1、INTERNATIONAL TELECOMMUNICATION UN ION ITU=T TELECOMMUNICATION STAN DAR DIZATI ON SECTOR OF ITU H.226 (09/98) SERIES H: AUDIOVISUAL AND MULTIMEDIA SYSTEMS Infrastructure of audiovisual services - Transmission multiplexing and synchronization Channel aggregation protocol for multilink operation on ci
2、rcuit-switched networks ITU-T Recommendation H.226 (Previously CCITT Recommendation) STD-ITU-T RECMN H.22b-ENGL 1778 D Li8b2571 Ob54313 708 ITU-T H-SERIES RECOMMENDATIONS AUDIOVISUAL AND MULTIMEDIA SYSTEMS Characteristics of transmission channels used for other than telephone purposes Use of telepho
3、ne-type circuits for voice-frequency telegraphy Telephone circuits or cables used for various types of telegraph transmission or simultaneous transmission H. I 0-H. 1 9 H.20-H.29 H.30-H.39 Telephone-type circuits used for facsimile telegraphy Characteristics of data signals H.40-H.49 H. 50-H. 99 H.
4、1 OO-H. 199 CHARACTERISTICS OF VISUAL TELEPHONE SYSTEMS INFRASTRUCTURE OF AUDIOVISUAL SERVICES General H.200-H.219 Trans wttiplexing and syn Systems aspects H.230-H.239 Communication procedures Coding of moving video H ,240-H.259 H .260-H .279 Related systems aspects H.280-H.299 Systems and terminal
5、 equipment for audiovisual services H.300-H.399 Supplementary services for multimedia H.450-H.499 For further details, please refer to ITU-T List of Recommendations. ITU-T RECOMMENDATION H.226 CHANNEL AGGREGATION PROTOCOL FOR MULTILINK OPERATION ON CIRCUIT-SWITCHED NETWORKS Summary This Recommendati
6、on is a channel aggregation protocol for multilink operation on circuit-switched networks. It is intended for use in applications where low latency and high efficiency are important considerations, such as H.324 multimedia telephony. Unlike synchronous time-division-multiplexing channel aggregation
7、protocols such as Recommendation H.221 and ISOAEC 13871 (“BONDING“), this Recommendation has the ability to operate with a changing number of channels with arbitrary, differing, or continuously varying bit rates (including modems according to Recommendation V.34) as well as fixed-rate channels (such
8、 as ISDN B-channels). Unlike packet-oriented channel aggregation protocols such as RFC 1990 (IPPP Multilink Protocol“), this Recommendation finely distributes payload octets across different channels, simultaneously minimizing latency and overhead. Source ITU-T Recommendation H.226 was prepared by I
9、TU-T Study Group 16 (1997-2000) and was approved under the WTSC Resolution No. 1 procedure on the 25th of September 1998. STD-ITU-T RECMN H-22b-ENGL 1798 48b257L Ob5Li315 780 m FOREWORD ITU (International Telecommunication Union) is the United Nations Specialized Agency in the field of telecommunica
10、tions. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of the ITU. The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The World Telecom
11、munication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics. The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in W
12、TSC Resolution No. 1. In some areas of information technology which fali within ITU-Ts purview, the necessary standards are prepared on a collaborative basis with IS0 and IEC. NOTE In this Recommendation, the expression “Administration“ is used for conciseness to indicate both a telecommunication ad
13、ministration and a recognized operating agency. INTELLECTUAL PROPERTY RIGHTS The ITU draws attention to the possibility that the practice or implementation of this Recommendatjon may involve the use of a claimed Intellectual Property Right. The ITU takes no position concerning the evidence, validity
14、 or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process. As of the date of approval of this Recommendation, the ITU had not received notice of intellectual property, protected by patents, which may be requ
15、ired to implement this Recommendation. However, impiementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. O ITU 1998 All rights reserved. No part of this publication may be reproduced or utilized in any form or
16、by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. 11 Recommendation H.226 (09/98) 1 2 3 4 4.1 4.2 4.3 5 5.1 5.2 5.3 CONTENTS Page Overview References Definitions Format conventions Numbering convention . Field-mapping convention
17、 Order of bit transmission Protocol definition Protocol overview . Header format . 5.2.1 FLAG 5.2.2 CONTROL 5.2.3 SEQUENCE NUMBER 5.2.4 CHANNEL TAG . 5.2.5 CHANNEL PROPORTION 5.2.6 DATA CRC . 5.2.7 HEADER CRC Procedures 5.3.1 Data transmission 5.3.2 Data reception . Appendix I . Example of Channel R
18、eference Model operation Recommendation H.226 (09/98) 1 1 1 2 2 3 3 3 3 4 6 6 7 7 7 7 8 8 8 12 15 . 111 STD-ITU-T RECMN H-22b-ENGL 1998 9 48b257L Ob54317 553 Recommendation H.226 CHANNEL AGGREGATION PROTOCOL FOR MULTILINK OPERATION (Geneva, 1998) ON CIRCUIT-SWITCHED NETWORKS 1 Overview This Recommen
