In the IEEE 802 network reference model, the controllink logic ( LLC ) layer of data communication protocol is the top layer of the data link layer (layer 2) of the seven layer OSI model. The LLC sublayer provides multiplexing mechanisms that allow for multiple network protocols (eg IP, IPX, Decnet and Appletalk) to coexist in multipoint networks and transported through the same network medium. It can also provide flow control and automatic error recovery (ARQ) management mechanisms.
The LLC sublayer acts as an interface between the media access control (MAC) sublayer and the network layer.
Video Logical link control
Operation
The LLC sublayer mainly deals with multiplexing protocols that are sent through the MAC layer (while transmitting) and demultiplexing them (upon receipt). It can also provide node-to-node flow control and fault management.
The flow control and error management capabilities of the LLC sublayer are used by protocols such as the NetBIOS Frames protocol. However, most protocol stacks that run on top of 802.2 do not use flow control and management of LLC sublayer errors. In this case flow control and error management are handled by transport layer protocols such as TCP or by some application layer protocols. These higher layer protocols work end-to-end, ie retransmissions are made from the original to the final destination, not the individual physical segment. For this protocol stack only multiplexer capabilities of the LLC sublayer are used.
Maps Logical link control
Application instance
X.25 and LAPB
A sublayer LLC is a key component in the initial packet switching networks such as X.25 network with LAPB data link layer protocol, in which the flow control and error management is done in a fashion node-to-node, which means that if an error was detected in the frame, the frame the retransmitted from one switch to the next instead. These extensive handshaking between nodes to create a network to be slow.
local area networks (LANs) and metropolitan area network ( MAN) protocol
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 on some non-IEEE 802 networks such as FDDI.
Ethernet
Because bit errors are very rare in wired networks, Ethernet does not provide flow control or automatic recurrence requests (ARQ), meaning that the wrong packets are detected but only canceled, not retransmitted (except in case of collisions detected by the MSM layer CSMA/CD protocol ). In contrast, retransmissions depend on higher layer protocols.
As EtherType in Ethernet frames using Ethernet framing II is used to multiplex different protocols over the MAC Ethernet headers it can be seen as an LLC identifier. However, the Ethernet Ethernet frames lack the LLC identifier in the Ethernet header, and vice versa, use the IEEE 802.2 LLC header after the Ethernet header to provide protocol multiplexing functionality.
Wireless LAN
In wireless communication, bit error is very common. In wireless networks such as IEEE 802.11, flow control and fault management are part of the CSMA/CA MAC protocol, and are not part of the LLC layer. The LLC sublayer follows the IEEE 802.2 standard.
HDLC
Some non-IEEE 802 protocols can be considered split into MAC and LLC layers. For example, while HDLC specifies both the MAC (packet framing) and LLC (multiplexed protocol, flow control, detection, and error control functions via retransmission packets when indicated), some protocols such as Cisco HDLC can use HDLC-like framing packages and their own LLC protocol.
PPP and modem
Through a telephone network modem, PPP link layer protocols can be regarded as an LLC protocol, providing multiplexing, but providing no flow control and fault management. In the telephone network, bit error may be common, meaning that fault management is very important, but that is currently provided by modern protocols. Current modem protocols have inherited the LLC features of the old LAPM link layer protocol, made for modem communication on old X.25 networks.
Mobile system
The GPRS LLC layer also encodes and decomposes SN-PDU (SNDCP) packets.
Power lines
Another example of a linked data link layer 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 networks through existing home cables (line power, telephone lines and coaxial cable).
See also
- Virtual Circuit Multiplexing (VC-MUX)
- Subnetwork Access Protocol (SNAP)
Source of the article : Wikipedia
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