ETSI TS 103 765-1 V1.1.0 (2025-11)

Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)

TECHNICAL SPECIFICATION
Rail Telecommunications (RT);
Future Railway Mobile Communication System (FRMCS);
Building Blocks and Functions;
Part 1: Transport Stratum
2 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)

Reference
DTS/RT-0083
Keywords
architecture, FRMCS, railways
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ETSI
3 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
Contents
Intellectual Property Rights . 5
Foreword . 5
Modal verbs terminology . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 8
3 Definition of terms, symbols and abbreviations . 9
3.1 Terms . 9
3.2 Symbols . 9
3.3 Abbreviations . 9
4 Principles and concept . 11
4.1 General . 11
4.2 Architecture . 11
4.2.1 Common Architecture Principles . 11
4.2.2 Architecture for a Standalone FRMCS Transport Domain . 12
4.2.3 Architecture for a FRMCS Transport Domain in interaction with other FRMCS Domains . 13
4.2.3.0 General . 13
4.2.3.1 FS reference point (at FRMCS Transport Domain level) . 14
NNI
4.2.4 FRMCS National Roaming Architecture (Home Routed) . 14
4.2.5 FRMCS Roaming Architecture (Local Breakout) . 15
4.3 FRMCS Transport Stratum Capabilities. 16
4.3.1 General . 16
4.3.2 Access Capabilities . 16
4.3.2.1 3GPP Radio Access. 16
4.3.2.1.1 General . 16
4.3.2.1.2 5G NR capabilities . 17
4.3.3 Core capabilities . 18
4.3.3.1 General . 18
4.3.3.2 3GPP NAS capabilities . 18
5 FRMCS Transport Stratum Constituents. 18
5.1 FRMCS Transport Stratum Access Domain Constitue nts . 18
5.1.1 General . 18
5.1.2 FRMCS Transport Wireless Access . 18
5.1.2.1 RMR Access . 18
5.1.2.2 MOCN Access . 18
5.2 FRMCS Transport - Core . 19
5.2.1 Core Architectural Element and Functions . 19
5.2.1.1 Access and Mobility Management Function (AMF) . 19
5.2.1.2 Session Management Function (SMF) . 20
5.2.1.3 User Plane Function (UPF) . 21
5.2.1.4 Policy Control Function (PCF) . 21
5.2.1.5 Authentication Server Function (AUSF) . 22
5.2.1.6 Unified Data Management (UDM) . 22
5.2.1.7 Unified Data Repository (UDR) . 23
5.2.2 Conditional Core Architectural Elements . 23
5.2.2.1 General . 23
5.2.2.2 5G-Equipment Identity Register (5G-EIR) . 23
5.2.2.3 Network Exposure Function (NEF) . 23
5.2.2.4 Network Repository Function (NRF) . 24
5.2.2.5 Binding Support Function (BSF) . 24
5.2.2.6 Service Communication Proxy (SCP) . 24
5.2.2.7 Security Edge Protection Proxy (SEPP) . 24
5.3 FRMCS Multipath Function . 24
ETSI
4 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
5.3.1 General . 24
5.3.2 FRMCS Multipath Function capability . 24
5.3.3 Architecture . 25
5.3.4 Multipath Gateway . 26
5.3.5 Multipath Client . 27
5.3.6 IP Address Assignment . 28
5.3.7 FRMCS Multipath Function procedures . 29
5.3.7.1 FRMCS Multipath Function discovery procedure . 29
5.3.7.2 FRMCS Multipath Function Multipath Datapaths handling and selection procedure . 29
5.3.7.3 FRMCS Multipath Function capability negotiation procedure . 30
5.3.7.4 FRMCS Multipath Function capability definition procedure . 30
5.3.7.5 FRMCS Multipath Function fallback procedure . 31
5.3.7.6 FRMCS Multipath Function cleanup procedure . 31
5.3.7.7 Release of FRMCS Multipath Dataflow . 31
6 Transport Stratum Procedures . 31
6.1 On-Board Radio Module Registration Procedure. 31
6.2 Transport path establishment for Mission Critical signalling procedure . 31
6.3 On-Board Radio Module deregistration procedure . 32
6.4 Roaming Home-Routed Transport Path Establishment . 32
6.5 Release of transport path . 32
7 Interaction of Service Stratum and Transport Stratum . 32
8 FRMCS Transport Stratum Quality of Service Framework . 32
8.1 FRMCS Quality of Service Principles. 32
8.2 FRMCS Unified Access Control (UAC) . 36
8.3 5QI and ARP Application per FRMCS Service . 36
8.4 5QI to DSCP Mapping . 37
9 Transport Stratum Synchronization . 37
9.1 Radio Network Synchronization . 37
9.2 Time-of-Day Synchronization . 38
10 Transport Stratum Identities . 38
Annex A (informative): Generalities on synchronization . 39
History . 40

ETSI
5 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
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Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
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Foreword
This final draft Technical Specification (TS) has been produced by ETSI Technical Committee Railway
Telecommunications (RT), and is now submitted for the Vote phase of the ETSI Standardisation Request deliverable
Approval Procedure (SRdAP).
