Unstructured Supplementary Service Data over IM CN subsystem

Description

Unstructured Supplementary Service Data over IM CN subsystem (USSI) is a 3GPP standard that adapts the legacy USSD service for operation within the IP Multimedia Subsystem (IMS) core network. It defines mechanisms to establish and manage USSD sessions using IMS protocols (primarily SIP and MSRP) instead of relying on the traditional circuit-switched signaling (MAP over SS7). In this architecture, the user equipment (UE) acts as a USSI client. When a user enters a USSD string (e.g., *123#), the UE does not send it via traditional NAS signaling to the MSC. Instead, it encapsulates the USSD information within a SIP MESSAGE request or establishes a SIP session with MSRP (Message Session Relay Protocol) for session-oriented dialogue and sends it to the IMS core.

The key network function for USSI is the USSI Application Server (USSI AS) residing in the IMS domain. The IMS Core (comprising P-CSCF, I-CSCF, S-CSCF) routes the SIP request containing the USSD payload to the USSI AS based on initial Filter Criteria (iFC) in the user's service profile. The USSI AS interprets the USSD string. It may contain the service logic itself or, more commonly, acts as a gateway/interworking function. In the latter case, it translates the SIP/MSRP messages into the legacy MAP/USSD format and communicates with the traditional USSD Service Center in the circuit-switched domain via an Interworking MRF (IM-MRF) or directly via an interworking interface. The response from the service logic follows the reverse path back to the UE, displayed as a USSD response.

USSI supports two modes of operation: session-based and transaction-based. The session-based mode uses a SIP INVITE to establish a session and MSRP for exchanging multiple USSD messages within that session, mimicking the classic USSD interactive menu experience. The transaction-based mode uses a single SIP MESSAGE request-response exchange for simple one-shot queries. This allows flexibility depending on the service complexity. The protocol details for carrying USSD payload are specified in SIP headers and body formats. USSI's role is critical for service continuity, allowing operators to decommission legacy circuit-switched cores while maintaining revenue-generating and essential USSD-based services for subscribers on pure IP access like VoLTE, VoWiFi, or eventually 5G Voice (VoNR).

Purpose & Motivation

USSI was created to solve the problem of USSD service continuity in the evolving mobile network landscape moving towards all-IP architectures, specifically with the deployment of IMS for multimedia services. As operators launched LTE (which initially lacked a circuit-switched voice core) and adopted IMS for Voice over LTE (VoLTE), traditional USSD services that relied on MAP signaling through the MSC/VLR became inaccessible to users camped on LTE. This created a service gap where subscribers could not access critical services like balance checks or prepaid top-ups when using their primary LTE data connection. USSI addresses this by providing a native IP-based transport for USSD within the IMS framework.

It directly addresses the limitation of the legacy USSD architecture, which was tightly coupled with the circuit-switched network domain. By defining a clean, SIP-based protocol for USSD, USSI enables the same interactive services to be delivered over any IP access network (LTE, 5G NR, WLAN) that has IMS connectivity. This was a necessary evolution to support the GSM Association's (GSMA) vision of Voice over LTE (VoLTE) and Rich Communication Services (RCS), which require all services to be IMS-based. Without USSI, operators would have been forced to implement complex fallback mechanisms (e.g., Circuit-Switched Fallback) just for USSD, degrading the user experience.

The historical motivation stems from 3GPP Release 11, where the industry recognized the need to fully converge services onto IMS. USSI allows operators to modernize their network core by migrating USSD application servers into the IMS cloud, reducing dependency on legacy SS7 networks and simplifying operations. It future-proofs the valuable USSD service ecosystem, ensuring that even as networks transition to 5G Standalone (SA) architectures without any circuit-switched elements, essential interactive services remain available to all users, including those with basic IMS-enabled devices.