In my latest book I have not only described how IMS works in mobile networks in Chapter 4 but I also gave a short introduction to the fixed line counterpart which was initially specified by TISPAN before the activities were merged in 3GPP. I've been wondering a bit lately if it's going to be easier to implement IMS in fixed line networks compared to mobile networks. Here's what I think is much easier:
- Legacy Device Reuse: In mobile networks, IMS requires mobile phones with IMS clients and a sophisticated packet switched radio network. In fixed line, things are much simpler. Customers just keep the analog phones they have today and either plug them into an analog to SIP converter that is part of their DSL or cable modem at home. If the analog to SIP converter is in the next street cabinet, the user doesn't have to do anything. To him IMS will be completely transparent.
- No interworking with a legacy network from a user point of view: Once a line is physically connected to the IMS system, it's just using that system. Period. In wireless, mobile phones often fall back to GSM where IMS is not available. In that case the legacy circuit switched system has to be used or at least a circuit bearer next to the IMS packet switched signaling. Not pretty and very complicated.
- No handovers to CS: A fixed line is a fixed line, no handover of an ongoing call to a circuit switched channel is required as a fixed line device never leaves it's place and network of origin. Compare that to wireless IMS where a mobile can leave the UMTS or LTE coverage area and has to be switched to a circuit switched bearer before contact is lost. A nightmare in practice.
- Different Mindset: In wireless, IMS is the IP MULTIMEDIA subsystem and many see voice telephony as just one service to run over it. In the fixed line world, I have the impression that for most people the IMS is there to put voice calls on an IP bearer (PSTN and ISDN emulation service) but that's pretty much it. No big dreams of IMS as the universal service platform for everyone and everything.