1、CCITT RECNN*F-721 92 4862591 0575739 682 m INTERNATIONAL TELECOMMUNICATION UNION CCITT THE INTERNATIONAL CONSULTATIVE COMMITTEE TELEGRAPH AND TELEPHONE TELEMATIC, DATA TRANSMISSION, ISDN BROADBAND, UNIVERSAL, PERSONAL TELECOMMUNICATIONS AND TELECONFERENCE SERVICES OPERATIONS AND QUALITY OF SERVICE F
2、.721 (08/92) VIDEOTELEPHONY TELESERVICE FOR ISDN I Recommendation F.721 CCITT RECMN*F-721 92 W 4862571 0575740 3T4 W FOREWORD The CCITT (the International Telegraph and Telephone Consultative Committee) is a permanent organ of the Intemational Telecommunication Union (ITU). CCIT is responsible for s
3、tudying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis. The Plenary Assembly of CCIT which meets every four years, establishes the topics for study and approves Recommendations prepared by its Study G
4、roups. The approval of Recommendations by the members of CCITT between Plenary Assemblies is covered by the procedure laid down in CCI” Resolution No. 2 (Melboume, 1988). Recommendation F.721 was prepared by Study Group I and was approved under the Resolution No. 2 procedure on the 4th of August 199
5、2. CcIT NOTES 1) telecommunication administration and a recognized private operating agency. 2) In this Recommendation, the expression “Administration” is used for conciseness to indicate both a A list of abbreviations used in this Recommendation can be found in Annex A. O ITU 1993 All rights reserv
6、ed. No part of this publication may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying and microfilm, without permission in writing from the ITU. CCITT RECMN*F=72L 92 m 4862591 0575741 230 m Recommendation F.721 VIDEOTELEPHONY TELESERVICE FOR ISDN
7、 (I 992) The CCIT, considering (a) that considerable efforts have been undertaken worldwide in order to develop videotelephone equipment based on rapid improvements in the quality of video codec algorithms; (b) that videotelephones of some manufacturers are already available on the market; (c) that
8、first trials of videotelephony have been performed nationally as well as internationally; (d) that a number of countries intend to introduce the videotelephone service as soon as possible; (e) that the ISDN will be an appropriate network for providing the videotelephony teleservice; (0 that ISDN is
9、offered in a number of countries, recognizes the need for a standardized international videotelephony teleservice which will guarantee the compatibility of videotelephones on a worldwide basis and therefore, recommends that the videotelephony teleservice, where implemented, respect the requirements
10、stated in this Recommendation. 1 Definition The videotelephony teleservice is a symmetrical, bidirectional, realtime, audiovisual teleservice in which speech and moving pictures are interchanged by means of one or two B-channels, using 64 kbids circuit-mode connections in the ISDN. The picture infor
11、mation transmitted is sufficient for adequate representation of fluid movements of a person displayed in head and shoulders view. 2 Description 2.1 General description The videotelephony teleservice is defined as a fully standardized ISDN teleservice following the principles given in Recommendation
12、1.210. Two cases can be identified for the videotelephony teleservice: Case I: Videotelephone based on using one circuit-mode 64 kbit/s connection. Case II: Videotelephone based on using two circuit-mode 64 kbids connections. For case I the 64 kbids connection carries both speech and video informati
13、on; for case II the fxst connection either carries speech or both speech and some video information and the second connection carries video information. The basic videotelephony teleservice is characterized by the transmission of moving pictures displayed continuously in colour, simultaneously with
14、the speech of the persons involved in the call (generally two in the case of a point-to-point connection). in the 64 kbids ISDN based on bandwidths of 3.1 kHz or 7 kHz respectively. The speech quality of this teleservice must be at least as good as that applicable to the telephony teleservice Recomm
15、endation F.721 (OW2) 1 CCITT RECMN*F-721 92 m 4862591 0575742 177 m The videotelephony teleservice shall allow the communication between: - two users (e.g. terminals) in a point-to point configuration via the ISDN over one or two circuit mode 64 kbitls connection(s); three or more users in a multipo
16、nt configuration as invoked by some supplementary services. - Videotelephone terminals must be capable of supporting the telephony teleservice. An essential feature of the service is that, besides videotelephony, it also provides to the user the possibility to communicate with other ISDN telephone o
17、r videotelephone terminals by using only the speech communication facility. It shall be possible to use videotelephone terminals to communicate with 3.1 kHz telephone terminals connected to the public switched telephone network (PSTN). 2.2 Specific terminology Full-back: Procedure performed either b
