A Few Salient IMS Features

Major Features
Runs over a Public Land Mobile Network (PLMN)
The PLMN is logical divided into Core Network (CN) and Access Network (AN) infrastructures
The AN is the radio area network stuff (BSS for GSM, RNS for UMTS)
The CN has Circuit Switched (CS) and Packet Switched (PS) domains
Note that "legacy" voice is circuit switched

Basically, IMS comprises all of the CN elements needed to provision IP services over the PS domain -->
Services Control Plane + Policy Engine + AAA
SIP-based control plane/signaling
Policy via COPS (SOAP/BEEP?)
AAA via Radius (Diameter?)