1、STD-ITU-T RECMN J-82-ENGL L79b H Li8b257L 062173b bb2 m INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU TRANSMISSION OF SOUND-PROGRAMME AND TELEVISION SIGNALS J .82 (07196) TRANSPORT OF MPEG-2 CONSTANT BIT RATE TELEVISION SIGNALS IN B-ISDN ITU-T Recommenda
2、tion 5.82 (P revi0 usiy ?CC IlT Recommendation?) STD-ITU-T RECMN J-AI?-ENGL 177b LIBb2571 Ob21737 5T9 FOREWORD The IT-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (IT). The IT-T is responsible for studying technical, operating and tar
3、iff questions and issuing Recommen- dations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the iT-T Study Groups which, in their turn, produ
4、ce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). IT-T Recommendation 5.82 was prepared by IT-T Study Group 9 (1993-1994) and was approved under the WTSC Resolut
5、ion No. 1 procedure on the 1 lth of July 1994. NOTE In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency. O ITU 1996 All rights reserved. No part of this publication may be reproduced or
6、utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the IT. 1 2 8 CONTENTS Scope References Abbreviations . Application aspects Service aspects . Specification of the recommended AALS Functions of the AAL type 1 for
7、 MPEG-2 transport . Handling of AAL user information Handling of Cell Delay Variation (CDV) . Handling of lost and misinserted cells Handling of the timing relationship Correction of bit errors and lost cells Functions of the AAL type 5 for MPEG-2 transport . Handling of AAL user information Handlin
8、g of Cell Delay Variation (CDV) . Handling of bit errors. lost cells and misinserted cells . Handling of the timing relationship Correction of bit errors and lost cells 7.1 7.2 7.3 7.4 7.5 8.1 8.2 8.3 8.4 8.5 Recommendation 5.82 (07/96) Page 1 1 1 2 2 3 4 4 4 4 4 4 5 5 5 5 5 6 i STD*ITU-T RECMN J-82
9、-ENGL 199b W Li8b2591 Ob21739 371 SUMMARY This Recommendation addresses the transport of television signals in ATM, in the case where they are WEG-2 coded with a constant bit rate. It recommends the utilization of the AAL type 1 with the convergence sublayer for the transport of video signals, or th
10、e utilization of the AAL type 5. INTRODUCTION A growing interest is currently being shown in the television community for the transport of digital television programmes in the Broadband Integrated Services Digital Network (Ei-ISDN). Reasons for this are twofold: From the application point of view, c
11、oding techniques for television signals have reached maturity, with the MPEG-2 standard (ITU-T Rec. H.222.0 I ISO/IEC 13818-1) increasingly recognized as the basic standard for coding of television signais. From the network point of view, the Asynchronous Transfer Mode (ATM) has been recommended as
12、the basic technique of B-ISDNs, providing the flexibility which future applications will need. Moreover, ITU-T Recommendations on B-ISDN have attained a high degree of stability and are near completion. The transport of television programmes in B-ISDN will undoubtedly be a useful service for carrier
13、s as well as for service providers. As initial ATM networks are currently under installation, the need to specify guidelines for the transport of MPEG-2 coded television signals in B-ISDN is recognized as urgent by standardization bodies. - - ii Recommendation 5.82 (07/96) Recommendation 5.82 TRANSP
14、ORT OF MPEG-2 CONSTANT BIT RATE TELEVISION SIGNALS IN B-ISDN (Geneva, 1996) 1 Scope The scope of this Recommendation covers the transport of television signais in Broadband-ISDN. In the context of this Recommendation television signals are encoded and transmitted in compliance with MPEG-2 systems sp
15、ecified in ITU-T Rec. H.222.0 I ISO/IEC 13818-1, with a constant bit rate. Broadband-ISDN is based on the Asynchronous Transfer Mode (ATM) (see Recommendation 1.121). Other ITU-T Recommendations which apply are those of the I-Series concerning ATM. 2 References The following Recommendations and othe
16、r references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; all users of this Recommendation are therefore encoura
17、ged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. IT-T Recommendation H.222.0 (1995) I ISO/IEC 13818-1:1996, Information technology - Generic cod
18、ing of moving pictures and associated audio information: Systems. CCITT Recommendation I. 12 1 ( 199 11, Broadband aspects of ISDN. ITU-T Recommendation I. 150 (1993), B-ISDN asynchronous transfer mode functional characteristics. IT-T Recommendation 1.361 (1993), B-ISDN ATM layer specification. ITU-
19、T Recommendation 1.362 (1993), B-ISDN ATM adaptation layer (AAL)functional description. ITU-T Recommendation 1.363.1 I), B-ISDN ATM Adaptation Layer specijication: Types I and 2 AAL. IT-T Recommendation 1.363.5 l), B-ISDN ATM Adaptation Layer specifcation: Type 5 AAL. ITU-T Recommendation 4.2931 (19
