The development stage of computer network can be divided into 4 stages:
1 Online terminal network
From the 1950s to the mid-1960s, when there were fewer computers,The remote terminal is connected to the computer host by dial-up and telephone communication lines, and multiple terminals share the resources of the host to form an online terminal network.
The characteristic of the on-line terminal network is one host, multiple terminals, with the host as the center, and no communication between the terminals.
2 Computer-Computer Network
Computer-computer network is a system that conforms to the definition of computer network. A typical network example is ARPANET built in 1969. ARPANET marked a milestone in the development of computer networks, which eventually evolved into the current Internet.
The computer-computer network is characterized by multiple hosts. The network design adopts a layered concept to divide the functions of the computer network into different levels. The peer layers of the communication parties adopt peer-to-peer network protocols that can be understood by each other. The layer function is realized by the network control protocol, and the network control protocol and the transmission data together constitute the protocol data unit (PDU). Using packet switching technology, the transmitted message is divided into packets to adapt to the bursty characteristics of computer data transmission.
This stage is also called the era of packet switching networks.The composition of a computer network includes a two-level subnet, namely, a communication subnet for communication control, transmission, and communication processing, and a resource subnet for data processing.
The computer network evolved from the first stage centered on the host to the communication subnet as the center.
3 Open computer network architecture
From the late 1970s to the early 1980s, different computer manufacturers developed and designed their own network architectures and network products, such as IBM's system network architecture SNA and DEC's DECNET. Computer network hardware and software products produced according to a certain manufacturer’s network architecture can only be interconnected between the network products produced by this manufacturer, and cannot be interconnected with other manufacturers’ network products. The computer network system composed of the network products of these manufacturers forms a "closed system."
Taking into account the development of computer networks, the International Organization for Standardization (ISO) needs to develop a computer network reference model framework that everyone follows. All network hardware and software produced according to this framework can be interconnected. In 1983, ISO gave the "Open System Interconnection Reference Model".OSI provides 7 levels of description, from low-level to high-level, followed by physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer.Each layer completes a specific function, and the same layers of different systems communicate using peer-to-peer protocols. The computer network system that follows the OSI design is an "open system".
At this stage, the TCP/IP protocol has made considerable progress, the formal protocol called ARPANET, which uses the IP protocol to realize the interconnection of heterogeneous computers and heterogeneous computer networks.
4 Internet
The Internet is a worldwide "Network of Networks". Networks includes multiple networks with different network architectures, such as various local area networks, metropolitan area networks, and wide area networks, as well as wireless networks and mobile networks. The reason for so many different networks is that no one type of network can meet the needs of all applications.Many different networks adopt different network technologies, network protocols, and address formats. It is necessary to provide a method for network interconnection so that data information of network users can be transmitted in the interconnected network.
The technical idea of realizing network interconnection is to use its own network protocol within each network. When each network communicates with other networks, use the TCP/IP protocol to shield the differences in the underlying physical network, and provide consistent services upward through the interconnection protocol IP , Realize the interconnection between different networks through interconnection equipment (such as routers), making the differences between various networks transparent to network users.
Internet applications have experienced three types of services: access services, provided by Internet service providers; content services, provided by Internet content providers; application services, provided by Internet application providers.