OSI 7层模型(OSI 7 layer model)

ISO 7 layer model

Physical

• The lowest layer of the OSI Model is concerted with electrically and optically transmitting raw unstructured data bits across the network from physical layer of the sending device to the physical layer of receiving device. It can include specifications such as voltages. pin layout, cabling and radio frequencies.

Data Link

• At the data link layer, directly connected nodes are used to perform node-to-node data transfer where data is packaged into frames. The data link layer also corrects errors that may have occurred at the physical layer.
• The data link layer encompasses two sub-layers of its own. The first, media access control(MAC), provides flow control and multiplexing for device transmission over a network. The second, the logical link control(LLC), provides flow and error control over the physical medium as well as identifies line protocols.

Network

• The network layer is responsible for receiving frames from the data link layer, and delivering them their intended destinations among based on addresses contained inside the frame. The network layer finds the destination by using logical addresses, such as IPO. At this layer, routers are a crucial component used to quite literally route information where is needs to go between networks.

Transport

• The transport layer manages the delivery and error checking of data packets. It regulates the size, sequencing and ultimately the transfer of data between systems and hosts. Once of the most common examples of transport layer is TCP or the Transmission Control Protocol.

Session

• The session layer controls the conversations between different computers. A session or connection between machines is setup, managed and terminated at layer 5. Session layer services also include authentication and reconnections.

Presentation

• The presentation layers formats or translates data for the application layer based on the syntax or semantics that the application accepts. Because of this, it at times also called the syntax layer. This layer can also handle the encryption and decryption required by the application layer.

Application

• At this layer, both the end-user and the application layer interact directly with the software application. This layer sees network services provided to end-user application such as a web browser or Office 365. The application layer identifies communication partners, resource availability and synchronizes communications.