The Input of the Transport Stream Multiplexer and the Output of the Transport Stream Demultiplexer are the Video and Audio Packetized Elementary Streams (PES)
MPEG-2 Transport Stream:
may contain one or multiple programs (even with independent time-base); is suitable to no error-free transmision; has a fixed length packet structure.

Transport Packet (188 bytes)
Header: 4 bytes

Payload: 184 bytes

Transport-Stream Packet and ATM transmission
The packet length is the output of a compromise: on one hand longer the packet, lower the overhead; on the other hand shorter the packet, easier the streams syncronization.
The 188 bytes transport stream packet can be easily divided into 4 ATM cells. Each ATM cell is composed by 5 bytes of header + 1 byte for the Adaptation Layer and 47 bytes for the payload (=188/4). So a transport stream can be transmitted through ATM with the minimum overhead.

Possible operations on the Transport-Streams
Program selection and demultiplexing of its elementary streams.
Recomposition of one or more Transport-Streams into one or more different Transport-Streams.
Extraction of a given Program from a Transport-Stream and production of a Program-Stream.
Transformation of a Program-Stream into a Transport-Stream.