1、INTERNATIONAL TELECOMMUNICATION UNION ITU=T TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU T.121 (07/96) SERIES T: TERMINAL EQUIPMENTS AND PROTOCOLS FOR TELEMATIC SERVICES Generic application template ITU-T Recommendation T.121 (Previously “CCITT Recommendation”) ITU-T T-SERIES RECOMMENDATIONS TERM
2、INAL EQUIPMENTS AND PROTOCOLS FOR TELEMATIC SERVICES For further details, please refer to ITU-TList of Recommendations. STD-ITU-T RECMN T*LZL-ENGL 199b D 48b2593 0622309 554 FOREWORD The IT-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union
3、 (IT). The IT-T is responsible for studying technical, operating and tariff 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
4、 the topics for study by the IT-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 WTSC Resolution No. 1 (Helsinki, March 1-12, 1993). ITU-T Recommendation T.121 was prepar
5、ed by IT-T Study Group 8 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 3rd 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 1
6、996 All rights reserved. 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. Recommendation T.121 (07/96) 1 1 2 3 4 5 6 7 8 CONTENTS Scope Normative Referenc
7、es Definitions Abbreviations . Overview 5.1 User application 5.2 Application Protocol Entity 5.3 Application Resource Manager 5.4 Application Service Element Generic Application Template . 6.1 Initialization 6.2 Enrollment 6.2.1 Registration session Standard base session 6.2.3 Non-standard base sess
8、ion 6.2.4 Public session 6.2.5 Private session . 6.3 Forming registry keys . 6.4 Capability negotiation . 6.5 Leaving a session Review of MCS resources and GCC services 7.1 Channels . 7.1.1 Static channel 7.1.2 Dynamic channel . 7.1.3 User ID channel 7.1.4 Assigned Channel . 7.1.5 private channel 7.
9、1.6 Potential problems with the use of MCS channels 7.1.7 Summary . 7.2 Data transfer 7.3 Tokens 7.3.1 Static tokens 7.3.2 Dynamic tokens 7.4 Application roster . 7.5 Application Registry . 7.7 Remote invoking . User application guidelines 8.1 Identification of user applications . 8.2 Use of collaps
10、ing and non-collapsing capabilities 8.3 Session context . 8.4 Selecting a session to participate in 8.5 Controlling the use of a session 8.6 . 6.2.2 7.6 Conductorship . Adding new participants to a session 8.7 Extension. modification and reuse of application protocols . Page 1 1 2 2 2 7 8 8 9 10 11
11、11 14 15 15 21 24 26 26 26 27 27 27 27 28 28 30 30 31 32 32 33 33 34 35 35 36 36 36 36 37 37 37 37 38 11 Recommendation T.121 (07/96) STD-ITU-T RECMN T-LZL-ENGL L99b 48b2591 Ob22LOb 327 SUMMARY This Recommendation provides guidance for application developers and application protocol developers on th
12、e correct and effective use of the T.120 infrastructure. It provides a generic model for an application that communicates using T.120 services and defines a Generic Application Template specifying the use of T.122 (MCS) and T.124 (GCC) services that are commonly required by application protocols mak
13、ing use of T. 120 services. This Recommendation provides a common structure to application protocol entities, ensuring that separate application protocol entities can coexist in the same conference. Application protocol entities that do not follow the operations defined in this Recom- mendation risk
14、 interfering with other application protocols. Recommendation T.121 (07/96) iii STD-ITU-T RECMN T.12L-ENGL L77b 48b2571 Ob22107 2b3 Recommendation T.121 GENERIC APPLICATION TEMPLATE (Geneva, 1996) 1 Scope Whilst user applications themselves are not the subject of standardization, the protocols emplo
15、yed by a user applica- tion need to be standardized in order to guarantee interworking. This Recommendation provides a generic model for a T.120 application with a view to defining a common framework (the Generic Application Template) to form the basis of standardized application protocols. It also
16、provides guidance to user application developers on how to exploit the T.120 infrastructure to achieve the desired application behaviour. The templates defined in this Recommendation are derived from terminal based application protocols; application protocols which incorporate an MCU component may r
17、equire different provisions. A T. 120 user application employs one or more Application Protocol Entities to communicate with user applications of similar functionality at other nodes in a conference. It is important to observe that, whilst it is not necessary to have identical implementations at eac
18、h node in order to achieve interoperability , groups of communicating user applications must employ the same application protocols for the same general purpose. Differences in capabilities are resolved through a capability exchange mechanism. Each Application Protocol Entity (APE) consists of two co
