IMS has made some headway in network infrastructures in part because, if you have an all-IP network, as with cable or WiMAX, for example, IMS provides specs for two key parts of the network architecture:
A secure carrier-grade signaling architecture based on SIP, an existing, widely used technology
Interconnection and inter-operation with the worldwide telephone network.
Neither IMS's Vogon ambitions to make charge-able events out of Web site visits, nor the potentially creative aspects of SIP signaling in enabling novel communications features helped IMS. It was purely practical matters that brought implementations of SIP that happened to be IMS capable into the telephone network.
Will IMS therefore win by default?
The answer will emerge from the way LTE (a.k.a. 4G) networks are engineered. LTE is tipped to be the dominant all-IP mobile wireless network technology, and is therefore the big prize for IMS vendors. You can think of LTE (4G) as 3G without the circuit switched telephony. LTE is faster and simpler than 3G.
The question is: Can you build an all-IP LTE network without IMS? Unfortunately for those network infrastructure vendors with a big investment in IMS, the answer appears to be “Yes.”
VoLGA is a spec related to LTE that describes how to do circuit switched voice with an LTE generic access network (GAN), and how to hand off between an LTE VoLGA call and a 3G network and 3G call. By teaching the 3G GAN controller (GANC) node a few things about how VoLGA handsets want to do circuit-switched signaling over IP, and how to set up gateways for VoLGA calls, you do away with the need for a new kind of signaling for IP calls.
That is, VoLGA is a specialized form of IP telephony built to get signaling and payload between an LTE endpoint and a mobile switching center (MSC), which is connected to the conventional telephone network. From that point, the LTE network and the phone calls on it look like a 3G circuit switched mobile network.
VoLGA is, therefore, a dagger at the heart of IMS. A VoLGA handset doesn't need SIP, much less IMS, to make phone calls on an all-IP LTE network. And an LTE network doesn't need to support SIP and IMS to support LTE handsets that support VoLGA.
This is both good and bad:
It's good because it extends the attractive simplicity of LTE: Fewer, simpler nodes make LTE networks cheaper to build and operate and faster to deploy.
It's good because it does not preclude IP communications services in the handset. In fact, it raises the importance of IP in the handset because the network does not take on responsibility for mediating IP communication.
It's bad because it closes the window on operators' ability to create new service products based on IMS capabilities. Cynics might say this is a deservedly unused non-opportunity.
Nevertheless, it does make adding features like, for example, HD voice more challenging in that they will have to be done in the circuit switched signaling domain for LTE voice calls.
For handset makers, VoLGA marks a clear boundary: LTE handsets are mobile Internet nodes, for sure, but they are also endpoints of the conventional PLMN, with 3G signaling. The IP side, with the possible exception of some messaging services, belongs to the Internet. The VoLGA phone calls belong to the telephone network.
Handsets are, therefore, IP communications devices: They are on an IP network, and anything beyond conventional circuit switched calls is going to be IP communication, and it is not going to be mediated by IMS.
Networks will be simpler, leaving more responsibility and opportunity for endpoint devices: Functions depending on multiple IM/VoIM protocols, one-to-many messaging, presence from multiple services, etc. will not have the excuse that “the IMS system will eventually gateway that stuff.” It's up to the endpoint developer to make it happen.
And what do you do if you have already placed a significant bet on IMS? You take the intent of IMS: To make all modes of communication available to the user and inter-operating, and you implement that intent in an endpoint device that takes the new modes of IP communication and makes them first-class citizens of the handset user experience, and does that using not just an IMS service, but using the services the user may want to use on the Internet.