Basic knowledge and terminology of computer network (2)---Hierarchical structure model

I. Overview

1.1 Layering principle

• 1. The principle of functional division of labor: that is, the functions of each layerdivideEach layer should have its own distinct layer that is different from other layers.basic skills。

• 2. The principle of isolation and stability: namelyThe structure of layers should be relatively independent and isolated from each other, so that changes in the content or structure of a certain layer have little impact on other layers, and the functions and structures of each layer are relatively stable.

• 3. The principle of branch expansion: that is, the public part and the branchable part are divided into different layers, which is conducive to the flexible expansion of the branch part and the relative stability of the public part. Appropriate technology is used for each layer.Reduce structural duplication。

• 4. Maintain the independence of the upper layer from the lower layer, and the upper layer uses the services of the lower layer in one direction .

• 5. The principle of convenient implementation: namelyFacilitate standardized technology implementation。

1.2 Noun

• Entity
  The active element at each level of the structure. It is a more abstract concept that represents any hardware or software process that can send or receive information.

• Peer-to-peer layer
  The same layer on different machines

• Peer entities
  refer to entities at the same level as the communicating parties. (That is, the corresponding entities on the same layer of structure between the communicating parties)

2. Protocols, interfaces, services

2.1 Agreement

A series of rules or conventions formulated for data exchange between peer entities in a computer network, which are horizontal , that is, onlyOnly peer entities will have an agreement. For example, there is an agreement between the presentation layers of two communication hosts, but the presentation layer of one party has no agreement with the session layer of the other party.

• Syntax: Specifies the format of transmitted data

• Semantics: Specifies the functions to be completed

• Synchronization: Specifies the order of various operations

2.2 Interface (Access Service Point SAP)

The joint part between each two levels is the connection point between two adjacent levels.

The interface isUsed to implement communication between layers, it is a more abstract concept that represents any hardware or software process that can send or receive information .

Two entities on the same layer communicate through interfaces, and entities on different layers also communicate through interfaces.

2.3 Services

Service refers to the function call provided by the lower layer of the network to the adjacent upper layer. The agreement is horizontal , i.e.The lower layer provides services to the upper layer one-way through the interface。

The specific implementation details of the services provided by the lower layer are completely shielded from the upper layer.

并非在一个层内完成的全部功能都称为服务，还要向第n+1层提供本层的服务，只有那些能够被高一层实体“看得见”的功能称为服务。

Can be divided into the following three categories:

1. Connection-oriented services and connectionless-oriented services

• Connectionless service
  means that the communicating parties do not need to establish a communication line in advance . Instead, each packet (message group) with a destination address is sent to the line, and the system independently selects a route for transmission.

• Connection-oriented service
  When communicating, both parties must establish a communication line in advance . The process is as follows:Three processes: establishing connection, using connection and releasing connection

2. Responding and unanswering services

• Response service
  means that the receiver gives a corresponding response to the sender after receiving the data.The response is automatically implemented within the transmission system, rather than implemented by the user . The response sent can be a positive response or a negative response , usually inSend a negative response when the received data contains errors。

• No-response service
  means that after the receiver receives the sent data,No feedback to the sender, the service is opaque, that is, it is not implemented by the user, but automatically implemented by the communication system. Therefore, the service cannot guarantee reliability

3. Reliable and unreliable services

• Reliable service
  refers to the networkWith error correction and detection mechanism, which can ensure that data is transmitted to the destination correctly and reliably .

• Unreliable service
  means that the network is onlytry your bestdelivered correctly and reliably, butNot guaranteedData is delivered to the destination correctly and reliably.
  For example, the file transfer service is a response service. The client does not respond after receiving the page file sent by the server, which is an unreliable service.

2.4 Data

2.4.1 Data format

• SDU Service Control Unit: Data that should be transmitted to complete the functions required by the user.

SDU (Service Data Unit), also called service data unit , is user service data at a designated layer. When transmitted to the receiver, the data does not change at the same protocol layer, that is,business part

• PCI protocol control information: Information that controls protocol operations. yesControl information added to the header or tail of the transmitted data to implement the protocol, such as address, error control information, serial number information, etc.

