Mobile networks
IMSAlso known as IP Multimedia Subsystem
Definition
IMS (IP Multimedia Subsystem) is the 3GPP architecture that operators use to deliver voice and multimedia services over IP. First specified in 3GPP Release 5, it uses SIP for signalling and is the core behind VoLTE, VoWiFi and 5G VoNR calls, as well as RCS messaging and interconnect with other networks.
Main IMS components
| Function | Role |
|---|---|
| P-CSCF (Proxy CSCF) | First SIP contact point for the phone; handles security and QoS requests |
| I-CSCF (Interrogating CSCF) | Entry point that finds the right S-CSCF for a subscriber |
| S-CSCF (Serving CSCF) | Registrar and session controller; applies service logic |
| HSS | Subscriber database with identities and authentication data |
| TAS / MMTel AS | Telephony application server: call forwarding, waiting, barring |
| MGCF / MGW, IBCF | Interworking with circuit-switched networks and other IP networks |
CSCF stands for Call Session Control Function. Together these elements do for a mobile operator what a Class 5 softswitch does for a fixed-line provider.
IMS and SIP
IMS is built on SIP with 3GPP extensions for mobility, security and charging. Phones register and set up calls with SIP, and voice travels as RTP. The phone’s SIP client is locked to the operator’s network and authenticated by the SIM, so IMS is not an open SIP service that third-party softphones can join.
Why IMS matters for gateways
IMS also handles interconnect between operators over IP through border functions such as the IBCF, and supports services such as RCS messaging and video calling over the same core.
With 2G and 3G being switched off, IMS is how mobile calls are made: over 4G (VoLTE), 5G (VoNR) or Wi-Fi (VoWiFi). A phone-based GSM gateway that uses the handset’s native calling benefits from all three. See VoLTE and Wi-Fi Calling for GSM gateways and Android SIP gateway.
Frequently asked questions
Is IMS the same as VoLTE?
No. IMS is the core network architecture; VoLTE is one service delivered through IMS over 4G access.
Can I connect my SIP phone directly to an operator’s IMS?
Generally no. IMS access is restricted to SIM-authenticated devices approved by the operator.
Does 5G use IMS for voice?
Yes. Voice over New Radio (VoNR) on 5G standalone networks uses the same IMS core as VoLTE.