The data link layer, or layer 2, is the second layer 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 ...

computer network 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 ...

ing. This layer is the protocol layer that transfers data between nodes on a network segment across the

physical layer In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip. Th ...

. The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer. The data link layer is concerned with local delivery of

frames A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (co ...

between nodes on the same level of the network. Data-link frames, as these protocol data units are called, do not cross the boundaries of a local area network. Inter-network routing and global addressing are higher-layer functions, allowing data-link protocols to focus on local delivery, addressing, and media arbitration. In this way, the data link layer is analogous to a neighborhood traffic cop; it endeavors to arbitrate between parties contending for access to a medium, without concern for their ultimate destination. When devices attempt to use a medium simultaneously, frame collisions occur. Data-link protocols specify how devices detect and recover from such collisions, and may provide mechanisms to reduce or prevent them. Examples of data link protocols are

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 ...

, Point-to-Point Protocol (PPP),

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 ...

and ADCCP. In the

Internet Protocol Suite The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the sui ...

(TCP/IP), the data link layer functionality is contained within the link layer, the lowest layer of the descriptive model, which is assumed to be independent of physical infrastructure.

Function

The data link provides for the transfer of

data frame A frame is a digital data transmission unit in computer networking and telecommunication. In packet switched systems, a frame is a simple container for a single network packet. In other telecommunications systems, a frame is a repeating structure s ...

s between hosts connected to the physical link. Within the semantics of the OSI network architecture, the protocols of the data link layer respond to service requests from 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 ...

, and perform their function by issuing service requests to the

. That transfer can be reliable or unreliable; many data link protocols do not have acknowledgments of successful frame reception and acceptance, and some data link protocols might not even perform any check for transmission errors. In those cases, higher-level protocols must provide flow control, error checking, acknowledgments, and retransmission. The frame header contains the source and destination addresses that indicate which device originated the frame and which device is expected to receive and process it. In contrast to the hierarchical and routable addresses of the network layer, layer 2 addresses are flat, meaning that no part of the address can be used to identify the logical or physical group to which the address belongs. In some networks, such as

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, the data link layer is described in more detail with

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 subla ...

(MAC) and

logical link control In the IEEE 802 reference model of computer networking, the logical link control (LLC) data communication protocol layer is the upper sublayer of the data link layer (layer 2) of the seven-layer OSI model. The LLC sublayer acts as an interface ...

(LLC) sublayers; this means that the IEEE 802.2 LLC protocol can be used with all of the IEEE 802 MAC layers, such as Ethernet,

Token Ring 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 ...

, IEEE 802.11, etc., as well as with some non-802 MAC layers 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 ...

. Other data-link-layer protocols, such as

, are specified to include both sublayers, although some other protocols, such as Cisco HDLC, use HDLC's low-level framing as a MAC layer in combination with a different LLC layer. In 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 a way to create a high-speed (up to 1 Gigabit/s)

local area network A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a larger ...

using existing home wiring ( power lines, phone lines and coaxial cables), the data link layer is divided into three sub-layers (application protocol convergence, logical link control and media access control).

Sublayers

The data link layer is often divided into two sublayers: ''

'' (LLC) and ''

'' (MAC).

Logical link control sublayer

The uppermost sublayer, LLC,

multiplexes 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 running at the top of the data link layer, and optionally provides flow control, acknowledgment, and error notification. The LLC provides addressing and control of the data link. It specifies which mechanisms are to be used for addressing stations over the transmission medium and for controlling the data exchanged between the originator and recipient machines.

Media access control sublayer

MAC may refer to the sublayer that determines who is allowed to access the media at any one time (e.g.

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 stati ...

). Other times it refers to a frame structure delivered based on

MAC address A media access control address (MAC address) is a unique identifier assigned to a network interface controller (NIC) for use as a network address in communications within a network segment. This use is common in most IEEE 802 networking tec ...

es inside. There are generally two forms of media access control: distributed and centralized. Both of these may be compared to communication between people. In a network made up of people speaking, i.e. a conversation, they will each pause a random amount of time and then attempt to speak again, effectively establishing a long and elaborate game of saying "no, you first". The Media Access Control sublayer also performs

frame synchronization In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals (i.e., a distinctive bit sequences or syncwords) are identified (that is, distinguished fro ...

, which determines the start and end of each frame of data in the transmission

bitstream A bitstream (or bit stream), also known as binary sequence, is a sequence of bits. A bytestream is a sequence of bytes. Typically, each byte is an 8-bit quantity, and so the term octet stream is sometimes used interchangeably. An octet may ...

