Snom 4S - Manual (Page 5)

snom technology AG • 5
1 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 is a SIP session border controller (SBC).
It enables non-NAT aware devices to operate in private networks. It also
allows operating the data center in a private network. It takes care about
translation of SIP messages with private network identities into identities
that can be addressed from the data center. When necessary or explicitly
required, it forwards media and changes the SDP attachments in the SIP
messages accordingly.
The SBC offers recording capabilities (depending on the licens-
ing). Through a separate management interface, operators can define
numbers and patterns that are silently recorded. Users may explicitly re-
quest 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 an appropriately configured web server, operators can
manage very large numbers of calls and, for example, forward them per
e-mail to users or authorities.
The SBC does not translate signaling, e.g., from SIP to H.323 or
to MGCP. It is a semi-transparent SIP proxy that takes care only about
known methods and leaves the rest unchanged. 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 advantage over back-to-back user agents (B2BUA) that operate on the
application level.
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