Link Layer
Encyclopedia
In computer network
ing, the link layer is the lowest layer in the Internet Protocol Suite
(commonly known as "TCP/IP"), the networking architecture of the Internet
(RFC 1122, RFC 1123). It is the group of methods or protocols
that only operate on a host's link. The link is the physical and logical network component used to interconnect hosts
or nodes
in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a Local area network
segment
or a wide area network
connection.
Despite the different semantics of layering in TCP/IP and OSI, the link layer is often described as a combination of the data link layer
(layer 2) and the physical layer
(layer 1) in the Open Systems Interconnection
(OSI) protocol
stack. However, TCP/IP's layers are descriptions of operating scopes (application, host-to-host, network, link) and not detailed prescriptions of operating procedures, data semantics, or networking technologies.
RFC 1122 exemplifies that local area network
protocols such as Ethernet and IEEE 802, and framing protocols such as Point-to-Point Protocol
(PPP) belong to the link layer.
specifications use terminology from the seven-layer OSI model
rather than the TCP/IP reference model. The TCP/IP model in general does not consider physical specifications, rather it assumes a working network infrastructure that can deliver media level frames on the link. Therefore RFC 1122 and RFC 1123, the definition of the TCP/IP model, do not discuss hardware issues and physical data transmission
and set no standards for those aspects, other than broadly including them as link-layer components. Some textbook authors have supported the interpretation that physical data transmission aspects are part of the link layer. That position will be held in the rest of this article. Others assumed that physical data transmission standards are not considered as communication protocols, and are not part of the TCP/IP model. These authors assume a hardware layer or physical layer below the link layer, and several of them adopt the OSI term data link layer
instead of link layer in a modified description of layering. In the predecessor to the TCP/IP model, the Arpanet Reference Model (RFC 908, 1982), aspects of the link layer are referred to by several poorly defined terms, such as network-access layer, network-access protocol, as well as network layer, while the next higher layer is called internetwork layer. In some modern text books, network-interface layer, host-to-network layer and network-access layer occur as synonyms either to the link layer or the data link layer, often including the physical layer.
to be placed into this layer are the Address Resolution Protocol
(ARP), its cousin, the Reverse Address Resolution Protocol
(RARP), and the Neighbor Discovery Protocol
(NDP), which is a facility delivering similar functionality as ARP for IPv6
. Since the advent of IPv6
, Open Shortest Path First
(OSPF) can be considered to operate on the link level as well, although the IPv4
version of the protocol was considered at the Internet layer
.
IS-IS
(RFC 1142) is another link-state routing protocol that fits into this layer when considering TCP/IP model, however it was developed within the OSI
reference stack (where it is a layer-3 protocol). It is not an Internet standard
.
The link layer also contains all hardware specific interface methods, such as Ethernet
and other IEEE 802
encapsulation schemes (see References).
and the physical layer
in the Open Systems Interconnection
(OSI) protocol
stack. Although they are congruent to some degree in technical coverage of protocols, they are not identical. The link layer in TCP/IP is still wider in scope and in principle a different concept and terminology of classification. This may be observed when certain protocols, such as the Address Resolution Protocol
(ARP), which is confined to the link layer in the TCP/IP model, is often said to fit between OSI's data link layer and the network layer. In general, direct or strict comparisons should be avoided, because the layering in TCP/IP is not a principal design criterion and in general considered to be "harmful" (RFC 3439).
Another term sometimes encountered, "network-access layer", tries to suggest the closeness of this layer to the physical network. However, this use is misleading and non-standard, since the link layer implies functions that are wider in scope than just network access. Important link-layer protocols are used to probe the topology of the local network, discover routers and neighboring hosts, i.e. functions that go well beyond network access.
