The TSG Core Network and Terminals WG4 (CT WG4) standardizes stage 2 and stage 3 aspects within the Core Network focusing on Supplementary Services, Basic Call Processing, Mobility Management within the Core Network, Bearer Independent Architecture, GPRS between network entities, Transcoder Free Operation, CAMEL, Generic User Profile, Wireless LAN - UMTS interworking and descriptions of IP Multimedia Subsystem. CT WG4 is also responsible as a "protocol steward" for the some IP related protocols (this involves analyzing, validating, extending if necessary, clarifying how they are used, specifying packages and parameter values).

Terms of reference 

The latest terms of reference were appoved at CT#72 in document CP-160372 

3GPP CT4’s mandate is to specify the protocols within the Core Network, defining specifications describing the protocol requirements.


3GPP TSG CT WG4 (CT4) is responsible for the standardization of the stage 2 aspects within the Core Network including:

• Supplementary Services in the CS Domain,
• Basic Call Processing in the CS Domain, 
• Mobility Management within the Core Network, 
• the Bearer Independent Architecture 
• and CAMEL.

CT4 is also responsible for the specification of the mobile specific protocol specifications within the mobile core network.

A number of protocols within the core network are specified by external bodies such as: the ITU-T and the IETF. CT4 are responsible for the specification of the  “profiling” of these protocols, i.e. describing how and which parts of these “external protocols” are to be used, setting their default behaviour, restricting unnecessary options and ensuring Backwards Compatibility.

CT4 is responsible for the following core network feature specifications:

• Stage 2 and stage 3 (between Core Network entities) of Mobility Management within the Core Network;
• Stage 2 and stage 3 (between Core Network entities) of Circuit-Switched Call Control within the Core Network, e.g. Basic Call Handling; 
• Stage 2 and (jointly with CT3) stage 3 specifications of the Bearer Independent Architecture;
• Stage 2 of the restoration procedures of the Core Network entities;
• Profiling of Call Control Protocols defined outside 3GPP to be used within the Core Network, e.g. BICC, SIP-I;
• Stage 2 (jointly with CT3) andStage 3 of MGW control protocol and profile definition (H.248 based);
• Stage 3 (between network entities) of GPRS (e.g. Interfaces based on GTP, MAP and Diameter);
• Stage 3 (between network entities) of EPC (e.g. Interfaces based on GTP, PMIP and Diameter);
• Stage 2 and stage 3 of Supplementary Services in the CS Domain;
• Stage 2 and stage 3 of Mobile Number Portability;
• Stage 2 and stage 3 of Subscriber Data Management, i.e. Numbering, Addressing and Identification and the Organisation of the Subscribers Data;
• Stage 2 and stage 3 of Transcoder Free Operation (TrFO) - OoBTC (in conjunction with SA4);
• Stage 2 and stage 3 of CAMEL;
• Stage 3 of Location Services;
• Stage 3 of Security (Authentication, Authorisation, Administration);
• Stage 3 of WLAN – UMTS, EPS - WLAN interworking;
• Stage 3 of Subscriber Certificates for Generic Authentication and Authorisation;
• Stage 2 (jointly with SA2) and Stage 3 of the Generic User Profile (GUP);
• Stage 3 (jointly with CT3) specifications of the IP Multimedia Subsystem (IMS); and
• Stage 3 on DNS procedures within 3GPP

CT4 is responsible as a “protocol steward” for the following IP related protocols. This involves: analyzing, validating,

extending (if necessary), clarifying how they are used and specifying packages with their parameter values:

• AAA protocols;
• Security protocols;
• SIGTRAN;
• Subscriber Data Management in the HSS and HSS-Core Network entities and the protocols to support it;
• Diameter protocol codes, in particular requesting Application IDs from IANA;
• Diameter AVP codes and result codes and, in particular, reserving AVP codes from the 3GPP specific range;
• Diameter Experimental result codes, in particular, reserving Experimental Result Codes from the 3GPP specific range.

The above list of standardization activities is not exhaustive and activities can be deployed within CT4, as long as they are inline with the generic mandate given by the CT plenary.

In general, 3GPP CT4 interacts with all 3GPP WGs but with the following specifically:

• 3GPP TSG SA WG1(SA1);

SA1 defines the feature requirements in the stage 1 specifications. The work of CT4 is based on the Stage 1 requirements.

• 3GPP TSG SA WG2 (SA2);

SA2 is responsible for the high-level architecture and functional partitioning specifications of the whole network (including the Core Network). CT4 is responsible for the detailed description of parts of this architecture the functional partitions related to the CT internal functions and protocols.

• 3GPP TSG SA WG6 (SA6);

SA6 is responsible for the Mission Critical Communication high-level architecture and functional partitioning specifications. CT4 is responsible for the detailed description of parts of this architecture the functional partitions related to the CT internal functions and protocols.

• 3GPP TSG CT WG1 (CT1);

CT1 is responsible for the Call Control, Mobility Management, and Session Management aspects across the radio interfaces.

These activities have impacts on the CT4 specifications, such as: the stage 2 Call Control, Supplementary Services, Handover and Session Management procedures and therefore CT4 closely collaborates with CT1 on these aspects.

• 3GPP TSG CT WG3 (CT3);

CT3 is responsible for the network interworking aspects and the user plane protocols (except for GTP); the network interworking of Media Control Protocols (MCP), Bearer Control protocols and for the PCC protocols. CT3 is also responsible for the Stage 2 aspects on the Mn and Ix interfaces. These activities have impacts on the activities within CT4 and therefore CT4 closely collaborates with CT3 on these aspects.

This list of 3GPP WGs is not exhaustive; CT4 will maintain a liaison with other 3GPP WGs and also other groups as needed, e.g. GSMA.

Useful links and information