The present document is part 1 of a multi-part deliverable covering Building Blocks and Functions for the Future
Railway Mobile Communication System (FRMCS), as identified below:
Part 1: "Transport Stratum";
Part 2: "Service Stratum";
Part 3: "Train On-Board functions and interfaces";
Part 4: "Trackside functions and interfaces";
Part 5: "User Equipment (UE) capabilities".
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.

ETSI
6 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
1 Scope
The present document specifies the FRMCS Transport Stratum for the Future Railway Mobile Communications System
(FRMCS). In general, the FRMCS Transport Stratum provides data connectivity and the related services.
The present document covers:
• 3GPP terrestrial access.
• Transport stratum interconnection scenarios in all aspects.
• 3GPP access sharing, mobility within transport stratum.
• QoS and policy control.
• Authentication and authorization for the use of the transport stratum.
• Transport stratum common features.
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found in the
ETSI docbox.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents are necessary for the application of the present document.
[1] ETSI TS 123 501 (V18.9.0): "5G; System architecture for the 5G System (5GS) (3GPP TS 23.501
version 18.9.0 Release 18)".
[2] ETSI TS 127 001 (V18.0.0): "Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS); General on Terminal Adaptation
Functions (TAF) for Mobile Stations (MS) (3GPP TS 27.001 version 18.0.0 Release 18)".
[3] ETSI TS 127 007 (V18.8.0): "Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS); LTE; 5G; AT command set for User
Equipment (UE) (3GPP TS 27.007 version 18.8.0 Release 18)".
[4] ETSI TS 138 201 (V18.0.0): "5G; NR; Physical layer; General description (3GPP TS 38.201
version 18.0.0 Release 18)".
[5] ETSI TS 138 300 (V18.5.0): "5G; NR; NR and NG-RAN Overall description; Stage-2 (3GPP
TS 38.300 version 18.5.0 Release 18)".
[6] ETSI TS 103 765-2: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Building Blocks and Functions; Part 2: Service Stratum".
[7] ETSI TS 103 765-3: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Building Blocks and Functions; Part 3: Train On-Board functions and
interfaces".
[8] FRMCS FFFIS-7950: "FRMCS FFFIS - Form Fit Functional Interface Specification", V2.1.0.
ETSI
7 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
[9] ETSI TS 124 501 (V18.10.0): "5G; Non-Access-Stratum (NAS) protocol for 5G System (5GS);
Stage 3 (3GPP TS 24.501 version 18.10.0 Release 18)".
[10] ETSI TS 123 502 (V18.9.0): "5G; Procedures for the 5G System (5GS) (3GPP TS 23.502
version 18.9.0 Release 18)".
[11] ETSI TS 138 211 (V18.6.0): "5G; NR; Physical channels and modulation (3GPP TS 38.211
version 18.6.0 Release 18)".
[12] ETSI TS 138 212 (V18.6.0): "5G; NR; Multiplexing and channel coding (3GPP TS 38.212
version 18.6.0 Release 18)".
[13] ETSI TS 138 213 (V18.6.0): "5G; NR; Physical layer procedures for control (3GPP TS 38.213
version 18.6.0 Release 18)".
[14] ETSI TS 138 214 (V18.6.0): "5G; NR; Physical layer procedures for data (3GPP TS 38.214
version 18.6.0 Release 18)".
[15] ETSI TS 138 321 (V18.5.0): "5G; NR; Medium Access Control (MAC) protocol specification
(3GPP TS 38.321 version 18.5.0 Release 18)".
[16] ETSI TS 138 322 (V18.2.0): "5G; NR; Radio Link Control (RLC) protocol specification (3GPP
TS 38.322 version 18.2.0 Release 18)".
[17] ETSI TS 138 323 (V18.5.0): "5G; NR; Packet Data Convergence Protocol (PDCP) specification
(3GPP TS 38.323 version 18.5.0 Release 18)".
[18] ETSI TS 138 331 (V18.5.1): "5G; NR; Radio Resource Control (RRC); Protocol specification
(3GPP TS 38.331 version 18.5.1 Release 18)".
[19] ETSI TS 129 522 (V18.9.0): "5G; 5G System; Network Exposure Function Northbound APIs;
Stage 3 (3GPP TS 29.522 version 18.9.0 Release 18)".
