In order to provide services to the Network Layer, the Data Link Layer must use the service provided to it by the Physical Layer. What the Physical Layer does is, accept a raw bit stream and attempt to deliver the data to the destination. The bit stream is not guaranteed to be error-free. the number of bits received may be less than or equal to or more than the number of bits that were transmitted and they have different values. It is the task of the Data Link Layer to provide the error-free transmission of the data. The usual approach is for the Data Link Layer to break the bitstream up into discrete frames and compute the checksum for each frame. When the frame arrives at the destination the checksum is recomputed. If the newly computed checksum is different from the one contained in the frame, the Data Link Layer knows that an error has occurred and takes the necessary steps to deal with it.
