NetMeeting 3.0 Resource Kit

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This chapter contains information about the T.120 standard, its architecture, and how Microsoft® Windows® NetMeeting® 3 supports T.120 for data conferencing. It also explains T.120 protocols and supported topologies.

What is the T.120 Standard?

The International Telecommunications Union (ITU) T.120 standard is made up of a suite of communication and application protocols developed and approved by the international computer and telecommunications industries. Using these protocols, developers can create compatible products and services for real-time, multipoint data connections and conferencing. With T.120-based programs, multiple users can participate in conferencing sessions over different types of networks and connections.

Depending on the type of T.120 product, the program can make connections, transmit and receive data, and collaborate using compatible data conferencing features, such as program sharing, whiteboard conferencing, and file transfer. Microsoft and more than 100 other major companies support the development of products and services using the T.120 standard.


T.120 products and services offer the following benefits to users:

  • T.120 ensures that many participants can send and receive data in real time without any errors in data transmission. Users can expect this reliability over many types of supported connections, including Transmission Control Protocol/Internet Protocol (TCP/IP).

  • For multipoint data conferencing, the T.120 standard supports a variety of common topologies, including cascaded, star, and daisy-chain connections. For more information about these topologies, see "Topologies" later in this chapter. 

  • Developers can create applications with T.120 alone, or in combination with other ITU standards, such as the H.323 standard for audio and video conferencing. 

T.120 Interoperability and Testing

One of the most important features of the T.120 infrastructure is the interoperability of products and services that support the standard. For more information about interoperability, see Chapter 9, "Product Interoperability."

T.120 Interoperability Testing

The interoperability of T.120 products is measured on two levels: networking and applications. The T.122, T.123, T.124, and T.125 standards make up the networking level of T.120. Products and services that meet these standards have the necessary infrastructure to do the following:

  • Establish and maintain conferences without any platform dependence. 

  • Manage multiple participants and programs. 

  • Send and receive data accurately and securely over a variety of supported networking connections. 

The T.126 and T.127 standards make up the applications level of T.120. These standards ensure that the electronic whiteboard and file transfer applications developed under T.120 can interoperate across platforms and networks, and within multiuser conferences. Microsoft runs test cases for any data conferencing function of other products that uses the same protocols and standards as NetMeeting. Test cases also verify the transfer of data at every level of the T.120 communications infrastructure.

How NetMeeting Uses the T.120 Standard

Microsoft developed NetMeeting data conferencing features based on the T.120 infrastructure, enabling NetMeeting to interoperate with other T.120 standards-based products. T.120 protocols are the building blocks for the following NetMeeting functions:

  • The ability to establish and maintain NetMeeting data connections 

    Two or more participants can establish a NetMeeting connection. Within the conference, T.120 protocols manage the sequencing and flow of data transported by NetMeeting connections. T.120 ensures that data is accurately and reliably transmitted between conference nodes. 

  • Built-in data conferencing applications 

    NetMeeting conference participants can use T.120-based applications. For example, file transfer is based on the T.127 standard and program sharing is based on T.128. 

  • Support for multiuser conferences 

    Many people can join a NetMeeting data conference for communication and collaboration. The top provider in NetMeeting provides the node controller that manages participants and their applications.

  • Capability to interoperate across networks and platforms 

    T.120 interfaces enable NetMeeting and other standards-based products to communicate across common networks using TCP/IP without platform restrictions.

T.120 Architecture

The following illustration shows the T.120 architecture. This architecture follows the Open Systems Interconnection (OSI) model, which specifies a series of layers, including lower level networking protocols for connecting and transmitting data, and interaction with higher level application protocols.


T.120 is an umbrella standard that encompasses the following communication and application standards and components:

  • T.121 

    This standard provides a generic application template (GAT), which specifies a common set of guidelines for building application protocols and the management facility that controls the resources used by the application. T.121 also describes how an application protocol, such as T.127 for file transfer, performs the following functions:

    • Registers itself with the conference. 

    • Applies its capabilities locally and remotely. 

    • Interoperates and negotiates capabilities with other applications.

    To ensure application consistency, T.121 is a required standard for products developed under T.120. The ITU also recommends that nonstandard applications incorporate T.121 to provide product interoperability.

  • T.122 

    This standard defines the multipoint services, which allow one or more participants to send data as part of a conference. These multipoint services are implemented by T.125, which provides the mechanism for transporting the data. Together, the T.122 and T.125 standards make up the T.120 multipoint communication services (MCS). T.122 supports various conference topologies; for more information, see "Topologies" later in this chapter. 

  • T.123 

    This standard is responsible for transporting and sequencing data, and for controlling the flow of data across networks, including connect, disconnect, send, and receive functions. For data transport, T.123 defines a series of network interface profiles. Also, T.123 provides an error-correcting mechanism that ensures accurate and reliable data delivery.

    T.123 Annex B, an addition to the T.123 data conferencing standard, also defines the protocol for secure data conferencing.

  • T.124 

    This standard provides the generic conference control (GCC) for initiating and administering multipoint data conferences. The GCC performs the following functions:

    • Serves as the information center, directing users and data in and out of conferences and monitoring progress so that the latest conference information is always available.

