A computer network is interconnection of various computer systems located at different places. In computer network two or more computers are linked together with a medium and data communication devices for the purpose of communication data and sharing resources.
The computer that provides resources to other computers on a network is known as server. In the network the individual computers, which access shared network resources, are known as nodes.
There are many different types of networks. However, from an end user's point of view there are two basic types:
Local-Area Networks (LANs)
LAN is a computer network that spans a relatively small area. Most LANs are confined to a single building or group of buildings. However, one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a wide-area network (WAN). Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it is also able to access data and devices anywhere on the LAN.
LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line; but the distance are limited, and there is also a limit on the number of computers that can be attached to a single LAN.
Wide-Area Networks (WANs)
A WAN is a computer network that spans a relatively large geographical area. Typically, A WAN consists of two or more local-area networks (LANs). Computers connected to a wide-area network are often connected through public networks, such as the telephone system. They can also be connected through leased lines or satellites. The largest WAN in existence is the Internet.
Topology is the geometric arrangement of the computers in a network. Common topologies include star, ring and bus.
The star network is frequently used to connect one or more small computers or peripheral devices to a large host computer or CPU. Many organizations use the star network or a variation of it in a time-sharing system, in which several users are able to share a central processor.
In a time-sharing setup, each terminal receives a fixed amount of the CPU's time, called a time slice. If you are sitting at a terminal and cannot complete your task during the time slice, the computer will come back to you to allow you to do so. Actually, because the CPU operates so much faster than terminals, you will probably not even notice that the CPU is away.
Star network is frequently used in a LAN to connect several microcomputers to a central unit that works as a communications controller. If the user of one microcomputer wants to send a document or message to a user at another computer, the message is routed through the central communications controller. Another common use of the star network is the feasibility of connecting several microcomputers to a mainframe computer that allows access to an organization's database.
Access and control of star network typically is maintained by a polling system. Polling means that the central computer, or communications controller "polls" or asks each device in the network if it has a message to send and then allows each in turn to transmit data.
The ring network is a Local Area Network (LAN) whose topology is a ring - can be as simple as a circle or point-to-point connections of computers at dispersed locations, with no central host computer or communications controller. That is, all of the nodes are connected in a closed loop. Messages travel around the ring, with each node reading those messages addressed to it.
One of the advantages of ring networks is that they can span larger distance than other types of networks, such as bus networks, because each node regenerates
messages as they pass through it. Access and control of ring networks are typically maintained by a "token-passing" system.
A Token-Ring network resembles a merry-go-round. To deliver a message, you would hand over your addressed note to a rider (the token) on the merry-go-round, who would drop it off at the appropriate place.
Bus networks are similar to ring network that the ends are not connected. All communications are carried on a common cable or bus and are available to each device on the network.
Access and control of bus networks are typically maintained by a method called contention, whereby if a line is unused, a terminal or device can transmit its message at will, but if two or more terminals initiate messages simultaneously, they must stop and transmit again at different intervals.
A protocol is an agreed-upon format for transmitting data between two devices. The protocol determines the following:
There are a variety of standard protocols from which programmers can choose. Each has it own particular advantages and disadvantages; for example, some are
simpler than the others, some are more reliable, and some are faster. The protocol can be implemented either in hardware or in software. Some of the popular protocols are TCP/IP, HTTP, FTP, SMTP, POP, Token-Ring, Ethernet, Xmodem, Kermit, MNP, etc.
The term architecture can refer to either hardware or software, or a combination of hardware and software. The architecture of a system always defines its broad outlines, and may define precise mechanisms as well.
An open architecture allows the system to be connected easily to devices and programs made by other manufacturers. Open architectures use off-the-shelf components and conform to approved standards. A system with a closed architecture, on the other hand, is one whose design is proprietary, making it difficult to connect the system to other systems.
Network architectures can be broadly classified as using either peer-to-peer or client/server architecture.
This is a type of network in which each workstation has equivalent capabilities and responsibilities. This differs from client/server architecture, in which some workstations are dedicated to serving the others. Peer-to-peer networks are generally simpler and less expensive, but they usually do not offer the same performance under heavy loads.
This is a network architecture in which each computer or process on the network is either a client or a server. Servers are powerful computers or processors dedicated to managing disk drives (file servers), printers (print servers), or network traffic (network servers). Clients are less powerful PCs workstations on which users run applications. Clients rely on servers for resources, such as files, devices, and even processing power.