| The main task of the data link layer is to take
a raw transmission facility and transform it into a line that
appears free of transmission 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 the receiver. 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.
The data link layer should provide error control between
adjacent nodes.
Another issue that arises in the data link layer (and most
of the higher layers as well) is how to keep a fast
transmitter from drowning a slow receiver in data. Some
traffic regulation mechanism must be employed in order to let
the transmitter know how much buffer space the receiver has at
the moment. Frequently, flow regulation and error handling are
integrated, for convenience.
If the line can be used to transmit data in both
directions, this introduces a new complication that the data
link layer software must deal with. The problem is that the
acknowledgment frames for A to B traffic compete for the use
of the line with data frames for the B to A traffic.
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