According to the IEEE 802 family of standards, Logical Link Control (LLC) is the upper sublayer of the OSI data link layer. The LLC is the same for the various physical media (such as Ethernet, token ring, and WLAN).
The LLC sublayer is primarily concerned with:
- Multiplexing protocols transmitted over the MAC layer (when transmitting) and demultiplexing them (when receiving).
- Providing flow and error control
The protocol used for LLC in IEEE 802 networks and in some non-IEEE 802 networks such as FDDI is specified by the IEEE 802.2 standard.
Some non-IEEE 802 protocols can be thought of as being split into MAC and LLC layers. For example, while HDLC specifies both MAC functions (framing of packets) and LLC functions (protocol multiplexing, flow control, detection, and error control through a retransmission of dropped packets when indicated), some protocols such as Cisco HDLC can use HDLC-like packet framing and their own LLC protocol.
An LLC header tells the Data Link layer what to do with a packet once a frame is received. It works like this: A host will receive a frame and look in the LLC header to find out where the packet is destined for - say, the IP protocol at the Network layer or IPX.
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