19、dation describes a protocol for aggregation of data over multiple independent channels. The protocol is designed to achieve the following goals: 8 low latency transmission even at very low bit rates (including POTS modem rates); low overhead even at very low bit rates (including POTS modem rates); t
20、he ability to accommodate an arbitrary number of channels; the ability to accommodate channels of arbitrary bit rate; the ability to accommodate channels of continuously varying bit rate; the ability to accommodate unidirectional or bidirectional channels; e e e a the ability to accommodate channels
21、 of differing bit rate; e e e independence from channel order; low implementation complexity. a the ability for channels to drop out or be added arbitrarily; 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute pro
22、visions 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 encouraged to investigate the possibility of applying the most recent edition of the Recomme
23、ndations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. i ISO/IEC 3309: 1993, Information Technology - Telecommunications and information exchange between systems - High-level data link control (HDLC) procedures - Frame Structure. 3 Def
24、initions This Recommendation defines the following terms: 3.1 aggregation involves transmission of data over more than one channel. 3.2 synchronization of the channel aggregation rule. 3.3 channel: A single unidirectional data stream over which data is transmitted. Channel channel reference model: A
25、 model used by the transmitter and receiver to maintain channel set: The set of Channels over which a given Data Set is transmitted. Recommendation H.226 (09/98) 1 STD.ITU-T RECMN H-22b-ENGL 1998 m 48b2591 Ob543LB 49T m 3.4 data set: A portion of the input data stream to be transmitted to which a si
26、ngle channel aggregation rule is applied. Transmission of a Header Set delineates a given Data Set fkom the previous Data Set and specifies the channel aggregation rule to be used. 3.5 header: A collection of parameters whose beginning is marked by one or more Flags. A Header may either be part of a
27、 Header Set, delineating the beginning of a Data Set, or may be used on one or more channels for fill insertion. 3.6 header set: The set of Headers that delineate the beginning of a Data Set. Each Header in a Header Set is transmitted over a different Channel in the Channel Set. 3.7 maximum header i
28、nterval: The maximum allowed duration of a Data Set. The value of the Maximum Header Interval is not specified in this Recommendation. 3.8 maximum inter-FLAG length: The maximum amount of data that is allowed to be sent between FLAGs on any one Channel. The data counted between FLAGs shall include t
29、he portion of the Header following a FLAG and any subsequent Samples prior to the next FLAG. This count shall not include any zeros inserted for transparency control. The Maximum Inter-FLAG Length applies to any FLAGs, including those used to mark the beginning of a Header Set as well as those used
30、for fill insertion. The default value for this Maximum Inter-FLAG Length is 65 534 octets, unless another value is specified by another Recommendation making use of this Recommendation. 3.9 maximum transmit skew: The largest allowed skew between Channels in a Channel Set at the transmitter. The skew
31、 is defined as the difference between the maximum and minimum, across all Channels, of the time duration that a bit of data takes from appearance from the higher layer protocol until it is transmitted over one of the Channels. 3.10 sample: The smallest unit of data which is always kept contiguous wh
32、en distributing data among multiple channels. The size of the Sample is 8 bits. 4 Format conventions 4.1 Numbering convention The numbering convention used in this Recommendation is shown in Figure 1. Bits are shown grouped into octets. The bits of a single octet are shown horizontally and numbered
33、from 1 through 8. Multiple octets are shown vertically. l4 e e IN TI60467088 Figure 1m.226 - Numbering convention 2 Recommendation H.226 (09/98) STD-ITU-T RECMN H-22b-ENGL 1998 m qBb2591 Ob54319 32b m 4.2 Order of bit transmission The octets are transmitted in ascending numerical order. Within each
34、octet, bit 1 is the first bit to be transmitted. 4.3 Field-mapping convention When a field is contained within a single octet, the lowest-numbered bit of the field represents the lowest-order value (or the least significant bit). When a field spans more than one octet, the highest-numbered bit of th
