Logical link control
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In the
IEEE 802 IEEE 802 is a family of Institute of Electrical and Electronics Engineers (IEEE) standards for local area networks (LAN), personal area network (PAN), and metropolitan area networks (MAN). The IEEE 802 LAN/MAN Standards Committee (LMSC) maintains ...
reference model of
computer networking A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ma ...
, the logical link control (LLC) data communication protocol layer is the upper sublayer of the
data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...
(layer 2) of the seven-layer
OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...
. The LLC sublayer acts as an interface between the
media access control In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...
(MAC) sublayer and the
network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of transfe ...
. The LLC sublayer provides
multiplexing In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...
mechanisms that make it possible for several network protocols (e.g. IP,
IPX Internetwork Packet Exchange (IPX) is the network layer protocol in the IPX/SPX protocol suite. IPX is derived from Xerox Network Systems' IDP. It also has the ability to act as a transport layer protocol. The IPX/SPX protocol suite was very p ...
and
DECnet DECnet is a suite of network protocols created by Digital Equipment Corporation. Originally released in 1975 in order to connect two PDP-11 minicomputers, it evolved into one of the first peer-to-peer network architectures, thus transforming DEC ...
) 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 Automatic repeat request (ARQ), also known as automatic repeat query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a packet) and timeouts ...
(ARQ) error management mechanisms.


Operation

The LLC sublayer is primarily concerned with
multiplexing In telecommunications and computer networking, multiplexing (sometimes contracted to muxing) is a method by which multiple analog or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource - a ...
protocols transmitted over the MAC layer (when transmitting) and demultiplexing them (when receiving). It can also provide node-to-node flow control and error management. The flow control and error management capabilities of the LLC sublayer are used by protocols such as the
NetBIOS Frames NetBIOS Frames (NBF) is a non- routable network- and transport-level data protocol most commonly used as one of the layers of Microsoft Windows networking in the 1990s. NBF or NetBIOS over IEEE 802.2 LLC is used by a number of network operating s ...
protocol. However, most protocol stacks running atop 802.2 do not use LLC sublayer flow control and error management. In these cases flow control and error management are taken care of by a
transport layer In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet protocol suite and the OSI model. The protocols of this layer provide end-to-end ...
protocol such as TCP or by some
application layer An application layer is an abstraction layer that specifies the shared communications protocols and Interface (computing), interface methods used by Host (network), hosts in a communications network. An ''application layer'' abstraction is speci ...
protocol. These higher layer protocols work in an end-to-end fashion, i.e. re-transmission is done from the original source to the final destination, rather than on individual physical segments. For these protocol stacks only the multiplexing capabilities of the LLC sublayer are used.


Application examples


X.25 and LAPB

An LLC sublayer was a key component in early packet switching networks such as
X.25 X.25 is an ITU-T standard protocol suite for packet-switched data communication in wide area networks (WAN). It was originally defined by the International Telegraph and Telephone Consultative Committee (CCITT, now ITU-T) in a series of drafts ...
networks with the
LAPB Link Access Procedure, Balanced (LAPB) implements the data link layer as defined in the X.25 protocol suite. LAPB is a bit-oriented protocol derived from HDLC that ensures that frames are error free and in the correct sequence. LAPB is specified i ...
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

The
IEEE 802.2 IEEE 802.2 is the original name of the ISO/IEC 8802-2 standard which defines logical link control (LLC) as the upper portion of the data link layer of the OSI Model. The original standard developed by the Institute of Electrical and Electronics Engi ...
standard specifies the LLC sublayer for all
IEEE 802 IEEE 802 is a family of Institute of Electrical and Electronics Engineers (IEEE) standards for local area networks (LAN), personal area network (PAN), and metropolitan area networks (MAN). The IEEE 802 LAN/MAN Standards Committee (LMSC) maintains ...
local area networks, such as IEEE 802.3/
Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
(when
Ethernet II In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a data unit on an Ethernet link transports an Ethernet frame as its payload. ...
frame format is not used),
IEEE 802.5 Token Ring network IBM hermaphroditic connector with locking clip. Screen contacts are prominently visible, gold-plated signal contacts less so. Token Ring is a computer networking technology used to build local area networks. It was introduc ...
, and
IEEE 802.11 IEEE 802.11 is part of the IEEE 802 set of local area network (LAN) technical standards, and specifies the set of media access control (MAC) and physical layer (PHY) protocols for implementing wireless local area network (WLAN) computer commun ...
. IEEE 802.2 is also used in some non-IEEE 802 networks such as
FDDI Fiber Distributed Data Interface (FDDI) is a standard for data transmission in a local area network. It uses optical fiber as its standard underlying physical medium, although it was also later specified to use copper cable, in which case it m ...
.


