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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Terminals<br />

Terminals are the LAN client endpoints in an <strong>IP</strong> telephony network. An H.323<br />

terminal can communicate with other H.323 terminals, an H.323 gateway, or an<br />

H.323 multipoint control unit (MCU).<br />

All terminals must support voice communication, though support for video <strong>and</strong><br />

data is optional. H.323 specifies the operations required for different types of<br />

terminals to work together, <strong>and</strong> all H.323 terminals must support the set of<br />

protocols described in 20.5.2, “H.323 protocol stack” on page 741.<br />

Additionally, H.323 terminals support multipoint conversations <strong>and</strong> provide the<br />

ability to initiate ad-hoc conferences. They also have multicast features that allow<br />

multiple people to participate in a call without centralized mixing or switching.<br />

Gateways<br />

Gateways enable st<strong>and</strong>ard telephones to use Vo<strong>IP</strong> services. They provide<br />

communication between H.323 terminals <strong>and</strong> terminals connected to either an<br />

<strong>IP</strong>-based network or another H.323 gateway. The gateway functions as a<br />

translator, providing the interface between the PSTN <strong>and</strong> the <strong>IP</strong> network.<br />

The gateway is responsible for mapping the call signaling <strong>and</strong> control protocols<br />

between dissimilar networks. It is also responsible for media mapping (for<br />

example, multiplexing, rate matching, audio transcoding) between the networks.<br />

Gatekeepers<br />

Gatekeepers are the most important component in an H.323 environment. A<br />

network of H.323 terminals <strong>and</strong> gateways under the control of a particular<br />

gatekeeper forms an integrated subnetwork within the larger <strong>IP</strong> network<br />

environment. The gatekeeper's functions include:<br />

► Directory server: Using information obtained during terminal registration, this<br />

function translates an H.323 alias address to an <strong>IP</strong> (transport) address. This<br />

enables the user to have meaningful, unchanging names to reference other<br />

users in the system. These names are arbitrary <strong>and</strong> can appear similar to<br />

those used in e-mail or voice mail applications.<br />

► Supervisory: The gatekeeper can grant permission to make a call. This can<br />

be used to apply b<strong>and</strong>width limits, reducing the likelihood of congestion within<br />

the network.<br />

► Call signaling: The gatekeeper can perform call routing functions to provide<br />

supplementary services. It can also provide multipoint controller functionality,<br />

supporting calls with a large number of participants.<br />

► Call management: The gatekeeper can perform call accounting <strong>and</strong> call<br />

management.<br />

740 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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