35、e first octet represents the highest-order value, and the lowest-numbered bit of the last octet represents the lowest-order value. An exception to the preceding field-mapping convention are the CRC parameters. In these cases, the lowest-numbered bit of the first octet is the highest-order term of th
36、e polynomial representing the CRC, and the highest-numbered bit of the last octet is the lowest-order term of the polynomial representing the CRC. 5 Protocol definition 5.1 Protocol overview This Recommendation acts as an additional protocol layer inserted below any protocol that would normally tran
37、smit data over a single data channel. The interface from the higher layer protocol is a stream of bits, emulating a synchronous serial channel. The result of the channel aggregation operation is to produce data to be transmitted on a number of separate output Channels. At the receiver, the data from
38、 each of these Channels is recombined to reconstruct the original data stream presented to the channel aggregation layer at the transmitter. The channel aggregation operation divides the input data stream into Samples of 8 bits each. Each Sample is transmitted on one of the available Channels. The c
39、hannel aggregation operation can proportionally distribute Samples among all available Channels - accounting for their relative data rates - so that as much as possible of the total available bandwidth is used and latency can be kept to a minimum. In order to reconstruct the original data stream at
40、the receiver, the receiver must be able to determine which Samples were placed on which Channel. This is done by means of a Channel Reference Model that operates in synchrony at both the transmitter and receiver. For the Channel Reference Model to operate effectively, it must depend on the relative
41、rates of all Channels in use. If the relative rates vary (or if the estimate of the relative rates vary) or if the number of Channels in use changes, the model must be updated with this new information. The Channel Reference Model is updated by transmission of overhead information. A portion of the
42、input data stream over which the Channel Reference Model operates autonomously without any updates is called a Data Set (see Figure 2). A Data Set may be of arbitrary length - a Data Set continues indefinitely until another Data Set begins in its place. The beginning of a Data Set is marked by trans
43、mission of a Header Set. A Header Set consists of a Header transmitted on each Channel to be used for this Data Set (the set of all Channels to be used by a Data Set is referred to as its Channel Set). The Header Set includes all of the information needed to synchronize the Channel Reference Models
44、at the transmitter and receiver for the duration of the Data Set. Recommendation H.226 (09198) 3 A Data Set may continue as long as the current Channel Reference Model accurately reflects the actual rates of the Channels in use (new Data Sets should also be started periodically to avoid significant
45、error propagation). NOTE -No explicit mechanism for extensibility of H.226 is provided within the protocol itself. It is assumed that the negotiation into H.226 is done using an out-of-band mechanism, and that this mechanism would also be able to negotiate the use of any future versions of this Reco
46、mmendation by mutual agreement between endpoints. Data Stream from Higher Layers n C h Channel O a n e 1 Channel 1 n . . i , L/ i u,/ T1604MIO-98 Header Set 1 Header Set 2 Samples Time 1 (8-bits) Figure 2H.226 - Transmission of channel aggregated data 5.2 Header format The Header contains the follow
47、ing parameters: 1) FLAG; 2) CONTROL; 3) SEQUENCE NUMBER, 4) CHANNEL TAG; 5) CHANNEL PROPORTION; 6) DATA CRC; 7) HEADER CRC. The format of the Header is shown in Figure 3. The shaded octets are mandatory in all Headers, while the presence of the unshaded octets depends on the use of the Header. 4 Rec
48、ommendation H.226 (09/98) Octet FLAG o, 1, 2, or 4 CHANNEL TAG 1 DATA CRC T16469-98 Figure 3hl.226 - Header format Recommendation H.226 (09/98) 5 5.2.1 FLAG The FLAG parameter of the Header shall contain one or more FLAG PATTERNS in contiguous succession. Each FLAG PATTERN is 8 bits in length and sh
49、all be set to the binary value O1 11 11 10. 5.2.2 CONTROL The CONTROL parameter contains a series of fields defined in Figure 4 and Table 1. I I I I D L CP CT SN Field Value Meaning SN CT O0 10 1 1 O0 A 2-octet SEQUENCE NUMBER parameter is included in this Header. A 4-octet SEQUENCE NUMBER parameter is included in this Header. This Header does not include a CHANNEL TAG parameter. This value shall not be used unless there is exactly one Header in the Header Set. A 1-octet CHANNEL TAG parameter is included in this Header. O1 o1 1 O 11 This Header does not include a SEQUENCE NUMB
