Friday, January 13, 2012

Network and computer science

Network (computer science)
Network Configurations
A star network connects many computers and computer resources, such as a printer, to a central computer called a server that directs the flow of information. A token-ring network uses a special message called a token that travels from computer to computer. The token designates which computer sends information through the network.

Network (computer science), a system used to link two or more computers. Network users are able to share files, printers, and other resources; send electronic messages; and run programs on other computers.
A network has three layers of components: application software, network software, and network hardware. Application software consists of computer programs that interface with network users and permit the sharing of information, such as files, graphics, and video, and resources, such as printers and disks. One type of application software is called client-server. Client computers send requests for information or requests to use resources to other computers, called servers, that control data and applications. Another type of application software is called peer-to-peer. In a peer-to-peer network, computers send messages and requests directly to one another without a server intermediary.
Network software consists of computer programs that establish protocols, or rules, for computers to talk to one another. These protocols are carried out by sending and receiving formatted instructions of data called packets. Protocols make logical connections between network applications, direct the movement of packets through the physical network, and minimize the possibility of collisions between packets sent at the same time.
Network hardware is made up of the physical components that connect computers. Two important components are the transmission media that carry the computer's signals, typically on wires or fiber-optic cables, and the network adapter, which accesses the physical media that link computers, receives packets from network software, and transmits instructions and requests to other computers. Transmitted information is in the form of binary digits, or bits (1s and 0s), which the computer's electronic circuitry can process.
A network has two types of connections: physical connections that let computers directly transmit and receive signals and logical, or virtual, connections that allow computer applications, such as e-mail programs and the browsers used to explore the World Wide Web, to exchange information. Physical connections are defined by the medium used to carry the signal, the geometric arrangement of the computers (topology), and the method used to share information. Logical connections are created by network protocols and allow data sharing between applications on different types of computers, such as an Apple Macintosh or a personal computer (PC) running the Microsoft Corporation Windows operating system, in a network. Some logical connections use client-server application software and are primarily for file and printer sharing. The Transmission Control Protocol/Internet Protocol (TCP/IP) suite, originally developed by the United States Department of Defense, is the set of logical connections used by the Internet, the worldwide consortium of computer networks. TCP/IP, based on peer-to-peer application software, creates a connection between any two computers.
The medium used to transmit information limits the speed of the network, the effective distance between computers, and the network topology. Copper wires and coaxial cable provide transmission speeds of a few thousand bits per second for long distances and about 100 million bits per second for short distances. (A million bits is equal to one megabit, and one megabit per second is abbreviated Mbps.) Optical fibers carry 100 million to 40 billion bits of information per second over long distances. (A billion bits is equal to one gigabit, and a billion bits per second is abbreviated Gbps.)
Wireless networks, often used to connect mobile, or laptop, computers, send information using infrared or radio-frequency transmitters. Infrared wireless local area networks (LANs) work only within a room, while wireless LANs based on radio-frequency transmissions can penetrate most walls. Wireless LANs using Wi-Fi technology have capacities of around 54 Mbps and operate at distances up to a few hundred meters. Wireless communications for wide area networks (WANs) use cellular radio telephone networks, satellite transmissions, or dedicated equipment to provide regional or global coverage. Although transmission speeds continue to improve, today’s wide area cellular networks run at speeds ranging from 14 to 230 kilobits per second. (A kilobit is equal to 1,000 bits, and one kilobit per second is abbreviated Kbps.) Some networks use a home’s existing telephone and power lines to connect multiple machines. HomePNA networks, which use phone lines, can transmit data as fast as 128 Mbps, and similar speeds are available on Power Line or HomePlug networks.
