Transport Streams are the combining (multiplexing) of multiple program channels (typically digital video channels) onto a signal communication channel (such as a satellite transponder channel). A MPEG transport stream (MPEG-TS) is also called a multi-program transport stream (MPTS).
MPEG transport streams (MPEG-TS) use a fixed length packet size and a packet identifier identifies each transport packet within the transport stream. A packet identifier in an MPEG system identifies the packetized elementary streams (PES) of a program channel. A program (such as a television show) is usually composed of multiple PES channels (e.g. video and audio).
Because MPEG-TS carry multiple programs, to identify the programs carried on a MPEG-TS, a program allocation table and program mapping table is periodically transmitted which provides a list of the programs contained within the MPEG-TS. These program tables provide a list of programs and their associated PIDs for specific programs which allows the MPEG receiver/decoder to select and decode the correct packets for that specific program.
MPEG transport packets are a fixed size of 188 bytes with a 4 byte header. The payload portion of the MPEG-TS packet is 184 bytes. The beginning of a transport packet includes a synchronization byte that allows the receiver to determine the exact start time of the packet. This is followed by an error indication (EI) bit that identifies there was an error in a previous transmission process. A payload unit start indicator (PUSI) flag alerts the receiver if the packet contains the beginning (start) of a new PES. The transport priority indicator identifies if the packet has low or high priority. The 13 bit packet identifier (PID) is used to define which PES is contained in the packet. The scrambling control flag identifies if the data is encrypted. An adaptation field control defines if an adaptation field is used in the payload of the transport packet and a continuity counter maintains a count index between sequential packets.
This figure shows an MPEG transport stream and a transport packet structure. This diagram shows that an MPEG-TS packet is fixed size of 188 bytes including a 4-byte header. The header contains various fields including an initial synchronization (time alignment) field, flow control bits, packet identifier (which PES stream is contained in the payload) and additional format and flow control bits.