Annex 3 to AVC-707R

SOURCE:	ITU-T SG15 Experts group for video coding and systems in ATM and other network environments
TITLE:	Report of the discussion on AAL issues

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1. Introduction

In the Grimstad meeting the H.32X Network Adaptation Protocol Reference Model (NA-PRM) used in the experts group has been revised as represented in TD-3 (Singapore).

The NA-PRM presents AAL1, AAL2 and AAL5 as candidate AAL types for the Audiovisual and Multimedia Services (AMS) supported by H.32X. In the Singapore meeting the experts group addressed the candidate AAL types in relation with the NA-PRM. 

The most relevant input documents include AVC-685, AVC-706 and TD-15 (ITU-T SG9 document DT/93). AVC-685 presents the preferred solutions in ETSI: AAL1 for CBR AMS servces and a new AAL2 (to be developed) based on AAL1 for VBR AMS services. AVC-706 presents the ATM-forum choice for their phase 1 workscope: video-on-demand with MPEG2-TS in CBR over AAL5. The ITU-T SG9 document DT/93 is a draft ITU-T recommendation for transport of MPEG2 CBR television signals in B-ISDN. It recommends the use of AAL type 1 including the convergence sublayer (CS) for video signals. This CS includes the so-called long interleaver error correction.

2. Candidate solutions

With these inputs, the SG15 experts group discussed:

	1. the necessity of the reduction of cell delay variation (CDV) before delivery to
	    H.222.0 and error correction,
	2. the candidate solutions for these functions, and
	3. their allocation to the layers of the NA-PRM as in TD-3,

We concentrated on AAL1 and AAL5 based possibilities. AAL2 remains as action item.

For item 3 we refined the layer structure of TD-3: H.222.0, H.222.1 specific part and:

	in AAL1: CS and SAR sublayers  
	in AAL5: SSCS and CPCS sublayers.       (as shown in Figure 1)

H.222.0	
============================== RTI ==============================	
H.222.1 specific, for AAL1
	H.222.1 specific, AAL5
CDV reduction 	
	error correction	                       error correction (?)		
				
AAL1 CS
CDV reduction
(long interleaver FEC)	AAL5 SSCS	
AAL1 SAR     1+47 byte
SN 4bit: CSi(1) + SCF(3)
SNP 4 bit	AAL5 CPCS   N+8 byte
8 Byte trailer:
reserved(2B), length(2B), CRC(4B)	

Figure 1  Function allocation in (sub)layers of NA-PRM 

No agreement for adopting a single solution for these (sub)layers was achieved. Therefore we have defined multiple service scenarios, with the objective to identify the candidate solutions and to minimize the number of different solutions. Table 1 shows the defined service scenarios, each for two network scenarios: End-to-end ATM and Hybrid ATM/circuit (circuit emulation). Figure 1 shows the agreed possibilities for allocation of the CDV reduction and the error correction functions.

Table 1   AAL options in service and network scenarios 
	End-to-End ATM	Hybrid ATM/circuit 
(circuit emulation)	
			
Conversational			
CBR MPEG2	*	AAL1	
VBR MPEG2	**	***	
H.320 and H.32Y	AAL1	AAL1	
			
Distribution			
CBR MPEG2	ITU-T SG9 (Broadcast): AAL1 with long interleaver	****	
CBR MPEG2	ATM-Forum (VoD): 
AAL5 without SSCS		
VBR MPEG2	**	*** / ****	
			
Interact. video retrieval			
CBR MPEG2	*	****	
VBR MPEG2	**	***	

*	AAL1 and AAL5 proposed	***	VBR over circuits to be discussed	
**	To be studied (AAL2 proposed)	****	Scenario for sending H.32X terminal source signals with CBR circuit in the hybrid transmission link.	

After discussion on the options for AAL1 and AAL5 we had the following conclusions:

   1 -	Cell loss recovery with short or long interleaver would not be required by the listed service scenarios except for the distribution scenarios as addressed by SG9.
   	The main reasons for this view were that the mean time between cell loss events in the expected network scenarios is considered to be large enough for the listed services (AVC-635, 10-6 worst case, 10-9 average, 10-11 best case) and that the impact of cell loss to the H.222.0 and higher layers is considered to be acceptable for the listed services. Contributions are invited for verification of these statements.

