Data communication protocols and how they work remain a mystery to many students and engineers. The sources of information are either books, which necessarily allow only limited interaction, and courses, which are usually expensive, and run only when sufficient interest is shown by customers. This paper describes PAT, a computer aided learning package which has been developed as a tool to help students and novice data communications engineers learn the basic concepts of data link protocols. The tool enables users to construct the frames of the HDLC standard data link level protocol. The frames can be combined in sequence to form a call, representing the execution of the protocol. At all stages, checks of both the syntax and semantics of the call are made.