18、y the network or by the calling videotelephone terminals to establish calls to 3.1 kHz telephone terminals. Call 1: The first call invoked in the videotelephony teleservice. It identifies the first 64 kbit/s connection between the subscribers. The ca is invoked for all the service cases. Call 2: The
19、 second call invoked in the videotelephony teleservice. It identifies tbe second 64kbit/s connection between the two subscribers. The call is invoked for case II only (2 x 64 kbitls). Retention timer: This timer specifies the amount of time that the network retains the call information of the origin
20、al call upon encountering busy or being released. This timer is a network provider option. The value of this timer is greater than 15 seconds. Videotelephone terminal: A terminal that supports the videotelephony teleservice. 3.1 Hz telephone terminal: A terminal that supports the telephony 3.1 kHz t
21、eleservice. 7 Wz telephone terminal: A terminai that supports the telephony 7 kHz teleservice. 3 Procedures 3.1 Provisiodwithdrawal 3.1.1 This teleservice may be provided after prior arrangement with the service provider or be generally available. 3.1.2 As a service provider option, the videotelepho
22、ny teleservice can be offered with several subscription options which apply separately to each ISDN number, all or a group of ISDN numbers on the interface. For each subscription option, only one value can be selected. Subscription option Value 2 Maximum number of information channels available at t
23、he called user Maximum number of total calls presented simultaneously at the called user m, where m is not greater than the number of information channels on the interface n, where n is not greater than the number of information channels on the interface The called user is identified by either one I
24、SDN number, all or a group of ISDN numbers on the interface. Recommendation F.721 (Oi92) CCITT RECNN*F.721 92 m 48b2591 0575743 003 = 3.2 Normal procedures 3.2.1 Activatioddeactivatiodregistration Not applicable. 3.2.2 Invocation and operation 3.2.2.1 Originating the service (call request) Call 1 pi
25、cture and sound are released simultaneously. 3.2.2.4 Chge of terminal communication mode As a consequence of end-to-end integrity on videotelephony teleservice, 7 kHz telephony teleservice and some 3.1 kHz telephony teleservice calls (i.e. where no speech processing is performed by the network), it
26、will be possible to use the B-channel protocols given in Recommendations G.725 and H.UZ. Depending on the terminal capabilities, it may be possible to change between the following communication modes according to Tables m.320 and 3B.320: - 3.1 kHz speech (Recommendation G.711); - 7 kI4z speech ecomm
27、eodation G.722); - different videotelephone terminal modes. Note I - The user may be required to establish additional calls in some cases. Note 2 - As an option, in some circumstances establishment of a videotelephone cail can be based on call set up as a 3.1 kHz telephone ca and then change to a vi
28、deotelephone cail if a change of service by using an end-to-end procedure is possible. In the case where a 3.1 kHz telephone cail is first established, on request of the calling user, the calling terminal will try to achieve framing and to exchange terminal capabilities on the existing B-channel. If
29、 it succeeds, this channel is used in the same way as a channel which is the result of a call 1 establishment. The change from the telephone call to a videotelephone call will not cause an interruption of the voice communication. If framing cannot be achieved on the existing B-channel, the calling u
30、ser has to release the telephone call and to request for call 1 establishment. This change cannot be performed without an interruption of the existing communication. 3.2.3 3.3 3.3.1 3.3.2 4 Interrogation Not applicable. Exceptional procedures Failure situations due to user error i) A user inputting
31、an improper service request shall be given an appropriate failure indication by the network and the call Set-up shall be ceased. ii) A user inputting a non-valid network number shall be given an appropriate failure indication by the network and the call set-up will be ceased. Failure situations due
32、to called user state i) A calling user attempting to establish a call to a user who is identified by the network to be busy either network determined user busy (NDUB) or user determined user busy (UDUB) shall be given an appropriate failure indication by the network and the call Set-up shail be ceas
33、ed. A user attempting to establish a call to a user whose terminal equipment fails to respond, shall be given an appropriate failure indication by the network and the call Set-up shall be ceased. ii) Recommendation P.721 (OW2) CCITT RECMN*F-721 92 4862591 0575745 98b Ui) On a call to a user whose te