[20] ETSI TS 133 501 (V18.9.0): "5G; Security architecture and procedures for 5G System (3GPP
TS 33.501 version 18.9.0 Release 18)".
[21] IETF RFC 9113: "HTTP/2".
[22] IETF RFC 8259: "The JavaScript Object Notation (JSON) Data Interchange Format".
[23] ETSI TS 123 289 (V18.10.0): "5G; Mission Critical services over 5G System; Stage 2 (3GPP
TS 23.289 version 18.10.0 Release 18)".
[24] ETSI TS 129 513 (V18.9.0): "5G; 5G System; Policy and Charging Control signalling flows and
QoS parameter mapping; Stage 3 (3GPP TS 29.513 version 18.9.0 Release 18)".
[25] ETSI TS 129 512 (V18.9.0): "5G; 5G System; Session Management Policy Control Service;
Stage 3 (3GPP TS 29.512 version 18.9.0 Release 18)".
[26] ETSI TS 129 511 (V18.3.0): "5G; 5G System; Equipment Identity Register Services; Stage 3
(3GPP TS 29.511 version 18.3.0 Release 18)".
[27] IETF RFC 8684: "TCP Extensions for Multipath Operation with Multiple Addresses".
[28] ETSI TS 124 193 (V18.8.0): "5G; 5G System; Access Traffic Steering, Switching and Splitting
(ATSSS); Stage 3 (3GPP TS 24.193 version 18.8.0 Release 18)".
[29] ETSI TS 129 514 (V18.9.0): "5G; 5G System; Policy Authorization Service; Stage 3 (3GPP
TS 29.514 version 18.9.0 Release 18)".
[30] ETSI TS 129 214 (V18.4.0): "Universal Mobile Telecommunications System (UMTS); LTE; 5G;
Policy and charging control over Rx reference point (3GPP TS 29.214 version 18.4.0 Release 18)".
[31] ETSI TS 138 104 (V18.9.0): "5G; NR; Base Station (BS) radio transmission and reception (3GPP
TS 38.104 version 18.9.0 Release 18)".
ETSI
8 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
[32] ETSI TS 138 133 (V18.9.0): "5G; NR; Requirements for support of radio resource management
(3GPP TS 38.133 version 18.9.0 Release 18)".
[33] Recommendation ITU-T G.8271.1: "Network limits for time synchronization in packet networks
with full timing support from the network".
[34] Recommendation ITU-T G.8271.2: "Network limits for time synchronization in packet networks
with partial timing support from the network".
[35] ETSI TS 103 793: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Radio Characteristics".
[36] ETSI TS 103 765-5: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Building Blocks and Functions; Part 5: User Equipment (UE) capabilities".
[37] ETSI TS 129 591 (V18.9.0): "5G; 5G System; Network Exposure Function Southbound Services;
Stage 3 (3GPP TS 29.591 version 18.9.0 Release 18)".
[38] ETSI TS 123 122 (V18.10.0): "Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS); LTE; 5G; Non-Access-Stratum (NAS)
functions related to Mobile Station (MS) in idle mode (3GPP TS 23.122 version 18.10.0
Release 18)".
[39] IETF RFC 8200: "Internet Protocol, Version 6 (IPv6) Specification".
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents may be useful in implementing an ETSI deliverable or add to the reader's
understanding, but are not required for conformance to the present document.
[i.1] ETSI TR 103 791: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Terminology for FRMCS specifications".
[i.2] IETF RFC 5905: "Network Time Protocol Version 4: Protocol and Algorithms Specification".
[i.3] IETF RFC 8915: "Network Time Security for the Network Time Protocol".
[i.4] ETSI TS 138 413 (V18.5.0): "5G; NG-RAN; NG Application Protocol (NGAP) (3GPP TS 38.413
version 18.5.0 Release 18)".
[i.5] ETSI TS 138 423 (V18.5.0): "5G; NG-RAN; Xn Application Protocol (XnAP) (3GPP TS 38.423
version 18.5.0 Release 18)".
[i.6] ETSI TS 138 463 (V18.0.0): "5G; NG-RAN; E1 Application Protocol (E1AP) (3GPP TS 38.463
version 18.0.0 Release 18)".
[i.7] ETSI TS 138 473 (V18.5.0): "5G; NG-RAN; F1 Application Protocol (F1AP) (3GPP TS 38.473
version 18.5.0 Release 18)".
[i.8] ETSI TS 136 423 (V18.4.0): "LTE; Evolved Universal Terrestrial Radio Access Network
(E-UTRAN); X2 Application Protocol (X2AP) (3GPP TS 36.423 version 18.4.0 Release 18)".
