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) maintai ...
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 ar ...
, 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 ...
(layer 2) of the seven-layer
OSI model. The LLC sublayer acts as an interface between the
media access control (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 trans ...
.
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 and
DECnet) 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.
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 protocol. However, most protocol stacks running atop
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 ...
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
TCP may refer to:
Science and technology
* Transformer coupled plasma
* Tool Center Point, see Robot end effector
Computing
* Transmission Control Protocol, a fundamental Internet standard
* Telephony control protocol, a Bluetooth communication s ...
or by some
application layer
An application layer is an abstraction layer that specifies the shared communications protocols and interface methods used by hosts in a communications network. An ''application layer'' abstraction is specified in both the Internet Protocol ...
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 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
The
IEEE 802.2 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) maintai ...
local area networks, such as
IEEE 802.3
IEEE 802.3 is a working group and a collection standards defining the physical layer and data link layer's media access control (MAC) of wired Ethernet. The standards are produced by the working group of Institute of Electrical and Electronics Eng ...
/
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 ...
(when
Ethernet II frame format 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
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 i ...
.
Ethernet
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.
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 standard.
HDLC
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
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 modulating one or more c ...
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 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) 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 ...
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 standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables).
See also
*
Subnetwork Access Protocol
The Subnetwork Access Protocol (SNAP) is a mechanism for multiplexing, on networks using IEEE 802.2 LLC, more protocols than can be distinguished by the 8-bit 802.2 Service Access Point (SAP) fields. SNAP supports identifying protocols by EtherT ...
(SNAP)
*
Virtual Circuit Multiplexing (VC-MUX)
References
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