Introduction to OSI Reference Model and Data Encapsulation Principles

The Application Layer is the highest layer of the OSI reference model, serving as the interface between computer users and various applications and networks. Its function is to directly provide services to users, completing various tasks users wish to accomplish over the network.

The Presentation Layer is the sixth layer of the OSI model. It interprets commands and data from the Application Layer, assigns meanings to various syntaxes, and transmits them to the Session Layer in a specific format. Its main function is to handle the representation of user information, such as encoding, data format conversion, and encryption/decryption.

The Session Layer is the fifth layer of the OSI model, serving as the interface between user applications and the network. Its main task is to provide methods for establishing and utilizing connections between the Presentation Layers of two entities. The connection between the Presentation Layers of different entities is called a “session.” Therefore, the task of the Session Layer is to organize and coordinate communication between two session processes and manage data exchange.

Main functions:

• The Transport Layer is an interface layer that connects high and low layers in the network architecture. The Transport Layer is not just a standalone structural layer, but the core of the entire analytical protocol system. The Transport Layer mainly provides End-to-End services to users, addressing issues such as datagram errors and packet sequencing. It is a critical layer in computer communication architecture, shielding higher layers from the communication details of lower layers, allowing users to ignore the detailed workings of the physical layer, data link layer, and network layer. The Transport Layer uses the network connection services provided by the Network Layer and can choose between connection-oriented or connectionless services based on system requirements.

Common:

• Transport data includes segments (Segment), TCP, UDP, processes, and ports.

Main functions:

• The Network Layer primarily creates logical links for data transmission between nodes, selecting the best path for packets through routing algorithms to achieve functions such as congestion control and network interconnection. The Network Layer is a key layer that oversees the network from the highest node, the router, responsible for routing packets from the source network to the target network. The Internet is a collection of multiple networks, and it is the routing path selection function of the Network Layer that allows for smooth connections between multiple networks, enabling information sharing.

Common:

• Transport data includes packets (Packet), routers, multilayer switches, firewalls, IP, IPX, RIP, OSPF.

Main functions:

• The Data Link Layer establishes data link connections between communication entities, with the basic unit of transmission being a “frame,” and provides error control and flow control services to the Network Layer. The Data Link Layer consists of the MAC (Media Access Control sublayer) and LLC (Logical Link Control sublayer). The main task of the Media Access Control sublayer is to specify how frames are transmitted over physical lines. The Logical Link Control sublayer manages communication between devices on the same network link. The Data Link Control sublayer is primarily responsible for logically identifying different protocol types and encapsulating them. In other words, the Data Link Control sublayer receives network protocol data, packet datagrams, and adds more control information to transmit the packet to its target device.

Common:

• Transport data includes frames (Frame), network cards, bridges, and layer 2 switches.

Main functions:

• The Physical Layer is the lowest layer of the reference model, primarily defining the electrical, mechanical, procedural, and functional standards of the system, such as voltage, physical data rates, maximum transmission distances, physical connectors, and other similar characteristics. The main function of the Physical Layer is to provide physical connections for the Data Link Layer using transmission media, responsible for the physical transmission of data streams. The basic unit of transmission for the Physical Layer is the bit stream, which consists of 0s and 1s, representing the most basic telecommunication signals or optical signals, and is the fundamental physical transmission characteristic.

Common:

• Transport data includes bits (Bit), repeaters, hubs, and network cables.