Table of contents
OSI model
The OSI model is a conceptual model of network communication, used to describe the communication and functions between different levels in a computer network. It divides network communication into seven different levels, each level is responsible for different tasks, making the design, development and management of network communication more modular and maintainable. Following are the seven layers of the OSI model and their main functions:
1. Physical Layer :
Function: Process the original bit stream on the physical transmission medium to ensure that the data can be transmitted in an appropriate manner on the transmission medium.
Example devices: hubs, repeaters, network cables, etc.
Main tasks: bit encoding, voltage specification, physical topology, etc.
2. Data Link Layer :
Function: Responsible for splitting the raw bit stream into frames and adding address information to transmit data between directly connected devices.
Example devices: switches, network cards, bridges, etc.
Main tasks: frame encapsulation, MAC address management, error detection and correction.
3. Network Layer :
Function: Responsible for transmitting data between different networks, routing data packets and ensuring that they reach the target device.
Example devices: routers, layer 3 switches, etc.
Main tasks: IP address allocation, routing, data packet forwarding, etc.
4. Transport Layer :
Function: Provide end-to-end data transmission services to ensure data reliability, flow control and error detection.
Example protocols: TCP (Transmission Control Protocol), UDP (User Datagram Protocol).
Main tasks: port management, flow control, data segmentation and reassembly, etc.
5. Session Layer :
Function: Manage the establishment, maintenance and termination of sessions, and handle session-level errors and synchronization issues.
Example features: session control, session recovery, etc.
6. Presentation Layer :
Function: Responsible for format conversion, encryption and decryption of data to ensure that applications can correctly interpret the data.
Example functions: data compression, encryption and decryption, data format conversion, etc.
7. Application Layer :
Function: Provide users with network application services, including file transfer, email, web browsing, etc.
Example protocols: HTTP, SMTP, FTP, SSH, etc.
Main tasks: application program interface, user authentication, data transmission, etc.
physical layer
The physical layer is the first layer in the OSI model (Open System Interconnection Model), which is the basic level of computer network communication. The main task of the physical layer is to process the raw bit stream on the physical transmission medium and ensure that the data can be transmitted to the connected devices in an appropriate manner. It focuses on how bit streams are sent and received over a transmission medium, without caring about the meaning or format of the data.
1. Bit encoding : The physical layer is responsible for converting digital data into analog signals or digital signals for transmission on the transmission media. This includes mapping digital 0s and 1s onto physical signals (such as voltages, light signals, etc.) so that data can be correctly interpreted between devices.
2. Transmission media : The physical layer focuses on the characteristics of different transmission media, including cables, optical fibers, radio waves, etc. It ensures that the selected transmission medium is suitable for the specific communication needs.
3. Physical topology : The physical layer involves defining and managing the physical topology of the network, such as bus topology, star topology, ring topology, etc. This determines how the device is connected for communication.
4. Transmission rate : The physical layer specifies the data transmission rate, usually expressed in bits per second (bps). Different transmission media support different transmission rates.
5. Signal transmission : The physical layer handles issues such as signal amplification, attenuation, noise and interference to ensure that data can be transmitted reliably.
6. Physical connection : The physical layer defines the physical interface between connected devices, including plugs, sockets, cable types, etc. This ensures that the device is properly connected to the network.
7. Physical layer equipment : Physical layer equipment usually includes repeaters and hubs. They are mainly used for signal amplification and physical layer signal transmission, but do not have higher-level intelligent routing functions.
data link layer
The data link layer is the second layer in the OSI model, located above the physical layer. Its main task is to manage the transmission of data frames on the physical transmission medium to ensure reliable point-to-point or point-to-multipoint data transmission. The work of the data link layer covers functions such as data encapsulation, frame transmission, address allocation, error detection and correction.
1. Encapsulation : The data link layer packages the data from the network layer into a data frame, adding header and tail information so that the receiver can identify the start and end of the frame and extract the data.
2. Address allocation : Each data link layer device has a unique hardware address, called MAC address (Media Access Control Address). The data link layer uses the MAC address to determine the recipient of the data frame to ensure that the data is only sent to the intended device.
3. Frame transmission : The data link layer is responsible for transmitting data frames from one physical layer device to another physical layer device, regardless of whether they are directly connected. This can involve direct point-to-point connections in a LAN or multi-hop transmission in a WAN.
4. Error detection and correction : The data link layer uses various technologies to detect and correct errors in transmission to ensure data integrity. Common error detection methods include parity check, CRC (cyclic redundancy check), etc.
