Network (computer science)

 

Network (computer science)

I  INTRODUCTION 

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.

II  NETWORK CONNECTIONS 

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. 

A  Media 

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.

B  Topology 

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. 

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.

C  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. 

III  NETWORK OPERATION AND MANAGEMENT 

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 in General on computer studies.

WRITTEN BY xl dansmich. @dm technic, all rights reserved.