ETSI
9 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the terms given in ETSI TR 103 791 [i.1] and the following apply:
gNodeB: base station that provides NR radio access for 5G networks
multipath dataflow: application flow of IP packets processed by the FRMCS Multipath Function, characterized by its
application-related attributes and identified via IP-5-tuples
multipath data path: network route within a Transport Domain
multipath policy: set of rules assessed by the FRMCS Multipath Function to determine the distribution of a Multipath
Dataflow to one or more Multipath Datapaths
multipath protocol: protocol operating over an IP transport network supporting the use of multiple network interfaces
(such as MP-TCP, MP-QUIC, etc.)
3.2 Symbols
Void.
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
5G NR 5G New Radio
5GC 5G Core Network
5G-EIR 5G-Equipment Identity Register
5GS 5G System
5QI 5G QoS Identifier
AF Application Function
AKA Authentication and Key Agreement
AMF Access and Mobility Function
API Application Programming Interface
APTS Assisted Partial Timing Support
ARP Allocation and Retention Priority
AUSF Authentication Server Function
AVP Attribute Value Pair
BSF Binding Support Function
CP Control Plane
CSC Communication Session Category
DL Downlink
DN Data Network
DNS Domain Name Server
DRB Dara Radio Bearer
DSCP Differentiated Service Code Point
EF Expedited Forwarding
ESP Encapsulating Security Payload
FFFIS Form Fit Functional Interface Specification
FQDN Fully Qualified Domain Name
FRMCS Future Railway Mobile Communication System
GBR Guaranteed Bit Rate
gNodeB Next generation Node B
GNSS Global Navigation Satellite System
GPSI Generic Public Subscription Identifier
GSM-R Global System for Mobile Communications - Railway
GW Gateway
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10 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
HPLMN Home PLMN
HR Home Routed
HTTP Hyper Text Transfer Protocol
ID Identity
IKEv2 Internet Key Exchange version 2
IM Infrastructure Manager
IMS IP Multimedia Subsystem
IMSI International Mobile Subscriber Identifier
IP Internet Protocol
IPSec Internet Protocol Security
IPv6 IP version 6
I-UPF Intermediate UPF
JSON JavaScript Object Notation
LBO Local Breakout
LIF Lawful Intercept Function
MAC Medium Access Control
MC Mission Critical
MCC Mobile Country Code
MCPTT Mission Critical Push To Talk
MCS Mission Critical Services
MCX Mission Critical Services
ME Mobile Equipment (in the 3GPP context)
MIMO Multi Input Multi Output
MNC Mobile Network Code
MNO Mobile Network Operator
MO Mobile Originated
MOCN Multi Operator Core Network
MP Multipath Protocol
MPF Multipath Function
MPLS Multiprotocol Label Switching
MPM Multipath Management
MP-QUIC Multipath QUIC
MP-TCP Multipath TCP
MPU Multipath User plane
MT Mobile Termination (in the 3GPP context)
N9HR N9 Home Routing
NAI Network Access Identifier
NAS Non-Access Stratum
NEF Network Exposure Function
NF Network Function
NR SA NR Stand-Alone
NR New Radio (In the context of 5G)
NRF Network Repository Function
NTP Network Time Protocol
O&M Operation & Management
OB On-Board
OTDOA Observed Time Difference of Arrival
PCC Policy and Charging Control
PCF Policy Control Function
P-CSCF Proxy Call Service Control Function
PDR Packet Detection Rule
PDU Protocol Data Unit
PFD Packet Flow Description
PLMN Public Land Mobile Network
PRTC-A Primary Reference Time Clock - Class A
PTP Precision Time Protocol
QFI QoS Flow identifier
QoS Quality of Service
QUIC Quick UDP Internet Connection
RAN Radio Access Network
RAT Radio Access Technology
RF Radio Frequency
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11 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
RLC Radio Link Control
RMR Railway Mobile Radio
RU Railway Undertaking
SBA NF Service-Based Architecture Network Function
SCP Service Communication Proxy
SDI Session Description Information
SEG Security Gateway
SEPP Security Edge Protection Proxy
SIB System Information Block
SIM Subscriber Identity/Identification Module
SIP Session Initiation Protocol
SLA Service Level Agreement
SM Session Management
SMF Session Management Function
SUCI Subscriber Concealed Identifier
SUPI Subscriber Permanent Identifier
TAP Transport Access Point
TCMS Train Control & Monitoring System
TCP Transport Control Protocol
TDD Time Division Duplex
TE Terminal Equipment (in the 3GPP context)
UAC Unified Access Control
UDM Unified Data Management
UDR Unified Data Repository
UE User Equipment (in the 3GPP context)
UICC Universal Integrated Circuit Card
UL Uplink
UPF User Plane Function
USIM Universal Subscriber Identity Module
VAS Voice Application Subsystem
VPLMN Visited PLMN
4 Principles and concept
4.1 General
Clause 4 describes the system architecture of the FRMCS Transport Stratum to support packet switched data
connectivity and service enabling deployments. The system architecture applicable for FRMCS is the 3GPP 5G System
(5GS) and the related 5GS architecture defined in ETSI TS 123 501 [1].
