The data link layer is the protocol layer that transfers data between adjacent network nodes in a wide area network or between nodes on the same local area network segment. 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, and may provide mechanisms to reduce or prevent them.
Delivery of frames by layer-2 devices is effected 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.
Other articles related to "link, data link layer, layer, link layer, data":
... Link, a single sausage, originally part of a chain. ...
... The uplink will typically be a port (or link aggregation group) connected to a router, firewall, server, provider network, or similar central resource ... Allowing direct data link layer communication between customer nodes would expose the local network to various security attacks, such as ARP spoofing ... Ports can be isolated from each other at the data link layer (for security, performance, or other reasons), while belonging to the same IP subnet ...
... 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 ... 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 ...
... This pattern is followed by up to 256 16-bit data frames ... Each data frame consists of a start bit of 1, an 8-bit data field, a 3-bit station address, and an even parity bit (which includes the start bit, so it ... the first frame in a message is a command byte, and following frames are associated data ...
... A data link layer technology, such as media access control (MAC) protocols on local area networks, has an address that has no significance beyond a single medium ... Besides Ethernet bridging based on MAC layer addresses, other data-link-layer technologies using forwarding tables include frame relay and Asynchronous Transfer Mode (ATM ... ATM has both link-local addresses and addresses that have end-to-end significance in the ATM domain ...
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