   2 -	In most cases there is a need for bit error correction capabilities. It was agreed that this capability should be part of the H.222.1 layer as it is an H.32X specific function (mandatory for encoders, optional for decoders).

   3 -	We will have a relatively simple low delay forward bit error correction mechanism. One possibility was to consider the BCH (511,493) code of H.261. An advantage of using the H.261 based FEC method is that it will facilitate interworking with existing H.320 and H.32Y terminals.  Other FEC options have been suggested, including a mechanism which will allow alignment with the ATM cell payload. Further contributions are invited.  

In Figure 1 the long interleaver FEC for AAL1 is shown between brackets. For the general H.32X terminal we prefer to have just bit error correction in the H.222.1 specific layer. It has been suggested that even this may be not needed if the transmission channel performs very well. This needs further consideration. Cell loss detection is provided by the AAL1-CS, detection of bit errors and cell losses in the AAL5 CPCS payload is provided by the CRC of AAL5.  

3. Reduction of CDV and the H.222.0 RTI

Reduction of the cell delay variation in the AAL1 can be included, to bring the jitter in the byte delivery times with respect to the intended arrival times within the range of the real time interface (RTI) of H.222.0 as shown in Figure 1. In this case no CDV reduction is required in the H.222.1 specific part. 

In the AAL5 scenario the CDV reduction will not be part of the AAL, but it will be incorporated in the H.222 layer (either H.222.1 specific or H.222.0 across RTI).

Concerning the question whether to specify the expected CDV across the RTI, the meeting expressed a preference to extend the jitter tolerance of the RTI interface to a value that would eliminate the need for the CDV reduction function in the AAL or H.222.1 layers in the expected ATM network configurations. 

The target value mentioned in the group was 1ms. This value is based on CDV studies as reported in AVC-635. Concern was expressed within the group that this value might not be sufficient for the currently unknown network configurations. An ATM pilot network configuration available today was reported to show 5 ms of jitter. CDV reduction below the RTI might therefore still be needed, but could be simpler with a higher jitter tolerance in the RTI interface. The conclusion was to keep 1 ms as target value.

4. Convergence of the AAL options to H.32X NA-PRM profiles

The candidate solutions for the NA-PRM as presented in Figure 1 don't show a major functional difference. The major arguments to include an AAL1 based solution in H.32X are the interworking capabilities for transmission over CBR circuits in a hybrid ATM + CBR circuit connection and in particular the interworking with H.320 terminals connected via N-ISDN. The arguments in favour of an AAL5 based solution are the simplicity of an AAL5 without SSCS and its early availability in hardware. 

As different terminal capabilities will already exist in H.222.1, the group adopted a profile approach for the NA-PRM in H.32X. The purpose of definition of a limited number of profiles is to provide a limited number of well defined interworking capability sets for the NA-PRM. A single main profile and two simple profiles cover the current NA-PRM proposals for CBR. 

The proposed H.32X main profile NA-PRM is a fully bi-directional terminal with both AAL1 and AAL5 based network adaptation as shown in Figure 2. This corresponds to terminal types A2 / B2. In case VBR also an AAL2 may become part of the main profile NA-PRM, resulting in a NA-PRM as in TD-3. The simple profiles are unidirectional for the audiovisual signals, receiving only, terminal type A1. Figures 3 and 4 show the simple profile 1 and the simple profile 5, based on AAL1 and AAL5 respectively.

Please note that Figures 2, 3 and 4 focus on the audio and video signals and on the AAL types. The arrows show the flow direction of the audiovisual information. The functions inside the layers are as in section 2 and 3, Figure 1.

Figure 2   H.32X main profile NA-PRM (terminal type A2/B2)  

Figure 3   H.32X simple profile 1 NA-PRM (terminal type A1)  

Figure 4   H.32X simple profile 5 NA-PRM (terminal type A1)  

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