 Terminals-A terminal is the end device of every connection. It provides real-time two-way communications with another H.323 terminal, gateway, or MCU. This communication consists of speech, speech and data, speech and video, or a combination of speech, data, and video. Gateways-Gateways establish the connections between the terminals in the H.323 network and the terminals that belong to networks with different pro- tocol stacks, such as the traditional PSTN network, SIP, or Megaco endpoints. Gatekeepers-Gatekeepers are responsible for translating between tele- phone numbers and IP addresses. They also manage the bandwidth and provide mechanisms for terminal registration and authentication. A gate- keeper also provides services such as call transfer and call forwarding. The main gatekeeper functions fall into two categories: mandatory services and optional services. Mandatory services address translation, admission con- trol, bandwidth control, and zone management. Optional services include Scope of H.323 Terminal H.323 H.323 Terminal Terminal H.324 Terminal H.323 Speech H.323 Terminal Gateway H.320 H.323 Terminal Gatekeeper H.323 Terminal Speech Terminal Figure 9.4 The H.323 architecture Chapter 9 IP Services call control signaling, call authorization, bandwidth management and reser- vation, call management, and directory services. Multipoint control units (MCUs)-MCUs establish multipoint confer- ences. An MCU consists of a mandatory multipoint control, which is for call signaling and conference control, and an optional multipoint processor, which is for switching or mixing of media streams and sometimes real-time transcoding of the received audio/video streams. Five types of information exchange are enabled in the H.323 architecture: audio; digitized voice; digitized video; data, including files or images; and communication control, which involves the exchange of supported functions, control of logic chan- nels, and control of connections and sessions (i.e., the setup and teardown). H.323 has a number of streng