bidirectional
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During the 1950s, most computers were similar in one respect. They had a main memory, a central processing unit (CPU), and peripherals. The memory and CPU were central to the system. Since then a new generation of computing has emerged in which computation and data storage need not be centralized. A user may retrieve a program from one place, run it on any of a variety of processors, and send the result to a third location.
A system connecting different devices such as PCs, printers, and disk drives is a network. Typically, each device in a network serves a specific purpose for one or more individuals. For
example, a PC may sit on your desk providing access to information or software you need. A PC may also be devoted to managing a disk drive containing shared files. We call it a file server. Often a network covers a small geographic area and connects devices in a single building or group of buildings. Such a network is a local area network (LAN). A network that covers a larger area such as a municipality, state, country, or the world is called a wide area network (WAN).
Generally speaking, most networks may involve many people using many PCs, each of which can access any of many printers or servers. With all these people accessing information, their requests inevitably will conflict[1]. Consequently, the devices must be connected in
a way that permits an orderly transfer of information for all concerned. A good analogy is a street layout in a large city. With only one person driving it matters little where the streets are, which ones are one-way, where the traffic signals are, or how they are synchronized. But with thousands of the cars on the streets during the morning rush hour, a bad layout will create congestion that causes major delays. The same is true of computer networks. They must be connected in a way that allows data to travel among many users with little or no delay. We call the connection strategy the network topology. The best topology depends on the types of devices and user needs. What works well for one group may perform dismally for another.
UNIT 16
Introduction to Computer Networks
New words and Technical Terms
peripheral congestion topology
dismally
mainframe bus
dart
muscle
EtheFig. 4-16. A common bus topology.
Some common network topologies are described as following. Fig. 4-16 shows a common bus topology (or simply bus topology) connecting devices such as workstations, mainframes, and file servers. They communicate through a single bus (a collection of parallel lines). A common approach gives each device an interface that listens to the bus and examines its data traffic. If an interface determines that data are destined for the device it serves, it reads the data from the bus and transfers it to the device. Similarly, if a device wants to transmit data, the interface circuit sense when the bus is empty and then transmit data. This is not unlike waiting on a freeway entrance ramp during rush hour. You sense an opening and either quickly dart to it or muscle