Common topologies used to arrange computers in a network are point-to-point, bus, star, ring, and mesh. Point-to-point topology is the simplest, consisting of two connected computers. The bus topology is composed of a single link connected to many computers. All computers on this common connection receive all signals transmitted by any attached computer. The star topology connects many computers to a common hub computer. This hub can be passive, repeating any input to all computers similar to the bus topology, or it can be active, selectively switching inputs to specific destination computers. The ring topology uses multiple links to form a circle of computers. Each link carries information in one direction. Information moves around the ring in sequence from its source to its destination. On a mesh network, topology can actually change on the fly. No central device oversees a mesh network, and no set route is used to pass data back and forth between computers. Instead, each computer includes everything it needs to serve as a relay point for sending information to any other computer on the network. Thus, if any one computer is damaged or temporarily unavailable, information is dynamically rerouted to other computers—a process known as self-healing. see Computer Architecture.
LANs commonly use bus, star, or ring topologies. WANs, which connect distant equipment across the country or internationally, often use special leased telephone lines as point-to-point links.
Sharing Information
When computers share physical connections to transmit information packets, a set of Media Access Control (MAC) protocols are used to allow information to flow smoothly through the network. An efficient MAC protocol ensures that the transmission medium is not idle if computers have information to transmit. It also prevents collisions due to simultaneous transmission that would waste media capacity. MAC protocols also allow different computers fair access to the medium.
One type of MAC is Ethernet, which is used by bus or star network topologies. An Ethernet-linked computer first checks if the shared medium is in use. If not, the computer transmits. Since two computers can both sense an idle medium and send packets at the same time, transmitting computers continue to monitor the shared connection and stop transmitting information if a collision occurs. When used on local area networks, Ethernet typically transmits information at a rate of either 10 or 100 Mbps, but newer wide-area technologies are capable of speeds as high as 10 gigabits per second (Gbps).
Computers also can use Token Ring MAC protocols, which pass a special message called a token through the network. This token gives the computer permission to send a packet of information through the network. If a computer receives the token, it sends a packet, or, if it has no packet to send, it passes the token to the next computer. Since there is only one token in the network, only one computer can transmit information at a time. Token Ring networks are now quite rare. Most LANs now use Ethernet technology. International Business Machines Corporation (IBM), the company that invented Token Ring in the early 1980s, no longer promotes the technology.
In the mid-1990s a new protocol called Asynchronous Transfer Mode (ATM) was introduced. This protocol encodes data in fixed-sized packets called cells rather than variable-sized packets used on an Ethernet network. It was designed as a way of merging old, circuit-switched telephone networks with more modern packet-switched computer networks in order to deliver data, voice, and video over the same channel. This can now be done with other protocols as well. Capable of speeds of nearly 10 Gbps, ATM is often used in wide area networks, but never really caught on with LANs.
Network management and system administration are critical for a complex system of interconnected computers and resources to remain operating. A network manager is the person or team of people responsible for configuring the network so that it runs efficiently. For example, the network manager might need to connect computers that communicate frequently to reduce interference with other computers. The system administrator is the person or team of people responsible for configuring the computer and its software to use the network. For example, the system administrator may install network software and configure a server's file system so client computers can access shared files.
Networks are subject to hacking, or illegal access, so shared files and resources must be protected. A network intruder could eavesdrop on packets being sent across a network or send fictitious messages. For sensitive information, data encryption (scrambling data using mathematical equations) renders captured packets unreadable to an intruder. Most servers also use authentication schemes to ensure that a request to read or write files or to use resources is from a legitimate client and not from an intruder. See Computer Security.
As of 2005, much of the Internet’s backbone—that is, its core infrastructure connecting so many of the world’s PCs—had been converted to 10 Gbps Ethernet, and researchers were working to develop even faster technologies. Ethernet speeds tend to grow by a factor of ten every half decade, and if this trend continues, speeds as high as 100 Gbps can be expected by 2008. Many researchers, however, believe that 100 Gbps is a bit further off, expecting an intermediate stop at 20 or 40 Gbps.
On the wireless side, some researchers are working on a new local area standard, known as 802.11n, which would double transmission speeds for wireless devices to nearly 200 Mbps. Other researchers are developing a new standard called Unlicensed Mobile Access (UMA), which would allow wireless mobile devices to seamlessly move from local area networks to wide area cellular networks and back again. Currently, there is no way for personal digital assistant (PDA) handhelds and cell phones to move automatically between a wireless LAN and a wireless WAN or vice versa. Service is interrupted, and a manual adjustment must be made on the device for wireless service to continue.

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