datalink

Datalink, or the data link layer, is the second layer of the OSI model. It provides the means for reliable transfer of data between adjacent network nodes over a physical link, and it defines the structure of data that is put on the wire in the form of frames. The datalink layer is responsible for node addressing on the local network segment, error detection, and the control of how frames are placed on and taken from the physical medium.

It is typically divided into two sublayers: the logical link control (LLC) sublayer and the media access control (MAC) sublayer. The LLC provides a common interface and multiplexing for network-layer protocols, while the MAC sublayer defines how devices access the shared medium and the format of the frame's MAC addresses.

Functions include framing, addressing, and error detection (for example, a frame check sequence using CRC). The layer may also provide flow control and, in some protocols, reliability through acknowledgments and retransmission. In multi-access networks, the MAC sublayer implements access methods to regulate who can transmit and when.

Common examples of datalink protocols include Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11), and PPP/HDLC in point-to-point links. Networking switches operate at the data link layer to forward frames between ports using MAC addresses; bridges perform similar functions in segmented networks. ARP maps IP addresses to MAC addresses within a local network, illustrating the interaction with the network layer.

The datalink layer interfaces with the physical layer to transmit raw bits and with the network layer to carry packets. It assumes lower-level reliability in some networks but can rely on higher-layer protocols for end-to-end reliability.