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
ing, the link layer is the lowest layer in the Internet Protocol Suite
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
(commonly known as "TCP/IP"), the networking architecture of the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
(RFC 1122, RFC 1123). It is the group of methods or protocols
Communications protocol
A communications protocol is a system of digital message formats and rules for exchanging those messages in or between computing systems and in telecommunications...
that only operate on a host's link. The link is the physical and logical network component used to interconnect hosts
Host (network)
A network host is a computer connected to a computer network. A network host may offer information resources, services, and applications to users or other nodes on the network. A network host is a network node that is assigned a network layer host address....
or nodes
Node (networking)
In communication networks, a node is a connection point, either a redistribution point or a communication endpoint . The definition of a node depends on the network and protocol layer referred to...
in the network and a link protocol is a suite of methods and standards that operate only between adjacent network nodes of a Local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
segment
Network segment
A network segment is a portion of a computer network. The nature and extent of a segment depends on the nature of the network and the device or devices used to interconnect end stations.-Ethernet:...
or a wide area network
Wide area network
A wide area network is a telecommunication network that covers a broad area . Business and government entities utilize WANs to relay data among employees, clients, buyers, and suppliers from various geographical locations...
connection.
Despite the different semantics of layering in TCP/IP and OSI, the link layer is often described as a combination of the data link layer
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....
(layer 2) and 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....
(layer 1) in the Open Systems Interconnection
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
(OSI) protocol
OSI protocols
The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T starting in 1977....
stack. However, TCP/IP's layers are descriptions of operating scopes (application, host-to-host, network, link) and not detailed prescriptions of operating procedures, data semantics, or networking technologies.
RFC 1122 exemplifies that local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...
protocols such as Ethernet and IEEE 802, and framing protocols such as Point-to-Point Protocol
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...
(PPP) belong to the link layer.
Definition in standards and text books
LAN standards such as Ethernet and IEEE 802IEEE 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...
specifications use terminology from the seven-layer OSI 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...
rather than the TCP/IP reference model. The TCP/IP model in general does not consider physical specifications, rather it assumes a working network infrastructure that can deliver media level frames on the link. Therefore RFC 1122 and RFC 1123, the definition of the TCP/IP model, do not discuss hardware issues and physical data transmission
Data transmission
Data transmission, digital transmission, or digital communications is the physical transfer of data over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibres, wireless communication channels, and storage media...
and set no standards for those aspects, other than broadly including them as link-layer components. Some textbook authors have supported the interpretation that physical data transmission aspects are part of the link layer. That position will be held in the rest of this article. Others assumed that physical data transmission standards are not considered as communication protocols, and are not part of the TCP/IP model. These authors assume a hardware layer or physical layer below the link layer, and several of them adopt the OSI term data link layer
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....
instead of link layer in a modified description of layering. In the predecessor to the TCP/IP model, the Arpanet Reference Model (RFC 908, 1982), aspects of the link layer are referred to by several poorly defined terms, such as network-access layer, network-access protocol, as well as network layer, while the next higher layer is called internetwork layer. In some modern text books, network-interface layer, host-to-network layer and network-access layer occur as synonyms either to the link layer or the data link layer, often including the physical layer.
Link-layer protocols
The core protocols specified by the Internet Engineering Task ForceInternet Engineering Task Force
The Internet Engineering Task Force develops and promotes Internet standards, cooperating closely with the W3C and ISO/IEC standards bodies and dealing in particular with standards of the TCP/IP and Internet protocol suite...
to be placed into this layer are the Address Resolution Protocol
Address Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
(ARP), its cousin, the Reverse Address Resolution Protocol
Reverse Address Resolution Protocol
The Reverse Address Resolution Protocol is an obsolete computer networking protocol used by a host computer to request its Internet Protocol address from an administrative host, when it has available its Link Layer or hardware address, such as a MAC address.RARP is described in Internet...
(RARP), and the Neighbor Discovery Protocol
Neighbor Discovery Protocol
The Neighbor Discovery Protocol is a protocol in the Internet Protocol Suite used with Internet Protocol Version 6 . It operates in the Link Layer of the Internet model and is responsible for address autoconfiguration of nodes, discovery of other nodes on the link, determining the Link Layer...
(NDP), which is a facility delivering similar functionality as ARP for IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
. Since the advent of IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
, Open Shortest Path First
Open Shortest Path First
Open Shortest Path First is an adaptive routing protocol for Internet Protocol networks. It uses a link state routing algorithm and falls into the group of interior routing protocols, operating within a single autonomous system . It is defined as OSPF Version 2 in RFC 2328 for IPv4...
