Polycom V2IU Converged Network Appliance 6400-S - Manual (Page 31)

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Configuring VoIP
An application-layer gateway provides basic proxy features for voice and
video over IP traffic. Serving as an ALG proxy, the 6400-S provides Network
Address Translation (NAT) services for the protected softswitch, gatekeeper
or other media devices. It maps multiple devices on the subscriber interface
(public) to a single IP address on the provider interface (private). The ALG
must first recognize and register a public network based device before it
presents traffic from the IP telephone, video endpoint or data device through
its provider port.
The 6400-S contains an MGCP, SIP, and H.323 call-control proxy ALG. VoIP
phones, video endponts and client adapters have to be configured to point to
the 6400-S as the call-control server, proxy, gatekeeper or gateway (depending
on protocol). The 6400-S then forward this traffic onto the actual call-control
server or gatekeeper.
For corporate customers with high-end routers and firewalls, the 6400-S can be
configured as a VoIP Application Layer Gateway only. This allows all of the
normal data traffic to continue to be handled by the existing network devices,
and only voice or traffic to be handled by the 6400-S. For this configuration, the
6400-S Subscriber Ethernet port is connected to the internet. The 6400-S
Provider Ethernet port is connected to a port on the local Ethernet switch.