. It entails one of several methods: timing-based detection, character counting, byte stuffing, and bit stuffing. * The time-based approach expects a specified amount of time between frames. * Character counting tracks the count of remaining characters in the frame header. This method, however, is easily disturbed if this field is corrupted. * Byte stuffing precedes the frame with a special byte sequence such as DLE STX and succeeds it with DLE ETX. Appearances of DLE (byte value 0x10) have to be escaped with another DLE. The start and stop marks are detected at the receiver and removed as well as the inserted DLE characters. * Similarly,

bit stuffing In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits. Bit stuffing is used for ...

replaces these start and end marks with flags consisting of a special bit pattern (e.g. a 0, six 1 bits and a 0). Occurrences of this bit pattern in the data to be transmitted are avoided by inserting a bit. To use the example where the flag is 01111110, a 0 is inserted after 5 consecutive 1's in the data stream. The flags and the inserted 0's are removed at the receiving end. This makes for arbitrary long frames and easy synchronization for the recipient. The stuffed bit is added even if the following data bit is 0, which could not be mistaken for a

sync sequence In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identif ...

, so that the receiver can unambiguously distinguish stuffed bits from normal bits.

Services

The services provided by the data link layer are: *Encapsulation of

data packets into

Frame synchronization In telecommunication, frame synchronization or framing is the process by which, while receiving a stream of framed data, incoming frame alignment signals (i.e., a distinctive bit sequences or syncwords) are identified (that is, distinguished fro ...

* In the

(LLC) sublayer: ** Error control (

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 timeout ...

, ARQ), in addition to ARQ provided by some transport-layer protocols, to

forward error correction In computing, telecommunication, information theory, and coding theory, an error correction code, sometimes error correcting code, (ECC) is used for controlling errors in data over unreliable or noisy communication channels. The central idea is ...

(FEC) techniques provided on the

, and to error-detection and packet canceling provided at all layers, including the

. Data-link-layer error control (i.e. retransmission of erroneous packets) is provided in wireless networks and V.42 telephone network modems, but not in LAN protocols such as

, since bit errors are so uncommon in short wires. In that case, only

error detection In information theory and coding theory with applications in computer science and telecommunication, error detection and correction (EDAC) or error control are techniques that enable reliable delivery of digital data over unreliable communic ...

and canceling of erroneous packets are provided. ** Flow control, in addition to the one provided on the

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 ...

. Data-link-layer flow control is not used in LAN protocols such as Ethernet, but in modems and wireless networks. * In the medium access control (MAC) sublayer: ** Multiple access methods for channel-access control, for example

protocols for

collision detection Collision detection is the computational problem of detecting the intersection (Euclidean geometry), intersection of two or more objects. Collision detection is a classic issue of computational geometry and has applications in various computing ...

and re-transmission in

bus networks and hub networks, or 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 ...

protocol for

collision avoidance In transportation, collision avoidance is the maintenance of systems and practices designed to prevent vehicles (such as aircraft, motor vehicles, ships, cranes and trains) from colliding with each other. Examples include: * Airborne collision av ...

in wireless networks. ** Physical addressing (

ing) **

LAN switching A network switch (also called switching hub, bridging hub, and, by the IEEE, MAC bridge) is networking hardware that connects devices on a computer network by using packet switching to receive and forward data to the destination device. A netw ...

(

packet switching In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...

), including

MAC filtering In computer networking, MAC address filtering is a security access control In the fields of physical security and information security, access control (AC) is the selective restriction of access to a place or other resource, while acces ...

Spanning Tree Protocol The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also ...

(STP), Shortest Path Bridging (SPB) and TRILL (TRansparent Interconnection of Lots of Links) ** Data packet queuing or

scheduling A schedule or a timetable, as a basic time-management tool, consists of a list of times at which possible tasks, events, or actions are intended to take place, or of a sequence of events in the chronological order in which such things are i ...

Store-and-forward Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in ...

switching or

cut-through switching Rat running (also known as rodent running, cut-through driving, or dive-bombing) is the practice by motorists of using residential side streets or any unintended short cut such as a parking lot, delivery service lane or cemetery road instead o ...

Quality of service Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...

(QoS) control ** Virtual LANs (VLAN)

Error detection and correction

In addition to framing, the data link layer may also detect and recover from transmission errors. For a receiver to detect transmission errors, the sender must add redundant information as an error detection code to the frame sent. When the receiver obtains a frame it verifies whether the received error detection code matches a recomputed error detection code. An error detection code can be defined as a function that computes the (amount of redundant bits) corresponding to each string of total number of bits. The simplest error detection code is the

parity bit A parity bit, or check bit, is a bit added to a string of binary code. Parity bits are a simple form of error detecting code. Parity bits are generally applied to the smallest units of a communication protocol, typically 8-bit octets (bytes) ...

, which allows a receiver to detect transmission errors that have affected a single bit among the transmitted bits. If there are multiple flipped bits then the checking method might not be able to detect this on the receiver side. More advanced methods than parity error detection do exist providing higher grades of quality and features. A simple example of how this works using metadata is transmitting the word "HELLO", by encoding each letter as its position in the alphabet. Thus, the letter ''A'' is coded as 1, ''B'' as 2, and so on as shown in the table on the right. Adding up the resulting numbers yields 8 + 5 + 12 + 12 + 15 = 52, and 5 + 2 = 7 calculates the metadata. Finally, the "8 5 12 12 15 7" numbers sequence is transmitted, which the receiver will see on its end if there are no transmission errors. The receiver knows that the last number received is the error-detecting metadata and that all data before is the message, so the receiver can recalculate the above math and if the metadata matches it can be concluded that the data has been received error-free. Though, if the receiver sees something like a "7 5 12 12 15 7" sequence (first element altered by some error), it can run the check by calculating 7 + 5 + 12 + 12 + 15 = 51 and 5 + 1 = 6, and discard the received data as defective since 6 does not equal 7. More sophisticated error detection and correction algorithms are designed to reduce the risk that multiple transmission errors in the data would cancel each other out and go undetected. An algorithm that can even detect if the correct bytes are received but out of order is the cyclic redundancy check or CRC. This algorithm is often used in the data link layer.

