Everything worth knowing, nothing that isn't.
What are the main parts of IMS?
Call Session Control Functions route the SIP signalling: the P-CSCF is the device's first point of contact, the I-CSCF finds the right serving node and the S-CSCF registers the user and applies their service profile. The HSS holds subscriber profiles and authentication data, which the CSCFs read over the Diameter Cx interface. Application servers then provide the services themselves — telephony, messaging and conferencing.
Which services run on IMS?
Voice over LTE (VoLTE) and voice over 5G New Radio (VoNR), video calling, Wi-Fi calling, RCS chat and SMS over IP. As operators switch off 2G and 3G, IMS becomes the only path for voice on many networks, and voice while roaming depends on IMS interconnection between operators.
How does SMS work on an IMS network?
A phone registered on IMS can receive SMS over IP: an IP Short Message Gateway (IP-SM-GW) sits between the SMSC and IMS and carries the message inside SIP, as specified in 3GPP TS 24.341. A2P traffic still enters the network through the operator's SMSC or A2P gateway, usually over SMPP, so businesses sending SMS do not need to integrate with IMS directly.
Why does IMS matter for network security?
IMS moves telephony onto IP and SIP, which brings IP-style threats: SIP flooding, caller-ID spoofing on interconnects and abuse of Diameter on roaming links. Operators protect it with session border controllers at the network edge and Diameter signalling firewalls on their roaming interfaces.