H.323 Architecture Protocols and Procedures

H.323 is an umbrella recommendation that depends on several other standards and recommendations to enable real-time multimedia communications. The following are the key H.323 reference recommendations.

Call Signaling and Control

• H.323: Packet-based multimedia communications systems

• H.225: Call control protocol

• H.235: Security

• H.245: Media control protocol

• Q.931: Digital subscriber signaling

• H.450.1: Generic functional protocol for the support of supplementary services in H.323

• H.450.2-11: Supplemental features: blind call transfer and call diversion, consulting (450.2); call forward, activation/deactivation, interrogation (450.3); call hold (450.4); call park, call pickup (450.5); call waiting (450.6); message waiting (450.7); name interrogation (450.3), call hold (450.4); call park/call pickup (450.5); call waiting (450.6); message waiting (450.7); name identification service (450.8); call completion (450.9); call offer (450.10); call intrusion (450.11)

H323 Annexes

• Annex D: Real time fax over H.323

• Annex E: Multiplexed call signaling

• Annex F: Simple Endpoint Terminal (SET)

• Annex G: Text SET

• Annex H: Mobility

• Annex I: Operation over low QoS networks

• Annex J: Secure SET

• Annex K: HTTP service control transport

• Annex L: Stimulus signaling

• Annex M: Qsig tunneling

• Annex N: QoS

Audio Codecs

• G.711: PCM audio codec 56/64 kbps

• G.722: Audio codec for 7 Khz at 48/56/64 kbps

• G.723: Speech codec for 5.3 and 6.4 kbps

• G.728: Speech codec for 16 kbps

• G.729: Speech codec for 8/13 kbps

Video Codecs

• H.261: Video codec for ≥ 64 kbps

• H.263: Video codec for ≤ 64 kbps

H.323 Version 1 was approved in 1996. Version 1 centered on multimedia communications, such as voice and video over IP data networks. Version 2 was approved in January 1998, and addressed deficiencies in Version 1 and introduced new functionality within existing protocols, such as Q.931, H.245, and H.225, and entirely new protocols. The most significant advances were in security, fast call set-up, supplementary services, and T.120/H.323 integration. There was more efficiency about getting media streams to transfer at a faster rate. The major functions introduced were Fast Connect (also known as Fast Start) and H.245 tunneling (transferring minimal call signals to more quickly establish connection). Version 3, approved in May 1999, had several new annexes or sections focused on H.323-compliant devices for large-scale production networks. It covered bandwidth management and QoS issues and focused on “smart” networks and “dumb” endpoints (master/slave relationship). Some specific areas addressed were:

• Connection over UDP

• Simple endpoint type

• Interdomain communications

• H.263 and packetization

• H.GCP decomposition architecture

H.323 Version 4 was approved in November 2000 and contained many enhancements in a number of important areas, including reliability, scalability, and flexibility. Many of the new features facilitate more scalable gateway and MCU solutions to meet the market requirements for IP telephony. Version 4 addressed the following issues:

• Gateway decomposition

• Multiplexed stream transmission

• Supplementary services

• Additive registrations

• Alternate gatekeepers

• Usage information reporting

• Endpoint capacity

• Tones and announcements

• Mapping aliases

• Indicating desired protocols

• Bandwidth management

• Reporting call status

• Real-time fax

• Call linkage

• Tunneling

• QoS

• H.245 in parallel with Fast Connect

• Generic extensibility framework

• H.323 URL

• Call credit-related capabilities

• DTMF relay via RTP