Description
The Signalling Gateway (SGW) is a critical interworking function in telecommunications networks, particularly within the 3GPP architecture for core network signalling. Its primary role is to act as a mediator between legacy signalling transport systems and modern IP-based signalling transport. Specifically, it often interfaces between the traditional SS7 (Signalling System No. 7) protocol stack, which typically runs over TDM (Time-Division Multiplexing) circuits like E1/T1, and the IP-based SIGTRAN (Signalling Transport) protocol suite defined by the IETF. The SGW performs signalling message conversion at the transport layer, allowing signalling entities (like MSCs, HLRs, or SCPs) that use SS7 to communicate with other entities that use SIGTRAN over IP networks, or vice-versa.
Architecturally, the SGW sits at the boundary between the TDM-based signalling network and the IP signalling network. It has physical interfaces for both: TDM links (e.g., for MTP2) on one side and IP network interfaces on the other. Internally, it implements the necessary protocol adaptation. For example, it receives SS7 messages via the Message Transfer Part (MTP) layers over a TDM link. The SGW then extracts the signalling message (the payload, like an ISUP or MAP message), encapsulates it within a SIGTRAN protocol such as M3UA (MTP3 User Adaptation), SCTP (Stream Control Transmission Protocol), and IP, and transmits it over the IP network to a destination like an IP-based MSC or Media Gateway Controller. Conversely, it receives SIGTRAN packets over IP, extracts the SS7 message payload, and delivers it via MTP over TDM to a legacy switch.
The SGW's operation is transparent to the higher-layer signalling applications (like MAP, CAP, or ISUP). It does not interpret or modify the application-layer content; it only adapts the transport mechanism. This allows legacy SS7-based network elements to be integrated into an evolving IP-based core network without requiring expensive hardware upgrades. The SGW is often deployed alongside a Media Gateway (MGW) for voice interworking, forming a complete gateway solution for migrating networks from TDM to all-IP. In 3GPP specifications, the SGW is referenced in contexts involving network interworking, legacy support, and migration paths, ensuring that critical signalling for call control, mobility management, and services can continue between old and new network domains.
Purpose & Motivation
The Signalling Gateway was created to solve the problem of network migration from legacy TDM-based signalling to modern IP-based signalling transport. As mobile networks evolved from 2G/3G to all-IP architectures like IMS and LTE, a major challenge was how to allow existing SS7-based network elements (e.g., legacy MSC, HLR) to communicate with new IP-based elements (e.g., softswitches, IMS nodes). Without an SGW, these networks would be isolated, breaking essential signalling for calls, SMS, and mobility.
The historical motivation stems from the industry's shift towards IP for cost, scalability, and flexibility. SS7 over TDM was robust but rigid and expensive to scale. SIGTRAN over IP offered a more efficient transport. The SGW bridges this gap, enabling a phased migration. It addresses the limitation of incompatible transport layers by performing the necessary adaptation, allowing operators to introduce IP-based nodes without immediately retiring all legacy equipment. This was crucial for the economic and technical feasibility of network evolution, ensuring service continuity during transition periods. Its specification across many 3GPP releases reflects its ongoing relevance in supporting legacy interfaces in increasingly IP-centric networks.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (28 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the SGW function was formally split into separate SGW-C (Control Plane) and SGW-U (User Plane) functions as part of the Control and User Plane Separation (CUPS) architecture. This introduced the Sxa interface between the SGW-C and SGW-U and defined the S11-U interface between the MME and the SGW-U specifically to support Control Plane CIoT EPS Optimisation. The release also detailed the functional split of SGW responsibilities and maintained support for a combined SGW/PGW architecture with separated planes.
- Enable SGW-C & PGW-C selection of UPF to take UE's NR capabilities into account TS 23.214CR0047
- Correcting the condition for selection of SGW-U for NR as secondary RAT TS 23.214CR0055
- Interface between MME and SGW-U for IoT data transmission TS 23.214CR0050
- SGW/PGW selection for NR TS 29.244CR0033
- Condition correction for SGW-U/PGW-U selection based on DCNR TS 29.244CR0069
- Selection of SGW-C/PGW-C for Dual Connectivity with NR TS 29.244CR0076
+ 9 more changes
In Release 16, the SGW function was enhanced with a new "Service parameter for SGW" capability. Furthermore, the release provided clarifications and corrections for specific procedures, including TEID allocation by the gateway user plane and the encoding of INFO messages for overlap signalling. These updates were made within the established architecture for the control and user plane separation (CUPS) of the SGW.
