Polycom MGC Administrator Administrator's Manual page 580

Appendix E: IP Network Components
E-6
With multimedia protocols, including H.323 and SIP, the local IP addresses
embedded in the packet payload are not routable in public networks. NAT
Traversal enables un-routable private addresses to be replaced with public,
routable ones in the packet payload, so that the media packets can find their
way through networks (both public and private) to reach the client devices.
Gateways
H.323
Optional
A gateway is a device that converts data from the format required for one type
of network to the format required for another, such as H.320 and H.323 or
SIP. Gateways are optional in an IP (H.323 or SIP) network if the network
does not include connections to other network types such as ISDN. A
gateway enables point-to-point multimedia calls, involving only two
endpoints using different signaling and media protocols, while an MCU
enables multipoint conferencing (three endpoints or more). The MGC unit
can be configured to include gateway functionality.
Gatekeepers
H.323
Optional
(Recommended)
To comply with H.323 standard, gatekeepers must perform two main
functions:
Translation of alias addresses of endpoints, MCUs and gateways to IP
•
addresses
• Bandwidth management
H.323 endpoints register with the gatekeeper at startup. When they register
they provide the gatekeeper with their IP address and a list of aliases by which
they are known to other devices. The gatekeeper maintains a database of
devices in the network that are active at any time.
SIP
Optional
SIP
N/A