The Voice over IP standard incorporates other components, including:
– G. standards, which specify analog-to-digital conversion and compression (as described earlier in this chapter).
– H.323 standard, which specifies call setup and interoperability between devices and applications.
– Realtime Transport Protocol (RTP), which manages end-to-end connections to minimize the effect of packets lost or delayed in transit on the network.
– Internet Protocol or IP, which is responsible for routing packets on the network.
ITU-T H.323 Standard
ITU-T H.323 is a standard approved by the ITU-T that defines how audiovisual conferencing data is transmitted across networks.
H.323 provides a foundation for audio, video, and data communications across IP networks, including the Internet.
H.323-compliant multimedia products and applications can interoperate, allowing users to communicate without concern for compatibility.
H.323 provides important building blocks for a broad new range of collaborative, LAN-based applications for multimedia communications.
H.323 sets multimedia standards for the existing infrastructure (for example, IP-based networks). Designed to compensate for the effect of highly variable LAN latency, H.323 allows customers to use multimedia applications without changing their network infrastructure.
By providing device-to-device, application-to-application, and vendor-to-vendor interoperability, H.323 allows customer’s products to interoperate with other H.323-compliant products. PCs are becoming more powerful multimedia platforms due to faster processors, enhanced instruction sets, and powerful multimedia accelerator chips.
Applications enabled by the H.323 standard include the following:
- – Internet phones
- – Desktop conferencing
- – Multimedia Web sites
- – Internet commerce
- – And many others
The H.323 standard specifies four kinds of components, which when networked together, provide the point-to-point and point-to-multipoint multimedia communication services: terminals, gateways, gatekeepers, multipoint control units (MCUs).
H.323 terminals are used for real-time bidirectional multimedia communications. An H.323 terminal can either be a PC or a standalone device running an H.323 and the multimedia applications. It supports audio communications and can optionally support video or data communications.
An H.323 gateway provides connectivity between an H.323 network and a non-H.323 network. For example, a gateway can connect and provide communication between an H.323 terminal and the Public Switched Telephone Network (PSTN). This connectivity of dissimilar networks is achieved by translating protocols for call setup and release, converting media formats between different networks, and transferring information between the networks connected by the gateway. A gateway is not required, however, for communication between two terminals on an H.323 network.
A gatekeeper can be considered the “brain” of the H.323 network. Although they are not required, gatekeepers provide important services such as addressing, authorization, and authentication of terminals and gateways, bandwidth management, accounting, billing, and charging. Gatekeepers may also provide call-routing services.
MCUs provide support for conferences of three or more H.323 terminals. All terminals participating in the conference establish a connection with the MCU. The MCU manages conference resources, negotiates between terminals for the purpose of determining the audio or video CODEC to use, and may handle the media stream. The gatekeepers, gateways, and MCUs are logically separate components of the H.323 standard, but can be implemented as a single physical device.
H.323 Gatekeeper Functionality
Gatekeepers provide call control services to network endpoints. A gatekeeper can provide the following services:
Address translation—Performs alias address to transport address translation. Gatekeepers typically use a translation table to perform the address translation.
Admissions control—Authorizes LAN access based on call authorization, bandwidth, or other criteria.
Call control signaling—The gatekeeper chooses to complete call signaling with endpoints or may process the call signaling itself. Alternatively, the gatekeeper may instruct endpoints to connect call signaling channel directly to another to bypass handling a signal channel.
Call authorization—A gatekeeper may reject calls from a terminal upon authorization failure.
Bandwidth management—Controls the number of terminals that are permitted simultaneous access to a LAN.
Call management—Maintains a list of active calls.