Logical link control
Logical Link Control (LLC) is the upper part of the data link layer in IEEE 802 networks. It sits between the MAC layer and the network layer and lets different network protocols share the same network medium by multiplexing them over one link. LLC can also provide optional flow control and error management, but many networks don’t use these features; reliability is usually handled by the transport layer (like TCP) or by higher-level protocols.
Historically, LLC’s flow and error control helped early networks (such as X.25 with LAPB), where error handling happened node-to-node and could slow things down. Today, IEEE 802.2 defines LLC for all 802 LANs (Ethernet, 802.5, 802.11) and it’s also used in some non-802 networks like FDDI. In Ethernet, protocol multiplexing is usually done with EtherType; if EtherType isn’t used, an 802.2 LLC header follows the Ethernet header.
In wireless networks (like 802.11), flow control and error management are handled by the MAC layer (CSMA/CA), not by LLC. Some non-802 protocols split MAC and LLC functions: for example, HDLC combines framing with LLC-like features, and PPP on telephone networks provides multiplexing but relies on higher layers for error handling. Modern systems borrow ideas from older LAPM used in X.25.
Other examples include GPRS, where the LLC layer also handles ciphering/deciphering of SN-PDU packets, and ITU-T G.hn, which splits LLC (flow/error control) from MAC (multiplexing) to run high-speed networking over existing home wiring.
This page was last edited on 2 February 2026, at 21:23 (CET).