(OSPF) can be considered to operate on the link level as well, although the IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
version of the protocol was considered at the Internet layer
Internet layer
The internet layer or IP layer is a group of internetworking methods in the Internet protocol suite, commonly also called TCP/IP, which is the foundation of the Internet...
.
IS-IS
IS-IS
Intermediate System To Intermediate System , is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices....
(RFC 1142) is another link-state routing protocol that fits into this layer when considering TCP/IP model, however it was developed within the OSI
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
reference stack (where it is a layer-3 protocol). It is not an Internet standard
Internet standard
In computer network engineering, an Internet Standard is a normative specification of a technology or methodology applicable to the Internet. Internet Standards are created and published by the Internet Engineering Task Force .-Overview:...
.
The link layer also contains all hardware specific interface methods, such as 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....
and other 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...
encapsulation schemes (see References).
Relation to OSI model
The link layer of the TCP/IP model is often compared directly with the combination of the 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....
and 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 Open Systems Interconnection
Open Systems Interconnection
Open Systems Interconnection is an effort to standardize networking that was started in 1977 by the International Organization for Standardization , along with the ITU-T.-History:...
(OSI) protocol
OSI protocols
The Open Systems Interconnection protocols are a family of information exchange standards developed jointly by the ISO and the ITU-T starting in 1977....
stack. Although they are congruent to some degree in technical coverage of protocols, they are not identical. The link layer in TCP/IP is still wider in scope and in principle a different concept and terminology of classification. This may be observed when certain protocols, such as the Address Resolution Protocol
Address Resolution Protocol
Address Resolution Protocol is a telecommunications protocol used for resolution of network layer addresses into link layer addresses, a critical function in multiple-access networks. ARP was defined by RFC 826 in 1982. It is Internet Standard STD 37...
(ARP), which is confined to the link layer in the TCP/IP model, is often said to fit between OSI's data link layer and the network layer. In general, direct or strict comparisons should be avoided, because the layering in TCP/IP is not a principal design criterion and in general considered to be "harmful" (RFC 3439).
Another term sometimes encountered, "network-access layer", tries to suggest the closeness of this layer to the physical network. However, this use is misleading and non-standard, since the link layer implies functions that are wider in scope than just network access. Important link-layer protocols are used to probe the topology of the local network, discover routers and neighboring hosts, i.e. functions that go well beyond network access.
Examples of functions supported
The link layer includes the following functionality:- Logical link-local networking methods
- Encapsulation of IP packets into frames
- Frame synchronizationFrame synchronizationWhile receiving a stream of framed data, frame synchronization is the process by which incoming frame alignment signals, i.e., distinctive bit sequences , are identified, i.e., distinguished from data bits, permitting the data bits within the frame to be extracted for decoding or retransmission...
- Error detection with removal of erroneous packets (Used in LANs and point-to-point fiber optical cables)
- Logical link controlLogical Link ControlThe logical link control data communication protocol layer is the upper sub-layer of the data link layer in the seven-layer OSI reference model...
(LLC) sublayer: (Used in modem protocols and wireless protocols)- Node-to-node error detection and automatic repeat request of erroneous packets
- Node-to-node flow controlFlow controlIn 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...
- Forward error correctionForward error correctionIn telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
(however much more common at the physical layer)
- Media access controlMedia 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...
(MAC) sublayer:- Multiple access protocols, for example with collision detection or avoidance
- Physical addressPhysical addressIn computing, a physical address, also real address, or binary address, is the memory address that is represented in the form of a binary number on the address bus circuitry in order to enable the data bus to access a particular storage cell of main memory.In a computer with virtual memory, the...
ing (MAC addressing) - LAN switching (packet switchingPacket switchingPacket switching is a digital networking communications method that groups all transmitted data – regardless of content, type, or structure – into suitably sized blocks, called packets. Packet switching features delivery of variable-bit-rate data streams over a shared network...
) including MAC filtering and spanning tree protocolSpanning tree protocolThe Spanning Tree Protocol is a network protocol that ensures a loop-free topology for any bridged Ethernet local area network. The basic function of STP is to prevent bridge loops and ensuing broadcast radiation... - Store-and-forward switching or cut-through switchingCut-through switchingIn computer networking, cut-through switching is a method for packet switching systems, wherein the switch starts forwarding a frame before the whole frame has been received, normally as soon as the destination address is processed...
