The data link layer as defined in the seven-layer OSI model is focused on achieving reliable
information transfer over the physical layer, (the data
modulation/demodulation process). This link layer includes the functions of error
detection and message/packet retransmission.
Two typesof link layer operation are usually defined - connectionless
and connection orientated.
Connectionless operation launches frames or packets into the network to be
transferred to the destination, but with no tracking of the frames to ensure correct
delivery, that is, if an error is detected in the frame or packet and it is discarded,
there is no means of provision for a retransmission to occur.
Connection-orientated operation, on the other hand, has all the built-in tracking
mechanisms and facilities for retransmission to make sure that guaranteed error-free
transmission occurs.
Most of the remainder of this book is concerned with the physical
layer of the OSI model the means by which binary data is best communicated over
the physical channel. Chapter 7, however, comes back to
discuss some of the error detection and retransmission issues involved in the link layer.
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