Overview - Snom 4S NAT Filter Admin Manual

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Network address translation (NAT) is a reality today. There
have been many discussions about the evil and the good of this network
topology and the replacement by IP version 6. However, operators and
business that want to offer VoIP services today and they must address
the problem.
The snom 4S NAT Filter enables non-NAT aware devices to
operate in private networks. The filter operates typically on a public IP
address. Non-NAT aware devices are automatically refreshed; NAT-aware
devices that operate behind symmetrical NAT may self-refresh their
bindings using the built-in STUN server of the filter, which is operating
on the same address as the SIP application. Devices on public Internet
traverse the filter without changes or refreshes. By supporting Interactive
Communications Establishment (ICE), the number of calls that must go
through the filter can be minimized.
The product also offers recording capabilities (depending on
the licensing). Through a separate management interface, operators
can define numbers and patterns that are silently recorded. Users may
explicitly request the recording of a call by pressing a key on the phone,
in this case the whole call will be recorded (even parts of the call before
pressing the key). The filter records in a compressed format where only
the voice part of the conversation is recorded in highly-compressed audio
format (13.2 kBit/s). By using the snom 4S recording studio, operators
can manage very large numbers of calls and for example forward them
per Email to the users or authorities.
The snom 4S NAT Filter is a session border controller. However,
we do not like the term "session border", because the program does
not control sessions nor is it on the border of a call. It does also not
translate signalling, e.g. from SIP to H.323 or to MGCP. If all user agents
are fully NAT-compliant or on public Internet, the filter can transparently
be removed from the network without changing of the call flows or
functionality. Also, the filter does not interfere with unknown applications.
This is tremendous advantage against session borders controllers that
operate on the application level.

Overview

snom technology AG • 5