Computer network is to get up between computer through joining medium interconnection , carries out data communication according to network agreement, realizing resource a kind of organization form of sharing.
What is to join medium? Join medium and play the part of the transportation of message as the transmission line in communication net and the connection of equipment the connection of role computer network medium kind is many, it may be the wired mediums such as cable, optical cable and twinning line , may be also the wireless mediums such as satellite microwave, the transmission medium adopted in this and communication net is the same basically.
On joining medium foundation, computer network must realize the communication between computer and computer resource share , therefore its structure, according to its function, may divide communication son net and two parts of resource son net. Is natural, basis the difference of hardware, divide it into host computer and communication son net two parts is also correct.
The concept of host computer is very important, the host computer been is the computer of every independence that forms network. In network, host computer runs application program. Here please notice difference host computer and the two terminal terminal fingers of main points person and network have dealings with necessary equipment, a keyboard adds a display can form a terminal, obviously, host computer is insufficient because of run application program and having only a keyboard and display , still has corresponding software and hardware. Therefore can not regard terminal as host computer, is but host computer sometimes as a terminal may.
What is agreement? Take cable to do comparison , must first stipulate the yard length of how many positions and the transmission form of message when cabling , expression starts what yard word, again, expression finishes what yard word, have made mistake what to be done, Zen place transmits address and the name of person, this kind of form and agreement decided in advance are agreements.
Such also the definition of network agreement: Make the different equipment in network can carry out the data communication that sinks establishes in advance the agreement and form that a complete communication both sides know and comply with together mutually.
Agreement is very important for computer in network , has no agreement in other words , is impossible to have computer net. It is supported that each kind of computer network has a set of agreement. Because of now when computer net kind is many, so, there are also the kind of many existing network communication agreements. Typical network communication agreement has open system to link mutual( OSI) agreement 1 , X, 25 agreements. TCP / IP is the agreement of communicating of Internet interconnection that can be mutually communicated and designed specially between various networks.
Evidently, because of the kind of different, computer network of the different, communication agreement that joins medium, items are numerous. But general, the geography that computer network may cover according to it scope, partition becomes local area network and wide area network. As local area network, point at the network that spreads in some kilometers of scopes, common department inside the country campus net and building net; Wide area network when minute scope offers the service of data communication very much in area, preceding NSFnet that mentions , domestic is communal as China to divide into groups to exchange net( CHINAPAC) and Chinese communal digital data net( CHINADDA), as well as the national educational and scientific research net( CERnet) etc. in suggestion belong to wide area network , CHINANET that builds will be also a wide area network.
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