Data Link layer

Layer 2: Data Link layer

This layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. Originally, this layer was intended for point-to-point and point-to-multi point media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multi access media, was developed independently of the ISO work,in the IEEE project802. IEEE work assumed sub layering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in modern data link protocols such as the point-to-point protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on Ethernet, and, on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the transport layers by protocols such as the TCP, but is still used in niches where the X.25 offers performance advantages.

Both WAN and LAN services arrange bits, from the physical layer, into logical sequences called frames. Not all physical layer bits necessarily go into frames, as some of these bits are purely intended for physical layer functions. For example, every fifth bit of the Fiber distributed data interface (FDDI) bit stream is not used by the data link layer.