a more efficient use of hard disk space because many users can access a single shared copy instead of storing separate copies on each user’s hard disk. Shared Peripheral Devices Perhaps the best incentive of r small business to link computers in a network is to share peripheral devices, especially expensive ones such as laser printers, large hard disks, and scanners. Sharing a laser printer on a network makes the cost much less prohibitive. By using a process called spooling, multiple users can send multiple print jobs to a printer. Personal Communication One of the most far-reaching applications of data communication in electronics mail (e-mail) a system of exchanging written messages (and increasingly, voice and video messages) through a network. E-mail in something of a cross between the postal system and a telephone answering system. In an e-mail system, each user has a unique address. To send someone an e-mail message, you enter the person’s e-mail address and then type the message. When you are finished, the massage is sent to the e-mail address. The next time that user accesses the e-mail system; it reports that mail has arrived. Some systems notify the recipient as ea h message arrives by flashing a message on the computer screen or beeping. After reading the message, the recipient can save it, delete it, forward it o someone else, or respond by sending back a reply massage. Easier Backup In business, data is extremely valuable, so making sure that employees back up their data is critical. One way to address this problem is to keep all valuable data on a shared storage device that employee’s access through a network. Often he person managing the network has the responsibility of making regular backups 1

INTRODUCTION TO COMPUTER LECTURE NO 13 of the data on the shared storage device from a single, central location. Network backup software is also available that enables backups to be made of files stored on employees’ hard drives.

HOW NETWORKS ARE STRUCTURED To understand the different types of networks and how they operate, it is important to know something about how networks can be structured. First, there are two main types of networks, distinguished mainly by geography: local area networks (LANs) and wide area networks (WANs).

Local Area Networks A network of computers located relatively near each other and connected by a cable (or a small radio transmitter) is a local area network (LAN). A LAN can consist of just two or three PCs connected together to share resources, or it can include several hundred computers of different kinds. Any network that exists within a single building, or even a group of adjacent buildings, is considered a LAN. A LAN permits all the computers connected to it to share hardware, software, and data. The most commonly shared resources are disk storage devices and printers.

Connecting Networks It is often helpful to connect different LANs. For example, two different departments in a large business may each have its own LAN, but if there is enough need for data communication between the departments, then it may be necessary to create link between the two LANs. To understand how this can be accomplished, you must firs know that, on a network, data is sent in small groups called packets. A packet is a group of bits that includes a header, payload, and control elements that are transmitted together. Each LAN is governed by a protocol, which is a set of rules and formats for sending and receiving data. If two LANs are built around the same communication rules, then they can be connected with a bridge or a router. A bridge is a relatively simple device that looks at the information in each packet header and rebroadcasts data that is traveling from on LAN t another. A router is a more complicated device that stores the addressing information of each computer on each LAN and uses this information to act like an electronic post office, sorting data and sending it along the most expedient route to its destination.

Wide Area Networks Typically, a wide area network (WAN) is two or more LANs connected together, generally across a wide geographical area using high-speed or dedicated telephone lines. For example, a company7 many have its corporate headquarters and manufacturing facility in one city and its marketing office in another. Each site needs resources, data, and programs locally, but it also needs to share data with the other site. To accomplish this feat of data communication, the company can attach a router each LAN to create a WAN. The Internet is the ultimate WAN because it connects many thousands of computer and LANs around the world. 2

INTRODUCTION TO COMPUTER LECTURE NO 13

File Server Networks Many networks include not only nodes but also a central computer with a large hard disk that is user of shared storage. This computer is known as the file server, network server, or simply, server. Files and programs used by more than one user (at different nodes) are generally kept on the server. One relatively simple implementation of a network with nodes and a file server is a file server network. This is a hierarchical arrangement in which each node can have access to the files on the server but not necessarily to files on other nodes.

Client/Server Networks One popular type of server-based network is client/server computing, a hierarchical strategy in which individual computers share the processing and storage workload with a central server. This type of arrangement requires specialized software for both the individual node and the network server. Client/server software is valuable to large, modern organizations because it distributes processing and storage workloads among resources efficiently. This means that users get the information they need faster.

Peer-to-Peer Computing A third arrangement is a peer-to-peer network, in which all nodes on the network have equal relationships to all others, and all have similar types of software. Typically, each node has access to at least some of the resources on all other nodes, so the relationship is nonhierarchical. Peer-to-peer LANs are commonly set up in small organizations (fewer than 50 employees) or in schools, where the primary benefit of a network is shared storage, printers, and enhanced communication. Where large databases are use, LANs are more likely to include client/server relationships.

NEWORK TOPOLOGIES FOR LANS Topology---the physical layout of the cables that connect the nodes of the network. There are three basic topologies: bus, star, and ring. Network designers consider a number of factors in determining which topology, or combination of topologies, to use.

The Bus Topology A bus network, like the bus of a computer itself, is a single conduit to which all the network nodes and peripheral devices are attached. Nodes on one type of bus network, Ethernet, transmit data at any time, regardless of any data being sent by other nodes. If one set of data happens to collide with another set of data transmitted by other nodes---the is, if two nodes try to send data at the same time----each node waits a small, random amount of time and then attempts to retransmit the data.

The Star Topology A star network places a hub in the center of the network nodes. Groups of data are routed through the central hub to their destinations. This scheme has an advantage in that the hub monitors traffic and prevents collisions, and a broken 3

INTRODUCTION TO COMPUTER LECTURE NO 13 connection does not affect the rest of the network. If you lose the hub, however, the entire network goes down.

The Ring Topology The ring topology connects the nodes of the network in a circular chain in which each node is connected to the next. The final node in the chain connects to the firs to complete the ring. With this methodology, each node examines data that is sent through the ring. If the at is not addressed to the node examining it, that node passes it along to the next node in the ring. The ring topology has a substantial advantage over the bus topology. There’s no danger of collisions because data lays flows in one direction. One drawback to the ring, however, is that if a connection is broken, the entire network goes down.

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