ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR   Document AVC-848

Study Group 15                                   October 1995

Experts Group for Video Coding and Systems
in ATM and other Environments

(Rapporteur's Group on part of Q.2/15)

                     Study Group 15 - CONTRIBUTION

Question: 2/15

SOURCE:   IBM

TITLE:    ATM Forum AMS Status

Purpose:  Information

ABSTRACT:  At the October 1 - 6th  ATM Forum meeting, the Audiovisual
and Multimedia Services (AMS) group (Service Aspects and Applications
(SAA) SWG) focused on preparing the AMS Video-on-Demand Specification
for final approval by the general membership of the ATM Forum.  The big
issue was the reopened MPEG-2 AAL5 packing issue.  After two days of
debate and 22 contributions, the group voted (43 YES, 22 NO, and 4
ABSTAIN) to change the MPEG-2 Encapsulation scheme from PCR aware to a
1 to N fixed packing scheme

After a section by section review of the document to resolve all of the
major and minor comments, the AMS Group voted (28 FOR, 1 AGAINST, 1
ABSTAIN) to forward the AMS Video-on-Demand. Specification to the
general ATM Forum membership for final vote.  The final document is
currently out of letter ballot.

Attached is the revised text for Section 5 describing the new 1-N fixed
packing scheme.






REVISED 1-N FIXED PACKING SCHEME.
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The following is the text from Section 5 as modified and adopted in the
October, 1995 ATM Forum SAA Meeting:

5. Network Adaptation  [Normative]
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All equipment conformant with this AMS Implementation Agreement shall
support the following network adaptation.

The MPEG-2 Single Program Transport Stream (SPTS) packets shall be
mapped into the ATM Adaptation Layer Type 5 (AAL5) with a NULL Service
Specific Convergence Sublayer.  In the mapping, one to N MPEG-2
Transport Streams (TS) packets are mapped into an AAL5-SDU.

For Switched Virtual Circuits (SVCs), the value of N is established via
ATM Signaling 4.0 at call setup using the AAL5 Maximum CPCS-SDU
negotiation procedure.  The AAL5 Maximum CPCS-SDU size that is signaled
is N*188 bytes (N being the number of TS packets).  This procedure is
defined in the ATM Forum Signaling 4.0 specification.  N used to form
the AAL5-SDUs shall be the Maximum CPCS-SDU Size / 188.

For Permanent Virtual Circuits (PVCs), the default value of N is two
(Maximum CPCS-SDU size = 376 bytes).  Other values of N may be selected
by bilateral agreement between the settop user and the server via
network provisioning. Furthermore, in order to insure a base level of
interoperability, all equipment shall support N = 2 (CPCSSDU size = 376
bytes).

In summary, the mapping shall be:

1.Each AAL5-SDU shall contain (the negotiated) N MPEG-2 SPTS
  packets, unless there are fewer than N packets left in the SPTS.  In
  the case when there are fewer than N packets left in the SPTS, the
  final CPCS-SDU contains all of the remaining packets.
2.The value of N is established via ATM signaling using N = the AAL5
  CPCS-SDU size divided by 188.  The default AAL5 CPCS-SDU size is 376
  octets, which is two TS packets (N = 2).
3.In order to ensure a base level of interoperability, all equipment
  shall support the value N=2 (AAL5 CPCS-SDU size of 376 octets).


5.1  Base level of N=2  [Normative]
----------------------------------

When N=2, the Network Adaptation shall be as follows:

ú ATM Adaptation Layer Type 5 (AAL5) with a NULL Service Specific
  Convergence Sublayer shall be used.
ú An AAL5 PDU shall contain two TS Packets unless it contains the
  last TS Packet of the SPTS.
ú An AAL5 PDU shall contain one MPEG2 SPTS Packet if that MPEG-2 TS
  Packet is the last TS Packet of the SPTS.
ú When an AAL5 PDU contains two SPTS Packets, which have length 188
  octets, the AAL5 CPCS-SDU has length 376 octets. This AAL5 CPCS-SDU,
  together with the CPCS-PDU Trailer of 8 octets, requires 384 octets and
  maps into 8 ATM cells with zero CPCS padding octets.  This is
  illustrated Figure 1.
                           See AMS VOD Specification for figure]

Figure 1 Format of AAL-5 PDU Containing 2 TS Packets


5.2 AAL-5 Action on Corrupted PDUs  [Normative]
------------------------------------------------

When a receiver receives a corrupted AAL5 CPCS-PDU that has a correct
length field, system performance may be improved by passing the
corrupted data, together with an indication that it is corrupted, from
the adaptation layer to the demultiplexer layer, rather than simply
discarding the data in the adaptation layer.  This is an end station
implementation option.