Description
RP-UI, or RP User Information, is the payload container within an RP-DATA message of the Relay Protocol. It encapsulates the complete Short Message (SM) as it is exchanged between the Short Message Service Centre (SMSC) and the User Equipment (UE) in an SMS over IMS (SMS-IP) system. The RP-UI does not contain just the raw text typed by the user; it contains the fully formatted Transport Protocol Data Unit (TPDU), which includes the SMS Transport Protocol (TP) layer header and the actual user data. The TP layer header contains fields like the TP-Protocol-Identifier, TP-Data-Coding-Scheme, and TP-Validity-Period, while the user data field contains the encoded text or binary information.
From an architectural perspective, the RP-UI is the innermost layer of the SMS protocol stack when carried over IMS. The stack can be visualized as: SIP message -> RP message (with RP-MTI, RP-SMEA) -> RP-UI field -> TP layer header and payload. When an IP-SM-GW receives an SMS from the legacy network via a MAP_MT_FORWARD_SM message, it extracts the SM (which is already a TPDU) and places it into the RP-UI field of a newly constructed RP-DATA message. This RP-DATA is then placed into the body of a SIP MESSAGE request. The SIP request is routed through the IMS core to the recipient UE. The UE's IMS client extracts the SIP body, identifies it as an RP-DATA via the RP-MTI, and then processes the RP-UI content by passing it to its SMS application layer, which decodes the TPDU to display the message to the user.
The handling of RP-UI is critical for supporting advanced SMS features. The TP layer within the RP-UI carries information necessary for concatenated SMS (long messages split into multiple parts), flash SMS, Unicode for international characters, and data coding for application ports (used for services like ringtone downloads or WAP push). Because the RP-UI transports the TPDU transparently, all these legacy SMS features work unchanged over the IMS transport. The IP-SM-GW does not need to interpret or modify the RP-UI content; its role is primarily to relay it between the different transport mechanisms (MAP/SCCP/SS7 and SIP/IP). This transparency ensures maximum compatibility and simplifies the gateway function.
Purpose & Motivation
The RP-UI field was created to provide a transparent and lossless carriage mechanism for the complete SMS payload (the TPDU) across the boundary between the legacy circuit-switched SMS network and the new IMS-based packet-switched network. Before IMS, the TPDU was carried within the MAP protocol. The move to an all-IP core required a method to embed this existing, well-defined TPDU structure into IP-based signaling without alteration. The RP layer, with its RP-UI field, serves as this standardized envelope.
This design solved the critical problem of service feature preservation. SMS is not just simple text; its TP layer supports a wide array of features (concatenation, port addressing, message classes, validity periods) that are embedded within the TPDU structure. By defining the RP-UI as a container that holds the entire, unmodified TPDU, 3GPP ensured that none of these features would be lost or broken when SMS traffic was routed via IMS. It allowed for a clean separation of concerns: the IMS layer (SIP) handles session control, discovery, and authentication, while the RP layer handles the SMS-specific dialog, and the RP-UI carries the actual message content. This approach minimized changes to the vast ecosystem of SMSCs and handset SMS clients, enabling a smooth migration path for operators and guaranteeing a consistent user experience regardless of the underlying transport network (CS or PS).
Key Features
- Encapsulates the complete SMS Transport Protocol Data Unit (TPDU).
- Transports the actual user message content (text, binary data) and its TP header.
- Transparently carried by the IP-SM-GW without interpretation or modification.
- Enables support for all legacy SMS features (concatenation, Unicode, flash SMS, port addressing) over IMS.
- Serves as the payload of the RP-DATA message within the Relay Protocol.
- Delivered end-to-end between the SMSC application layer and the UE's SMS application layer.
Evolution Across Releases
Defined in the inaugural version of TS 29.338 as the RP User Information field. Its purpose was to serve as the transparent container for the SMS TPDU within the newly specified Relay Protocol for IMS-based SMS transport. This established the fundamental principle of TPDU transparency across the CS-IMS interworking boundary.
Defining Specifications
| Specification | Title |
|---|---|
| TS 29.338 | 3GPP TS 29.338 |