The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 40

tor. This packet must now no longer be decoded by the MPEG decoder
which, instead, has to mask the error which, in most cases, can be seen as a
type of “blocking“ in the picture.
It may be necessary occasionally to transmit more than 4 bytes of header
per transport stream packet. The header is extended into the payload field
in this case. The payload part becomes correspondingly shorter but the to-
tal packet length remains a constant 188 bytes. This extended header is
called an “adaptation field” (Fig. 3.10.). The other contents of the header
and of the adaptation field will be discussed later. “Adaptation control
bits” in the 4 byte-long header show if there is an adaptation field or not.

188 byte

4 byte
header

184 byte
payload

Optional
adaptation
field

Header

Optional
fields
dep. on
flags

Adaption
field
length

Discontinuity
indicator

... ... ...

5 flags PCR

8 1 5 4 2 b i t

Fig. 3.10. Adaptation field

The structure and especially the length of a transport stream packet are
very similar to a type of data transmission known from telephony and
LAN technology, namely the “asynchronous transfer mode” or ATM in
short. Today, ATM is used both in long-haul networks for telephony and
Internet calls and for interconnecting computers in a LAN network in
buildings. ATM also has a packet structure. The length of one ATM cell is
53 bytes containing 5 bytes of header and 48 bytes of payload. Right at the
beginning of MPEG-2 it was considered to transmit MPEG-2 data signals
via ATM links. Hence the length of an MPEG-2 transport stream packet.
Taking into consideration one special byte in the payload part of an ATM
cell, this leaves 47 bytes of payload data. It is then possible to transmit 188
bytes of useful information by means of 4 ATM cells, corresponding ex-
actly to the length of one MPEG-2 transport stream packet. And indeed,
MPEG-2 transmissions over ATM links are nowadays a fact of life. Ex-