Annex 8 to AVC-496R
Comparison of MPEG-2 Systems and AAL/ATM
1.	Introduction
MPEG 2 systems will specify syntax for a program stream and a transport stream.  Figure 1 shows the relationship between these two streams.  Figure 1 illustrates that a service, or program, may consist of multiple elementary streams i.e multiple video, audio, and data streams.  

Figure 1.  Relationship between MPEG 2 system program stream and transport stream.
The program stream is for error free transmission mediums.  It has similar functionality and syntax to the MPEG 1 system.  The program stream carries elementary streams for one program only.  Program stream description tables hold parameters relating to each elementary stream.  A program stream directory may also be defined.
The transport stream is designed for use on lossy transmissions mediums.  The transport stream consists of packets with payloads which are relatively short, compared to an MPEG 1 system packet.  Elementary streams belonging to one program, or service, are identified by a mulitplex map, which lists programs and the elementary streams that make up each program.
The terminology of Figure 1 is used here for explanation and is not necessarily that used by the MPEG systems group.  In addition the transport stream model shown in Figure 1 is not yet definite.  Alternatives include a one layer structure in which all functions are carried out at the transport layer.
2.	Transport Stream
A transport packet consists of a fixed length packet prefix (header) and a payload.  The packet length is fixed on one data link, but may be different on different links.  The first part of the payload in some packets may be a variable length adaptation field.
Figure 2 shows the structure of the MPEG 2 transport packet as currently defined.

Figure 2.  Current structure of MPEG 2 transport packet
The functions of the transport layer as currently defined include
packet synchronization - facilitated by a synchronization byte in the packet prefix
packet loss detection - uses a 4 bit packet continuity count
multiplexing - the packet prefix contains a packet identification field.  A packet identification field value is uniquely associated with one elementary stream only
priority indication - some bits are available in the packet prefix for explicit packet priority indication
timing - the program clock reference (PCR) field is contained in the application field.  PCR provides the same function as the system clock reference (SCR) in MPEG 1
error indication  - a field is provided to convey packet error indication to the decoder
Figure 3 illustrates a general requirement of a transport layer multiplexer.  Transport packets are of fixed length on a physical link, but may be different on different links.  This allows optimisation of packet length for a particular physical medium.

Figure 3.  General requirement of a transport layer multiplexer.
The transport layer PCR field, or equivalently a PCR delta field, may require modification at each multiplexing and demultiplexing point, even where packet lengths do not change.  It may be possible to deal with small changes in packet position i.e delay jitter, through use of a phase locked loop and a packet buffer at the decoder.
3.	Packetised Elementary Layer
The packetised elementary layer resembles the MPEG 1 packet layer.  The MPEG 1 packet layer performs the following functions 
multiplexing - supported by the stream_id field
stream synchronization - supported by presentation time stamps (PTS) and decoder time stamps (DTS)
decoder buffer signalling - buffer size requirements are transmitted at this level
In the MPEG 2 transport model no multiplexing is performed at the packetised elementary layer.  All multiplexing is done at the transport layer.  This supports simple drop and insert capabilities in the transport stream.  Currently the packetised elementary stream does not contain pack headers i.e. the packetised elementary stream does not support system clock reference (SCR).  The equivalent field is the transport layer PCR field.
The packetised elementary stream may or may not be synchronized with the transport layer packets.
4.	MPEG 2 system transport model and B-ISDN protocol model relationship
The MPEG 2 transport layer is a packet multiplex that may be viewed as possessing similar functionality to the B-ISDN ATM layer.  The transport layer also contains a continuity counter, a function that in B-ISDN is typically included in the Segmentation and Reassembly (SAR) sublayer.  Figure 4 shows approximate equivalence between the B-ISDN Protocol Reference Model (PRM) and an implied MPEG 2 system transport model.  The relationship between the lower layers of the two models is not clear.  These boundaries are shown shaded.

Figure 4.  Approximate relationship between B-ISDN PRM and implied MPEG 2 system transport model.
5.	MPEG 2 system and B-ISDN interaction
A number of scenarios are possible as to the interaction of MPEG 2 systems and B-ISDN.  Possible ones are 
MPEG 2 on B-ISDN - figure 5 illustrates protocol models for connection between two B-ISDN terminals using MPEG video and audio elementary streams.  In this case only that functionality not provided by an AAL needs to be provided by MPEG 2 systems (min_sys).  This could be provided by the MPEG 2 program stream, with one elementary stream per program stream, using a common clock between program streams, or if appropriate, by the packetised elementary stream.
MPEG 2 transport stream connected to B-ISDN for service delivery to B-ISDN terminal - in this case the MPEG 2 transport stream may need to be terminated at an interworking unit, such that one of the reference points described above is available.
MPEG 2 network interconnection using B-ISDN - in this case the MPEG 2 transport or program stream might be transported transparently by the B-ISDN.  Figure 6 illustrates this for the case of the transport stream.  Minimal SAR functionality of error detection might be required.

Figure 5.  Possible use of MPEG 2 systems for connection between B-ISDN terminals.

Figure 6.  Possible MPEG 2 network transport stream interconnection using B-ISDN.
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