This is yet another reason why we may not see full SIP-to-SIP, IMS-to-IMS
interoperability without gateways and other devices to interface to legacy platforms
and systems we have already discussed.
This is truly an evolutionary approach to IMS implementation, and for any operator
who already has a legacy network in place, it is the only approach that makes any business
sense. The long-term goal is to support SIP-enabled devices in a pure IMS environment,
but the road to get there will be long and require transitional approaches.
Figure 2.8 The pure IMS network
MSC PDSN
I-CSCF
P-CSCF
P-CSCF
P-CSCF
P-CSCF
MSC
BSC
AN
Node
A
Node
B
Node
A
Node
B
AN
PCF
VLR
HLR
BTS BTS BTS
Simplified IMS network
HSS
AS
AS
AS
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Chapter 3
Session Initiation Protocol (SIP)
The 3GPP had many protocols to choose from when creating implementation standards
for VoIP. There already existed many different call control protocols, such as MEGACO,
Media Gateway Control Protocol (MGCP), H.323 protocols, and many more. This, in
fact, is one of the problems when trying to implement VoIP networks; there are many
different protocols that have to interoperate, yet there lacks an implementation and
interoperability standard for VoIP.
The SIP protocol was actually derived from existing protocols developed to support
other forms of media.
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