Annex 7 to AVC-743R

Network adaptation for the broadband audiovisual communication

SOURCE:	ITU-T SG15 Experts group for video coding 
		and systems in ATM and other environments
TITLE:	Network adaptation for the broadband audiovisual communication
		systems and terminals
PURPOSE:	Information

Annex 1:	Report of the discussion on AAL issues in November 1994 (Annex 3 to AVC-707R)
Annex 2:	Complete network adaptation protocol reference model for H.32X (Annex 9 to AVC-743R) 
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1. Introduction

In its January meeting, the ITU-T SG15 AVC experts group reconsidered the H.32X Network Adaptation Protocol Reference Model (NA-PRM) with respect to the AAL and H.222.1 functionalities. Inputs for the discussion were the report of the discussion on AAL issues from the Experts group meeting in November 1994 and the input documents AVC-741 and AVC-742. 

AVC-741 was the outcome of the ETSI NA5 SAM group discussion on AAL2 and presented a proposal for a generic AAL for VBR services. AVC-742 proposed similar functionalities for inclusion in the H.222.1 specific layer of  the NA-PRM.
The result of the discussion is presented here.

2. Allocation of the functions to the layers

Figure 1 presents the agreed function allocation in the NA-PRM model.
This is a simplified model for the purpose of discussion that doesn't show signalling (the complete version is in Annex 2). It focusses on the AAL and H.222.1 specific layer functionalities.

The physical layer is considered to be a high quality transmission channel.  This is usually achieved by inclusion of channel specific error correction for the physical transmission medium. The bit error rate available to the ATM layer is assumed to be BER < 10-7 (AVC-635: ATM performance assumptions). Although these BER figures may be sufficient for many services, we consider them as too high for high bit rate audiovisual services. Table 1 shows an indication of the mean time between errors at BER = 10-7 and BER = 10-8.

Table 1
	BER = 10-7	BER = 10-8	
Service bit rate [Mbit/s]	1	5	10	20	1	5	10	20	
Mean time between errors [s]	10	2	1	0.5	100	20	10	5	

We therefore need a service specific forward error correction for bit errors. The lowest layer in the model where it can be placed is either the AAL-CS or the H.222.1 specific layer. As the AAL-SAR and AAL-CPCS should be generic, we concluded that the H.222.1 specific layer is the appropriate one. It should be noticed that the namegiving of this layer is arbitrary: it may be indicated as H.222.1 specific layer (H.222.1 SL) or  the Service Specific part of an AAL-CS (AAL-SSCS).  In this document we will further indicate it as H.222.1 SL/SSCS.

a) Definition of H.222.1 / common AAL1-5 SSCS

+------+--------+----------+---------------------------------+-----+
|  RS  | Option | Optional |  MPEG PS or TS bytes            | RS  |
| Sync | flags  | fields   |                                 | FEC | 
+------+--------+----------+---------------------------------+-----+
|<----1 byte--->|<--0..5-->|                                 |<-4->|
|<-------------------------------N bytes-------------------------->|

b) Details of the FEC frame

Figure 1

The FEC and FEC framing are conditional: they are mandatory in the case of transmit and receive terminals (conversational), H.32x terminal types A2 and B2. For receive only terminals A1 and B1 and interworking mode between A2/B2 and A1/B1 further study is needed.

We concluded that in our model also the compensation for cell delay variation (CDV) is appropriate in the H.222.1 SL/SSCS. The purpose of the CDV compensation function in the receiving terminal is to deliver the user information - MPEG transports stream (TS) or program stream (PS) - to the MPEG/H.222.0 decoder, conforming to the MPEG/H.222.0 real time interface (RTI) specification. In the constant bit rate (CBR) case, this may be achieved by an adaptive clock method once the bit rate of delivery is known. 

For MPEG/H.222.0 we also considered the case of piecewise continuous bit rate (PCBR), as an MPEG specific case for VBR. In this case an indication is needed for the intended delivery times of the user bytes to the RTI. Although the intended delivery schedule is in principle provided by the MPEG/H.222.0 stream itself, an independent delivery function in the H.222.1 SL/SSCS layer could be advantageous. This could be supported by optional time stamp and delivery bit rate fields in the H.222.1 SL/SSCS FEC frames.  To avoid possible long framing delays in the PCBR case, partially filled FEC frames may be transmitted, with use of the lenth indication field.
  
