The physical layer defines the mechanical characteristics, electrical characteristics, functional characteristics and process characteristics between activation, maintenance and shutdown of communication endpoints. This layer provides a physical medium for the upper layer protocol to transmit data. This is just a specification. At this level, the unit of data is called bit.
Typical specifications that belong to the definition of physical layer include EIA/TIA RS-232, EIA/TIA RS-449, V.35, RJ-45, fddi token ring network, etc.
Layer 2: Data Link Layer
The data link layer provides reliable transmission through unreliable physical media. The functions of this layer include: physical address addressing, data framing, flow control, data error detection and retransmission. At this layer, the unit of data is called a frame. Representatives of data link layer protocols are: ARP, RARP, SDLC, HDLC, PPP, STP, Frame Relay, etc.
Layer 3: Network layer
The network layer is responsible for routing packets between subnets. The network layer can also realize congestion control, Internet interconnection and other functions. At this layer, the unit of data is called a packet. The representatives of network layer protocols are: IP, IPX, RIP, OSPF, etc.
The fourth layer: the transport layer
The transport layer is the first end-to-end, that is, the host-to-host hierarchy. The transport layer is responsible for segmenting the upper layer data and providing reliable or unreliable end-to-end transmission. In addition, the transport layer must deal with end-to-end error control and flow control. At this level, the unit of data is called a data segment. Representatives of transport layer protocols are: TCP, UDP, SPX, etc.
The fifth floor: the conversation floor.
The session layer manages the session process between hosts, that is, it is responsible for the establishment, management and termination of the session between processes. The session layer also realizes data synchronization by inserting checkpoints in the data. Representatives of session layer protocols are RPC, SQL, NFS, X WINDOWS and ASP.
The sixth layer: presentation layer
The presentation layer transforms the upper data or information to ensure that the application layer information of one host can be understood by the application program of another host. Data conversion in presentation layer includes data encryption, compression and format conversion. Representatives of presentation layer protocols include ASCII, PICT, TIFF, JPEG, MIDI and MPEG.
The seventh layer: the application layer
The application layer provides an interface for the operating system or network applications to access network services. The representatives of application layer protocols are: Telnet, FTP, HTTP, SNMP, etc.
Extended data:
When it comes to networks, we can't talk about the OSI reference model. OSI Reference Model (OSI/RM) is the full name of Open System Interconnection (OSI/RM), which is a network system interconnection model proposed by ISO. Although the practical application of OSI reference model is not very significant, it does help to understand the internal operation of network protocols and provides a good reference for us to learn network protocols.
Seven levels of understanding:
Physical layer: physical interface specification, transmission bit stream, network card working in the physical layer.
Data layer: framing, ensuring error-free transmission of frames and MAC addresses, forming EH Ethernet frames.
Network layer: routing, flow control, IP address, forming IP packets.
Transport layer: port address, such as HTTP corresponding to port 80. TCP and UDP work at this layer, as well as error checking and flow control.
Session layer: organizes the communication between two session processes, and uses NETBIOS and WINSOCK protocols to manage data exchange. QQ and other software used for communication should work at the session level.
Presentation layer: makes communication between different operating systems possible.
Application layer: corresponding to each application software.
Reference: Baidu Encyclopedia-Seven-layer Model