Logical Link Control

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The Logical Link Control (LLC) data communication protocol layer is the upper sub-layer of the Data Link Layer (which is itself layer 2, just above the Physical Layer) in the seven-layer OSI reference model. It provides multiplexing mechanisms that make it possible for several network protocols (IP, IPX) to coexist within a multipoint network and to be transported over the same network media, and can also provide flow control mechanisms.

The LLC sub-layer acts as an interface between the Media Access Control (MAC) sublayer and the network layer.

As the Ethertype in an Ethernet II framing formatted frame is used to multiplex different protocols on top of the Ethernet MAC header it can be seen as LLC identifier.

Operation

The LLC sublayer is primarily concerned with:

  • Multiplexing protocols transmitted over the MAC layer (when transmitting) and decoding them (when receiving).
  • Providing flow and error control

The protocol used for LLC in IEEE 802 networks, such as IEEE 802.3/Ethernet (if the EtherType field isn't used), IEEE 802.5, and IEEE 802.11, 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.

Another example of a Data Link Layer which is split between LLC (for flow and error control) and MAC (for multiple access) is the ITU-T G.hn standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables).

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 - for example, the IP protocol at the Network layer or IPX.

The GPRS LLC layer also does ciphering and deciphering of SN-PDU (SNDCP) packets.

See also


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