I have just read the whitepaper "IMS - IP Multimedia Subsystem" by Ericsson, and here is my executive summary for those who don't have time to read this 25-page white paper.
IMS (IP Multimedia Subsystem) is an international standard (3GPP, 3GPPs) that enable IP-based multi-media communications across wireless operators.
Today, many wireless communications (such as voice calls) are not IP-based. IMS helps wireless operators to migrate more services to IP-based serviecs (such as "voice over IP" instead of "circuit switched voice call").
Many advanced services (such as Push-to-Talk or Video-phone) are operator specific services and do not work across operators (e.g., Nextel user can not talk to Verizon user using Push-to-Talk service, DoCoMo user can not make a videl call to Vodafone user). IMS does not simply make it possible, but also makes it easy to enable inteoperability of those advanced multimedia services - with a single service-independent inter-operator relationship, eliminating service-specific network-to-network interfaces.
IMS also unifies variety of communication services (voice call, push-to-talk, video call, text message, picture message, instant message, ...) with a single authentication, session control, and routing mechanism - making it possible to offer much more integrated services (such as switching from voice call to vide call in the middle, or sending pictures while making a voice call).
People from web or software industry may say "why do they need such a standard? There is an internet standard, SMTP for e-mail. AOL, Microsoft and Yahoo provide IM services and Skpe offers great VoIP service. " - very good point.
The answer is "wireless operators don't want to be a dumb pipe". There are still significant values they have beyond just access to the network - such as "phone-number-based routing", "billing service", "connectivity to traditional phone system", and most importantly "the relationship with subscribers". Wireless operators want to leverage these strength and become one of important service providers (competing Skype, AOL, MSN and Yahoo in a certain degree) - not only over wireless network but also over wirelines and broadband.
IMS infrastructure providers like Ericsson, Lucent and Nokia are working very hard to convince wireless operators to buy into this IMS vision, and help those operators to migrate to IP-based services quickly, and even help them to become significant players if and when the wireless/wired convergence happens.
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