    • Maintains lists of conference participants and their applications; the GCC identifies compatible applications and features so that products can interoperate. 

    • Tracks MCS resources so that conflicts do not occur when conference participants use multiple application protocols, such as T.127 for file transfer and T.128 for application sharing. 

  • T.125 

    This standard specifies how data is transmitted within a conference. T.125 defines the private and broadcast channels that transport the data, and ensures accurate and efficient communication among multiple users. T.125 also implements the multipoint services defined by T.122. 

  • T.126 

    This standard specifies how an application sends and receives whiteboard information, in either compressed or uncompressed form, for viewing and updating among multiple conference participants. The role of T.126 is to manage the multiuser workspace provided by the whiteboard. 

  • T.127 

    This standard defines how files are transferred simultaneously among conference participants. T.127 enables one or more files to be selected and transmitted in compressed or uncompressed form to all or selected participants during a conference.

  • T.128 

    This standard was proposed by Microsoft as an addition to the T.120 standard and is accepted by the International Telecommunication Union, Telecommunication Standardization Sector (ITU-T). T.128 specifies the program sharing protocol, defining how participants in a T.120 conference can share local programs. Specifically, T.128 enables multiple conference participants to view and collaborate on shared programs.

  • Node Controller 

    The node controller is the command and control entity for T.120 and is responsible for administering network-level events, including the management of conference connections, participants, and conference data. This controller takes command of the other T.120 layers, particularly the transport layer, and uses the GCC, MCS, and other protocol services to manage the entire conference. The node controller acts as the translator, ensuring that events are interpreted and ordered correctly. 

For more information about the T.120 architecture, see the International Multimedia Teleconferencing Consortium (IMTC) Web site: 


The T.120 architecture supports several topologies that define how users connect to a conference and transmit data during the conference. The following illustration shows the topologies typical of NetMeeting conferences: star, daisy-chain, and cascaded.


One of the participants in the conference is assigned as the top provider (conference host). The top provider manages the resources in the conference, including meeting participants and any shared programs or features, such as Whiteboard and Chat.

The top provider is determined in the initial connection between the first two participants. Then, the relationship between the location of the top provider and that of any additional participants differentiates the various topologies.

When NetMeeting initiates a call for a person, it determines whether a conference is already running, and, if not, it creates one locally with that person as the top provider. If another conference is already running, NetMeeting gives the person the option of joining that conference. NetMeeting also provides a Host Meeting feature (found on the Call menu), which determines the top provider automatically, based on the selected conference host rather than on the caller order.

Data flows according to the conference topology, which is determined by how each connection in the conference is established. For example, in the following illustration, the following order of calls establishes the conference topology:

  • A (top provider) initiates calls to B and C. 

  • Then, B calls D and E (or D and E each call B). 

  • C calls F and G (or F and G each call C). 


There is only one top provider in a conference. After the top provider (A) is established, that computer remains the top provider throughout the conference.

Note Two conferences cannot be joined together. Therefore, if C called F and G first, it would not be possible for them to join the conference with A, B, D, and E.

During data conferencing, if B shared a program with the other conference participants, data would flow simultaneously to the adjoining connections (A, D, and E). Then, data would continue to flow outward to the remaining connections (C, F, and G).

If B hangs up or is removed from the conference, D and E are also removed. D and E may maintain a conference together, though, if they were connected with audio and video in the original session. D and E would not have data conferencing, however, because this function is removed when B hangs up.

T.120 Conferencing Products and Services

Because T.120 is purely a data standard, T.120-compliant products and services that operate with NetMeeting provide data conferencing and collaboration features rather than audio or video support. NetMeeting can potentially interoperate with any T.120 product or service that uses the same TCP/IP transport. These products and services can be separated into the following categories:

  • Data conferencing clients 

    The T.120 infrastructure gives products and services the ability to connect and send data. This means that NetMeeting is capable of connecting to any data conferencing client that supports ITU T.120. Also, these clients can potentially interoperate, depending on the compatibility of application layer protocols supported by the other product. Any incompatibility in protocols would be at the application layer, and not with the T.120 protocols. 

  • Conferencing servers or bridges 

    Otherwise known as a Multipoint Control Unit (MCU), conferencing servers or bridges host multipoint data conferences that could also be held in conjunction with audio or video conferencing. The MCU has the necessary components to handle multiple nodes from multiple computers in a single conference. For the MCU provider, conferencing services require an investment in server hardware, software, and technical support. Users benefit from guaranteed performance and scalability that can cross platforms, manufacturers, and networks. For more information about MCUs, see Chapter 11, "Understanding the H.323 Standard." 

    Note Using a bridge to host a meeting between computers with conferencing products from different manufacturers does not guarantee compatibility, particularly in the case of products that do not support the same application layer protocols. The conferencing bridge does not provide a compatibility layer that enables different products to interoperate. 

    Other manufacturers provide multipoint bridging and management services that enable many sites to collaborate over the Internet and enable users to schedule, create, administer, join, and track conferences in one convenient place. You will need to use NetMeeting hardware requirements through a conferencing bridge, which limits the conference to the speed of the slowest connection.