• PDU (Protocol Data Unit): a data unit transmitted between peer levels. It refers to the data unit transmitted by the communication protocol between the sender and the receiver, including packets (or datagrams) and responses (or response messages)
  . ), is the basic unit for transmitting data in the network , which may include data and necessary information for synchronization.

2.4.2 Data encapsulation and decapsulation

For detailed explanation, please refer to the articles of other excellent bloggers

3. Wireless communication transmission method

3.1 Simplex

data transmissionCan only be done in one direction, using a transmission line.

For example, radios, speakers in broadcasting rooms, etc.

3.2 Half duplex

Data transmission can be over a signal carrierTransmit in both directions, but not simultaneously。

For example, a walkie-talkie.

3.3 Full duplex

data transmissionAllows data to be transferred in both directions simultaneously, which is equivalent to the combination of two simplex communication methods in terms of capabilities.

It is equivalent to the way signals are transmitted when making phone calls today.

4. Reference model

4.1 OSI/ISO model

The OSI reference model is divided into seven layers, as shown in the figure

The OSI/ISO model explains the communication process:

• Intermediate system: The model function of the intermediate system usually only has the bottom three layers of the OSI model, including switches, routers and other intermediate devices.

• End-to-end communication method:
  End-to-end communication: refers to the fact that before data transmission, the 4G router and 4GDTU pass through various switching devices, and a link is established between the two end devices. After the link is established, the sending end can send data until the data is sent and the receiving end confirms that the reception is successful.

• Point-to-point communication method:
  Point-to-point communication: refers to the sending end transmitting data to the device directly connected to it, and this device transmits the data to the next device directly connected to it at the appropriate time . One directly connected device transmits data to the receiving end .

1. Application layer

• Definition: It is the interface between users and the network. Includes all programs that can interact with users and generate network traffic.

• Function:
  Provides services directly for application processes. Its function is to realize mutual communication between multiple system application processes and complete a series of services required for business processing.

• The application layer 数据传输单位ismessage

• Typical application layer services:
  File transfer (FTP)
  Email (SMTP)
  World Wide Web (HTTP)
  DNS protocol
  …

2. Presentation layer

• Definition
  A representation (syntax, semantics) for handling information exchanged between two communication systems

• Function:

  • 1. Data format conversion (translator)
    converts the 01 data stream into pictures and other information

  • 2.Data encryption and decryption
    WeChat payment password

  • 3. Data compression and recovery
    video calls, etc.

• main agreement

  • 1.JPEG
  • 2.ASCII

3. Session layer

• definition
  

• Function

  • 1. Establish, manage and terminate sessions
  • 2. Using checkpoints allows the session to resume communication from the checkpoint/synchronization point when communication fails, achieving data synchronization (suitable for transferring large files)

• Main Agreement:

  • 1. ADSP
  • 2.ASP

4. Transport layer

• Definition Communication
  in the host , that is , communication.两个进程端到端
• Transmission unit: message segment or user datagram
• Function
  • 1. Reliable transmission and unreliable transmission
  • 2. Error control
  • 3. Flow control
  • 4. Reuse and deuse

• main agreement
  • 1.TCP
  • 2.UDP

5. Network layer

• Definition:
  The main task is to transmit packets from the source to the destination and provide communication services for different hosts on the packet switching network.

• Transmission unit: data packet

• Function

  • 1. Routing (choosing the best path)
  • 2. Flow control
  • 3. Error control
  • 4.Congestion control

• main agreement
  • 1.IP protocol
  • 2.IPX
  • 3.ICMP
  • 4.IGMP
  • 5.ARP
  • 6.RARP
  • 7.Routing protocol

6. Data link layer

• Transmission unit: frame

包含Mac地址相关内容。

• main agreement
  • 1. SDLC
  • 2. HDLC
  • 3. PPP
  • 4. STP
  • 5. 802.11（wifi）
  • 6. Ethernet

7. Physical layer

• Definition
  The main task is tophysical mediarealized onTransparent transmission of bitstream。

• Transmission unit: bit

• Function

  • 1. Define interface characteristics
  • 2. Define the transmission mode:单工，半双工，双工
  • 3. Define the transmission rate
  • 4. Bit synchronization
  • 5. Bit encoding

• main agreement

  • 1.Rj45
  • 2.802.3

4.2 TCP/IP model

4.3 Comparison between OSI model and TCP/IP model

4.4 Five-layer reference model