1、STD*ITU-T RECMN F-723-ENGL 177b LiAb257L Ub23LiLi5 707 INTERNATIONAL TELECOMMUNICATION UNION ITU-T TELECOMM UN I CATION STANDARDIZATION SECTOR OF ITU F.723 (07/96) SERIES F: NON-TELEPHONE TELECOMMUNICATION SERVICES Audiovisual services Videophone service in the Public Switched Telephone Network (PST
2、N) ITU-T Recommendation F.723 (Previously “CCITT Recommendation“) ITU-T F-SERIES RECOMMENDATIONS NON-TELEPHONE TELECOMMUNICATION SERVICES TELEGRAPH SERVICE Operating methods for the international public telegram service The gentex network Message switching The intemational telemessage service The in
3、ternational telex service Statistics and publications on international telegraph services Scheduled and leased communication services Phototelegraph service Mobile services and multidestination satellite services MOBILE SERVICE TELEMATIC SERVICES Public facsimile service Teletex service Videotex ser
4、vice General provisions for telematic services MESSAGE HANDLING SERVICES DIRECTORY SERVICES DOCUMENT COMMUNICATION Document communication Programming communication interfaces F. 1-F. 109 F. 1-F. 19 F.20-F.29 F.30-F.39 F.40-F.58 F.59-F.89 F.90-F.99 F. 100-F. 104 F. 105-F. 1 O9 F. 1 10-F. 159 F. 110-F
5、. 159 F. 160-F.399 F. 160-F. 199 F.200-F.299 F.300-F.349 F.350-F.399 F.400-F.499 F.500-F.549 F.550-F.599 F.550-F.579 F.580-F.599 DATA TRANSMISSION SERVICES F.600-F.699 ISDN SERVICES F.800-F. 849 UNIVERSAL PERSONAL TELECOMMUNICATION Il HUMAN FACTORS F.850-F.899 F.900-F.999 For further details, please
6、 refer to ITU-T List of Recommendations. STDmITU-T RECMN F.723-ENGL 1796 Li86259L Ob23447 58T FOREWORD The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The ITU-T is responsible for studying technical, operating and tariff q
7、uestions 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 ITU-T Study Groups which, in their turn, produce R
8、ecommendations 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). iTLJ-T Recommendation F.723 was prepared by ITU-T Study Group 1 (1993-1996) and was approved under the WTSC Resolut
9、ion No. 1 procedure on the 19th of July 1996. 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 u
10、tilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. Recommendation F.723 (07/96) 1 1 2 3 4 5 6 7 8 9 10 CONTENTS Introduction Normative references . Definitions Terminology . Description . 5.1 General descri
11、ption . 5.3 Service categories . 5.2 Basic functionalities 5.4 Possible applications . 5.5 Optional features . Procedures 6.1 Normal procedures Network aspects . Terminal aspects . Quality of service . 9.1 Picture quality . 9.2 Audio quality 9.3 Overall delay . Interworkinglintercommunication Interc
12、ommunication with mobile videotelephony Intercommunication with ISDN videotelephony 10.1 Intercommunication with telephony . 10.2 10.3 Annex A . Attributes and values A.l Low layer attributes A.2 Access attributes . A.3 High layer attributes . A.4 General attributes 11 Recommendation F.723 (07/96) P
13、age 1 1 2 2 2 2 3 3 3 3 3 4 4 4 4 4 4 5 5 5 5 5 5 5 6 6 6 Recommendation F.723 VIDEOPHONE SERVICE IN THE PUBLIC SWITCHED TELEPHONE NETWORK (PSTN) (Geneva, 1996) 1 Introduction This Recommendation contains the descripti on and network specific service requirements for videophone services offered in t
14、he Public Switched Telephone Network (PSTN). The substance of this Recommendation complements the main body of the Supplement to Recommendation F.720, which deals with network independent service requirements for respective Low Bit Rate (LBR) videophone services provided in networks such as PSTN and
15、 digital mobile telecommunication networks across Low Bit Rate (LBR) channels. The difference between the service requirements in these two network domains stems from variations in access rates, mobility, robustness of digital wireless transmission and different terminal environments. In addition to
16、 network specific requirements, the network independent requirements for LBR videophone services and general requirements for all videophone services, included in Recommendation F.720, apply for the service as well. Due to bandwidth and technical restrictions, the quality of service is limited and m
17、ay be inadequate for many applications, in particular in the professional domain. Hence, it is essential that users of the service can utilize the reduced network capabilities as efficiently as possible with a flexibility in the channel allocation between voice, video, image and data. This service c
18、an be used on a stand-alone basis or as part of a multimedia application. In the latter case, the same requirements apply. 2 Normative references The following Recommendations and other references contain provisions which, through reference in the text, constitute provisions of this Recommendation.
