LinkSchicht

LinkSchicht, commonly known as the data link layer, is the second layer of the OSI networking model. It governs the node-to-node transfer of data across a physical link and provides mechanisms to detect and sometimes correct errors introduced by the physical medium.

Its main responsibilities include framing of data into discrete units called frames, addressing of devices on the local link using physical addresses (MAC addresses, in many networks), and error detection using techniques such as a frame check sequence. It may also manage flow control to prevent overwhelming a receiver and regulate access to shared media in networks where multiple devices contend for the same channel.

Many implementations divide the data link layer into two sublayers: the MAC (Media Access Control) sublayer and the LLC (Logical Link Control) sublayer. The MAC sublayer handles access to the physical medium and defines addressing at the link layer; the LLC sublayer provides multiplexing of higher-layer protocols and may offer error detection and flow control independent of the MAC. In practice, different standards reorganize these functions.

Examples of data link layer technologies include Ethernet (IEEE 802.3), Wi-Fi (IEEE 802.11), and point-to-point protocols such as PPP and HDLC. Bridges and switches operate at this layer, using MAC addresses to forward frames and, in the case of switches, learn and maintain a forwarding table. The data link layer often assumes responsibility for reliable delivery on some networks, while others leave reliability to higher layers.

The data link layer interfaces with the physical layer below and the network layer above. It abstracts the details of the physical medium and provides a stable interface for node-to-node communication that supports framing, addressing, and error detection. In non-OSI contexts, its functions are implemented within standards for specific technologies, and terminology can vary between protocols.