- Clarification of TEID allocation by gateway user plane TS 23.214CR0074
- Correction for the encoding of the INFO message for overlap signalling using the in-dialog method TS 29.163CR1054
- Signalling to the UPF that an access of a MA PDU session is unavailable TS 29.244CR0381
- Service parameter for SGW TS 29.303CR0127
In Release 17, specific enhancements were made for the Signalling Gateway function, particularly concerning the operation of a combined SGW/PGW set. The updates introduced procedures for the restoration of PDN connections served by such a combined node and defined mechanisms for moving these connections to a new SGW IP address within the set. Furthermore, corrections were applied to the Lawful Interception architecture for the SGW/PGW.
- Restoration of PDN connections served by a combined SGW/PGW in a Set TS 29.274CR2029
- New SGW IP Address when moving PDN connections for a combined SGW/PGW/SMF set TS 29.274CR2042
- Correction to LI Architecture for the SGW/PGW TS 33.127CR0132
- IMS signalling optimization with HSS GID TS 23.228CR1240
- Passing PSCell ID to SGW for EN-DC TS 29.274CR2025
In Release 18, the specification introduced new details and corrections for the Bootstrap Data Channel Setup Signalling Procedure. This procedure is a key part of the signalling between the control and user plane functions within the SGW architecture. The changes focused on modifying, correcting, and clarifying the steps of this specific setup signalling process.
In Release 19, the SGW function was updated with corrections to the signalling procedure for the Application Data Channel Interworking via the Mobility Function (MF). This change specifically addressed the interworking functionality for networks applying control and user plane separation, ensuring proper operation as documented in the existing SGW, PGW, and TDF specifications. The update maintained the architectural principles where split network entities, like the SGW with separated control (SGW-C) and user plane (SGW-U) functions, can interwork with non-split entities.
- Corrections to Signalling Procedure of Application Data Channel Interworking via MF TS 23.228CR1659
Explore further
Broader topics and technologies where SGW plays a role.
Defining Specifications
3GPP specifications that define or reference SGW, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.214 vj00 | Control and User Plane Separation for EPC | Rel-19 |
| TS 23.221 vj00 | 3GPP System Architectural Requirements | Rel-19 |
| TS 23.228 vj50 | IMS Stage-2 Service Description | Rel-19 |
| TS 23.236 vj00 | Intra Domain Connection of RAN Nodes to Multiple CN Nodes | Rel-19 |
| TS 23.380 vj10 | IMS Restoration Procedures | Rel-19 |
| TR 23.799 ve00 | Study on Next Generation System Architecture | Rel-14 |
| TS 23.857 vb00 | EPC Node Failure & Restoration Study | Rel-11 |
| TS 25.467 vj00 | UTRAN Architecture for 3G Home Node B | Rel-19 |
| TR 26.924 vj00 | MTSI QoS Improvement Study | Rel-19 |
| TS 28.702 vj00 | Core Network NRM IRP Information Service | Rel-19 |
| TS 29.163 vj00 | Interworking between 3GPP IM CN and CS networks | Rel-19 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.273 vj10 | AAA Protocols for Non-3GPP Access in EPS & 5GS NSWO | Rel-19 |
| TS 29.274 vj50 | GTPv2-C Control Plane Protocol Specification | Rel-19 |
| TS 29.281 vj20 | GTPv1-U Protocol Specification | Rel-19 |
| TS 29.303 vj10 | DNS Procedures for Evolved Packet System | Rel-19 |
| TS 29.863 v820 | IMS-CS Multimedia Interworking Feasibility Study | Rel-8 |
| TS 32.102 vj00 | Telecom Management Physical Architecture Framework | Rel-19 |
| TS 32.632 vb00 | Core Network Resources IRP: Network Resource Model | Rel-11 |
| TS 32.732 vb00 | IMS Network Resource Model IRP: Information Service | Rel-11 |
| TS 32.867 vf10 | Management Impacts of EPC CUPS | Rel-15 |
| TS 33.127 vj50 | Lawful Interception Architecture and Functions | Rel-19 |
| TR 43.901 vj00 | Generic Access to A/Gb Interface Feasibility Study | Rel-19 |