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I am beginning to dive into IMS (IP Multimedia Subsystem) technologies. Where can I get some good background/introductory information about the related technologies?

• A PDF overview from 2007: https://www.rennes.enst-bretagne.fr/~gbertran/files/IMS_an_overview.pdf.
  Was linked to from the Wikipedia article.
   
• Wikipedia currently (June 3, 2010) has an article on the technologies: http://en.wikipedia.org/wiki/IP_Multimedia_Subsystem
  A whole lot of Acronym Soup.
   
• OpenIMSCore (https://www.openimscore.org/) has some Vmware VMs you can download to start playing with the technologies.