34、nninai equipment has responded that the called user is being informed of the cal, but has failed to answer within a defined period, the calling user attempting to establish the call shall be given an appropriate failure indication by the network and the call Set-up shall be ceased. 3.3.3 Failure sit
35、uation due to network conditions A user attempting to establish a call but meeting call failure situations due to network conditions (e.g. congestion), sha be given an appropriate failure indication by the network. 3.3.4 Failure situations due to called user state dor network condition A user attemp
36、ting to establish a cai1 but meeting call failure situations due to network conditions (e.g. congestion) or called user state (e.g. NDUB or UDUB), can have call information retained for the duration of the retention timer. 3.3.5 Decreasing of service quality due to fall-back In the case of decreasin
37、g of service quality due to audio andlor visual fall-back, appropriate indication should be given to both users, even if problems occur in one direction only. 4 5 5.1 5.1.1 Network capabilities for charging Charging capabilities are outside the scope of this Recommendation. Intercommunication and in
38、terworking considerations Intercommunicatiodinterworking with other terminals Intercommunication with 3.1 IrHz and 7 kHz ISDN terminals or interworking with PSTN shail be offered. General principles The videotelephony service shall include voice encoding according to Recommendation G.711. It may inc
39、lude additional speech encoding as an optional feature. The following fundamental requirements shall be fulfilled i) The user of a videotelephone terminai shall be able to establish caiis to 3.1 kHz and 7 kHz telephone terminals (if the 7 kHz capability is supported) connected to the ISDN and to tel
40、ephone terminals connected to the PSTN, Optionally, it should be able to reach other ISDN audiovisual terminals. ii) A videotelephone terminai shall be able to accept calls from 3.1 kHz and 7 kHz (if 7 kHz capability is supported) telephone terminals connected to the ISDN and from 3.1 kHz telephone
41、terminals connected to the PSTN. Optionally, it should be able to accept caiis from other ISDN audiovisual terminals. As an option, videotelephone terminals may be pre-programmed to receive incoming videotelephone cas only. This latter functions may be requested by users possessing, e.g. both a vide
42、otelephone terminal and a 3.1 kHz telephone terminal connected to the same access arrangement. 5.1.2 Fall back procedure 5.1.2.1 Fall back in the destination network Fall-back to 3.1 kHz telephony shall be an inherent feature of videotelephony teleservice and shail be provided as a default procedure
43、. Recommendation F.721 (OWZ) 5 CCITT RECMNtF-72L 92 m 4862593 0575746 812 m The user shall be offered the possibility of indicating whether be requires inkrworking/fall-back to the 3.1 kHz telephony teleservice. A request for the videotelephony teleservice without fall-back (if indicated by the call
44、ing terminal) shall be possible. The foilowing procedure shall apply: - If the calling user has indicated that fall-back is allowed, the network may offer the call to the called user at all videotelephone and 3.1 kJ3z telephone terminals if possible. The called user can accept the call either as a v
45、ideotelephone or a 3.1 kHz telephone call at any teminal where the call is offered. Note - The called terminals may recognize the fall-back situation and indicate it to the user. The calling user shall be informed of the resultant telecommunication service, i.e. the videotelephony or 3.1 kHz telepho
46、ny teleservice. If no terminal accepts the call, this shall be indicated to the calling user. If a busy condition is met at the terminals, supplementary services, e.g. completion of cails to busy subscriber shall apply. Note - Echo cancellation will be disabled for videotelephone calls. If fall-back
47、 occurs, there is no current signalling mechanism for re-enabling the echo cancellers. When fall-back is not implemented by the network (possible short-term situation), fall-back may be performed end-to-end by the calling videotelephone terminal by originating a 3.1 kHz telephone ca. - - - - 5.1.2.2
48、 Fall-back when the ISDN does not offer the videotelephone teleservice If the calling user has indicated that fall-back is allowed but the destination network does not support the videotelephone capabilities the calling user shall receive an indication that fall-back has occurred and an indication o
49、f the resultant telecommunication service. The called user shall be offered the incoming ca as a telephony 3.1 kHz call. 5.2 Interworking with private ISDNs If the called user is on private ISDN, the fali-back procedures will be performed by the private ISDN. The result of call presentation (videotelephony or 3.1 kHz telephony) within the private ISDN shall be indicated to the public ISDN. 6 6. Attributeslvalues Application of the attribute method Depending on the case that applies the videotelephony teleservice description is based on invocation of one or two calls: call
