If a subscriber wants to download a music file, SIP messages
(an INVITE initially) must be exchanged with the music download server. Once the
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server has authenticated and authorized the subscriber for the download, the server
can then send the file using the File Transfer Protocol (FTP). At the conclusion of the
download, SIP is used to release the session.
You can read more details about establishing a session in previous chapters. The
main point here is that SIP identifies each and every transaction in the network, and
it confirms whether or not service delivery was successful. If there was a problem with
service delivery, SIP will return an error code indicating that failure.
This is why SIP (or any other form of signaling) is so important to operators when
auditing subscriber billing and preventing repudiation. It??™s surprising that more operators
have not implemented this simple process already.
Access Security
The first step to securing any network is ensuring only those subscribers authorized
to use the network services are provided access. This requires a few procedures and
certainly can be challenging for any operator.
The most important is authentication. This process is required to ensure the subscription
is valid (they are who they say they are), and the device requesting services
is authorized to access the network.
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