The H.323 Alg - D-Link NetDefend DFL-210 User Manual

6.2.9. The H.323 ALG

InboundBypassProxy
6.2.9. The H.323 ALG
H.323 is a standard approved by the International Telecommunication Union (ITU) to allow
compatibility in video conference transmissions over IP networks. It is used for real-time audio,
video and data communication over packet-based networks such as the Internet. It specifies the
components, protocols and procedures for providing such multimedia communication, including
Internet phone and voice-over-IP (VoIP).
H.323 Components
H.323 consists of four main components:
Terminals
Gateways
Gatekeepers
Multipoint Control Units
H.323 Protocols
The different protocols used in implementing H.323 are:
H.225 RAS signalling and Call
Control (Setup) signalling
H.245 Media Control and
Transport
Action Src Interface
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Devices used for audio and optionally video or data
communication, such as phones, conferencing units, or
"software phones" such as the product "NetMeeting".
An H.323 gateway connects two dissimilar networks and
translates traffic between them. It provides connectivity
between H.323 networks and non-H.323 networks such as
public switched telephone networks (PSTN), translating
protocols and converting media streams. A gateway is not
required for communication between two H.323 terminals.
The Gatekeeper is a component in the H.323 system which is
used for addressing, authorization and authentication of
terminals and gateways. It can also take care of bandwidth
management,
gatekeeper may allow calls to be placed directly between
endpoints, or it may route the call signalling through itself to
perform functions such as follow-me/find-me, forward on
busy, etc. It is needed when there is more then one H.323
terminal behind a NATing device with only one public IP.
MCUs provide support for conferences of three or more
H.323 terminals. All H.323 terminals participating in the
conference call have to establish a connection with the MCU.
The MCU then manages the calls, resources, video and audio
codecs used in the call.
Used for call signalling. It is used to establish a connection
between two H.323 endpoints. This call signal channel is
opened between two H.323 endpoints or between a H.323
endpoint and a gatekeeper. For communication between two
H.323 endpoints, TCP 1720 is used. When connecting to a
gatekeeper, UDP port 1719 (H.225 RAS messages) are used.
Provides control of multimedia sessions established between
two H.323 endpoints. Its most important task is to negotiate
opening and closing of logical channels. A logical channel
could be, for example, an audio channel used for voice
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