D-Link DFL-860 Product Manual - Page 276
H.323 Protocols, H.323 ALG features, Gateways, Gatekeepers, Multipoint Control Units
UPC - 790069900204
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6.2.9. The H.323 ALG Chapter 6. Security Mechanisms Gateways Gatekeepers Multipoint Control Units 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, 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. 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: 276