IMS CSCF Description

Definitions and abbreviations

  • UE: User Equipment
  • HSS: Home Subscriber Server
  • P-CSCF: Proxy - Call Session Control Function
  • S-CSCF: Serving - Call Session Control Function
  • MAR: Diameter Cx Multimedia-Auth-Request: Used by S-CSCF to request authentication data from HSS
  • SAR: Diameter Cx Server-Assignment-Request: Used by S-CSCF to notify registration status (or just request user profile) to HSS
  • PPR: Diameter Cx Push-Profile-Request: Used by HSS to push user profile changes to S-CSCF
  • RTR: Diameter Cx Registration-Termination-Request: Used by HSS to request user (or a subset of user's public identities) registration termination to S-CSCF

Network Architecture

Toward UE YateUCN acts as a P-CSCF. UE uses SIP protocol to communicate with YateUCN.
Toward HSS YateUCN acts a a S-CSCF using Diameter Cx/Dx interface.


User authentication and registration

User registration starts with UE sending a SIP REGISTER request to P-CSCF.
YateUCN will identify the HSS related data (local node to use, peer connection …) based on UE’s private identity, require authentication vectors from HSS (using MAR command) and challenge the UE.
When UE will answer and it’s response validated YateUCN will notify the HSS about user registration using SAR command.
User profile is requested from HSS on first registration only (if requested it is returned in the response to SAR). If a tel URI is returned by the HSS in user profile YateUCN will store it as MSISDN to be used later (e.g. route voice calls to it).

User registration subscription

UEs may subscribe to registration status by sending a SIP SUBSCRIBE request.
YateUCN will create to subscription and notify the UE when registration status changes.

User registration termination by UE

UE request unregister by sending a SIP REGISTER request with expire time set to 0.
YateUCN will remove user registration data, and notify HSS using Diameter SAR command.

User registration termination by HSS

HSS may request user registration termination using Diameter RTR command.
YateUCN will remove user registration data and notify UE on registration termination (if there is a subscription to registration status).

User profile update by HSS

HSS may request user profile update by using Diameter PPR command.
YateUCN will update stored data from user profile.

Voice calls

YateUCN can handle:

  • UE to UE calls
  • Calls from/to interconnect gateway

See also


  • ETSI TS 129 228 IP Multimedia (IM) Subsystem Cx and Dx Interfaces; Signalling flows and message contents
  • ETSI TS 129 229 Cx and Dx interfaces based on the Diameter protocol; Protocol details
  • ETSI TS 124 229 IP multimedia call control protocol based on SIP and SDP; Stage 3
  • ETSI TS 133 203 3G security; Access security for IP-based services

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