4.2 Architecture
4.2.1 Common Architecture Principles
The Transport Stratum consists of the access domain and the 5G core network domain (see Figure 4.2.2-1).
The transport stratum User Equipment (UE) is part of the transport stratum and requires appropriate credentials
necessary to make use of the transport stratum. FRMCS Transport Stratum identities are described in clause 10.
The FRMCS Transport Stratum utilizes terrestrial wireless 3GPP access based on 5G NR RAT and shall be
implemented as described in ETSI TS 138 201 [4], ETSI TS 138 300 [5] and ETSI TS 123 501 [1].
For 5GC, the UE supports N1-reference point for Non-Access Stratum (NAS) signalling for the types of access.
ETSI
12 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
The 5G core domain used for FRMCS Transport Stratum shall provide the convergence between the types of access for
the control plane N2-reference point and for the user plane N3 - reference point. In the case of devices (e.g. VAS
controller) integrated in IM/RU internal IT/LAN structures, FRMCS transport access control is granted using best
practice methods, and FRMCS service access control is granted based on FRMCS SIP Core and MCX procedures. QoS
within the underlying trackside IP network (for both gNodeB - UPF connection or outside of 5G) is based on
implementations that map FRMCS QoS towards QoS mechanisms in the IP network. FRMCS SIP Core is as specified
in ETSI TS 103 765-2 [6].
In general, a UE in transport stratum is considered to be a logical block is subdivided into several domains separated by
reference points. It may consist of one or multiple physical units for different types of access. A UE is subdivided into
the UICC and the Mobile Equipment (ME), which can be further subdivided into Mobile Termination (MT) and
Terminal Equipment (TE) in accordance with - ETSI TS 127 001 [2] and ETSI TS 127 007 [3]. In a broader sense, the
UE allows a service client to obtain transport services for its communication. The FRMCS Transport Stratum shall
support IPv6 IP addressing as specified in IETF RFC 8200 [39].
Accordingly, two Transport Access Points (TAPs) shall be provided for interaction between the 5G transport stratum
and the service stratum:
• TAP Service Client forms the transition to the potential service clients.
• TAP Service Agent is located at the edge of the 5G core domain. N5/Rx-reference point or N33-reference
point (if NEF is deployed in 5GC) shall be used for the control plane and N6-reference point shall be used for
the user plane in accordance with ETSI TS 123 501 [1].
4.2.2 Architecture for a Standalone FRMCS Transport Domain
The Standalone FRMCS Architecture represents a FRMCS network without external connections, neither to foreign
FRMCS Domain, nor to a GSM-R system or a Mobile Network Operator's 5G Core domain.
FRMCS On-Board FRMCS Trackside
TAP
TAP
Service
Service
FRMCS Agents FRMCS
User Equipment MNO Access Domain
Clients
5GC Domain MC System
3GPP capable and
NG-RAN
non-3GPP via WAN
Radio Access
N2
Network
Serving
N3 AUSF N5/Rx
N14
(See NOTE 2)
MC Service
NR Uu
N13
Server)
N12
PCF
AMF UDM
Service Agents
Radio Module
IP N10
FRMCS Access Domain
N11
Board
N2
N16
N7
IP
NR Uu
SMF
IP
N4
NG-RAN
Data Network
N6
Radio Access
UPF
N3
Network
IP
IP
Service
Service
Clients Clients
Transport Stratum
NOTE 1: N2 and N3 reference points are protected by ESP and IKEv2 certificates-based authentication as per
clauses 9.2 and 9.3 in ETSI TS 133 501 [20]. IPsec is mandatory to implement on the gNodeB. On the
core network side, a SEG may be used to terminate the IPsec tunnel.
NOTE 2: Use of MOCN is subject to subject to FRMCS operator policy.

Figure 4.2.2-1: FRMCS Standalone Transport Domain Architecture (non-roaming)
The minimum 5G System (5GS) for a standalone FRMCS Domain Architecture shall be based on the non-roaming
reference architecture as outlined in clauses 4.2.2 (Network Functions and entities) and 4.2.3 (Non-roaming reference
architecture) of ETSI TS 123 501 [1] and shall contain the logical following building blocks:
• Authentication Server Function (AUSF).
• Access and Mobility Management Function (AMF).
