Computer networks Now every almost have computers， and computer good does have its downside - it depends on how you If you want to ask me: I on the computer do what， I can tell you， I will do a lot of things in the May first and have out-of-town relatives chat， then can check information， listening to music， watching movies， My favorite on the computer music and watching movies， because it very relaxing my But the most people used compter to surf the Now Let us know about the computer networkⅠIntroduction Computer networks，the wide spread sharing of information among groups of computers and their users，are a central part of the information The popular adoption of the personal computer(PC) and the local area network (LAN) during the 1980s has led to the capacity to access information on a distant database;download an application from overseas;send a message to a friend country;and share files with a colleague-all from a personal The network that allow all this to be done so easily are sophisticated and complex They rely for their effectiveness on many cooperating The design and deployment of the wordwide computer network can be viewed as one of the great technological wonders of recent Nowadays Intemet is very popular all over the world， especially in some big Do you know when the Intemet was first established? Built in 1960s， the Internet was a crude network of a few computers which shared If one of the computers broke down， the whole networks would be unable to work， causing continual At first， just the government had access to the Internet， using it for communications among different However， by 1970s the Internet had been used in universities， banks， and At the beginning of 1990s computers became affordable for common people and this affordability increased the use of the Internet by people，It is said that each day tens of millions of people log off， making it the most important part of people's ⅡLocal Area Network One of the most dramatic events in computer networking has been the introduction and rapid growth of local area network(LAN) as a way to standardize the system of linking computers used in office As the name suggests，this is a means of connecting a number of computing elements At the simplest level，a LAN provides no more more than a shared medium (such as a coaxial cable to which all computers and printers are connected) along with a set of rules that govern the access to that The most widely used LAN，Ethernet，used a mechanism called Carrier Sense Multiple Access-Collosion Detect(CSMA-CD)This means that each connected device can only use the cable when it has established that each connected device is using If there is contention， the device looking for a connection backs off and tries again The Ethernet transfers data at 10M bits/sec，which is fast enough to make the distance between devices They appeat to be connected directly to their There are many different layouts(such as bus ，star，ring，see Figure 6A-2)and a number of different access protocols for LANDespire this variety，all LANs share the feature that they are limited in range (typically they cover one building) and are fast enough to make the connecting network invisible to the devices that use In addition to providing shared access，modern LANs can also give users a wide range of sophisticated Managenment software packages are available to control the way in which devices are configured on the local area network (LAN)，how users are administered，and how network resources are A widely adopted structure o local network is to have a number of servers that are available to a (usuallu much greater) nember of The former，usually powerful computers，provide service such as print control，file sharing，and mail to the latter，which are usually personal ⅢRouters and brideges The facilities on most local area network (LAN) are very Most organizatonns do not wish to have small isolated islands of computing They usually want to extend facilities over a wider area so that groups can wok wtthout having to be Routers and bridges arespecialized devices that allow two or more local area network (LAN) to be The bridges is the more basic device and can only connect local area network (LAN) of the same The router is a more intelligent component that can interconnent many different types of computer Many large companies have corporate adta network that are founded on a collection of local area network (LAN) and From the user’s point of view，this arrangement provides them with a physically diverse network that looks like one coherent ⅣWide Area Networks At some point，it becomes impractical to extend a local area network (LAN) any Physical limitation sometimes drives this，but more often than not there are more concenient or cheaper ways to extend a computer Two major components in most real computer networks are the public telephone and data These provide long-distance links that extend a local area network (LAN) into a wide area network (WAN)Nearly all of the national network operators offer services for the interconnection of computer These services range from simple， low speed data links that work over the public telephone network to sophisticaied high speed data services that are ideally suited to the interconnection of LANThese high speed data services are usually referred to as broadband It is anticiopated that they will provide the necessary links between LANs that make what is called the information superhighway a
