Triple-Play Service Deployment

Chapter 6: Troubleshooting Video in the Headend
transport and the MPEG-2 transport standard for delivering
compressed video and associated audio in real-time. This packet
based MPEG-2 transport standard was revolutionary. As IP
transport was gaining momentum at this time, IP was a proven
and widely accepted means of transporting packetized data, yet it
had no means for providing needed synchronization. The MPEG-2
transport standard accomplished this with such efficiency that it
remains the global medium for delivery of digital television. Even
today, every set top box is an MPEG-2 receiver, regardless of the
type of network or the video codec used to decode the video ES
itself. Every piece of broadcast equipment that modifies the
program stream in any way today is an MPEG-2 device, creating a
new MPEG-2 transport stream at its output so as to preserve the
unique identification of program elements and the integrity of the
Program Clock Reference (PCR), the mechanism used to transfer
the encoder’s STC.
MPEG-2 Transport
MPEG-2 transport begins by taking the compressed ES and
packetizing it at one video frame per packet, as illustrated in Figure
6.4. The packet payload is the video frame and the header holds
the Presentation and Decode Time Stamps (PTS and DTS). The
audio ES is similarly packetized, although no DTS is used in the
header. Each of these packets then is divided into uniform MPEG-
2 transport packets of 188 bytes. The payload can be up to 184
bytes, the header at least 4 bytes. Within the header is a number
called the Packet Identifier (PID). PIDs are used to identify the type
of payload in each packet, allowing any downstream device to
rapidly sort the incoming packets.The process of combining these
188 byte packets to create an MPEG-2 transport stream is
performed in an MPEG-2 multiplexer.
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