A frame typically resides at Layer 2 , the data link layer, of the OSI (Open System Interconnection) seven layer model. The frame relates to the header and trailer that surround the higher layer PDU (Protocol Data Unit). Typical frames include HDLC (High Level Data Link Control) in which the frames are delineated by flags which consist of zero followed by six ones and then a zero. No other bit sequence in the flow will have this combination. To avoid this combination at any other time bit stuffing is used.