Protocol examples

ARCnet Attached Resource Computer NETwork (ARCNET or ARCnet) is a communications protocol for local area networks. ARCNET was the first widely available networking system for microcomputers; it became popular in the 1980s for office automation tasks. It w ...

ATM ATM or atm often refers to: * Atmosphere (unit) or atm, a unit of atmospheric pressure * Automated teller machine, a cash dispenser or cash machine ATM or atm may also refer to: Computing * ATM (computer), a ZX Spectrum clone developed in Mo ...

* Cisco Discovery Protocol (CDP) *

Controller Area Network A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol, designed originally for ...

(CAN) * Econet *

* Ethernet Automatic Protection Switching (EAPS) *

Fiber Distributed Data Interface 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 ...

(FDDI) *

Frame Relay Frame Relay is a standardized wide area network (WAN) technology that specifies the physical and data link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated ...

* High-Level Data Link Control (HDLC) * IEEE 802.2 (provides LLC functions to IEEE 802 MAC layers) * IEEE 802.11

wireless LAN A wireless LAN (WLAN) is a wireless computer network that links two or more devices using wireless communication to form a local area network (LAN) within a limited area such as a home, school, computer laboratory, campus, or office building ...

I²C I2C (Inter-Integrated Circuit, ), alternatively known as I2C or IIC, is a synchronous, multi-controller/multi-target (master/slave), packet switched, single-ended, serial communication bus invented in 1982 by Philips Semiconductors. It is w ...

* LattisNet * Link Layer Discovery Protocol (LLDP) *

LocalTalk LocalTalk is a particular implementation of the physical layer of the AppleTalk networking system from Apple Computer. LocalTalk specifies a system of shielded twisted pair cabling, plugged into self-terminating transceivers, running at a rate ...

MIL-STD-1553 MIL-STD-1553 is a military standard published by the United States Department of Defense that defines the mechanical, electrical, and functional characteristics of a serial data bus. It was originally designed as an avionic data bus for use w ...

Multiprotocol Label Switching Multiprotocol Label Switching (MPLS) is a routing technique in telecommunications networks that directs data from one node to the next based on labels rather than network addresses. Whereas network addresses identify endpoints the labels identi ...

(MPLS) * Nortel Discovery Protocol (NDP) * Point-to-Point Protocol (PPP) * Profibus *

SpaceWire SpaceWire is a spacecraft communication network based in part on the IEEE 1355 standard of communications. It is coordinated by the European Space Agency (ESA) in collaboration with international space agencies including NASA, JAXA, and RKA. W ...

* Serial Line Internet Protocol ( SLIP) (obsolete) * Split multi-link trunking (SMLT) * IEEE 802.1aq - Shortest Path Bridging *

* StarLan *

* TRILL (TRansparent Interconnection of Lots of Links) *

Unidirectional Link Detection Unidirectional Link Detection (UDLD) is a data link layer protocol from Cisco Systems to monitor the physical configuration of the cables and detect unidirectional links. UDLD complements the Spanning Tree Protocol which is used to eliminate swi ...

(UDLD) * UNI/O * 1-Wire * and most forms of

serial communication In telecommunication and data transmission, 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 ...

e.g. USB,

PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe or PCI-e, is a high-speed serial computer expansion bus standard, designed to replace the older PCI, PCI-X and AGP bus standards. It is the common m ...

Relation to the TCP/IP model

In the

(TCP/IP), OSI's data link layer functionality is contained within its lowest layer, the link layer. The TCP/IP link layer has the operating scope of the link a host is connected to, and only concerns itself with hardware issues to the point of obtaining hardware (MAC) addresses for locating hosts on the link and transmitting data frames onto the link. The link-layer functionality was described in RFC 1122 and is defined differently than the data link layer of OSI, and encompasses all methods that affect the local link. The TCP/IP model is not a top-down comprehensive design reference for networks. It was formulated for the purpose of illustrating the logical groups and scopes of functions needed in the design of the suite of internetworking protocols of TCP/IP, as needed for the operation of the Internet. In general, direct or strict comparisons of the OSI and TCP/IP models should be avoided, because the layering in TCP/IP is not a principal design criterion and in general, considered to be "harmful" (RFC 3439). In particular, TCP/IP does not dictate a strict hierarchical sequence of encapsulation requirements, as is attributed to OSI protocols.

References

* *

External links

{{Commons category
DataLink layer simulation, written in C#

DataLink Layer, Part 2: Error Detection and Correction
OSI model Link protocols de:OSI-Modell#Schicht 2 – Sicherungsschicht