The data link layer is concerned with transmitting reliable data through the physical layer. As such, it defines such things as physical addressing (for example, the MAC address of a machine or device), network topology, line discipline, error notification, ordered delivery of frames, and flow control.
The IEEE has divided this layer into two sublayers: the MAC sublayer and the LLC sublayer.
Continuing the rail road analogy, this layer is concerned with moving cargo onto and off of the railroad cars.
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