It should be realized that implementers may consider the combination of the CDV compensation function in the H.222.1 SL/SSCS layer and the clock recovery in MPEG/H.222.0 layer. 

3. Protocol data unit in the H.222.1 specific or AAL-SSCS layer

The agreed proposal for the protocol data unit (PDU) in terminal types A2 and B2 is a Reed-Solomon (RS) FEC frame. Its length and number of FEC redundancy bytes have not been fully agreed yet. The advantage of the byte oriented RS code is that it is capable to correct random bit errors and short bursts of bit errors.

Document AVC-742 proposes a FEC frame length of 255 byte, with 1 byte for RS sync + options flags and 4 RS FEC redundancy bytes. This allows the correction of any two errorred bytes in the block of 255 with a FEC overhead of only 1.96 % (= (1+4)/255 ).
Considering the hardware availability we identified no advantage in a FEC frame length longer than 255 byte. 

The FEC frame has the following structure: 

+------+--------+----------+---------------------------------+-----+
|  RS  | Option | Optional |  MPEG PS or TS bytes            | RS  |
| Sync | flags  | fields   |                                 | FEC | 
+------+--------+----------+---------------------------------+-----+
|<----1 byte--->|<--0..5-->|                                 |<-4->|
|<-------------------------------N bytes-------------------------->|

Presence of the optional fields is indicated by the option flags. An illustration of possible optional fields for the support of piecewise continuous bit rate delivery is presented in AVC-742.

The experts group considered the desirability of alignment of the FEC frame in the AAL SAR-PDU. The group concluded that such alignment would be desirable to allow for easier implementation of the FEC frame synchronization, but it is not essential, since synchronization is primarily supported by the RS sync word. There was no proposal for common and useful value of N that would align well in both AAL1 and AAL5 SAR PDU's. 

4. The AAL SAR and CPCS layers

The proposed FEC framing in H.222.1 / AAL-SSCS is intended to be generic for both AAL type 1 and 5 SAR and CPCS. Both of them are suitable for CBR, PCBR. Their suitability for VBR in general is for further study.

The fundamental differences between the two AAL types are:

-The AAL1 SAR supports detection of cell losses by its cell sequence counter. The generic common part CS function needed is a cell loss detection/insertion function operating on the received sequence count values. This allows to specify the exact position and number of missing bytes when cell loss is detected and for padding of the correct number of dummy bytes at this position.

-The AAL5 supports error detection in its PDU by its CRC field. In case of errors the complete PDU becomes invalid, without indication of the number of bytes that are lost or invalid. Use of a fixed PDU size would assist in padding the correct number of bytes if a PDU is invalid.

-The AAL1 SAR-PDU is fixed to a single ATM cell payload: 47 + 1

-The AAL5 SAR-PDU can be variable in length. This length will in general cover several to many ATM cells. If the CRC check is done in the receiving terminal, delivery of the PDU to the higher layer can only take place after reception of the CPCS trailer and the CRC processing. This implies a higher delay than in the AAL1.

Several views were expressed in the meeting concerning the hardware complexity and availability of AAL1 and AAL5 chips. We concluded that there is no significant difference in complexity for AAL1 SAR + sequence number processing or AAL5 CPCS including CRC check. The major difference is in the fact that AAL1 chips normally include CS functions like adaptive clock recovery or error correction, which are not essential in our NA-PRM.

5. Impact on signalling protocol

The proposed NA-PRM is not reflected in the signalling options that are currently present in the ATM Adaptation Layer parameters information element for AAL1 in Q.2931. The requested AAL parameters for AAL1 currently contain a fixed CBR value only. A new entry in the CBR rate field table for AAL1 indicating PCBR would give the correct indication. Verification is needed whether such a new entry is completely consistent in Q.2931.

6. Conclusion

There currently is no agreed opinion in the experts group on the preferred AAL type for SAR and CPCS. As we agreed that FEC is needed in the H.222.1 / AAL-SSCS, we defined a PDU for the H.222.1 / AAL-SSCS layer that is a RS FEC frame of max. 255 bytes that is independent of the choice of AAL types.

END