19、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 Recommendations and other references li
20、sted below. A list of the currently valid IT-T Recommendations is regularly published. ITU-T Recommendation G.723.1 (1996), Dual rate speech coder for multimedia communications transmitting at 5.3 and 6.3 kbit/s. - ITU-T Recommendation H.223 (1996), Multiplexingprotocol for low bit rate multimedia c
21、ommunication. - ITU-T Recommendation H.245 (1 996), Control protocol for multimedia communication. - ITU-T Recommendation H.263 (1996), Video coding for low bit rate communication. - ITU-T Recommendation H.281 (1994), A far-end camera control for videoconferences using H.224. - ITU-T Recommendation
22、H.324 (1996), Terminal for low bit rate conversational multimedia - communication. - - - ITU-T Recommendation T. 120 (1996), Data protocols for multimedia conferencing, ITU-T Recommendation T. 126 (1996), Multipoint still image and annotation protocol. ITU-T Recommendation V.8 bis (1996), Procedures
23、 for the identrfication and selection of common modes of operation between DCEs and between DTEs over the general switched telephone network and on leased point-to-point telephone-vpe circuits. ITU-T Recommendation V.34 (1994), A modem operating at data signalling rates of up to 28 800 bids for use
24、on the general switched telephone network and on leased point-to-point 2-wire telephone-Spe circuits. - Recommendation F.723 (07196) 1 3 Definitions For the purposes of this Recommendation, the following definitions apply. 3.1 channel aggregation: The capability to aggregate basic PSTN data connecti
25、ons in given discrete data rate increments to establish one higher bit rate data channel. 3.2 dynamic channel allocation: The principle of allocating in a flexible way the available channel capacity between various types of data, comprising audio, video, control information, still pictures and other
26、 application specific data to maximize the quality of service. 3.3 fall-back): A procedure, initiated by either the videophone or the network that reduces the bit rate of the voiceband connection. 3.4 in-band signalling: Signalling between videophone units that support user control functions, monito
27、ring functions, user alerts or advisory information about the status of the videophone units or the network connection. 3.5 basic videophone: A videophone containing mandatory features only and conforming to performance parameters contained in the Recommendations without any optional features nor an
28、y improved performance parameters. 3.6 enhanced videophone: A videophone containing enhanced operational features and conforming with performance parameters defined as “optional” in the Recommendations. 3.7 motion rendition: The ability of the receiving entity to reproduce motion from the transmitte
29、d video. 4 Terminology For the purposes of this Recommendation, the following abbreviations are used. CIF See Recommendation F.720 (Supplement) PSTN Public Switched Telephone Network QCIF See Recommendation F.720 (Supplement) SQCF See Recommendation F.720 (Supplement) 5 Description 5.1 General descr
30、iption The general service description characteristics to all LBR videophone services is included in the Supplement of the F.720 Recommendation. The service offers a real-time conversational two-way audiovisual end-to-end communication, comprising video, audio and optional*) in-band data transfer ca
31、pabilities. As a rule, the audiovisual information is transferred along a single PSTN connection, based on LBR data channels. The aggregation of two PSTN connections for achieving increased overall capacity and QOS may be offered as an option (further studies are required). For the video the service
32、 should support the coding scheme defined by Recommendation H.263 and a spatial resolution conforming to QCIF and the terminal requirements covered by Recommendation H.324. Besides at Set-up, fall-back may also occur under degraded network conditions through bit-rate reduction. 2, As a terminal capa
33、bility in-band data transfer is optional, but as a service functionality it is mandatory. 2 Recommendation F.723 (07/96) 5.2 Basic functionaiities The basic functionalities, characteristic of videophone services in general as well as those of the LBR videophone services have to be supported. The bas
34、ic PSTN speech communication facility, i.e. that of an analogue telephone, must be included in the terminal, so that the user shall be able to use the terminal as a normal telephone as well. Concerning the fall-back, a slow repetition rate video mode, manually controllable from the receiving end, mu
