H.323 Standard

H.323 was one of the first standards for asynchronous and packet-based multimedia data transmission. This standard remains the backbone of various technologies. Mainly, the protocol stack of the standard ensures real-time voice and video transmission over IP networks, enabling audio/video call services, including multipoint conferencing.

A key innovation in the penultimate version of the H.323 standard released in 2006 was the concept of the "assigned gatekeeper," an extension of the alternative gatekeeper concept.

With the "assigned" gatekeeper, an endpoint can register without referring to the list of alternative gatekeepers and switch to an alternate controller upon failure, according to selection procedures. The endpoint, or "current" gatekeeper, continues to monitor the state of the "assigned" gatekeeper, switching the system back upon restoration. This helps operators preemptively maintain network readiness. If alternate gatekeepers are geographically dispersed, endpoints should register with the nearest one. In any case, the "assigned" gatekeeper concept provides operators with greater flexibility in using network resources.

Another significant improvement in H.323 is related to security procedures. Earlier versions supported security via H.235—Security and Encryption for H.323 (and Other H.245-Based) multimedia terminals. This applies to any terminals used in point-to-point and multipoint conferences that use H.245 for control. H.235 ensures H.323 systems' authentication, secrecy, and integrity, focusing more on user rather than terminal identification.

H.235 security profiles allow the use of either user passwords or digital certificates with public key encryption or both. Using these profiles isn't mandatory.

H.235 allows for flexible service and function combinations regarding cryptographic algorithm choices (both standard and proprietary) and their operations. Each H.3xx device should support enough algorithms to ensure compatibility.

In H.323 version 6, security was enhanced by overhauling H.235, resulting in a family of recommendations from H.235.0 to H.235.9. Manufacturers could offer a standardized version of RTP—SRTP, ensuring IP media stream security.

This H.323 version also includes new documents and applications for H.245, permitting various codes like GSM iLBC and H.264; more detailed procedures for call transparency, error handling, etc.; clarifications in H.225 on field use, QoS support (H.361), and broadcast messages (H.460.21); improvements for real-time text and data combined transmission to conform with V.151.

New functions in H.323:

H.235.0–H.235.9 - H.323 system security definitions, including SRTP protocol support;

H.239 - managing stream data type indication, Role Management;

H.241 - Extended Video Procedures;

H.249 - Extended User Input Indications (e.g., mouse clicks or cursor movements);

H.361 - End-to-End Quality of Service (instead of H.323 Appendix N) and Service Priority Signaling;

H.460.10 - Call Party Category (ISUP field transport through H.323);

H.460.11 - Delayed Call Establishment (ensuring media stream presence before the incoming call signal);

H.460.12 - Glare Control Indicator;

H.460.13 - Called User Release Control (important for critical calls like emergency services to know how and when the connection was dropped);

H.460.14 - Multi-Level Precedence and Pre-Emption (adding MLPP support to H.323);

H.460.15 - Call Signaling Transport Channel Suspension and Redirection;

H.460.16 - ensuring reliable disconnection via Multiple-Message Release Sequence Capability;

H.460.17 - RAS Tunneling Through H.225.0;

H.460.18 - Traversal of H.323 Signaling across NAT and Firewalls;

H.460.19 - Traversal of H.323 Media Across NAT and Firewalls;

H.460.20 - Location Number for H.323;

H.460.21 - Message Broadcast for H.323 Systems.

Let's also look at two other standards in the H.323 family: video coding protocols and the T.120 conference protocols.

Video coding standardization has come a long way from the early H.261 and MPEG-1 to the modern-day H.264/265, with the latter continually improving. The latest video coding standard is H.265 HEVC (High Efficiency Video Coding), known for its increased compression efficiency and interactive/broadcast video application support.

T.120 is a set of telecommunication and application protocols for real-time multipoint conferencing. T.120 products can establish connections, transfer and receive data, and work together using software and file sharing, etc.

Key features of T.120 include organizing and maintaining conferences on any platform, managing many participants and programs, and securing data exchange in various network scenarios.

T.120 protocol family:

H.323 is used for setting up sessions and multimedia streams in IP networks, while T.120 provides the protocol stack for collaboration and data exchange during these sessions. T.120 includes:

T.121: Basis for application protocols;

T.122, together with T.125: Available multipoint services;

T.123: Specifies transport profiles for PSTN, ISDN, CSDN, PSDN, Novell NetWare IPX, and TCP/IP networks;

T.124: Generic Conference Control (GCC), a toolkit for organizing and managing conferences;

T.125: Multipoint Communication Service Protocol (MCS) for signal information and data transfer between MCS providers;

T.126: Procedures for viewing and annotating still images between two or more applications;

T.127: File sharing service between conference attendees with bidirectional prioritization and optional compression before transport;

T.128: Call aspect specifications;

T.129: Interaction profile between T.120 and T.610/T.611;

T.130: Multimedia exchange session management;

T.131: Quality of service management in T.120;

T.132: Audio conference service protocol;

T.133: Data conference service protocol;

T.134: Video conference session management protocol.

T.120's architecture is a two-level structure with predefined interaction protocols. Lower-level protocols (T.122, T.123, T.124, and T.125) describe an application-independent mechanism for multipoint communication, while upper-level protocols (T.126 and beyond) are application protocols, allowing standardized and non-standardized applications to coexist within one conference. These additions expand T.120's functionality and applicability in various multimedia conference and data exchange scenarios.