2.1. Network Basics

2.1.1. Composition of computer communication network

A computer network consists of communication subnets and resource subnets. The communication subnet is responsible for error-free and orderly transmission of data, and its processing functions include error control, flow control, routing, and network interconnection.

The resource subnet is the local system environment for computer communication, including hosts, terminals, and applications. The main functions of the resource subnet are user resource configuration, data processing and management, software and hardware sharing, and load balancing.

In general, a computer communication network is a system that is carried by the communication subnet, transmits and shares various information of the resource subnet.

2.1.2. Communication Protocol

In order to complete the orderly exchange of information between computers, the concept of communication protocol is proposed, which is defined as a set of rules that must be followed by both (or multiple parties) communicating with each other on how to exchange information.

The agreement involves three elements, namely:

  • Grammar: Grammar is the structure and format of user data and control information, and the meaning of the order in which the data appears

  • Semantics: used to explain the meaning of each part of the bitstream

  • Timing: A detailed description of the order in which events are implemented

2.1.3. OSI seven-layer model

2.1.3.1. Introduction

OSI (Open System Interconnection) is divided into seven layers: physical layer, data link layer, network layer, transport layer, session layer, presentation layer, and application layer. Its specific functions are as follows.

2.1.3.2. Physical Layer

  • Provide the mechanical, electrical functions and procedures required to establish, maintain and release physical links

  • Physical transmission, fault monitoring and physical layer management of data streams (bit streams) over transmission media

  • Receives frames from the data link layer and converts the bitstream into a signal on the underlying physical medium

2.1.3.4. Network layer

  • Responsible for the routing or switching of end-to-end data, establishing connections for transparent data transmission

  • Addresses and resolves all issues related to the transfer of data between heterogeneous networks

  • Use the functions of the transport layer above and the data link layer below

  • Formatted messages are called packets

2.1.3.5. Transport Layer

  • Provides error-free data transmission

  • Receive data from the session layer, split the data into smaller packets if necessary, deliver the packets to the network layer and ensure that the packets arrive at their destination intact and correctly

  • Provides reliable and transparent data transmission between systems, providing end-to-end error recovery and flow control

2.1.3.6. Session Layer

  • Provides coordination of communication processes between nodes

  • Responsible for enforcing session rules (such as whether the connection allows half- or full-duplex communication), synchronizing data flow, and re-establishing connections in the event of failures

  • Use the functions of the presentation layer above and the transport layer below

2.1.3.7. Presentation layer

  • Provide data format, transformation and encoding conversion

  • Involves the syntax and semantics of the data being transferred

  • Encode the message in a format suitable for electronic transmission

  • Perform data compression and encryption at this layer

  • Receive messages from the application layer, convert the format, and pass to the session layer, which is often merged into the application layer

2.1.3.8. Application Layer

  • Includes various protocols that define specific user-facing applications: e.g. email, file transfer, etc.

2.1.3.9. Summary

The lower three-layer model belongs to the communication subnet, which involves providing transparent connections between users. The operation is mainly based on each link (hop-by-hop), and communication is performed on each data link between nodes. Communication on each link is controlled by the network layer, but depends on the coordinated operation of other nodes.

The upper three layers belong to the resource subnet and are mainly involved in ensuring that information is transmitted in a correct and understandable form.

The transport layer is the interface between the upper three layers and the lower three layers. It is the first end-to-end layer, which guarantees a transparent end-to-end connection, meets the user’s quality of service (QoS) requirements, and provides appropriate information to the upper three layers. form.