The OSI model was established by ISO. This is a theoretical model, and no actual product is fully compliant with the OSI model. The establishment of the OSI model is only a theory introduced to analyze the convenience of network communication. It also lays the foundation for future development of practical agreements or products.
The OSI model is divided into seven layers: from top to bottomThe application layer refers to the network operating system and the specific application, corresponding to the WWW server, FTP server and other application software presentation layer data syntax conversion, data transmission and other sessions to establish a conversational relationship between the two ends, and responsible for the data transmission layer responsible for the error inspection and repair, to ensure the quality of transmission, is the place where TCP works. The network layer provides the address scheme, where the IP protocol works (packet) Data link layer will be sent from the physical layer of unprocessed bits of data to the physical layer of data frame corresponding network cable, network card, interface and other physical equipment (bits)
layer Seventh -Application layer Function: refers to the network operating system and the specific application, corresponding to the WWW server, FTP server application software 1, the term "application layer" does not mean a special application running on the network, but instead provides a set of convenient program developers in their own applications to use the network features of the service. 2, the application layer provides services including file transfer (FTP), file management and e-mail information Processing (SMTP) and so on.Sixth Floor-Presentation LayerFunctions: Internal code conversion, compression and decompression, encryption and decryption, acting as the "translator" role between the application and the network. 1, in the presentation layer, the data will be in accordance with the network can understand the format of the scheme, this format is different from the type of network used. For example, IBM hosts use EBCDIC encoding, while most PCs use ASCII code. In this case, the presentation layer is required to complete this transformation 2, the Presentation layer protocol also decodes and encodes the image and file format information. 3, the presentation layer management data decryption and encryption, such as System password processing. If you are querying your bank account on the Internet, you are using a secure connection.
Layer Fifth -session layer Function: Responsible for establishing and maintaining communication between two nodes in the network. 1, the function of the session layer includes: Establish the communication link, keep the communication link of the session process unblocked, synchronize the dialogue between the two nodes, decide whether the communication is interrupted and when the communication is interrupted, decide where to resend the example: using full-duplex or half-duplex mode, how to initiate transmission, how to end the transmission, how to set the transmission The session layer sets the communication period by determining the priority of the node communication and the length of the communication time.
Layer Fourth -Transport layer Function: Fixed serial number, control data flow, error detection and fault handling, to ensure that the data reliably, sequentially, without error from point A to transmission to point B 1, because if there is no transport layer, the data will not be validated or interpreted by the recipient, so the transport layer is often considered to be the most important layer o S I model. 2, the transmission protocol at the same time flow control or the receiver can receive data based on the degree of speed to specify the appropriate transmission rate. 3, the transport layer according to the maximum size of the network can be processed by the long packet is forced to split and number. For example, Ethernet cannot receive packets larger than 1 5 0 0 bytes. The transport layer of the sender node divides the data into smaller pieces of data and arranges a sequence number for each piece of data, so that the data can be reorganized in the correct order when it reaches the transport layer of the receiving node. The process is called sorting. 4, in the network, the transport layer sends a C K (answer) signal to notify the sender that the data has been received correctly. If the data is wrong or the data is not answered at a given time period, the transport layer will request the sender to resend the data. Note: One service that works on the transport layer is the T-C p (Transfer Control Protocol Transmission Protocol) in the TCP/IP protocol, and the other Transport Layer service is the Ipx/spx protocol set S P X (Serial package Exchange sequence Packet switching)
layer three -Network layer Functions: Addressing, selecting Route 1, the network layer determines the best path from Node A in one network to Node B in another network by considering the cost of sending priority, network congestion, quality of service, and optional routing. 2, in the network, "Routing" is based on the addressing scheme, usage patterns and accessibility to guide the transmission of data. 3. The Network layer protocol can compensate for the imbalance of data transmitting, transmitting and receiving equipment. To accomplish this task, the network layer fragments and reorganizes the packets. 4. Segmentation and reorganization refers to the process of reducing the size of a data unit when data is transferred from a network segment capable of processing a large data unit to a network segment that can handle only smaller data units. Reorganization is the refactoring of segmented data units. Note 1, the network layer segment refers to the reduction of the data frame size, and network segmentation refers to a network divided into smaller logical fragments or physical fragments.
second Layer -Data link layer Functions: Synchronization, error detection, development of Mac Method 1, its main function is to divide the data received from the network layer into a specific frame can be transmitted by the physical layer. 2, frame is used to move the data structure package, which not only includes raw (unprocessed) data, or "effective load", but also includes the sender and receiver of the network address and error correction and control information. The address determines where the frame will be sent, while error correction and control information ensures that the frame arrives without error. 3. Typically, the sender's data link layer waits for a response from the receiving party to receive the data correctly.
first layer -Physical layer Function: The media specification for transmitting information, the specifications of the data being presented and transmitted in entity, the Connector Specification 1, the layer includes physical network media, such as cable connection, connector, network card and so on. 2. The physical layer protocol generates and detects voltages to transmit and receive signals that carry data. 3, although the physical layer does not provide error correction services, but it can set the data transfer rate and monitor the number of cases: on your desktop P C plug in the network interface card, you have established the basis of computer networking. In other words, you provide a physical layer. The data is generated by the topmost (usually the application) of the transmitting end and is transmitted from the upper layer down. Each layer will be added to the front end of the special information on the layer, known as the "header", and then passed to the next layer, we may wish to "add a header" as "put on a layer of envelopes." So at the bottom of the page, the original data was already set on a 7-layer envelope. And then through the network lines, telephone lines, optical cables and other media, transmission to the receiving end. After receiving the data, it will be transmitted from the lowest level to the upper layer, each through the level of the removal of a layer of envelopes (that is, to remove the layer identified by the header), until the top, the data will be restored to the original from the top of the transmission end of the original appearance. The common method used for the correct order of memory layer (application layer, presentation layer, session layer, Transport layer, network layer, data link layer, physical layer) is one of countless networks to express its application by transmitting voice signals.
iOS Model Layered Network protocol
