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LLC (logical link control)

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Introduction:

Logical Link Control (LLC) is one of the two sub-layers of the Data Link Layer (Layer 2) in the Open System Interconnection (OSI) model. The LLC is responsible for providing a reliable and error-free communication link between devices, even when they use different protocols, network topologies, and data formats. This layer is present in both wired and wireless networks and is used to control the data flow between devices.

The LLC layer provides services to the upper layers of the OSI model, especially to the Network Layer (Layer 3) and the Transport Layer (Layer 4). In this article, we will explain what LLC is, how it works, and its functions.

What is LLC?

The LLC is a sub-layer of the Data Link Layer that operates on top of the Media Access Control (MAC) sub-layer and below the Network Layer. The LLC is responsible for providing a reliable, error-free, and flow-controlled communication link between devices that use different network protocols.

The LLC sub-layer is the upper part of the Data Link Layer, which is responsible for framing data into frames, error detection, and correction, and flow control. The MAC sub-layer is the lower part of the Data Link Layer, responsible for media access control, addressing, and physical transmission.

LLC is primarily responsible for:

  • Providing a logical link between two devices or nodes on the same network segment or LAN (Local Area Network).
  • Providing flow control and error detection and correction services to the Network Layer.
  • Supporting different network protocols and topologies.

LLC operates independently of the physical layer and the network protocols, allowing different network protocols to use the same LLC sub-layer.

How LLC Works?

The LLC operates on top of the MAC sub-layer and provides a uniform interface to the Network Layer. The LLC uses the service access point (SAP) to communicate with the upper layers of the OSI model. The SAP is a logical address that identifies the LLC service interface to the upper layers.

The LLC sub-layer encapsulates the data received from the Network Layer into frames and appends a header and a trailer to each frame. The header contains the source and destination address, the LLC control information, and the protocol identifier. The trailer contains the error detection and correction codes and the frame delimiter.

When the LLC receives a frame from the MAC sub-layer, it checks the destination address and the protocol identifier to determine if the frame is intended for the local node. If the frame is intended for the local node, the LLC removes the header and the trailer, checks for errors, and passes the data to the Network Layer. If the frame is not intended for the local node, the LLC discards the frame.

If the LLC detects errors in the frame, it discards the frame and requests retransmission. The LLC uses flow control mechanisms to prevent the Network Layer from overwhelming the LLC sub-layer with data. The LLC uses a sliding window protocol to control the flow of data between devices.

Functions of LLC:

The LLC sub-layer provides several functions to the Data Link Layer and the upper layers of the OSI model. These functions include:

  1. Frame Delimitation: The LLC sub-layer is responsible for framing the data into frames and inserting the frame delimiter at the beginning and end of each frame. The frame delimiter is a unique bit pattern that marks the start and end of a frame.
  2. Addressing: The LLC sub-layer is responsible for providing the source and destination addressing in the frame header. The source and destination address are used to identify the devices that are communicating with each other.
  3. Error Control: The LLC sub-layer is responsible for detecting and correcting errors in the frames. The LLC uses error detection codes such as cyclic redundancy check (CRC) to detect errors and retransmission mechanisms to correct errors. If the LLC detects errors in a frame, it discards the frame and requests retransmission of the frame.
  4. Flow Control: The LLC sub-layer uses flow control mechanisms to prevent the Network Layer from overwhelming the LLC sub-layer with data. The LLC uses a sliding window protocol to control the flow of data between devices. The sliding window protocol allows the LLC to control the number of frames that can be sent by the Network Layer at any given time.
  5. Multiplexing: The LLC sub-layer allows multiple network protocols to use the same LLC sub-layer. The LLC uses the protocol identifier field in the frame header to identify the network protocol that is being used.
  6. Logical Link Management: The LLC sub-layer is responsible for managing the logical link between two devices or nodes on the same network segment or LAN. The LLC uses the service access point (SAP) to communicate with the upper layers of the OSI model.
  7. Protocol Conversion: The LLC sub-layer can convert between different network protocols, allowing devices that use different network protocols to communicate with each other.

LLC and Ethernet:

Ethernet is the most widely used LAN technology that uses the LLC sub-layer. Ethernet is a popular LAN technology that is used in many organizations and enterprises. The LLC sub-layer is an essential part of the Ethernet protocol.

The Ethernet protocol uses the LLC sub-layer to provide flow control, error detection and correction, and logical link management services. The Ethernet protocol uses the LLC sub-layer to provide a reliable and error-free communication link between devices on the same network segment or LAN.

The Ethernet protocol uses the Media Access Control (MAC) sub-layer to provide media access control and addressing services. The MAC sub-layer is the lower part of the Data Link Layer, and the LLC sub-layer is the upper part of the Data Link Layer.

Conclusion:

The Logical Link Control (LLC) sub-layer is an essential part of the Data Link Layer in the OSI model. The LLC is responsible for providing a reliable, error-free, and flow-controlled communication link between devices that use different network protocols.

The LLC sub-layer provides several functions to the Data Link Layer and the upper layers of the OSI model, including frame delimitation, addressing, error control, flow control, multiplexing, logical link management, and protocol conversion.

The LLC sub-layer is used in many LAN technologies, including Ethernet, to provide a reliable and error-free communication link between devices on the same network segment or LAN. The LLC sub-layer is an essential part of the Ethernet protocol and is used to provide flow control, error detection and correction, and logical link management services.