Introduction to Computer network

I. The emergence and development of computer network

1. Terminal-oriented computer network

One representative of the first generation of computer networks was Sabrei, a the early 1960s American airline company that invested
Air ticket booking system consisting of a central computer and over 2000 terminals across the United States.

2. Computer Communication Network

This computer group, which is connected by multiple host systems and transmits information as the main purpose, is called the Computer Communication network, which is called the second generation computer network, and its typical representative is ARPANET.

3. Computer network

In the late the 1970s, the computer network changed from the original information transmission to shared network resources. In this context protocol generation (Protocol), under the control of the Protocol, to achieve the main purpose of sharing resources (hardware, software and data, etc.), the collection of multiple computers connected by a communication system is called a computer network, or a third generation computer network.

two. The definition and application of computer network

1. Definition: communication lines will be dispersed in different locations with independent computer systems connected to each other , and by the network protocol
A collection of computers for data Communication and resource sharing, called a computer network.

"Communication lines" can be twisted pair, coaxial cable (coarse, thin), optical fiber, microwave, communication satellites, infrared and laser, etc.
"Different places" gives the differences in the geographical location of each computer, connecting them to form a "net", and according to the scope of the coverage of the local (such as the building, campus, etc.), wide-ranging (such as national, International) computer network.
"Independence" means that there is no obvious master-slave relationship between computers on the Internet, that is, any computer on the Internet cannot be forced to start, stop and control another computer on the internet, so the terminal-oriented network is not a
Envelope

"Interconnected" in order to enable the exchange of information between computers on the Internet, and by the connection of different ways to produce
Different types of computer networks on the structure.

"Network protocol" can be simply described as "the communication process in the whole network to comply with
Code of Practice. "

"Data" can include text, graphics, sounds, images, and so on.

"Resources" means devices that provide services to users for a limited period of time, including soft devices such as various language handlers, service programs and applications, and hard devices
(such as the processing power of mainframe computers, ultra-large capacity memory, high-speed printers, etc.) and data (data files, public
Common database, etc.).

"Sharing" means that these "resources" can be used by all users on the Web, and users do not have to consider their location in the network and resources in the network.

2. Application

Network functions:

• Provide resource sharing.

• Provide a quick exchange of information.

• Provides distribution processing capabilities.

• Achieve centralized control and management.

• Improve system reliability.

• Improve the performance and price ratio of the system.

Web applications: E-mail, remote login (telnet), File transfer (FTP), network newsgroups (USENET),
WWW browsing, electronic bulletin Board (BBS), virtual reality, IP telephony, IRC and ICQ, e-commerce and network entertainment.

three. The composition and classification of computer network

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In order to reduce the complexity of the network, reduce the workload and facilitate the interconnection of the heterogeneous machine, and consider to make full use of the communication line resources, improve the integrity and reliability of the network, simplify the design, the "Data processing" and "Communication processing" task separation, thus dividing the "resource subnet" and "communication subnet" two parts. 1 Sing 3 The outside of the box is the "resource subnet", and the inner part of the box
Is "communication subnet", that is, the computer network is composed of these two subnets. The clear division of labor in this kind of function makes both the connection, the need to cooperate, and the independence, which is beneficial to the whole network efficiency.

Classification of computer networks:

1. According to the network topology, it can be divided into star-type network, ring-type network, bus network, tree-type network and network-type network.

• Star-shaped network structure

A network of dedicated mini-switch PBXs is a star topology. A LAN consisting of hubs and twisted pair is also a star-shaped structure. Star-Shaped Network

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• Bus network structure

Only one site is allowed to occupy the bus at any one time, that is, the information can only be sent by that site, while the other site is in a blocked send state, allowing only information to be received. Bus network

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• Ring network structure

A ring network is a closed loop in which each host is made up of a point link of its own repeater or transponder.

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• Tree-shaped mesh

The tree-type network is the node that the nodes are connected hierarchically, and the higher the high-level nodes are, the more reliability requirements are.

• 
  

  • Network Type Network

The network type Network general also has the rule type and the irregular type, this structure's biggest characteristic is the high reliability. Because there is a redundant link between nodes, you can choose a different link even if one of the links fails. But obviously the disadvantage of this structure is that the communication line is long, the cost is high and the path control is complicated.

