logical link control (LLC) layer

Definition of logical link control (LLC) layer in The Network Encyclopedia.

What is LLC Layer (logical link control Layer)?

One of the two sublayers into which the data-link layer of the Open Systems Interconnection (OSI) reference model is subdivided for data-link protocols used on local area networks (LANs). The logical link control (LLC) layer is part of the IEEE Project 802 specifications.

The LLC protocol is based on the earlier High-level Data Link Control (HDLC) protocol. “LLC” sometimes refers to the IEEE 802.2 protocol itself, which is the most common LAN protocol implemented at the LLC layer.

How it works

For LAN data-link protocols such as Ethernet, the data-link layer is divided into an upper layer called the logical link control (LLC) layer and a lower layer called the media access control (MAC) layer. The MAC layer coordinates access to the physical layer according to a media access control method, which for Ethernet is the Carrier Sense Multiple Access with Collision Detection (CSMA/CD) scheme. The MAC layer thus provides services to the LLC layer so that protocol data units can be transferred to the medium without any concern about the broadcast, framing, addressing, or error-detection schemes used. The LLC uses the MAC services to provide two types of data-link operations to the network layer above it: LLC1 for connectionless and LLC2 for connection-oriented data-link communication services (known as Type 1 and Type 2, respectively). These LLC services are grouped into two classes:

  • Class 1 services:
    Connectionless services used by applications that do not require error detection or flow control.

     

  • Class 2 services:
    Either connectionless (Type 1) or balanced-mode connection-oriented (Type 2) data transfer services. The LLC provides the error detection and recovery, flow control, and resequencing services needed for connection-oriented data transfer.