ETSI
13 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
rd
• Data Network (DN), e.g. operator services, Internet access or 3 party services.
• Policy Control Function (PCF).
• Session Management Function (SMF).
• Unified Data Management (UDM).
• Unified Data Repository (UDR).
• User Plane Function (UPF).
• User Equipment (UE).
• (Radio) Access Network ((R)AN).
4.2.3 Architecture for a FRMCS Transport Domain in interaction with other
FRMCS Domains
4.2.3.0 General
A FRMCS Transport Domain in interaction with other FRMCS Domains shall implement a home routed interconnected
domain architecture depicted in Figure 4.2.3.0-1, which enables the UE (from the own FRMCS Transport Domain or
from another FRMCS Transport Domain) in a visited FRMCS network to communicate with the service stratum of its
home network.
FRMCS On-Board FRMCS Trackside
Foreign FRMCS
User Equipment Foreign FRMCS
3GPP capable and Access Domain 5GC domain
non-3GPP via WAN
TAP N15
AMF
Service N11
N2
Clients
V-SMF V-PCF
N4
V-UPF
Home FRMCS
NG-RAN
N12 N16
Radio Access N3 N8 N24 MC System
NR Uu TAP
Network
Service
V-SEPP V-NRF
N27 Agents
Serving
N9
N32
FS MC Service
NNI
Server)
H-SEPP
H-NRF AUSF
Radio Module
IP N27
N13
Board
Service agents
N12
N8
UDM
N24
N16 N10
N5/Rx
IP
H-SMF
H-UPF N4
N7
IP
H-PCF
Data Network
N6
IP
Home FRMCS 5GC Domain
Service
Clients
IP
Transport Stratum
Service
Clients
NOTE: N2 and N3 reference points are protected by ESP and IKEv2 certificates-based authentication as per
clauses 9.2 and 9.3 in ETSI TS 133 501 [20]. IPsec is mandatory to implement on the gNodeB. On the
core network side, a SEG may be used to terminate the IPsec tunnel.

Figure 4.2.3.0-1: Architecture for a FRMCS Transport Domain in interaction with
a foreign FRMCS Domain, MC services provided by the Primary (Home) FRMCS MC system
ETSI
14 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
The minimum 5G system (5GS) for a FRMCS Transport Domain in interaction with other FRMCS Domains shall be
based on the roaming reference architecture as outlined in clauses 4.2.2 (Network Functions and entities) and 4.2.4
(Roaming reference architecture) of ETSI TS 123 501 [1] and shall comprise of the building blocks listed in
clause 4.2.2 of the present document and the following additional logical building blocks for security reasons, mostly:
• Network Repository Function (NRF).
• Security Edge Protection Proxy (SEPP).
FS in Figure 4.2.3.0-1 refers to the Network-to-Network Interconnection (NNI), connecting different FRMCS
NNI
Domains.
4.2.3.1 FSNNI reference point (at FRMCS Transport Domain level)
The FS reference point is the reference point between FRMCS Domains. At FRMCS Transport Domain level, it
NNI
enables mobility of FRMCS Users between FRMCS Domains.
The FS reference point is composed of the 5GC reference points listed in Table 4.2.3.1-1 for Local Breakout (LBO),
NNI
Home Routing (HR) or both.
Table 4.2.3.1-1: FS basic reference points
NNI
Reference point Between Purpose LBO/HR
N8 AMF (VPLMN) ↔ UDM (HPLMN) Subscription retrieval Both
N9 UPF (VPLMN) ↔ UPF (HPLMN) User-plane traffic forwarding HR
N10 SMF (VPLMN) ↔ UDM (HPLMN) Session-related subscription data LBO
N12 AMF (VPLMN) ↔ AUSF (HPLMN) Authentication Both
N16 SMF (VPLMN) ↔ SMF (HPLMN) Inter-PLMN session management Both
N24 PCF (VPLMN) ↔ PCF (HPLMN) Mobility-related policies Both
N32 SEPP (VPLMN) ↔ SEPP (HPLMN) Secure inter-PLMN control-plane communication Both

NOTE: N14 can be implemented to enable smooth mobility and network selection across different PLMNs in 5G,
by allowing AMFs to exchange all necessary information to maintain UE service continuity.
A FRMCS Transport Domain shall support the reference points listed in Table 4.2.3.1-1.
A FRMCS Transport Domain should implement the N27 reference point to enable SBA NF discovery (under
communication model B and C as described in ETSI TS 123 501 [1], clause E.1).
