Logical Link Control
Encyclopedia
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
, Decnet
and Appletalk
) to coexist within a multipoint network and to be transported over the same network media, and can also provide flow control
and automatic repeat request (ARQ) error management mechanisms.
The LLC sub-layer acts as an interface between the media access control
(MAC) sublayer and the network layer
.
In today's networks, flow control and error management is typically taken care of by a transport layer protocol such as the TCP
protocol, 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.
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.
standard specifies LLC sublayer for all IEEE 802
local area 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
.
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. However, If Ethernet is used without EtherType field, Ethernet is considered as lacking LLC sublayer.
standard.
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.
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 modem 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.
) packets.
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).
Data link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
(which is itself layer 2, just above the physical layer
Physical layer
The physical layer or layer 1 is the first and lowest layer in the seven-layer OSI model of computer networking. The implementation of this layer is often termed PHY....
) in the seven-layer OSI reference model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...
. It provides multiplexing
Multiplexing
The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
mechanisms that make it possible for several network protocols (IP
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
, IPX
IPX
Internetwork Packet Exchange is the OSI-model Network layer protocol in the IPX/SPX protocol stack.The IPX/SPXM protocol stack is supported by Novell's NetWare network operating system. Because of Netware's popularity through the late 1980s into the mid 1990s, IPX became a popular internetworking...
, Decnet
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 into a networking powerhouse in the 1980s...
and Appletalk
AppleTalk
AppleTalk is a proprietary suite of protocols developed by Apple Inc. for networking computers. It was included in the original Macintosh released in 1984, but is now unsupported as of the release of Mac OS X v10.6 in 2009 in favor of TCP/IP networking...
) to coexist within a multipoint network and to be transported over the same network media, and can also provide flow control
Flow control
In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with...
and automatic repeat request (ARQ) error management mechanisms.
The LLC sub-layer acts as an interface between the media access control
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC) sublayer and the network layer
Network layer
The network layer is layer 3 of the seven-layer OSI model of computer networking.The network layer is responsible for packet forwarding including routing through intermediate routers, whereas the data link layer is responsible for media access control, flow control and error checking.The network...
.
Operation
The LLC sublayer is primarily concerned with:- MultiplexingMultiplexingThe multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred...
protocols transmitted over the MACMedia Access ControlThe media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
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 the TCP
Transmission Control Protocol
The Transmission Control Protocol is one of the core protocols of the Internet Protocol Suite. TCP is one of the two original components of the suite, complementing the Internet Protocol , and therefore the entire suite is commonly referred to as TCP/IP...
protocol, 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
IPX
Internetwork Packet Exchange is the OSI-model Network layer protocol in the IPX/SPX protocol stack.The IPX/SPXM protocol stack is supported by Novell's NetWare network operating system. Because of Netware's popularity through the late 1980s into the mid 1990s, IPX became a popular internetworking...
. 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.25X.25
X.25 is an ITU-T standard protocol suite for packet switched wide area network communication. An X.25 WAN consists of packet-switching exchange nodes as the networking hardware, and leased lines, Plain old telephone service connections or ISDN connections as physical links...
networks with the LAPB
LAPB
Link Access Procedure, Balanced 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 right sequence. LAPB is specified in and ISO/IEC 7776...
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 802.2IEEE 802.2
IEEE 802.2 is the IEEE 802 standard defining Logical Link Control , which is the upper portion of the data link layer of the OSI Model. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer...
standard specifies LLC sublayer for all IEEE 802
IEEE 802
IEEE 802 refers to a family of IEEE standards dealing with local area networks and metropolitan area networks.More specifically, the IEEE 802 standards are restricted to networks carrying variable-size packets. IEEE 802 refers to a family of IEEE standards dealing with local area networks and...
local area networks, such as IEEE 802.3
IEEE 802.3
IEEE 802.3 is a working group and a collection of IEEE standards produced by the working group defining the physical layer and data link layer's media access control of wired Ethernet. This is generally a local area network technology with some wide area network applications...
/Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
(if the EtherType
EtherType
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the PayLoad of an Ethernet Frame. This field was first defined by the Ethernet II framing networking standard, and later adapted for the IEEE 802.3 Ethernet networking standard.EtherType...
field isn't used), IEEE 802.5, and IEEE 802.11
IEEE 802.11
IEEE 802.11 is a set of standards for implementing wireless local area network computer communication in the 2.4, 3.6 and 5 GHz frequency bands. They are created and maintained by the IEEE LAN/MAN Standards Committee . The base version of the standard IEEE 802.11-2007 has had subsequent...
, and in some non-IEEE 802 networks such as FDDI
Fiber distributed data interface
Fiber Distributed Data Interface provides a 100 Mbit/s optical standard for data transmission in a local area network that can extend in range up to . Although FDDI logical topology is a ring-based token network, it does not use the IEEE 802.5 token ring protocol as its basis; instead, its...
.
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
EtherType
EtherType is a two-octet field in an Ethernet frame. It is used to indicate which protocol is encapsulated in the PayLoad of an Ethernet Frame. This field was first defined by the Ethernet II framing networking standard, and later adapted for the IEEE 802.3 Ethernet networking standard.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. However, If Ethernet is used without EtherType field, Ethernet is considered as lacking LLC sublayer.
Wireless LAN
In wireless communications, bit errors are very common. In wireless networks such as IEEE802.11, 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 802.2IEEE 802.2
IEEE 802.2 is the IEEE 802 standard defining Logical Link Control , which is the upper portion of the data link layer of the OSI Model. The LLC sublayer presents a uniform interface to the user of the data link service, usually the network layer...
standard.
HDLC
Some non-IEEE 802 protocols can be thought of as being split into MAC and LLC layers. For example, while HDLCHigh-Level Data Link Control
High-Level Data Link Control is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization...
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
Cisco HDLC is an extension to the High-Level Data Link Control network protocol created by Cisco Systems, Inc. HDLC is a bit-oriented synchronous data link layer protocol that was originally developed by the International Organization for Standardization...
can use HDLC-like packet framing and their own LLC protocol.
PPP and modems
Over telephone network modemModem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s, PPP
Point-to-Point Protocol
In networking, the Point-to-Point Protocol is a data link protocol commonly used in establishing a direct connection between two networking nodes...
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 modem protocols. Today's modem protocols have inherited LLC features from the older LAPM
LAPM
Link Access Procedure for Modems is part of the V.42 error correction protocol for modems.LAPM is an error control protocol defined in ITU-T recommendations V.42. Like many data link layer protocols, it is a variant of HDLC...
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 (SNDCPSNDCP
SNDCP stands for Sub Network Dependent Convergence Protocol.This 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 protocol at the bottom....
) packets.
Power lines
Another example of a data link layerData link layer
The data link layer is layer 2 of the seven-layer OSI model of computer networking. It corresponds to, or is part of the link layer of the TCP/IP reference model....
which is split between LLC (for flow and error control) and MAC (for multiple access) is the ITU-T
ITU-T
The ITU Telecommunication Standardization Sector is one of the three sectors of the International Telecommunication Union ; it coordinates standards for telecommunications....
G.hn
G.hn
G.hn is the common name for a home network technology family of standards developed under the International Telecommunication Union's Standardization arm and promoted by the HomeGrid Forum...
standard, which provides high-speed local area networking over existing home wiring (power lines, phone lines and coaxial cables).