DATA

            Support for data sharing is an optional component of H.323. When supported, data conferencing enables collaboration sharing, whiteboard sharing, file transfer, fax transmission, and instant messaging. The T.120 standard provides this capability to H.323.

T.120 is a real-time data communication protocol designed specifically for conferencing needs. Like H.323, Recommendation T.120 is an umbrella for a set of standards that enable the real-time sharing of specific applications data among several clients across different networks.

T.120 Architecture

            Figure 5. T.120 Architecture

The T.120 architecture may be seen as a composition of two layers. The lower layer provides the ability to control the hardware and software systems participating in a data conference.  The upper layer, on the other hand, provides the ability to share user applications among conference participants. 

The lower layer is composed of the following standards: 

1. T.122 – T.122 is the Multipoint Communication Service (MCS) for Audiographics Conferencing - Service Definition. MCS is a generic service designed to support highly interactive multimedia conferencing. It gives full-duplex multipoint communication between a number of applications, over a variety of networks.

2. T.123 – The T.123 standard basically prescribes the means by which participants in a conference communicate. T.123 defines the Protocol Stacks for Audiographic and Audiovisual Teleconference Applications. It specifies transport profiles for different types of networks such as LAN, ISDN, PSTN, etc.

3. T.124 - T.124 is referred to as the Generic Conference Control (GCC). It provides a framework for conference management and control of audiographic and audiovisual terminals and Multipoint Control Units (MCUs). It involves functions such as conference establishment and termination, managing the participation of those involved in the conference, managing the role of application protocols and application capabilities, etc.

4. T.125 - T.125 is the Multipoint Communication Service Protocol Specification. It defines a protocol that operates through a multipoint communication domain. It specifies the format of messages and the procedures for their exchange over transport connections.

Together, T.122 and T.125 form the multipoint “engine” of T.120 known as an MCS or Multipoint Control System.  MCS relies on T.123 to deliver data.

The upper layer is composed of the following standards:

1.                  T.121 - T.121 is referred to as the Generic Application Template (T.GAT). It describes a generic model of a T.120 application and defines a template that encompasses operations that are common to T.120 application protocols. It is intended to provide a common structure for T.120 application protocols. The GAT is a conceptual model and does not impose rules on the structure of the application.

2.                  T.126 - T.126 is referred to as Multipoint Still Image and Annotation Protocol. It defines a protocol that can be used by applications that require interoperable graphical information exchange in a multi-vendor environment. It provides the functionality required for whiteboarding, annotated image exchange and hard copy image exchange. Whiteboarding is a facility that enables one participating party to load an image onto a conceptual whiteboard which can then be viewed and annotated by other participants on-line during the conference.

3.                  T.127 - T.127 is concerned with Multipoint Binary File Transfer. It defines a protocol that supports the transfer of files within a conferencing group or any group communication where the T.120 series of protocols is being used. It provides the facilities to distribute one or more files simultaneously using the primitives provided by T.122 (Multipoint Communication Service). T.127 is a versatile, lightweight protocol which is designed to work between both sophisticated and relatively simple applications.

Interoperability

            There are two levels of interoperability that may be attained between T.120 products: network-level interoperability and application-level interoperability. Network-level interoperable products have the ability to:

1.      Establish and maintain conferences without any platform dependence.

2.      Manage multiple participants and programs.

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

Application-level interoperable products can utilize the whiteboarding and multi-point file transfer features of T.120. Application-level interoperable products must also be network-level interoperable.

Advantages 

The following is a list of major benefits associated with the T.120 standard.

 1.Multipoint Data Delivery – T.120 provides an elegant abstraction for developers to create and manage a multipoint domain with ease. From an application perspective, data is seamlessly delivered to multiple parties in real-time.

 2.Interoperability - T.120 allows endpoint applications from multiple vendors to interoperate. T.120 also specifies how applications may interoperate with (or through) a variety of network bridging products and services that also support the T.120 standard.

 3.Reliable Data Delivery – Error-corrected data delivery ensures that all endpoints will receive each data transmission.

 4.Multicast Enabled Delivery – In multicast enabled networks, T.120 can employ reliable (ordered, guaranteed) and unreliable delivery services. Unreliable data delivery is also available without multicast. By using multicast, the T.120 infrastructure reduces network congestion and improves performance for the end user. The T.120 infrastructure can use both unicast and multicast simultaneously, providing a flexible solution for mixed unicast and multicast networks. The Multicast Adaptation Protocol (MAP) is expected to be ratified in early 1998.

5. Network Independence – The T.120 standard supports a broad range of transport options, including the Public Switched Telephone Networks (PSTN or POTS), Integrated Switched Digital Networks (ISDN), Packet Switched Digital Networks (PSDN), Circuit Switched Digital Networks (CSDN), and popular local area network protocols (such as TCP/IP and IPX via reference protocol). Furthermore, these vastly different network transports, operating at different speeds, can easily co-exist in the same multipoint conference.

8.Application Independence - Although the driving market force behind T.120 was teleconferencing, its designers purposely sought to satisfy a much broader range of application needs. Today, T.120 provides a generic, real-time communications facility that can be used by many different applications. These applications include interactive gaming, virtual reality and simulations, real-time subscription news feeds, and process control applications.

9.Co-existence with Other Standards - T.120 was designed to work alone or within the larger context of other ITU standards, such as the H.32x family of video conferencing standards. T.120 also supports and cross-references other important ITU standards, such as V.series modems.

10.Extendability - The T.120 standard can be freely extended to include a variety of new capabilities, such as support for new transport stacks (like ATM or Frame Relay), improved security measures, and new application-level protocols.