Uses of Computer Networks

Resource Sharing: programs (O.S., applications), data, equipment (printers, disks) are available to all users of the network regardless of location.

Many organizations have a substantial number of computers in operation, often located far apart. A company with many factories may have a computer at each location to keep track of inventories, monitor productivity, and do the local payroll. That is Resource sharing, and the goal is to make all programs, equipment, and especially data available to anyone on the network without regard to the physical location of the resource and the user.

High reliability: By replicating files on different machines and having spare CPUs; users are more immune from hardware/software failure.

A second goal is to provide high reliability by having alternative sources of supply. In addition having multiple CPU’s means that if one goes down, the others may be able to take over its work, although reduced performance. Military, banking, air traffic Control, Nuclear reactor safety, and many other applications.

Less cost: Small machines have about 1/10 the power of a mainframe but 1/1000 the cost. By using such machines with file server machine(s), a local area network LAN can be cheaply installed. It is easy to increase the capacity by adding new machines.

Another goal is to save money. Small computers have a much better price/performance ratio than large ones. In this model, the users are called clients, and the whole arrangement is called the client-server model. Another networking goal is scalability, the ability to increase system performance gradually as the workload grows just by adding more processors. With centralized mainframes, when the system is full, it must be replaced by a larger one, usually at great expense and even greater disruption to the users. A computer network can provide a powerful Communication medium among widely separated employees.

Communications in Computer Networks

Users have access to email and the Internet

• Access to remote information
• Person-to-person communication
• Interactive entertainment

Many people pay their bills, manage their bank accounts, and handle their investments electronically. Home shopping is being popular. Newspaper will go on-line and be personalized. The next step beyond newspapers is the on-line digital library. Another application that falls in this category is access to information systems like the current World Wide Web, which contains information about the arts, business, cooking, government, health and too many other topics to even mention

valume of computer network

computer network is an interconnected collection of autonomous computers.As the volume of information in most fields of interest grows so issues of information dissemination and access become more significant. Networking holds the key to this by providing the means to exchange information and to share the resources involved. A network can link people through Computers, related equipment and other devices using media such as coaxial cable, telephone lines and microwave and satellite links for the purpose of information exchange and resource sharing. Information exchange may, for example, take the form of electronic mail or participation in a public computer-based ‘conference’. Resource sharing may include the use of a distant computing facility, or access to a locally sited laser printer or the retrieval of information from a shared data base where the information itself is a resource.

Initially, computer networks were set up to promote the time sharing of large mainframe computers. Typically, the terminals were of the same type and there were as many physical connections to the mainframe as terminals on the network. The next step was to accommodate peripheral devices such as line printers and file stores, as well as extending the range of terminal devices and expanding the number of physical connections. With more terminal connections on the network than accesses to the mainframe, users competed for ports and the use of the mainframe. The merging of computer and communication technologies has significant influence on the way in which computer systems are organized. Networks are being set up, not only to share computing power, but also, and even primarily, to improve the flow of information within organizations and beyond.

Network technology may be discussed in terms of three different methods of information transfer: voice, data and video. Organizations seeking to provide efficient and cost effective communication links may well wish to use all three. The technologies are essentially different but it is not uncommon for a network to combine two of the three and in some cases over parts of a network all three. Voice networking is most common and an account for most of today’s networking traffic

Basic Computer Network Architecture

Basic Computer Network Architecture

NODE : node is any piece of hardware on the system that can be addressed by a message from another node, i.e., a computer, fax, modem, or CD-ROM drive.

HUB : A hub is also known as concentrator in which all nodes are connected whose purpose is to simplify the wiring of the nodes to each other and to route signals between the nodes.

BACKBONE : A Backbone is a high capacity link to which many nodes or hubs can connect and it’s designed to carry lots of traffic.

SERVER : A server is any network computer that can be accessed by others to receive files, send print jobs, or connect to the Internet. The computers that are connected to the server are called clients because they depend on the server for programs, data, or connections to other computers and devices.

Local Area Networks

They are generally called LANs, are privately-owned networks within a single building or campus of up to a few kilometers in size. They are widely used to connect personal computers and workstations in company offices and factories to share resources and exchange information.
LANs are distinguished from other kinds of networks by three characteristics

• their size
• their transmission technology and
• their topology.

