| OSI Model | |
|---|---|
| 7 | Application Layer |
| 6 | Presentation Layer |
| 5 | Session Layer |
| 4 | Transport Layer |
| 3 | Network Layer |
| 2 | Data Link Layer
|
| 1 | Physical 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.
The Data Link Layer is the protocol layer which transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment[1]. The Data Link Layer provides the functional and procedural means to transfer data between network entities and might provide the means to detect and possibly correct errors that may occur in the Physical Layer. Examples of data link protocols are Ethernet for local area networks (multi-node), the Point-to-Point Protocol (PPP), HDLC and ADCCP for point-to-point (dual-node) connections.
The Data Link Layer is concerned with local delivery of frames between devices on the same LAN. Data Link frames, as these protocol data units are called, do not cross the boundaries of a local 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.
When devices attempt to use a medium simultaneously, frame collisions occur. Data Link protocols specify how devices detect and recover from such collisions, but it does not prevent them from happening.
Delivery of frames by layer 2 devices is affected through the use of unambiguous hardware addresses. A frame's header contains 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.
The data link thus provides data transfer across the physical link. 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 have any form of checksum to check for transmission errors. In those cases, higher-level protocols must provide flow control, error checking, and acknowledgments and retransmission.
In some networks, such as IEEE 802 local area networks, the Data Link Layer is described in more detail with Media Access Control (MAC) and Logical Link Control (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, IEEE 802.11, etc., as well as with some non-802 MAC layers such as FDDI. Other Data Link Layer protocols, such as HDLC, 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 G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) Local area network 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 Medium Access Control).
Within the semantics of the OSI network architecture, the Data Link Layer protocols respond to service requests from the Network Layer and they perform their function by issuing service requests to the Physical Layer.
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The uppermost sublayer is Logical Link Control (LLC). This sublayer multiplexes protocols running atop 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.
The sublayer below it is Media Access Control (MAC). Sometimes this refers to the sublayer that determines who is allowed to access the media at any one time (usually CSMA/CD). Other times it refers to a frame structure with MAC addresses 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, we look for clues from our fellow talkers to see if any of them appear to be about to speak. If two people speak at the same time, they will back off and begin a long and elaborate game of saying "no, you first".
The Media Access Control sublayer also determines where one frame of data ends and the next one starts -- frame synchronization. There are four means of frame synchronization: time based, character counting, byte stuffing and bit stuffing.
The Data Link Layer is often implemented in software as a "network card driver". The operating system will have a defined software interface between the data link and the network transport stack above. This interface is not a layer itself, but rather a definition for interfacing between layers.
In the frame work of the TCP/IP (Internet Protocol Suite) model, OSI's Data Link Layer, in addition to other components, is contained in TCP/IP's lowest layer, the Link Layer. The Internet Protocol's Link Layer only concerns itself with hardware issues to the point of obtaining hardware addresses for locating hosts on a physical network link and transmitting data frames onto the link. Thus, the Link Layer is broader in scope and encompasses all methods that affect the local link, which is the group of connections that are limited in scope to other nodes on the local access network.
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.