Appendix 1. References.

3GPP TS 22.090 — USSD (Stage 1): service requirements and user-facing behavior. 

3GPP TS 23.090 — USSD (Stage 2): functional architecture and procedures between network entities. 

3GPP TS 24.090 — USSD (Stage 3): protocol details for USSD signaling procedures (Layer 3 aspects relevant to USSD service logic). 

3GPP TS 29.002 — MAP (Stage 3): normative MAP operations used for USSD over SS7/MAP (e.g., ProcessUnstructuredSS, UnstructuredSS, Notify), including parameters and error handling.

3GPP TS 24.390 — USSI (USSD over IMS): USSD service support over IMS (USSD encapsulated in SIP signaling; includes the USSD XML MIME type used for IMS USSD). 

3GPP TS 23.038 — Alphabets and language-specific information: GSM 7-bit default alphabet, UCS2 usage, and data coding scheme conventions commonly applied to USSD payload encoding/decoding.

3GPP TS 24.008 — Mobile radio interface Layer 3; Core network protocols (Stage 3): broader Layer 3 procedures and related coding conventions often referenced alongside DCS handling in core signaling contexts. 

ITU-T Q.711 / Q.714 — SCCP: SCCP routing/addressing concepts (PC/SSN/GT) used underneath TCAP/MAP. 

ITU-T Q.771 / Q.775 — TCAP: TCAP transaction/dialogue structure (Begin/Continue/End/Abort) that underpins MAP USSD dialogues. ITU+1

RFC 4960 — SCTP: transport protocol commonly used by SIGTRAN stacks.

RFC 4666 — M3UA: SS7 MTP3 User Adaptation Layer (widely used for carrying MTP3 signaling over IP).

RFC 3868 — SUA: SCCP User Adaptation Layer (alternative SIGTRAN adaptation depending on deployment).

RFC 3261 — SIP: base SIP protocol used by IMS.

RFC 3428 — SIP INFO method: INFO method definition (commonly referenced because USSI/USSD over IMS can be carried in SIP signaling).

RFC 6086 — SIP INFO package framework: framework around INFO usage and packages.

SMPP v3.4 — Short Message Peer-to-Peer Protocol: commonly used by legacy/value-added platforms; some deployments map USSD interactions via SMPP conventions.