| The main task of the data link layer
is to take a raw transmission facility and transform it into a line that
appears free oftransmission errors in the network layer. It accomplishes
this task by having the sender break the input data up into data frames(typically
a few hundred bytes), transmit the frames sequentially, and process the
acknowledgment frames sent back by thereceiver. Since the physical layer
merely accepts and transmits a stream of bits without any regard to meaning
of structure, it is up to the data link layer to create and recognize frame
boundaries. This can be accomplished by attaching special bit patterns
to the beginning and end of the frame. If there is a chance that these
bit patterns might occur in the data, special care must be taken to avoid
confusion. |