Ethernet

Since bit errors are very rare in wired networks, Ethernet does not provide flow control or
automatic repeat request Automatic repeat request (ARQ), also known as automatic repeat query, is an error-control method for data transmission that uses acknowledgements (messages sent by the receiver indicating that it has correctly received a packet) and timeouts ...
(ARQ), meaning that incorrect packets are detected but only cancelled, not retransmitted (except in case of collisions detected by the
CSMA/CD Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other statio ...
MAC layer protocol). Instead, retransmissions rely on higher layer protocols. As the
EtherType EtherType is a two-Octet (computing), octet field in an Ethernet frame. It is used to indicate which Communications protocol, protocol is Encapsulation (networking), encapsulated in the payload of the frame and is used at the receiving end by th ...
in an Ethernet frame using
Ethernet II framing In computer networking, an Ethernet frame is a data link layer protocol data unit and uses the underlying Ethernet physical layer transport mechanisms. In other words, a network packet, data unit on an Ethernet link transports an Ethernet frame ...
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.


Wireless LAN

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 Carrier-sense multiple access with collision avoidance (CSMA/CA) in computer networking, is a network multiple access method in which carrier sensing is used, but nodes attempt to avoid collisions by beginning transmission only after the channel ...
MAC protocol, and not part of the LLC layer. The LLC sublayer follows the
IEEE 802.2 IEEE 802.2 is the original name of the ISO/IEC 8802-2 standard which defines logical link control (LLC) as the upper portion of the data link layer of the OSI Model. The original standard developed by the Institute of Electrical and Electronics Engi ...
standard.


HDLC

Some non-IEEE 802 protocols can be thought of as being split into MAC and LLC layers. For example, while
HDLC High-Level Data Link Control (HDLC) is a bit-oriented code-transparent synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The standard for HDLC is ISO/IEC 13239:2002. HDLC provides both c ...
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 Cisco HDLC (cHDLC) is an extension to the High-Level Data Link Control (HDLC) network protocol, and was created by Cisco Systems, Inc. HDLC is a bit-oriented synchronous data link layer protocol that was originally developed by the International ...
can use HDLC-like packet framing and their own LLC protocol.


PPP and modems

Over telephone network
modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...
s, 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 Access Procedure for Modems (LAPM) is part of the V.42 error correction protocol for modems.ITU-T Recommendation V.42Error-correcting procedures for DCEs using asynchronous-to-synchronous conversion/ref> LAPM is an error control protocol defin ...
link layer protocol, made for modem communication in old X.25 networks.


Cellular systems

The GPRS LLC layer also does ciphering and deciphering of SN-PDU (
SNDCP SNDCP, Sub Network Dependent Convergence Protocol, is part of layer 3 of a GPRS protocol specification. SNDCP interfaces to the Internet Protocol at the top, and to the GPRS-specific Logical Link Control (LLC) protocol at the bottom. In the spirit ...
) packets.


Power lines

Another example of a
data link layer The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer p ...
which is split between LLC (for flow and error control) and MAC (for multiple access) is the
ITU-T The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
G.hn G.hn is a specification for home networking with data rates up to 2 Gbit/s and operation over four types of legacy wires: telephone wiring, coaxial cables, power lines and plastic optical fiber. A single G.hn semiconductor device is able to n ...
standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables).


See also

* Subnetwork Access Protocol (SNAP) *
Virtual Circuit Multiplexing Virtual circuit multiplexing or VC-MUX is one of the two (the other being LLC encapsulation) mechanisms for identifying the protocol carried in ATM Adaptation Layer 5 (AAL5) frames specified by {{IETF RFC, 2684, Multiprotocol Encapsulation over ...
(VC-MUX)


References

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