19、mponents: 0 A generic portion which includes elements common to most or all application protocols, for example initialization and resource management. Such operations need only be specified once (in this Recommen- dation) and then referenced by each protocol specification. A function specific portio
20、n which enables user applications of like functionality to interwork. This part may be standardized in which case it is specified in a T.120 application protocol Recommendation, or non-standard. This Recommendation describes the components identified above and defines a Generic Application Template
21、(GAT) containing those operations that are expected to be common to most T. 120 application protocols. The Generic Application Template eases the task of the application protocol developer and provides a common structure for application protocol entities. This modular approach allows the function sp
22、ecific elements of a protocol specification to be clearly identified, and thus enables it to be reused more readily in other protocol specifications. This Recommen- dation defines a rigorous protocol which is expressed through a sequence of primitive operations. Furthermore, this Recommendation can
23、be made normative by reference from standardized application protocol Recommendations. Use of the Generic Application Template in the definition of T.120 application protocols is encouraged. 2 Normative References The following Recommendations and other references contain provisions which, through r
24、eference 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 encouraged to investigate the possibility of applying t
25、he most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. - - ITU-T Recommendation T. 120 (1996), Data protocols for multimedia conferencing. ITU-T Recommendation T.122 (1993), Multipoint communication
26、 service for audiographics and audio- visual conferencing service definition. ITU-T Recommendation T. 124 (1995), Generic conference control. ITU-T Recommendation T. 125 (1994), Multipoint communication service protocol speccation. ITU-T Recommendation T.50 (1992), International Reference Alphabet (
27、IRA) (Formerly International Alphabet No. 5 or IA5) -Information Technology - 7-bit coded character set for information interchange. - - - Recommendation T.121 (07/96) 1 3 Definitions For the purposes of this Recommendation, the following definitions apply: 3.1 for managing .GCC and MCS resources. a
28、pplication resource manager: The part of an Application Protocol Entity that provides generic functionality 3.2 specific functionality, e.g. message formats and sequences. application service element: The part of an Application Protocol Entity that provides Application Protocol 3.3 without requiring
29、 user intervention. As such it is similar to a standard base session used by standard APEs. non-standard base session: An Application Protocol Session that can be located by non-standard APEs 3.4 session. private session: An Application Protocol Session with restricted membership controlled by the c
30、reator of that 3.5 relevant base session for the protocol is already in use and a user wishes to establish further independent sessions. public session: An Application Protocol Session with unrestricted membership that is typically used if the 3.6 conference. A Registration Session makes use of the
31、T.124 default session in which there is no Session ID. registration session: An Application Protocol Session used by applications to advertise their presence to the 3.7 concept of a session creator only applies to public and private sessions. session creator: The Application Protocol Entity that enr
32、olls in a new Application Protocol Session. The 3.8 concept of a session member only applies to public and private sessions. session member: An Application Protocol Entity that enrolls in an existing Application Protocol Session. The 3.9 requiring user intervention. Standard base session identifiers
33、 are specified in Recommendation T. 120. standard base session: An Application Protocol Session that can be located by standard APEs without 4 Abbreviations For the purposes of this Recommendation, the following abbreviations are used: ARM Application Resource Manager APE Application Protocol Entity
34、 ASE Application Service Element GAT Generic Application Template GCC Generic Conference Control GCC SAP GCC Service Access Point MCS Multipoint Communication Service MCSAP MCS Service Access Point MCU Multipoint Control Unit PDU Protocol Data Unit SAP Service Access Point 5 Overview Figure 1 illust