20、93, Broadband Integrated Services Digital Network (B-ISDN) - Digital Subscriber Signalling No. 2 (DSS 2) - User-Network Integace (UNI) layer 3 specifcation for basic calUconnection control. - - - - - - - - 3 Abbreviations For the purposes of this Recommendation, the following abbreviations are used:
21、 AAL ATM Adaptation Layer ATM Asynchronous Transfer Mode CBR Constant Bit Rate CDV Cell Delay Variation CLP Cell Loss Priority CPCS Common Part Convergence Sublayer l) Presently at the stage of draft. Recommendation 5.82 (07196) 1 CRC cs CSI FEC IDU MPEG PCR PDU QOS RS SAP SAR sc SDU sscs TS STD-ITU
22、-T RECMN J*BZ-ENGL 199b W 48b2591 0bZ1741 T2T Cyclic Redundancy Check Convergence Sublayer Convergence Sublayer Indication Forward Error Correction Interface Data Unit Moving Pictures Expert Group Programme Clock Reference Protocol Data Unit Quality of Service Reed Solomon Service Access Point Segme
23、ntation and Reassembly sublayer Sequence Count Service Data Unit Service Specific Convergence Sublayer Transport Stream 4 Application aspects This Recommendation concerns MPEG-2 coded television signals. It takes into consideration the case where signals are coded with a constant bit rate. This is a
24、 mandatory requirement for this Recommendation to apply. MPEG-2 signals are assumed to be coded in the form of a “Transport Stream” as it is defined in Recommenda- tion H.222.0, since, as mentioned in that Recommendation, the “Transport Stream is designed for use in environments where errors are lik
25、ely, such as storage or transmission in lossy or noisy media”. However, the presentation of signals in the form of the Transport Stream is not mandatory. As specified in Recommendation H.222.0, the MPEG-2 Transport Stream (TS) is organized in fixed-length packets sized to 188 octets. 5 Service aspec
26、ts Recommendation 1.362 gives a classification of services based on the attributes of the timing relationship between source and destination, bit rate and connection mode. In particular, Recommendation 1.362 states that Class A services have the following attributes: - - constant bit rate; - connect
27、ion-oriented mode. timing relationship between source and destination required; The transport of television signals can have the attributes corresponding to those of Class A, so the transport of television signals may be considered as a Class A service in relation to B-ISDN. In addition, in Recommen
28、dation 1.362, examples given for Class A services include constant bit rate video. Recommendation 1.362 also gives the relationship between service classes and AAL types. In Recommendation 1.342, it is stated that the provision of constant bit rate services can utilize the AAL type 1. It should be n
29、oted that the MPEG-2 TS has means Programme Clock Reference (PCR) to recover the clock of the transmitting AAL user. As a result, MPEG-2 coded television signals can also be transported using the AAL type 5 when connections are assigned an appropriate QOS. 2 Recommendation 5.82 (07196) The Cell Dela
30、y Variation (CDV) at the ATM-SAP depends on the network size, the volume and the characteristics of accommodated traffic, among other factors. It is assumed that the CDV value may be in the order of about 1-3 ms for CBR traffic. If, for example, the H.222.0 decoder implementation can only accept CDV
31、 less than the above value at its input, and if the CDV has not been compensated for at the AAL-SAP, then Network Adaptation (see Figure i) should provide the necessag CDV reduction. The method to reduce the CDV in the Network Adaptation function, if required, is implementation specific. 1 Network I
32、 Adaptation AAL-SAP 4- ATM-SAP -9- I I T932C-Q/dO1 FIGURE UJ.82 The submitted Cell Loss Priority parameter (CLP) is set to high priority. The CLP bit is defined in Recommenda- tion 1.361 and its usage for constant bit rate services is addressed in Recommendation 1.150. 6 Specification of the recomme
33、nded AALS Basically, the aim of the AAL is to enhance the quality of service provided by the ATM layer in order to fulfill service requirements. For the distribution of television signals, in addition to real-time requirements, it may be necessary to improve robustness to cell loss. If not countered
34、, cell loss events might lead to unacceptable artifacts for the users. Therefore the AAL used for the transport of television signals may have to provide cell loss correction capabilities. The AAL type 1 and the AAL type 5 are specified in Recommendations 1.363.1 and 1.363.5. In the description of t
