The computer application early, every user is exclusive hardware resources. However, with the expansion of application, computer each other more and more need to exchange information. If the computer to exchange information between directly, we call them is the Internet. When a person or a group of computers with other people's computer to share information or some computer hardware resources (such as disk, printer, tape machine, communications processor), then it needs to them through a medium, running a system software, according to some topology structure connected up, this is a computer network.
The definition of computer network: the computer network can describe from different sides, so give it down to an exact definition of complete is difficult. Generally, and said, all that will be different and geographical position with independent function or independent of the operating system DuoGe computer system, through the communication equipment and lines will link up the, the perfect function of network software (network protocol, information exchange ways, control program and network
operating system) realize network resource sharing system called computer network.
From this definition, the traditional host/terminal type system cannot be completely in the sense of computer network. Relative to the host time-sharing system-terminal modes for, the network every computer is independent, mutually no primary and secondary of the points. For example, any computer on the network cannot enforce or shut down the other computer network.
Based on the PC LAN, the server is a network central core. According to the server on the network, the role of, also can be further divided into will file servers, to print the server and communication server. The file server can make it has a large capacity disk storage space supply of the client to use online, receiving a client data processing, access request; To print the server accepted from client print tasks, and will be printed content in the print queue, when the mission's turn in the queue is sent to the printer to print output; Responsible for the communication server network to the main computer contact client, nets and nets of communication between the client and the Shared between expensive high-speed modem or fax communication equipment, etc. In short, the server on the network only provide services, provide and management on the disk drive, printer and communication equipment such multiple concurrent access to at the same time.
Entities (Entities) entity may be defined as the server, client, equivalence machine and they are running the aggregation of software.
Asynchronous transfer: be transmitted data coding into a sequence of pulse, and transmit a ASCII format, the first send start bit, then the data bits, parity, and stop bits. Asynchronous transfer is that according to the agreed upon the fixed format, a frame of a frame of transmission, because each character with start bit, stop a, thus transmit efficiency low, mainly used for low and middle communication circumstance;
Synchronous transfer: use 1 or 2 synchronous transfer the beginning of the process character says, then is n characters data block, does not allow to have the gap between characters, the sender sends, the first to send to the original data coding and sent out. Due to send a server code bring their own clock, realize the received or sent between the two sides of the self synchronous function. The receiving end after decoding, it can get the original data. Then each character starting, stopping, who doesn't need, greatly reduce overhead, the data transmission efficiency higher than asynchronous transfer, often used for high speed communications occasion. But the hardware is relatively complicated
Network operating system (NOS) is like a computer operation must have it independent operating system as support, computer network also must have corresponding network operating system support. Network operating system is a DuoZhong system software component, in the basic system have DuoZhong configuration options and above, the user can according to need to make optimal combination. At present the computer network operating system mainly UNIX, Windows NT, NetWare, etc. UNIX operating system is a computer network across, wow, mainframe systems; Windows NT is Microsoft rolls
out, can run in the microcomputer and the work site, the distributed graphic of the network operating system; Novell NetWare is launched, mainly to computer LAN network operating system.
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