1. Packaging framing (added before and after each piece of data header and trailer, and constitutes a frame, the frame is determined limits)
2. transparent transmission (one character "ESC", met "EOT" or "SOH" in the data control characters inserted "ESC" escape character control character, and the receiver in the data link layer data delete this insert escape characters before being sent to the network layer)
3. The error detection (Cyclic Redundancy Check)