Avaya Nortel Communication Server 1000 Manual page 118

118 Features
If a compatible NAT router is detected, successful IP Phone registration
occurs and the software invokes the NAT Mapping Keep Alive function to
prevent loss of the IP connection. If an incompatible NAT is detected, an
error is displayed on the IP Phone display and the IP Phone is not allowed
to register.
Mapping
When an IP Phone is used in a private network behind a NAT device, the
NAT router strips the IP Phone private IP address and private port number
and assigns it a public IP address and public port number.
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.
The NAT device performs private-to-public mapping for the signaling port
for each IP Phone behind it to support multiple IP Phones. The TPS uses
fixed port numbers for signaling. The NAT device must perform consistent
private-to-public mapping for these port numbers.
Ports" (page 119)
Copyright © 2003–2008, Nortel Networks
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lists the UDP port number used.
Nortel Communication Server 1000
IP Line Fundamentals
NN43100-500 02.02 Standard
Release 5.5 4 February 2008
Table 27 "Signaling UDP