Data Link Layer
In the OSI (Open Systems Interconnection) model, the data link layer lies in the second layer and in the TCP / IP (Transmission Control Protocol / Internet Protocol) reference model, it corresponds to the link layer.
The data link layer is a protocol layer which transfers data between the adjacent network nodes of the wide area network or between nodes that lie on the same local area network segment. The data link layer provides the procedural and functional means of transferring data between the network entities. It might also provide the means of detecting and then possibly correcting the errors that may happen in the physical layer. Some of the examples of the data link protocols are Ethernet for local area networks (multi – node), the point to point protocol (PPP), High Level Data link Control (HDLC) and Advanced Data Communication Control Protocol (ADCCP) for point – to – point (dual – node) connections.
The data link layer is actually concerned with the local delivery of the frames between the devices that lie on the same network i.e., Local Area Network (LAN). These protocol data units are called Data – link frames and they do not or can be said as they cannot cross the boundaries of any local network. The Inter – network routing and global addressing are the higher layer functions, and thus allows the data – link protocols to focus on the local delivery of the messages, the addressing, and the media arbitration. In this way, the data link layer is almost similar to the neighborhood traffic cop. It also endeavors the data to arbitrate between the parties which are contending for their access to a medium, without the concern for their ultimate destruction.
When the devices attempt to use a medium simultaneously, the frame collisions occur. The Data – link protocols specifies how the devices detect and recover from such collisions, and thus may provide mechanisms to reduce or prevent them from such situations. The delivery of the frames by the second layer devices is effected through the use of some unambiguous hardware addresses. A frame’s header contains the source and destination addresses which indicates those devices that get originated from the frames and those devices which are expected to receive and process those. In contrast to the hierarchical and routable addresses of the network layer, the second layer addresses are flat, which means that no part of the address can be used to identify the logical or physical group to which the address belongs.
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