2. Depending on the area covered by the network, it can be divided into LAN and WAN.

Local area network (LAN), wide area network (WAN)

four. Computer network architecture and Protocols

• International Organization for Standardization ISO

OSI/RM uses seven levels of architecture, the bottom is the first layer, the top layer is the seventh level, bottom in turn is called: Physical layer, Data link layer, network layer, transport layer,

The session layer, the presentation layer, and the application layer are represented by each layer name in the English initials PH, DL, N, T, S, P, and A.

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The main functions of each layer are as follows:

• The physical layer , which is the lowest layer of the OSI model, is directly connected to the transmission medium and is the only layer in which information is exchanged between peers. It transmits the raw data ——— bitstream, regardless of the meaning and structure of the bit stream. The physical layer defines the mechanical, electrical, functional, and process four interface characteristics for physical connections that establish, maintain, and terminate the transmission of data bitstream on the transmission medium.

• Data Link Layer It is to deal with raw primitive bit streams from the physical layer, in the specified format to form the data transfer unit of this layer ——— frame, and on the link between adjacent two nodes error-free transmission, that is, through the calibration, The method of confirming and re-sending changes the original unreliable physical connection to the error-free data link. This layer is to complete the establishment, maintenance and release of the link, data encapsulation and disassembly, frame synchronization, error control and data flow control. For shared channel networks, this layer should also have a mechanism for resolving contention channels.

• The data transfer units at this layer of the network layer are groupings or packets (packet). By controlling the operation of the communication subnet, the path selection algorithm is used to make the packet select an optimal path from the source side to the destination. This layer is to solve the congestion that the packet may encounter in the transmission, and when it needs to cross multiple communication subnets, it also solves the problem of internetwork interconnection.

• The transport layer is also referred to as the transfer layer, transport layer, or forwarding layer. This layer of information transmission Unit is the message (message), the layer and above the level of each peer layer protocol is an end-to-end (End-to-end), that is, the host to the host, does not involve communication within the network communication details. It is the software interface between the lower layer of communication processing and the high-level data processing-oriented. Key features include the establishment, maintenance and release of this connection, end-to-end sequencing, flow and error control, multiplexing (multiplexing) and shunt (splitting), and end-to-end reliable, transparent data transfer.

• The session layer is also called a meeting layer or a dialog layer. At this level and above, the units of the data transfer do not have additional names, which are generally referred to as messages. The purpose of this layer is to represent/apply entity organizations to collaborate and synchronize their conversations, and to provide the necessary means for managing the exchange of data between them, such as allowing conversations to be full-duplex or half-duplex. When a conversation is allowed to be half-double, this layer controls which party has the right to send data by providing a data token.

• Presentation Layer This layer handles the syntax (syntax) transformation of data exchange between two application entity AE (Application entity), which involves the concepts of abstract syntax, transfer syntax, etc. Solve the inconsistency of data structure and transform the different encoding of data. At the same time to save money, often to minimize the actual transmission of the number of bits, so this layer provides the function of data compression. To prevent malicious eavesdropping and tampering, this layer also provides data encryption capabilities.

• application tier It is the highest level in osi/rm, providing a means for user processes to access the OSI environment, which is responsible for application management, semantic representation of user information, and execution of applications.

TCP/IP Architecture

The Internet is based on the TCP/IP protocol. TCP/IP is a set of protocols, because TCP and IP are well-known two protocols, so the entire protocol family is called TCP/IP, the following is a brief introduction to the entire TCP/IP architecture.

The architecture model of TCP/IP is a four-layer model, which is divided into application layer, Transport layer (TCP), Network interconnect layer (IP) and network interface layer.

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• The application layer ——— this layer as the highest tier, providing several applications for user invocation. The application uses the Transport Layer service to send or receive data. The data form can be a string of messages, or it can be seen as a byte stream.

• The fundamental task of the Transport Layer ——— this layer is to provide reliable end-to-end communication. This layer ensures reliable transmission, no chaos, and flow control. To do this, the acknowledgement and request re-send function is received. The transport layer divides the data into packets and passes it to the next layer.

• The network interconnect layer ——— This layer encapsulates the packet of TCP packets into an IP datagram, plus the IP header, using a routing algorithm to determine whether the datagram is delivered directly to the host or to the next gateway. The layer sends the datagram to the network interface layer.

• The network interface layer ——— This layer is the lowest layer of Internet software. It is responsible for receiving datagrams sent from IP,
  and submit the data to the specified network.

There are a lot of relevant knowledge is not listed, the computer network is the actual computer Network + protocol + Service + ideas.

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Introduction to Computer network