&%PAGE& - &%page& - Annex 5 to Doc. AVC-177R Annex 5 to Doc. AVC-177R ADDENDUM TO LIAISON STATEMENT FROM SGXV EXPERTS GROUP TO SGXVIII Source : Experts Group Purpose: Action ---------------- 1. Introduction At its fourth meeting in Yokosuka, Japan, November 1991, the following issues were discussed by the CCITT SGXV Experts Group for ATM Video Coding, and recognized as items about which SGXVIII could provide information. This addendum to the liaison statement prepared at the third meeting in Santa Clara, August 1991, seeks response from SGXVIII on the following issues. 2. UPC and NPC The choice of algorithm used to perform UPC and NPC functions has an impact on the video coding scheme, since compliance will affect the rate control strategy, particularly for VBR video. Standardization of the UPC/NPC algorithm(s) is essential to ensure that terminal equipment can be used on any network and that inter-network (e.g, international) connections can be established. The Experts Group wishes to advise SGXVIII of the impact that the UPC/NPC algorithm may have on the video coder, and seeks information on the program of standardization efforts in this area. 3. Clock recovery While simple synchronization strategies such as frame repeat or frame dropping are possible for lower quality video services, such methods are not acceptable if high quality is to be maintained. Therefore, many video services will require a means of synchronizing encoder and decoder video clocks. While the availability of a universal, synchronous, network based reference clock could provide a solution, the Experts Group has a concern that this may not always be available, particularly if network access is via a LAN and PBX. In this latter case, an explicit means of providing clock synchronization is required. The Experts Group notes, form the report of the SGXVIII Rapporteurs Meeting on AAL (Ottawa, 1-3 October 1991), that this is an area being studied by CCITT SGXVIII. Both SFET and Time Stamping techniques are discussed in that report. We believe that Time Stamping is more appropriate for VBR services than is SFET, and have previously expressed our strong desire for commonality of AAL for the support of VBR and CBR video. We request that SGXVIII take this into account during its work in this area, and request early advice of progress. 4. Quality of Service guarantees The Experts Group understands that, as yet, no agreement has been reached within SGXVIII on whether a guaranteed QoS will be offered on Low Priority channels. As the QoS (CLR, delay) has a major impact on the video coding algorithms appropriate for video service support, the Experts Group urges SGXVIII to give urgent attention to this issue and request early advice of progress. The Experts Group also perceives a risk if no values of QoS are standardized. If the same QoS cannot be provided on different networks or in different regions, this would compromise the viability of large terminal markets and communications spanning different regions. 5. ATM Header availability at receiving terminal In discussion within the Experts Group and elsewhere, it has been suggested that certain bits within the ATM header (in particular, CLP and PT), if available to a receiving terminal, could provide useful information regarding the cell content and avoid the need for duplicate labeling at a higher layer. The viability of using this technique is dependent on the ATM header being passed up from the network to the receiving terminal device. Can SGXVIII advise whether this will always be possible? 6. Burstiness characteristics of cell loss The length of a cell loss burst has a significant impact on the means of error protection/correction in a video coding system, and could even influence the basic coding architecture. While we appreciate the SGXVIII's information on this matter provided by WPXVIII/6 in November 1990, a quantitative description of the expected cell loss burst behavior is desired at the earliest opportunity. The Experts Group notes, from the SGXVIII Rapporteurs Meeting on AAL (Ottawa, 1-3 October 1991), that a modulo 8 sequence number is considered adequate to detect error in a CBR, AAL Type 1 application, would this also be appropriate for a VBR video transmission using AAL Type 2? Any information that would give a quantitative description of cell loss burst behavior would be welcome. Are any measures of cell loss burst length likely to be included in the QoS parameters agreed between user and network? 7. Multimedia multiplexing Some advantages of multiplexing the components of a multimedia call on different VCs (i.e. cell-based multiplexing) have been recognized by the Experts Group. To use this approach, however, requires determination of the media type by the receiving terminal at the time the connection/call is established. What is the mechanism by which this media identification is envisioned to occur? 8. Conclusion In this addendum to the liaison statement to SGXVIII prepared at the August meeting in Santa Clara, the SGXV Experts Group for ATM Video Coding has provided information on the items it sees as important in progressing its video coding standardization activity. Consideration of, and response to the issues raised above are requested. END