5. Flow control : The data link layer can perform flow control to ensure that the sender does not send too much data to the receiver, thereby preventing data loss or overflow.
6. MAC protocol : The data link layer includes different types of MAC protocols for managing access on shared media. Common MAC protocols include CSMA/CD (Carrier Sense Multiple Access with Collision Detection) and CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance).
7. LAN topology : The data link layer defines the topology of the LAN, such as bus topology, star topology, ring topology, etc., and how to transmit data in these topologies.
8. Logical link control : The data link layer also includes logical link control, which is used to establish, maintain and terminate logical links to ensure the orderly transmission of data.
Network layer
The network layer is an important part of a computer network. It is responsible for transmitting data packets between different computers and ensuring that these data packets can reach the destination host from the source host. The main functions of the network layer include routing, addressing, and packet forwarding.
The network layer is the third layer in the OSI model and is responsible for passing data from the transport layer (layer 4) to the data link layer (layer 2) and the physical layer (layer 1), and it is a key component in building the Internet part one.
1. Role : The main role of the network layer is to select an appropriate path for data packets so that they can reach the destination host from the source host, even if there are multiple intermediate nodes (such as routers) in the network.
2. IP address : At the network layer, each device is assigned a unique IP address. This address is used to identify the location of the device in the network. IPv4 and IPv6 are two common versions of IP addresses. IPv4 uses 32-bit addresses, while IPv6 uses 128-bit addresses.
3. Routing : Routing is one of the core functions of the network layer. It involves choosing the best path in a network to send packets from source to destination. Routers are network layer devices that are responsible for determining the next hop of data packets based on the destination IP address.
4. Subnets : The network layer also involves dividing the network into different subnets. This helps organize and manage large networks and allows routing between different subnets.
5. TTL (Time-to-Live) : TTL is a field in the data packet that is used to prevent data packets from looping indefinitely in the network. Each time a packet passes through a router, the TTL value is decremented by 1. When the TTL value reaches zero, the packet is discarded.
6. Protocol : At the network layer, common protocols include IP (Internet Protocol), ICMP (Internet Control Message Protocol), and IGMP (Internet Group Management Protocol). These protocols define the format of data packets, routing algorithms, and other related rules.
7. IPv4 and IPv6 : IPv4 is the most common version of IP address, but due to the problem of address exhaustion, IPv6 is gradually replacing it. IPv6 provides more IP addresses to support the growing number of Internet devices.
8. VPN (Virtual Private Network) : The network layer is also related to VPN. VPN allows to securely connect networks in different geographical locations through encryption and tunneling technology.
transport layer
The transport layer is the fourth layer in the OSI model, located above the network layer. Its main task is to provide end-to-end data transmission services to ensure data reliability, flow control and error detection. The function of the transport layer is to establish, maintain, and terminate communication sessions between two computers in the network to efficiently transmit data.
1. End-to-end communication : The transport layer is responsible for establishing a communication session between the source host and the target host. It identifies different applications or services by port numbers to ensure that data can be delivered to the correct target application.
2. Flow control : The transport layer can perform flow control to ensure that the sender does not send too much data to the receiver, thereby avoiding data loss or overflow. This ensures that the data transfer rates between the sender and receiver are reasonably matched.
3. Reliability : The transport layer provides reliable data transmission services, ensuring that data reaches the destination in the correct order, and retransmitting lost or damaged data packets when necessary. This is usually achieved through acknowledgment and retransmission mechanisms in the protocol.
4. Data segmentation and reassembly : The transport layer divides the data received from the application layer into smaller data segments and reassembles them into complete messages at the receiving end. This helps manage and transfer large amounts of data efficiently.
5. Port number : The transport layer uses port numbers to identify different applications or services. The source and destination hosts use the port number to determine which application the data should be passed to. Common transport layer protocols such as TCP and UDP use port numbers.
6. Multiplexing and demultiplexing : The transport layer allows multiple applications to share the network connection at the same time, mixing their data together through multiplexing, and distributing the data to the receiving end through demultiplexing. The right application.
7. Error detection and correction : The transport layer can detect and correct errors in data transmission to ensure data integrity.
8. Protocol : At the transport layer, common protocols include TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). TCP provides reliable, connection-oriented transmission, while UDP provides unreliable, connectionless-oriented transmission.
session layer
The session layer is the fifth layer in the OSI model, located above the transport layer and below the presentation layer. It is mainly responsible for establishing, managing and terminating sessions in order to establish a logical connection between two communicating nodes. The main task of the session layer is to ensure that the exchange of data occurs in an end-to-end communication session, while handling session-level errors and synchronization issues.