35、st be supported. Dynamic channel allocation shall be provided as a mandatory system capability. Dynamic channel allocation is executed by means of a framing, synchronization and terminal negotiation mechanism in conformance to Recommen- dations H.245 and H.324 and other relevant IT-T Recommendations
36、. 5.3 Service categories As a rule, the service employs a single PSTN connection. The possibility to aggregate two or more PSTN connections to set up a higher bit rate connection for improving the QOS (multilink operation) as well as the possibility to temporarily extend or reduce the bearer capabil
37、ities during the call without disconnecting the call (in-call modification) are for further study. 5.4 Possible applications In the consumer/residential segment the envisaged applications are real-time human-to-human interaction, based on head-and-shoulders view, and remote surveillance and event mo
38、nitoring e.g. for baby-sitting, security, as well as other non-conversational applications. In business/institutional segment the foreseen applications are remote expert consultation, requiring audiovisual support, remote surveillance and recognition, remote trouble-shooting, remote inspection and a
39、ccessing videoconferences. The above applications may be offered on a stand-alone basis or as part of a value-added multimedia application requiring extended terminal capabilities. From the service point of view the basic videophone terminal has to support QCIF only for the motion video. Some termin
40、als, i.e. enhanced videophones may support also SQCIF3) format or at least they are capable of receiving SQCIF video frames. 5.5 Optional features The facilities and functionalities defined in Supplement 1 to Recommendation F.720 in general apply. Furthermore the possibility to access a multipoint v
41、ideophone call has to be provided in accordance with T. 120-Series protocols. For a possible still picture data transfer in multipoint calls, as well as for data transfer of telepointer and annotation, Recommendation T. 126 applies. In case a far-end camera control is provided, Recommendation H.281
42、shall apply. 6 Procedures The service is envisaged to address the consumer/residential market using low-cost stand-alone terminals with a minimum set of functionalities. 3, According to the current Recommendation H.263, the capability of receiving SQCIF video frames is supported by all H.263 video c
43、odecs as a mandatory feature. Recommendation F.723 (07196) 3 6.1 Normal procedures 6.1.1 Caii procedures The call Set-up and call initialization procedures are defined in the Recommendation on network independent requirements for LBR videophone services (see Supplement to Recommendation F.720). When
44、 a videophone call has been originated and the called terminal is incompatible or unable to comply with the call request, the calling subscriber is given an appropriate indication. The call will progress automatically as a telephone call if the calling user accepts the call. 7 Network aspects The tr
45、ansfer rate achieved on a given PSTN line is strongly dependent on the transmission quality, i.e. crosstalk and noise level, which may have a substantial impact on the overall QOS with the consequence that operation on only the low repetition rate mode is possible. Therefore, maintaining appropriate
46、 signallinterference ratio in the local loop enabling the maximum data rate or a rate close to it is important to guarantee adequate overall QOS necessary for gaining user acceptance. The service can be offered also on leased point-to-point telephone type circuits. 8 Terminal aspects The service is
47、envisaged to address the consumerhesidentiai market using low-cost stand-alone terminals with a minimum set of functionalities. The terminals with enhanced functional capabilities may consist of PCs equipped with auxiliary plug-in units providing the needed terminal capabilities for PSTN videoteleph
48、ony (see Supplement to Recommendation F.720). The bearer capabilities for transferring the digital audiovisual data over analogue PSTN lines are implemented using V.34 modems with bit rates up to 28.8 kbit/s and beyond4). 9 Quaiity of service The QOS is dependent on the quality of the obtained telep
49、hone line and networks involved, which may result in a reduced modem transfer rate and impaired audiovisual quality as well as delayed call Set-up due to prolonged modem start-up phase. 9.1 Picture quality The picture quality depends on the available modem transfer rate, amount of movements in the picture and the selected channel rate. Increased amount of changes in the image content causes degraded performance of motion rendition. For further study5). 9.2 Audio quality See Recommendation F.720 (Supplement). 4, The V.34 modems are envisaged to be upgraded in the near future resulting in a
