In today's digital world, network communication has become an important part of our daily lives and business activities. To better understand and manage network communications, network engineers and administrators use different models to organize and explain network protocols and communication processes. This article will introduce the two most important network models: the OSI seven-layer model and the TCP/IP five-layer model.
OSI seven-layer model
The OSI (Open Systems Interconnection) model is a theoretical network communication model developed by the International Organization for Standardization (ISO) in the 1980s. This model divides the network communication process into seven levels, each level is responsible for different functions, thereby achieving modular and hierarchical design of network communication. The following are the seven levels of the OSI model:
Physical Layer : This is the lowest layer and is responsible for the transmission of data on physical media. It includes physical properties such as voltage, current, cables and optical fibers. The main task of the physical layer is to transmit the bit stream from one node to another.
Data Link Layer : This layer is responsible for the transmission of data frames, as well as identifying and correcting errors in data transmission. It usually includes the management of MAC (Media Access Control) addresses.
Network Layer : The network layer is responsible for routing data packets to ensure that the data reaches the destination node from the source node. It uses IP addresses to identify devices on the network and control the forwarding of data packets.
Transport Layer : The transport layer provides end-to-end data transmission services, but it usually does not include error detection and correction functions. The transport layer is mainly responsible for reliable transmission of data and flow control. TCP (Transmission Control Protocol) is a reliable protocol that ensures data reaches its destination in order and retransmits lost packets when necessary. UDP (User Datagram Protocol) is another protocol at the transport layer, but it does not provide reliability guarantees and does not perform error detection and correction.
Session Layer : The session layer is responsible for establishing, managing and terminating sessions (or connections) to ensure the reliability of data transmission. It also handles synchronization and checkpointing of data.
Presentation Layer : The presentation layer handles data format conversion, data encryption and decryption, and data compression. It ensures data format compatibility between different devices.
Application Layer : The application layer contains network applications such as web browsers, email clients, and file transfer protocols. This layer provides users with an interface to access network services.
The OSI seven-layer model is both complex and impractical: so the OSI seven-layer model has not been implemented or implemented.
When actually building a network, only some layers in the OSI seven-layer model design are used, that is, the following TCP/IP five-layer (or four-layer) model is implemented.
TCP/IP five-layer model
Different from the OSI model, the TCP/IP five-layer model is the model used in actual Internet communication and is often called the Internet model. It's simpler, but also closer to how the internet actually works. The following are the five levels of the TCP/IP model:
Application Layer : The application layer is responsible for communication between applications and provides high-level services, such as email transmission (SMTP), File Transfer Protocol (FTP), Telnet, etc. Network programming is usually done at the application layer.
Transport Layer : The transport layer is responsible for data transmission between two hosts, ensuring that data is sent reliably from the source host to the target host. Transmission Control Protocol (TCP) is a commonly used transport layer protocol that provides reliability and flow control.
Network Layer : The network layer is responsible for address management and routing, using IP addresses to identify hosts, and planning data transmission routes through routing tables. Router works at the network layer.
Data Link Layer : The data link layer is responsible for the transmission and identification of data frames between devices, including frame synchronization, conflict detection and data error checking. Ethernet, Token Ring and Wireless LAN are all data link layer standards. Switch works at the data link layer.
Physical Layer : The physical layer is responsible for the transmission methods of optical/electrical signals, including network cables, coaxial cables, optical fibers, wireless electromagnetic waves, etc. The capabilities of the physical layer affect the maximum transmission rate, transmission distance and interference immunity. Hub works at the physical layer.
The layer where the network device is located
Host : The operating system kernel of a host usually implements the lower four layers of the TCP/IP five-layer model, from the transport layer to the physical layer. This includes functions that handle the transport layer (such as TCP and UDP), network layer (such as IP), data link layer (such as Ethernet), and physical layer (such as network cable interface).
Router : Router implements the lower three layers of the TCP/IP five-layer model, from the network layer to the physical layer. It is mainly responsible for routing and forwarding of data packets, thus covering the functions of the network layer, data link layer and physical layer.
Switch : The switch implements the lower two layers of the TCP/IP five-layer model, from the data link layer to the physical layer. Its main task is to exchange data frames based on MAC addresses, so it covers the functions of the data link layer and the physical layer.
Hub : Hub only implements physical layer functions. It copies data from one port to all other ports and does not handle data frames or addresses intelligently, so it is only responsible for the physical layer transmission.