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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Multipoint control units (MCUs)<br />

A multipoint control unit (MCU) is an endpoint in the network. It provides the<br />

ability for three or more terminals or gateways to participate in a multipoint<br />

conference. An MCU provides conference management, media switching, <strong>and</strong><br />

multipoint conferencing.<br />

There are three types of multipoint conferences:<br />

► Centralized: All terminals have point-to-point communication streams with the<br />

MCU. The MCU is responsible for management of the conference. It<br />

receives, processes, <strong>and</strong> sends the voice packets to other terminals.<br />

► Decentralized: Terminals communicate directly with each other. An MCU is<br />

not directly involved.<br />

► Mixed multipoint: This represents a mixture of centralized <strong>and</strong> decentralized<br />

conferences. The MCU ensures operations of the conference are transparent<br />

to the each terminal.<br />

20.5.2 H.323 protocol stack<br />

The H.323 specification is an umbrella recommendation. Various components of<br />

H.323 are defined by other ITU-T st<strong>and</strong>ards. Figure 20-4 on page 742 shows the<br />

suite of H.323 definitions:<br />

► H.225 call control: This protocol provides call signaling <strong>and</strong> control functions.<br />

In networks without a gatekeeper, call signaling messages are passed<br />

directly between the calling <strong>and</strong> called endpoints. In networks that contain a<br />

gatekeeper, these messages are initially exchanged between the calling<br />

endpoint <strong>and</strong> the gatekeeper. H.225 call control messages use <strong>TCP</strong> transport<br />

services.<br />

► H.225 registration, admission, <strong>and</strong> signaling (RAS) control: The RAS channel<br />

is used for communication between the endpoints <strong>and</strong> the gatekeeper. The<br />

protocol defines st<strong>and</strong>ard procedures for gatekeeper discovery, endpoint<br />

registration, endpoint location, <strong>and</strong> admissions control. H.225 RAS messages<br />

use UDP transport services.<br />

► H.245 conference control: This specifies the protocol used after a call<br />

establishment phase has completed. H.245 negotiates the media channels<br />

used by RTP <strong>and</strong> R<strong>TCP</strong>. The protocol defines st<strong>and</strong>ard procedures for<br />

exchanging capabilities, determining master <strong>and</strong> subordinate assignments,<br />

<strong>and</strong> controlling conferences.<br />

► The other protocols are already described in 20.2.3, “S<strong>IP</strong> protocol<br />

architecture” on page 734.<br />

Chapter 20. Voice over Internet Protocol 741

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