- Data packet queueing or scheduling algorithms
- Help protocols not encapsulated in IP packets:
- Some routing protocolRouting protocolA routing protocol is a protocol that specifies how routers communicate with each other, disseminating information that enables them to select routes between any two nodes on a computer network, the choice of the route being done by routing algorithms. Each router has a priori knowledge only of...
s - IP address to/from physical address resolution protocols
- Some routing protocol
- Physical networking aspects: (It is disputable if and to which extent these are included)
- Bit-by-bit or symbol-by-symbol delivery
- Net bit rate
- Digital modulation
- Line coding
- Carrier sense and collision detection utilized by some level 2 multiple-access protocols
- Circuit switchingCircuit switchingCircuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel through the network before the nodes may communicate. The circuit guarantees the full bandwidth of the channel and remains connected for the...
, establishing circuit switched connections - 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...
- Forward error correctionForward error correctionIn telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
codes - SerialSerial communicationIn telecommunication and computer science, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are sent as a whole, on a link with several parallel channels...
or parallel communication - Bit synchronization
- Start-stop signalling and flow controlFlow controlIn 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...
in asynchronous serial communication - Signal strengthSignal strengthIn telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
(voltageVoltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
and powerElectric powerElectric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
levels), impedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit... - Forward error correctionForward error correctionIn telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
, bit-interleaving and other channel coding - EqualizationEqualizationEqualization, is the process of adjusting the balance between frequency components within an electronic signal. The most well known use of equalization is in sound recording and reproduction but there are many other applications in electronics and telecommunications. The circuit or equipment used...
filtering, training sequences, pulse shapingPulse shapingIn digital telecommunication, pulse shaping is the process of changing the waveform of transmitted pulses. Its purpose is to make the transmitted signal better suited to the communication channel by limiting the effective bandwidth of the transmission. By filtering the transmitted pulses this way,...
and other signal processingSignal processingSignal processing is an area of systems engineering, electrical engineering and applied mathematics that deals with operations on or analysis of signals, in either discrete or continuous time...
of physical signals - Mechanical specification of electrical connectorElectrical connectorAn electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
s and cableCableA cable is two or more wires running side by side and bonded, twisted or braided together to form a single assembly. In mechanics cables, otherwise known as wire ropes, are used for lifting, hauling and towing or conveying force through tension. In electrical engineering cables are used to carry...
s, for example maximum cable length - Electrical specification of transmission lineTransmission lineIn communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...
signal level and impedanceElectrical impedanceElectrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit... - Radio interface, including electromagnetic spectrumElectromagnetic spectrumThe electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
frequency allocationFrequency allocationUse of radio frequency bands of the electromagnetic spectrum is regulated by governments in most countries, in a Spectrum management process known as frequency allocation or spectrum allocation. Radio propagation does not stop at national boundaries...
and specification of signal strengthSignal strengthIn telecommunications, particularly in radio, signal strength refers to the magnitude of the electric field at a reference point that is a significant distance from the transmitting antenna. It may also be referred to as received signal level or field strength. Typically, it is expressed in...
, analog bandwidth, etc. - Specifications for IR over optical fiberOptical fiberAn optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
or a wireless IR communication link
RFC references
- RFC 1122, "Requirements for Internet Hosts -- Communication layers," IETF, R. Braden (Editor), October 1989
- RFC 1123, "Requirements for Internet Hosts -- Application and Support," IETF, R. Braden (Editor), October 1989
- RFC 893, "Trailer Encapsulations," S. Leffler and M. Karels, April 1984
- RFC 826, "An Ethernet Address Resolution Protocol," D. Plummer, November 1982
- RFC 894, "A Standard for the Transmission of IP Datagrams over Ethernet Networks," C. Hornig, April 1984
- RFC 1042, "A Standard for the Transmission of IP Datagrams over IEEE 802 Networks," J. Postel and J. Reynolds, February 1988
- RFC 2740, "OSPF for IPv6", R. Coltun, et al., December 1999
See also
Other layers defined in the OSI model of networking that relate to the link layer:- Network layerNetwork layerThe 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...