The MPEG Data Stream - Digital Video and Audio Broadcasting Technology - page 41

amples of this are found, e.g. in Austria where all national studios of the
Austrian broadcasting institution ORF (Österreichischer Rundfunk) are
linked via an ATM network (called LNET). In Germany, too, MPEG
streams are exchanged over ATM links.

ATM = Asynchronous Transfer Mode

53 byte

5 byte
header

48 byte
payload

(48 1) * 4 = 188 byte
= MPEG2 transportstrom
stream packet length

188 byte MPEG2 TS packet

47 byte
payload

47 byte
payload

47 byte
payload

47 byte
payload

4 ATM cells

1 byte spec. information

5 byte header

Fig. 3.11. ATM cell

When MPEG signals are transmitted via ATM links, various transmis-
sion modes called ATM Adaptation Layers can be applied at the ATM
level. The mode shown in Fig. 3.11. corresponds to ATM Adaptation
Layer 1 without FEC (i.e. AAL1 without FEC (forward error correction)).
ATM Adaptation Layer 1 with FEC (AAL1 with FEC) or ATM Adapta-
tion Layer 5 (AAL5) are also possible. The most suitable layer appears to
be AAL1 with FEC since the contents are error-protected during the ATM
transmission in this case.
The fact that the MPEG-2 transport stream is a completely asynchro-
nous data signal is of particularly decisive significance. There is no way of
knowing what information will follow in the next time slot (= transport
stream packet). This can only be determined by means of the PID of the
transport stream packet. The actual payload data rates in the payload can
fluctuate; there may be stuffing to supplement the missing 184 bytes. This
asynchronism has great advantages with regard to future flexibility, mak-