What are the primary functions of the logical link control layer?

The LLC layer is responsible for:





- Multiplexing (splittling multiple messages over one stream) protocols transmitted over the MAC layer and demultiplexing them ( putting the messages back together)


- Providing flow and error control





The LLC multiplexes information by splitting it into 'frames' of data - short bursts -, sending the frames across the line, and arranging the frames back in order.





This is akin to writing notes on differently-colored pieces of paper, ripping them in pieces, and throwing all the pieces to a friend, who can then arrange the notes by color and in the correct order.





The LLC specifies the order in which frames are to be assembled with a header, a tag of information that says what to do with the information that is sent once it is received.





This is akin to adding another small piece of each colored paper into the bunch, with each of these outlining how that note is to be put together. This would speed your friend's task up greatly.





The LLC's other responsibility is to control errors. The LLC does this using a CRC, or Cyclic Redundancy Check. Each frame has a trailer that contains a few bits of information on how the information in that frame is to be put together. The receiving node compares the information in the trailer with the information that it has assembled, to see if they match up.





In our friends-passing-notes analogy, this would be like you sending pieces of paper along with the rest that show a picture of what the completed notes should look like. Your friend could compare what s/he had assembled to the picture you made, in order to verify that what s/he put together was correct.





The LLC provides only for error detection, however, not for error repair.