The data link layer is the second layer in the OSI reference model, between the physical layer and the network layer. The data link layer provides services to the network layer based on the services provided by the physical layer. Its most basic service is to reliably transmit data from the network layer to the target network layer of adjacent nodes.
The data link layer defines how data is transmitted over a single link. These protocols are relevant to the various media in question. Example: ATM, FDDI, etc. (Recommended learning:phpstorm)
The data link layer must have a series of corresponding functions, mainly including: how to combine data into data blocks, which is called this in the data link layer The data block is a frame, and the frame is the transmission unit of the data link layer;
How to control the transmission of frames on the physical channel, including how to deal with transmission errors, and how to adjust the sending rate to match the receiver; and Provides management of the establishment, maintenance and release of data link paths between two network entities.
Physical link (physical line): It is composed of transmission media and equipment. Original physical transmission lines refer to basic physical transmission media and equipment that do not use high-level error control.
Data link (logical line): On a physical line, the transmission of these data is controlled through some procedures or protocols to ensure the correctness of the transmitted data.
The hardware and software that implement these procedures or protocols are added to the physical lines, thus forming a data link, a transmission path from the data sending point to the data receiving point. When multiplexing technology is used, there can be multiple data links on one physical link.
The data link layer mainly has two functions: frame coding and error correction control. Frame encoding means defining a packet that contains information frequency, bit synchronization, source address, destination address, and other control information.
The data link layer protocol is divided into two sub-layers: Logical Link Control (LLC) protocol and Media Access Control (MAC) protocol.
The most basic function of the data link layer is to provide transparent and reliable basic data transmission services to users of this layer.
Transparency means that there are no restrictions on the content, format and encoding of data transmitted on this layer, and there is no need to explain the meaning of the information structure; reliable transmission frees users from the need to worry about lost information, interfering information and inconsistent order. Worry about waiting correctly.
These situations may occur in the physical layer, and error correction codes must be used in the data link layer to detect and correct errors.
The data link layer enhances the function of the physical layer to transmit the original bit stream. It transforms the possible error-prone physical connections provided by the physical layer into logically error-free data links, so that it can perform better on the network layer. for an error-free line.
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