Traditional LANs run at speeds of 10 to 100 Mbps. Newer LANs may operate at higher speeds, up to hundreds of megabits/sec

Bus Topology

In a bus network, at any instant one machine is the master and is allowed to transmit. Computers on an Ethernet can transmit whenever they want to; if two or more packets collide, each computer just waits a random time and tries again later

Ring Topology

In a ring, each bit propagates around on its own, not waiting for the rest of the packet to which it belongs. Typically each bit circumnavigates the entire ring in the time it takes to transmit a few bits, often before the complete packet has ever been transmitted

Star Topology

Star architecture has the nodes connected to a central, or host, computer. When one of the computers on the network sends a signal, the host routes it to the node it’s addressed to. There are no direct connections between the nodes on the network expect through the host computer

Ethernets

computers to laser printers. Ethernet employs Carrier Sense Multiple Access/ Ethernet is probably the best known example of a bus structured LAN. It consists of one 0r more segments of coaxial cable to which devices are known as transceivers. Each device requiring a connection to the network is joined through a transceiver which monitors network traffic and a controller which provides the interface between the equipment and the network A connected device is able to send data if its controller finds that there is no other transmission in progress. The controller circuit is, of course designed to guard against mishap should two or more devices believe the highway, or data channel, is free. Each transceiver contains a collision detect circuit which continuously monitors activity on the highway. If a collision is detected, the appropriate controllers are notified and the transmission ceases at that point. After random delays, the controllers involved look again to see if the channel is clear and retransmit the same data (starting at the beginning again). Destination controllers are also informed when any collision occurs and thus are alerted to ignore any corrupt packets which may arrive. In reality, because of the very high data transfer rate and well controlled mode of operation, there is rarely a need to retransmit packets.

The first network access system, called Ethernet, was developed by Xerox in the 1970s to link Collision Detection (CSMA/CD).The three most popular implementations of Ethernet are called 10Base-5, 10Base-2,10Base-T.

The number 10 indicates the speed in megahertz,” Base” indicates that it’s based on Ethernet, and the last number indicates the cable used to connect the node. Ethernet can use any of the network architectures, star, bus, or ring. 10Base-5 and 10Base-2 use bus architecture and 10Base-T use a star architecture.

The latest version, Gigabit Ethernet, delivers data at gigabit speeds making it possible to deliver multimedia applications over the network

Intranets

Using internet technology, corporations are building company wide intranets which are accessible only to their employees and other invited users. Intranets are used not only for posting documents within an organization But also for linking software, databases, and hardware into a universal network.
These private intranets share the same software and hardware as internet and speak the same language, But they are fenced off from others on the internet by defensive barriers called FIREWALLS

Popularity of Intranets:

1. The resources on the intranet are accessible to anyone, anywhere as long as they have a computer, a modem, and a password.
2. Complex data and applications can be used over the intranet with ease.
3. Learning to use an intranet is simple. If you know how to use the web, you know how to use an intranet.
4. Costs are lower because so much software and so many connections are available from so many sources.

Metropolitan Area Networks

A metropolitan area network or MAN is basically a bigger version of a LAN and normally uses similar technology. A MAN can support both data and voice, and might even be related to the local cable television network. A MAN just has one or two cables and does not contain switching elements, which shunt packets over one of several potential output lines. It is called DQDB

Wide area networks

A wide area network or WAN, spans a large geographical area, often a country or continent. We will call this machines host. The hosts are connected by a communication subnet, the job of the subnet is to carry messages from host to host, and just as the telephone system carries words from speaker to listener. The subnet consists of two distinct components: Transmission lines and Switching elements.

Transmission lines move bits between machines. The switching elements are specialized computers used to connect two or more transmission lines. They are called routers. The collection of communication lines and routers form the subnet.

Internet Works

Internet works:

Many networks exist in the world, often with different hardware and software. People connected to one network often want to communicate with people attached to a different one. This desire requires connecting together different, and frequently incompatible networks, sometimes by using machines called gateways to make the connection and provide the necessary translation, both in terms of hardware and software. A collection of interconnected networks is called an internet work or internet. A common form of internet is a collection of LANs connected by a WAN.

Wireless Networks

Wireless Networks:

Mobile computers, such as notebook computers and personal digital assistants (PDA’s), are the fastest-growing segment of the computer industry. Wireless networks have many uses. A common one is the portable office. Another use is for rescue workers at disaster sites where the telephone system has been destroyed. Wireless networks are important to the military. Wireless networking and mobile computing are often related, they are not identical.