snom technology AG • 5
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Overview
Network address translation (NAT) is a reality today. There have
been many discussions about the evil and the good of this network topol-
ogy and the replacement by IP version 6. However, operators and busi-
ness want to offer VoIP services today and therefore must address the
problem.
The snom 4S NAT Filter enables non-NAT aware devices to oper-
ate in private networks. The filter typically operates on a public IP ad-
dress. Non-NAT aware devices are automatically refreshed; NAT-aware
devices that operate behind symmetrical NAT may self-refresh their bind-
ings 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 Commu-
nications 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 de-
fine 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 e-mail to
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 neither
controls sessions nor is it on the border of a call. It also does not trans-
late 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 the call flows or functional-
ity. Also, the filter does not interfere with unknown applications. This is
a tremendous advantage over session border controllers that operate on
the application level.
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