35、he AAL type 1, four Convergence Sublayers (CSs) have been defined. In particular, a CS is dedicated to the transport of video signals. This CS is appropriate for the transport of television signals. The AAL type 1 can be used for all MPEG-2 CBR coded video applications. The AAL type 5 can be used fo
36、r MPEG-2 CBR coded video applications having less stringent QOS requirements. The AAL type 5 may require some functions to be performed in higher layers to compensate for a lower QOS at the AAL-SAP. 3 Recommendation 5.82 (07/96) STD-ITU-T RECMN J-82-ENGL 199b W 48b2591 Ob21743 8T2 7 Functions of the
37、 AAL type 1 for WEG-2 transport The size of the MPEG-2 TS packets has been specified in relation to the SAR-PDU payload of the AAL type 1, Le. 47 octets. As a result, a TS packet matches exactly four SAR-PDU payloads when the AAL type 1 is used. Functions and protocols of the AAL type 1 to be used a
38、re those described in Recommendation 1.363.1. Functions of the CS for the transport of video signais are described in Recommendation 1.363.1. These functions are listed in detail below. 7.1 Handling of AAL user information The length of the AAL-Service Data Unit (AAL-SDU) is one octet. At the AAL-SA
39、P (AAL-Service Access Point), the “Structure” parameter is not used. The “Status” parameter is used in the receiving AAL-SAP whenever data is judged to be errored, thus permitting adequate counteractions in higher layers, for example to set the “transport error indicator” bit in the TS packet layer.
40、 7.2 Handling of Cell Delay Variation (CDV) It is mandatory to perform this function in order to keep residual CDV values at the AAL-Service Access Point (AAL-SAP) within limits compatible with specifications concerning WEG-2 time stamps in Recommendation H.222.0. A buffer is used to perform this fu
41、nction. In the event of buffer underflow, it is mandatory for the CS to maintain bit count integrity by inserting the appropriate number of dummy bits. In the event of buffer overflow, it is mandatory for the CS to maintain bit count integrity by dropping the appropriate number of dummy bits. 7.3 Ha
42、ndling of lost and misinserted cells Detection of lost and misinserted cell events in the AAL type 1 is performed by using a 3-bit Sequence Count (SC) enabling detection of up to six consecutive cell loss occurrences and of one misinserted cell occurrence. The CS processing for SC operations is desc
43、ribed in Recommendation 1.363.1. 7.4 Handling of the timing relationship In order to provide a timing relationship between the source and destination, end-to-end synchronization is required. The method to be used is the adaptive clock method described in Recommendation 1.363.1. This method is approp
44、riate because it is not necessary in the case of the transport of video programmes to comply with the wander specifications of Recommendation G.823. Furthermore, the adaptive clock method does not rely on the availability of an external reference clock. 7.5 Correction of bit errors and lost cells In
45、 the CS for video transport described in Recommendation 1.363.1, a method based on Forward Error Correction (FEC) is proposed optionally for unidirectional video transport. This method relies on a combination of Reed Solomon (RS) codes and octet interleaving: the size of the interleaver is 128 SAR-P
46、DU payloads (Le. 128 x 47 octets, called the CS-PDU), whereas RS (128 and 124) codes are used. This method shall be used for correction of bit errors and lost cells in the case of the transport of video programmes in ATM. Correction capabilities are up to four cell losses (consecutive or not) in a g
47、roup of 128 cells, and up to two errored octets in a block of 128 octets. As specified in Recommendation 1.363.1, the use of the erasure mode is mandatory. In the case where uncorrected errors have been detected, the “Status” parameter is used at the AAL-SAP to inform higher layers, as stated in 7.1
48、 above. The CS-PDU structure is synchronized by the Convergence Sublayer Indication (CSI) bit which is set to “1” for the first SAR-PDU payload of the CS-PDU. 4 Recommendation 5.82 (07196) - STD-ITU-T RECMN JmBZ-ENGL 177b m 48b2571 Ob21744 737 The processing delay depends on the bit rate and corresp
49、onds to a duration of 124 x 47 x 2 octets (at the AAL-SAP). Furthermore, the beginning of the interleaver corresponds to the beginning of a TS packet (the interleaver contains exactly 3 1 TS packets). 8 Functions of the AAL type 5 for MPEG-2 transport Functions and protocols of the AAL type 5 to be used are those described in Recommendation 1.363.5. Both the SAR and the CPCS are used. The SSCS is null. The MPEG-2 TS packets are mapped into the AAL type 5. In the mapping, 1 to N MPEG-2 TS packets are mapped into the AAL type 5 SDU, where N is an integer. Padding may b
