Logical data link

The above content is reviewed and announced by the National Science and Technology Term Review Committee


The data link layer is the first part of the OSI reference model. The second layer, between the physical layer and the network layer, provides services to the network layer on the basis of the difference between the physical layer. The main function of the data link layer is to achieve reliable data transmission on less reliable physical links through some data link layer protocols and link control procedures.

"Line", "Link" and "Data Link" are different concepts. There is no switching node in the middle of the line, and the link is a passive end-to-end physical line segment. In data communication, the communication link between two computers is often formed by many lines connected in series.

Adding hardware and software that implement some procedures for controlling data transmission to the link constitutes a data link like a data pipeline. Sometimes the link is often called a physical link, and the data link is called a logical link, that is, the physical link plus the necessary communication procedures are the data link.

When using multiplexing technology, there can be multiple logical data links on one physical link. In order to realize the correct transmission of data frames between adjacent nodes, the data link layer must include basic functions such as link management, frame synchronization, flow control, error checking and recovery.

Logical data link standard

Logical data link standard, similar to HDLC and ADCCP. In order to adapt to multiple forms of local area network access, the IEEE 802 standards committee divides the data link layer of OSI into two sublayers: logical link control (LLC) sublayer and media access control (MAC) sublayer. The definition of LLC control process and X in CCITT equilibrium mode. 25 Link access procedures HDLC (LAPB) are basically the same. The balanced mode is used in a peer-to-peer network. In this kind of network, any workstation can actively communicate with other workstations directly.

LLC provides two types of basic services: one type is "connectionless" operation without confirmation. Even if the logical connection for transmitting sequence control information and confirmation messages has not been established, the workstation can also send to other workstations Messages, suitable for transmitting less important messages or systems with higher-level error recovery and sequence functions; the other is the traditional balanced data communication service, which is responsible for establishing a logical connection between two LLCs, each LLC can send and Accept the message or confirmation, and ensure that the transmission of the message sent to the outside is complete and accurate.

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