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Gateways
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.
Gatekeepers
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, accounting, billing and charging. The
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.
Multipoint Control Units
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.
H.323 Protocols
The different protocols used in implementing H.323 are:
H.225 RAS signalling and Call
Control (Setup) signalling
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.
H.245 Media Control and
Transport
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
communication. Video and T.120 channels are also called
logical channels during negotiation.
T.120
A suite of communication and application protocols.
Depending on the type of H.323 product, T.120 protocol can
be used for application sharing, file transfer as well as for
conferencing features such as whiteboards.
H.323 ALG features
The H.323 ALG is a flexible application layer gateway that allows H.323 devices such as H.323
phones and applications to make and receive calls between each other when connected via private
networks secured by NetDefend Firewalls.
The H.323 specification was not designed to handle NAT, as IP addresses and ports are sent in the
payload of H.323 messages. The H.323 ALG modifies and translates H.323 messages to make sure
that H.323 messages will be routed to the correct destination and allowed through the NetDefend
Firewall.
The H.323 ALG has the following features:
6.2.9. The H.323 ALG Chapter 6. Security Mechanisms
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