NOTE-This Recommendation was originally written to describe the general aspects of the protocol and procedures specified in Recommendation Q.921 [1]. With the introduction of Recommendation Q.922 [2], it is necessary for Q.920 to evolve to a general introduction to the data link layer protocols and procedures specified in the Q.920-Series of Recommendations. It is for further study to place into proper perspective those aspects of Q.920 which apply uniquely to Q.921 in the current text.