IMS¶

The IP Multimedia System is a 3GPP derivative of IETF standards for providing IP based multimedia (voice, video, messaging, ...).

IMS is designed to be universal and indepentendt of the underlying IP transport network (called the IP-CAN).

IMS is most widely adopted in the context of VoLTE and VoWiFi.

IMS Core Network¶

The IMS core network primarily consist sof three CSCF (Call Session Control Functions):

• The P-CSCF (Proxy CSCF)
• The I-CSCF (Interrogating CSCF)
• The S-CSCF (Serving CSCF)

P-CSCF¶

The P-CSCF is the "gateway" to the IMS core network. To this P-CSCF all IMS clients (e.g. VoLTE phones) register. It performs cryptographic authentication of the clients and also acts as an transport mode IPsec gateway. Only properly authenticated SIP signaling is forwarded from clients to the other IMS core network elements.

I-CSCF¶

The I-CSCF helps the P-CSCF to resolve which S-CSCF is responsible for a given subscriber. There can be multiple different S-CSCF for different types of subscribers, or simply for load balancing. Also, in case if IMS roaming, the S-CSCF is at a different network operator (the home operator).

S-CSCF¶

The S-CSCF is the anchor for the IMS client / subscriber.