35、rates the T.120 system model in which user applications employ both standard and non-standard application protocols in order to communicate with their peers at other nodes within the same conference. The scope of a user application is restricted to local functions (e.g. user interface) that do not a
36、ffect communication between peer user applications. Each user application makes use of one or more Application Protocol Entities (APEs) that implement interworking protocols as shown in Figure 2. In the model described here, an APE can be further subdivided into two 2 Recommendation T.121 (07/96) ST
37、D-ITU-T RECMN TnLZL-ENGL L77b 48b2571 Ob22330 858 W elements: an Application Resource Manager (ARM) and an Application Service Element (ASE). The ARM is responsible for the generic management of GCC and MCS resources whilst the ASE provides interworking of specific functionality; for example, an ASE
38、 may provide general purpose file transfer functionality as specified in Recommen- dation T.127, or still image exchange functionality as specified in Recommendation T.126. User Application(s) (Using both standard and nowstandard Application Protocols) User Appiicatii(s) (Using std. Appl. Protocds)
39、User Application(s) (Using non-std Protocols) A v I I . I Non-standard Application Protocol Entities -i Recommendation T.121 (07/96) 3 - STD-ITU-T RECMN T-121-ENGL L99b m 4862593 Ob22111 774- m User Application A Node Control ler Generic Conference Control (GCC) Rec. T.124 Rec. T.122r.125 TO82501 th
40、is is referred to as the registration session. The following session types exist: Registration Session: may be used by both standard and non-standard application protocols. The purpose of a registration session is to enable applications to advertise their presence to the conference, without having t
41、o become active. This enables applications to determine which nodes in the conference have similar functionality, whilst at the same time consuming minimal resources. The registration session also allows applications to advertise application protocol specific functionality to the conference as non-c
42、ollapsing capabilities. Applications would typically remain enrolled in the registration session for the duration of the conference in order to receive notification of any changes in functionality advertised by other nodes. An application enrolled in the registration session for a specific applicati
43、on protocol receives information on ali sessions in the conference using that particular application protocol. Recommendation T.121 (07/96) 5 STD-ITU-T RECMN T-121-ENGL 17qb LiBb257L Ob22113 5b7 W Site A 4 4 Application 1 Site C I + 4 l l FIGURE 4f.121 Example of applications using multiple protocol
44、s Use of a registration session for actual application activity is not part of these models and is discouraged. This ensures a clean segregation of active and dormant applications. This segregation is important because: Any collapsing capabilities advertised by inactive applications may have an adve
45、rse impact on the capability set of the session. Defunct registry entries in a registration session resulting from application activity are likely to persist until the end of the conference. 0 0 For each application protocol within a conference there can be at most one registration session. It is ch
46、aracterized by having a Session Key comprised of the appropriate Application Protocol Key and no Session ID. 6 Recommendation T.121 (07/96) STD-ITU-T RECMN T-121-ENGL L77b Li8b259L Ob22LL4 4T3 W Standard Base Session: reserved for use by standardized application protocols. Access to a standard base
47、session is unrestricted and ail participants within the session have equal status, that of session member; they may join and leave the session at will. As the Session Identifier for a standard base session of a standardized application protocol is predefined (in Recommendation T.120), an application
48、 may join the session without any user intervention. A standard base session uses a static channel as the Session ID channel. The set of resources that need to be identified during the enrollment process for a standard base session may include further static or dynamic channels and tokens, as specif
49、ied by the application protocol. Dynamic channels and tokens may be employed if additional resources are required beyond this basic set. Typically, the standard base session would be the preferred choice for an application using a standard application protocol that wishes to communicate with all peer applications within a conference. For each standardized application protocol within a conference, there is at most one standard base session. Non-Standard Base Session: reserved for use by non-standard application protocols . The non-standard base session allows applications which e