4.2.4 FRMCS National Roaming Architecture (Home Routed)
National roaming allows the use of Mobile Network Operator spectrum in a National Roaming architecture. This
architecture uses the FRMCS Access Domain radio module, which is equipped to handle Mobile Network Operator
(MNO) spectrum as well, or a separate MNO radio module in the On-Board FRMCS, which is configured to roam on
the MNO network. The FRMCS Multipath Function, specified in clause 5.3, determines which On-Board FRMCS
Radio Module is used for the Multipath Datapath of the application.
In the home routed national roaming architecture (depicted in Figure 4.2.4-1), FRMCS On-Board Service Clients can
use FRMCS On-Board UE that is attached to MNO roaming partner's radio access. Home routed FRMCS bearers
traverse both the VPLMN MNO's 5G Core and the Home FRMCS 5G core. Serving MC System's FRMCS Service
Agents and wireline Service Clients are connected to the Home FRMCS Operator's domain.
ETSI
15 Final draft ETSI TS 103 765-1 V1.1.0 (2025-11)
On-Board Trackside
MNO roaming partner
User Equipment VPLMN
VPLMN (MNO’s 5G Core Domain)
3GPP capable and (MNO Access
TAP N15
AMF
non-3GPP via WAN Domain)
Service N11
N8
Clients
FRMCS MC
N2 V-UPF N4
V-SMF V-PCF
NG-RAN
N12
NR Uu N3 System
Radio Access
N14
Network
N16
N24
V-SEPP
N27
TAP
Serving
V-NRF
Service
MC Service
Agents
Server)
N9 FS
ONI
N32
Service agents
H-NRF AUSF
Radio Module
Home FRMCS
IP
N27 N13
Board
N12
H-SEPP
Access Domain
N8 UDM
IP
N16
N10
IP
NG-RAN N5/Rx
H-SMF
NR Uu N3
Radio Access H-UPF N4
Data Network
N7
Network N24
H-PCF
N14
N6
IP
N2
AMF N11
Service
Clients
Home FRMCS 5G Core Domain
IP
Service
Clients
Transport Stratum
NOTE: N2 and N3 reference points are protected by ESP and IKEv2 certificates-based authentication as per
clauses 9.2 and 9.3 in ETSI TS 133 501 [20]. IPsec is mandatory to implement on the gNodeB. On the
core network side, a SEG may be used to terminate the IPsec tunnel.

Figure 4.2.4-1: FRMCS National Roaming Architecture
(Home Routed, as per Figure 4.2.4-6 in ETSI TS 123 501 [1])
The interface functionalities and interactions between an MNO core and an FRMCS core is known as the FS
ONI
reference point.
The following interdomain reference points shall be supported for national roaming with home routing:
• N32: Between the SEPPs of each 5G Core Domain.
• N9: Connecting visited UPF and home UPF.
4.2.5 FRMCS Roaming Architecture (Local Breakout)
In roaming architecture with Local Breakout (Figure 4.2.5-1), FRMCS On-Board Service Clients use FRMCS
On-Board UE that is attached to a foreign FRMCS (LBO roaming partn
...

SLOVENSKI STANDARD
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
01-september-2025
Železniške telekomunikacije (RT) - Prihodnji sistem radijskih komunikacij za
železnice (FRMCS) - Gradniki in funkcionalnosti - 1. del: Transportna raven
Rail Telecommunications (RT) - Future Railway Mobile Communication System
(FRMCS) - Building Blocks and Functions - Part 1: Transport Stratum
Ta slovenski standard je istoveten z: ETSI TS 103 765-1 V1.0.0 (2025-07)
ICS:
33.060.01 Radijske komunikacije na Radiocommunications in
splošno general
45.020 Železniška tehnika na Railway engineering in
splošno general
oSIST-TS ETSI/prTS 103 765-1 en
V1.0.0:2025
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025

oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
Draft ETSI TS 103 765-1 V1.0.0 (2025-07)

TECHNICAL SPECIFICATION
Rail Telecommunications (RT);
Future Railway Mobile Communication System (FRMCS);
Building Blocks and Functions;
Part 1: Transport Stratum
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
2 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)

Reference
DTS/RT-0083
Keywords
architecture, FRMCS, railways
ETSI
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ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
3 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
Contents
Intellectual Property Rights . 5
Foreword . 5
Modal verbs terminology . 5
1 Scope . 6
2 References . 6
2.1 Normative references . 6
2.2 Informative references . 8
3 Definition of terms, symbols and abbreviations . 9
3.1 Terms . 9
3.2 Symbols . 9
3.3 Abbreviations . 9
4 Principles and concept . 11
4.1 General . 11
4.2 Architecture . 11
