Features Page 161 of 910
IP Line Description, Installation and Maintenance
To support multiple IP Phones behind one NAT device, NAT must map
between public/private IP addresses, and ports for each IP Phone behind it.
There is a mapping for both a signaling port and a media (voice) port.
Placing an IP Phone behind Multiple NAT devices is an unsupported
configuration. If it is necessary to have a configuration with multiple NATs
between the IP Phone and the Voice Gateway Media Card, all NATs on the
path must follow the rules described in the following sections for signaling
and media streams.
Mapping is configured and implemented using the NAT device. The IP Line
application does not implement any of the mappings.
NAT and signaling
NAT hides the true identity of the IP Phone from the LTPS. The LTPS is only
aware of any IP Phone based on the public IP address and port of the signaling
messages. A signaling message originates from the IP Phone on the private
side from port 5000. That signaling message is then mapped from the private
side to a public IP and port; that is the IP address seen by the LTPS.
Signaling messages between the Voice Gateway Media Card and IP Phones
are carried by RUDP. Each RUDP connection is distinguished by its IP
address and port number.