It is a 13-bit code in the transport packet header. PID 0 indicates that the packet contains a PAT PID. PID 1 indicates a packet that contains CAT. The PID 8191 (all 1s) indicates null (stuffing) packets. All packets belonging to the same elementary stream have the same PID. MPEG-2 transmits transport stream data in packets of 188 bytes. At the start of each packet is a packet identifier (PID) that tells the receiver what to do with the packet. Since the MPEG-2 data stream might be in MCPC mode, the receiver has to decide which packets are part of the current channel being watched and pass them onto the video decoder for further processing. The packets that aren't part of the current channel are simply discarded. There are typically four types of PIDs used by satellite receivers. The VPID is for the video stream and the APID is for the audio stream. Usually this data is embedded into the video stream, though occasionally a PCR (program clock reference) PID is used to synchronize the video and audio packets. The fourth PID is used for data such as the program guide and information about other frequencies that make up the total package.