Data Link Layer

The Data Link Layer (layer 2) is the next layer in the reference model. This layer is responsible for the provision of both connection-oriented and connectionless communications among network protocols, through the transfer of data link SDUs. The connections in this layer are also facilitated by addresses called (unsurprisingly) data link addresses. These addresses provide a means for Network Layer protocols to identify each other, and to establish data link connections between themselves. In addition to setting up these connections, the data link layer also provides error notifications, sequence control (ordering of bits of data), and Quality of Service (QoS) parameters. These QoS parameters may allow a network protocol to specify certain requirements, such as the minimum throughput (speed at which data is transferred across a link) or the maximum tolerable error rate on the link. This layer can be further segmented into two sub layers—the Medium Access Control (MAC) and the Logical Link Control (LLC) sub layers. The LLC is responsible for providing addressing, flow control, error detection, and identification of which Network Layer protocol is utilizing the services at the Data Link Layer, while the MAC controls how hosts access the physical media. Devices that operate at this layer include switches and bridges. One example of a Data Link Layer protocol is the X.212 data link service protocol.