IP multimedia subsystem is a brand-new form of multimedia services, which can meet the needs of terminal customers for more novel and diversified multimedia services. It is an important way to introduce differentiated services such as the triple integration of voice, data, and video, and to implement VoLTE and VoNR high-definition voice services.

The design of IMS architecture and CSCF utilizes the SoftSwitch technology to realize the separation of business and control, and separation of call control and media transmission. Although IMS is a system developed by 3GPP for mobile users to access multimedia services, it is an important part of the solution to the integration of mobile and fixed networks because it fully integrates the technologies of the IP domain.

Overview

• Use VoNR, VoLTE or VoIP technology for voice calls
• Use VoNR, VoLTE or VoIP technology for video calls
• Interactive call between LTE and PSTN or 2/3G CS via E1/T1 connection
• Interaction between IMS networks
• Instant messaging
• Generate prepaid or postpaid billing events
• Support IPSec ESP for signal transmission
• Support AKAv1 / AKAv2 / Http Digest / SIP Digest authentication
• Supplementary business:
  • Demonstration of calling line identification
  • Call line identification restriction
  • Unconditional call forwarding
  • Call forwarding when the mobile user is busy
  • No answer call forwarding
  • Unable to reach the call forwarding on the mobile subscriber
  • Call waiting
  • Call hold
  • Restrictions determined by the operator
• Third-party call
• T.38 fax service
• Work with RCS to support status information
• Interact with SMC&SMGW to support SMS service

Capacity

• 300K online users
• 84,000 concurrent calls
• Up to 2M busy call attempts (BHCA)

Reliability and scalability

• 99.999% availability
• Easily scale by adding more servers
• Geographically distributed VoIP platform
• Simple failover and redundancy

OM

• Configuration management
• Performance management
• Fault management
• Web-based management

S-CSCF

• Process registration requests by acting as a registrar defined in [RFC3261].
• Authenticate users through IMS authentication and key agreement (AKA) mode.
• During registration or when processing requests for unregistered users, download user information and service configuration files from HSS.
• To route mobile terminal traffic to P-CSCF, and route mobile source traffic to I-CSCF, branch gateway control function (BGCF) or application server (AS).
• S-CSCF enhanced iFC processing
• Interaction with RCS
• Send SIP OPTIONS to achieve remote peer-to-peer availability
• Use the Domain Name System (DNS) conversion mechanism to convert E.I64 numbers into SIP Universal Resource Identifiers (URI)
• Send accounting-related information to CCF for offline billing and send it to the online billing system

P-CSCF

• To allow detection of INVITE requests sent to a special address, trigger a Cx LIR query to use the IMPU in the P-Asserted-Identity header.
• Modify the request URI type, for example, from TEL URI or SIP URI to SIP URI with user = phone.
• The first entry point (from the P-CSCF in the local network or from the P-CSCF in the visited network)
• Based on the allocation in the HSS, S-CSCF allocation is performed by selection

I-CSCF

• In order to allow detection of INVITE requests sent to a special address, a Cx LIR query is triggered to use the IMPU in the P-Asserted-Identity header.
• Modify the request URI type, for example, from TEL URI or SIP URI to SIP URI with user = phone.
• The first entry point (from the P-CSCF in the local network or from the P-CSCF in the visited network)
• Based on the allocation in the HSS, S-CSCF allocation is performed by selection

PCEF

PCEF (Policy and Charging Enforcement Function) is the execution unit of the PCC system policy. The function is integrated in the GGSN gateway of GPRS, PGW of EPC, PDG of WLAN:

• Detection of business data flow
• Strategy execution
• Billing function based on data flow

PCRF

PCRF (Policy and Charging Rule Function) is the management unit of the PCC system policy:

• Maintain the association between the GW (GateWay) control session and the IP-CAN (IP Connection Access Network) session through the Gx interface
• As the information exchange point between BBERF (Bearer Binding and Event Reporting Function) and PCEF, it uses the event trigger forwarding between BBERF and PCEF