This is because the early versions of SIP were developed for use in delivering voice
over the Internet. The concept of secure communications over the Internet could be a
Establishing a Session in the IMS 107
bit of an oxymoron in the ???Wild West??? of this space. The Internet has always used minimal
restrictions on the users in terms of connectivity and security.
The 3GPP has recognized that SIP, according to RFC 3261, did not provide the robust
controls needed for their wireless networks, and therefore added many enhancements
and extensions to the protocol for use in traditional telecom models.
The first step for any session establishment is establishing a connection to the IMS
network. This does not mean connecting to the IP network, or the GSM network, but
the call control network. The subscriber must first establish the physical connection
into the access network (such as a landline or a wireless connection). Then they can
begin communicating via SIP to the IMS.
Depending on the device type, and the location of the subscriber, there may be direct
access into the IMS or the connection may rely on various gateways and other networks
to gain access. For example, if a subscriber has a SIP-enabled device, the service provider
may be able to provide a direct connection to the P-CSCF within their network
(the P-CSCF function is described in greater detail later).
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