Logical Link Control (LLC)

The Logical Link Control (LLC) is the upper sublayer of the data link layer in the OSI reference model. It acts as an interface between the network layer and the medium access control (MAC) sublayer of the data link layer.

The LLC sublayer is primarily used for its multiplexing capabilities, allowing multiple network protocols to operate simultaneously within a multipoint network over the same network medium.

LLC Layer in the OSI Model

The data link layer is divided into two sublayers to handle different aspects of data transmission:

• Logical Link Control (LLC) sublayer − Handles protocol multiplexing, flow control, and error recovery

• Medium Access Control (MAC) sublayer − Controls access to the physical transmission medium

Functions of LLC Sublayer

• Protocol Multiplexing − Multiplexes protocols over the MAC layer during transmission and de-multiplexes them during reception

• Flow Control − Provides hop-to-hop flow control to prevent buffer overflow at the receiver

• Error Control − Implements error detection and recovery mechanisms for reliable data transmission

• Frame Sequencing − Assigns sequence numbers to frames for proper ordering and duplicate detection

• Acknowledgment Tracking − Manages acknowledgments in connection-oriented services to ensure reliable delivery

LLC Service Types

Conclusion

The LLC sublayer provides essential data link services including protocol multiplexing, flow control, and error recovery. It enables multiple network protocols to share the same physical medium while maintaining reliable communication between adjacent network nodes.