1. Executive Summary & Standards Compliance

The RBC <-> Train interface governs all air-gap Euroradio communication between the wayside Radio Block Centre (RBC) and the onboard ETCS On-Board Unit (EVC / OBU) in an ERTMS/ETCS Level 2 signaling system.

Applicable Standards:

UNISIG Subset-026: System Requirements Specification (SRS) Chapter 3: Principles (Movement Authority, Modes, Transitions, Emergency Stop) Chapter 7: ERTMS/ETCS Language (Variables, Packets, Data Types) Chapter 8: Messages & Variable Dictionary UNISIG Subset-039: FIS for the Euroradio Interface UNISIG Subset-040: Euroradio on Radio Network (GSM-R / FRMCS / LTE) UNISIG Subset-098 / Subset-099: RBC System & Interface Requirements CENELEC EN 50128 / EN 50159: Software for Railway Control & Safe Communication in Open Systems (SIL 4)

Note

Simulation Testbed Status (Simplified Mode): In the current test simulator environment (RBC_Test_Sim_Train / RBC_Test_Sim_Core), messages are exchanged using the compact simplified wire encoding (P0, M136, M146 for Train-to-Track and M24_M15, M3 for Track-to-Train) as documented in RBC_Test_Env/doc/design/SIM_INTERFACES.dox. This simplified wire layer maps directly into the formal Subset-026 structures detailed below.

2. Radio Message Protocol Architecture

The communication link is established over a safe, connection-oriented Euroradio transport channel (GSM-R circuit-switched data / GPRS packet-switched data or FRMCS over LTE/5G).

sequenceDiagram
    autonumber
    participant EVC as Train On-Board (EVC)
    participant RBC as Radio Block Centre (RBC)

    Note over EVC,RBC: 1. Session Establishment (Subset-026 §3.5)
    EVC->>RBC: Msg 155: Initiation of Communication Session
    RBC-->>EVC: Msg 24: General Message (Session Accept)
    EVC->>RBC: Msg 159: Session Established (ETCS Baseline Version)

    Note over EVC,RBC: 2. Train Data & Registration (Subset-026 §3.18)
    EVC->>RBC: Msg 129: Validated Train Data (Length, Speed, Brake curves) + Packet 0
    RBC->>EVC: Msg 8: Acknowledgement of Train Data

    Note over EVC,RBC: 3. Cyclic Position Reporting & MA Issuance (Subset-026 §3.6, §3.8)
    loop Every T_CYCLOC (e.g. 500ms - 2s)
        EVC->>RBC: Msg 136: Position Report (LRBG, D_LRBG, Speed, Mode, Level)
    end
    EVC->>RBC: Msg 132: MA Request (Q_MARQSTREASON)
    RBC->>EVC: Msg 3: Movement Authority (Packet 15, Packet 21, Packet 27) [M_ACK=1]
    EVC->>RBC: Msg 146: Acknowledgement (T_TRAIN_ACK)

3. Radio Message Catalogue

3.1 Track-to-Train Messages (RBC $\to$ Train)

All Track-to-Train messages share a common standard 12-byte header followed by message-specific variables and optional packets:

$$\text{Header} =\text{NID_MESSAGE (8b)}, \text{L_MESSAGE (10b)}, \text{T_TRAIN (32b)}, \text{M_ACK (1b)}, \text{NID_LRBG (24b)}