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SIP really doesn't do telephony all that well. Feature transparency is still a work in progress. Common features like 1A2 key system emulation are incredibly complex to implement. The whole presence concept is half-baked. SIP is just an evolving set of protocols
Now that is talking!
SIP does not do telpehony very well because traditional telephony features were designed for the late 70's and early 80's technological environment when they were developed. In particular, they were not designed for an environment of ubiquitous personal and device mobility. SIP was designed for just such a communications environment and more than that was designed to create applications within that environment. The traditional telpehony feature set has been made obsolete along with the ystems that provide them.
I wonder what use a 1A2 key system will be in a world in which SOHO users will be connected to DSL lines. I do not have the same puzzlement when I think about SIP services and SOHO users.
Feature transparency is an artifact of the technological past. It is a means to try to synchronize the workings of isolated call control feature logics. It has obvious limitations in the types of features that can be supported. One gets the standard lowest common denominator features and is unable to create any feature that would be of specific use either personally or in respect to an enterprise need.
The sooner that feature transparency is dropped as a goal the sooner this inustry will be able to create applications that will be actually ueful and actually attract customer interest. No more check box feautrez that nobody uses in other words.
The SIP purist on seeing the 3GPP architecutre would wonder why it took them so long to create so very little. The 3GPP architecture is just the SIP architecture dressed up in endless acronyms. SIP has no problem with application servers. Indeed it was designed to cater to the needs of application servers. It can route and reroute audio and data almost at will to coordinate the operation of multiple application servers
As an aside, ISDN was not about integrating the backbone, it was intended to create a single network for all services that would be accessed via a single socket type. The ISDN S bus was a long way both physically and conceptually from SONET rings.
ISDN failed for two main reasons. Firstly it failed because the telephone compnaies realized that the best palce for ISDN (like all digital network systems) was at the periphery. This was a threat to their monopoly and so they made sure that ISDN could not provide services at the periphery. In doing so, they made sure that ISDN could offer no useful services and so ensured that ISDN could find no customer base. Secondly, ISDN was developed at the time of the Internet expansion and just as it became available the web was developed. ISDN could not hope to compete agaisnt these two technologies.