Logical link control
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In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer (layer 2) of the seven-layer OSI model. The LLC sublayer provides multiplexing mechanisms that make it possible for several network protocols (IP, IPX, Decnet and Appletalk) to coexist within a multipoint network and to be transported over the same network medium. It can also provide flow control and automatic repeat request (ARQ) error management mechanisms.
The LLC sublayer is primarily concerned with:
- Multiplexing protocols transmitted over the MAC layer (when transmitting) and decoding them (when receiving).
- Providing node-to-node flow and error control
In today's networks, flow control and error management is typically taken care of by a transport layer protocol such as TCP, or by some application layer protocol, in an end-to-end fashion, i.e. retransmission is done from source to end destination. This implies that the need for LLC sublayer flow control and error management has reduced. LLC is consequently only a multiplexing feature in today's link layer protocols. 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 to what protocol stack the packet is destined - for example, the IP protocol at the network layer or IPX. However, today most non-IP network protocols are abandoned.
X.25 and LAPB
An LLC sublayer was a key component in early packet switching networks such as X.25 networks with the LAPB data link layer protocol, where flow control and error management were carried out in a node-to-node fashion, meaning that if an error was detected in a frame, the frame was retransmitted from one switch to next instead. This extensive handshaking between the nodes made the networks slow.
Local area network (LAN) and metropolitan area network (MAN) protocols
The IEEE 802.2 standard specifies the LLC sublayer for all IEEE 802 local area networks, such as IEEE 802.3/Ethernet (if the EtherType field is not used), IEEE 802.5, and IEEE 802.11. IEEE 802.2 is also used in some non-IEEE 802 networks such as FDDI.
Since bit errors are very rare in wired networks, Ethernet does not provide flow control or automatic repeat request (ARQ), meaning that incorrect packets are detected but only cancelled, not retransmitted (except in case of collisions detected by the CSMA/CD MAC layer protocol). Instead, retransmissions rely on higher layer protocols.
As the EtherType in an Ethernet frame using Ethernet II framing is used to multiplex different protocols on top of the Ethernet MAC header it can be seen as an LLC identifier. However, Ethernet frames lacking an EtherType have no LLC identifier in the Ethernet header, and, instead, use an IEEE 802.2 LLC header after the Ethernet header to provide the protocol multiplexing function.
In wireless communications, bit errors are very common. In wireless networks such as IEEE 802.11, flow control and error management is part of the CSMA/CA MAC protocol, and not part of the LLC layer. The LLC sublayer follows 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.
PPP and modems
Over telephone network modems, PPP link layer protocols can be considered as a LLC protocol, providing multiplexing, but it does not provide flow control and error management. In a telephone network, bit errors might be common, meaning that error management is crucial, but that is today provided by modern protocols. Today's modem protocols have inherited LLC features from the older LAPM link layer protocol, made for modem communication in old X.25 networks.
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).