1. Session establishment : The session layer is responsible for establishing a session or connection between two communicating nodes. This can involve a handshake process, where the session layer ensures that both communicating parties are ready to exchange data.
2. Session maintenance : Once the session is established, the session layer is responsible for maintaining the status of the connection, including the order of data transmission, confirmation and retransmission of data packets, etc. It also handles synchronization issues within the session to ensure data consistency on both sides of the communication.
3. Session termination : When communication ends, the session layer is responsible for terminating the session, clearing related resources, and notifying both communicating parties of the end of the session. This ensures the correct release of resources and the normal end of the communication.
4. Dialogue control : The session layer supports the dialogue control function, allowing both communicating parties to send data alternately without conflict or data confusion.
5. Error handling : The session layer handles session-level errors, such as communication interruption, connection loss, or session timeout. It can attempt to reestablish the connection or take other steps to restore communication.
6. Synchronization : The session layer can handle synchronization issues in communication to ensure that data transmission between communicating parties is carried out at the correct time to avoid data confusion or loss.
7. Session identification : The session layer can assign a unique identifier to the session to distinguish different sessions. This helps track and manage multiple sessions happening at the same time.
8. Logical link management : The session layer can also manage logical links so that multiple applications can share the same physical connection.
presentation layer
The presentation layer is the sixth layer in the OSI model, located above the session layer and below the application layer. Its main task is to handle the representation, encoding, encryption and decryption of data to ensure the interoperability of data between different computers and applications. The job of the presentation layer is to convert data from the application format into a common format for transmission over the network, while also being responsible for the security and readability of the data.
1. Data format conversion : The presentation layer is responsible for converting data from an application-specific format into a common format that the network can understand. This facilitates data exchange between different applications.
2. Data encryption and decryption : The presentation layer can provide data encryption and decryption functions to ensure the confidentiality of data during transmission. Encryption makes data unreadable to unauthorized visitors.
3. Data compression : The presentation layer can compress data to reduce the bandwidth requirements and transmission time of data transmission. Data is compressed before transmission, and the receiving end needs to decompress the data to restore the original information.
4. Character encoding : The presentation layer is responsible for character set conversion and character encoding processing to ensure that computers in different countries and regions can correctly interpret and display text data.
5. Data format check : The presentation layer can check whether the format of the data is correct and make necessary repairs to ensure the integrity of the data.
6. Graphics and image conversion : In the presentation layer, graphics and image data can be converted into different formats to adapt to different display devices or applications.
7. Data annotations and tags : The presentation layer can add additional information, annotations, or tags to the data to provide context during transmission and interpretation.
8. Data compression : The presentation layer can compress data to reduce the amount of data transmitted over the network, thereby improving efficiency.
Application layer
The Application Layer is the seventh and highest layer in the OSI model, located on top of all other layers. The application layer is the top layer of the computer network and provides the interface between users and network services. It includes a variety of applications and protocols for a variety of different network communication needs, such as file transfer, email, web browsing, video conferencing, etc.
1. User interface : The application layer provides the interface between users and computer networks, allowing users to interact with the network through applications. These applications can be browsers, email clients, file transfer tools, etc.
2. Network services : The application layer includes various network services, such as file transfer, email, remote access, domain name resolution, web browsing, instant messaging, etc. These services provide users with different network features.
3. Application layer protocols : In the application layer, there are many specific application layer protocols, each of which is designed for specific application scenarios. For example, HTTP (Hypertext Transfer Protocol) is used for web page transmission, SMTP (Simple Mail Transfer Protocol) is used for email transmission, FTP (File Transfer Protocol) is used for file transfer, etc.
4. Data interaction : The application layer protocol is responsible for defining the format, exchange method and communication rules of data. These protocols ensure that data is passed and interpreted correctly between the source and destination.
5. Security : The application layer also involves security issues, including authentication, data encryption, and access control to protect the confidentiality and integrity of data and communications.
6. Network management : Some application layer protocols and applications are used for network management, helping administrators monitor and maintain network performance, configure equipment, etc.
7. User authentication : The application layer allows users to authenticate to ensure that only authorized users can access specific network resources.
8. Error handling : Application layer protocols and applications can handle application-level errors, such as handling lost packets, timeouts, and retransmissions.