Both the Simple Mail Transport Protocol (SMTP) and the
Hypertext Transport Protocol (HTTP) were used to create SIP.
SIP was chosen because while it may not have been the perfect protocol, it did support
all forms of media, and with some modifications it could be made much more robust and
secure. The 3GPP has defined many extensions to SIP through the IETF for this very
reason.
The SIP protocol within the IMS is used to control everything that a subscriber does.
This includes voice, messaging, e-mail, and data transfers. This is a big advantage over
conventional VoIP implementations because it allows the operator to support all media
types using one common protocol. It also means the operator can invest in one set of
back office applications supporting SIP rather than having to invest in many different
systems supporting disparate protocols.
The use of the SIP protocol for all things IMS simplifies the implementation of an all-
IP network considerably for many different reasons, including those already discussed.
Yet SIP is not the perfect answer, as there are still things missing from SIP that are
needed in the IMS. These are the extensions just mentioned.
SIP is not a new protocol. In fact, SIP was created from two well-known protocols
widely used on the Internet today: Simple Mail Transport Protocol (SMTP) and
Hypertext Transport Protocol (HTTP).
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