4.2.1 Common Architecture Principles . 11
4.2.2 Architecture for a Standalone FRMCS Transport Domain . 12
4.2.3 Architecture for a FRMCS Transport Domain in interaction with other FRMCS Domains . 13
4.2.3.0 General . 13
4.2.3.1 FS reference point (at FRMCS Transport Domain level) . 14
NNI
4.2.4 FRMCS National Roaming Architecture (Home Routed) . 14
4.2.5 FRMCS Roaming Architecture (Local Breakout) . 15
4.3 FRMCS Transport Stratum Capabilities. 16
4.3.1 General . 16
4.3.2 Access Capabilities . 16
4.3.2.1 3GPP Radio Access. 16
4.3.2.1.1 General . 16
4.3.2.1.2 5G NR capabilities . 17
4.3.3 Core capabilities . 17
4.3.3.1 General . 17
4.3.3.2 3GPP NAS capabilities . 18
5 FRMCS Transport Stratum Constituents. 18
5.1 FRMCS Transport Stratum Access Domain Constitue nts . 18
5.1.1 General . 18
5.1.2 FRMCS Transport Wireless Access . 18
5.1.2.1 RMR Access . 18
5.1.2.2 MOCN Access . 18
5.2 FRMCS Transport - Core . 19
5.2.1 Core Architectural Element and Functions . 19
5.2.1.1 Access and Mobility Management Function (AMF) . 19
5.2.1.2 Session Management Function (SMF) . 20
5.2.1.3 User Plane Function (UPF) . 21
5.2.1.4 Policy Control Function (PCF) . 21
5.2.1.5 Authentication Server Function (AUSF) . 22
5.2.1.6 Unified Data Management (UDM) . 22
5.2.1.7 Unified Data Repository (UDR) . 22
5.2.2 Conditional Core Architectural Elements . 23
5.2.2.1 General . 23
5.2.2.2 5G-Equipment Identity Register (5G-EIR) . 23
5.2.2.3 Network Exposure Function (NEF) . 23
5.2.2.4 Network Repository Function (NRF) . 23
5.2.2.5 Binding Support Function (BSF) . 23
5.2.2.6 Service Communication Proxy (SCP) . 24
5.2.2.7 Security Edge Protection Proxy (SEPP) . 24
5.3 FRMCS Multipath Function . 24
ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
4 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
5.3.1 General . 24
5.3.2 FRMCS Multipath Function capability . 24
5.3.3 Architecture . 25
5.3.4 Multipath Gateway . 26
5.3.5 Multipath Client . 27
5.3.6 IP Address Assignment . 28
5.3.7 FRMCS Multipath Function procedures . 28
5.3.7.1 FRMCS Multipath Function discovery procedure . 28
5.3.7.2 FRMCS Multipath Function Multipath Datapaths handling and selection procedure . 29
5.3.7.3 FRMCS Multipath Function capability negotiation procedure . 29
5.3.7.4 FRMCS Multipath Function capability definition procedure . 30
5.3.7.5 FRMCS Multipath Function fallback procedure . 30
5.3.7.6 FRMCS Multipath Function cleanup procedure . 30
5.3.7.7 Release of FRMCS Multipath Dataflow . 30
6 Transport Stratum Procedures . 30
6.1 On-Board Radio Module Registration Procedure. 30
6.2 Transport path establishment for Mission Critical signalling procedure . 31
6.3 On-Board Radio Module deregistration procedure . 31
6.4 Roaming Home-Routed Transport Path Establishment . 31
6.5 Release of transport path . 31
7 Interaction of Service Stratum and Transport Stratum . 32
8 FRMCS Transport Stratum Quality of Service Framework . 32
8.1 FRMCS Quality of Service Principles. 32
8.2 FRMCS Unified Access Control (UAC) . 35
8.3 5QI and ARP Application per FRMCS Service . 35
8.4 5QI to DSCP Mapping . 36
9 Transport Stratum Synchronization . 36
9.1 Radio Network Synchronization . 36
9.2 Time-of-Day Synchronization . 37
10 Transport Stratum Identities . 37
Annex A (informative): Generalities on synchronization . 38
History . 39

ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
5 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
Intellectual Property Rights
Essential patents
IPRs essential or potentially essential to normative deliverables may have been declared to ETSI. The declarations
pertaining to these essential IPRs, if any, are publicly available for ETSI members and non-members, and can be
found in ETSI SR 000 314: "Intellectual Property Rights (IPRs); Essential, or potentially Essential, IPRs notified to
ETSI in respect of ETSI standards", which is available from the ETSI Secretariat. Latest updates are available on the
ETSI IPR online database.
Pursuant to the ETSI Directives including the ETSI IPR Policy, no investigation regarding the essentiality of IPRs,
including IPR searches, has been carried out by ETSI. No guarantee can be given as to the existence of other IPRs not
referenced in ETSI SR 000 314 (or the updates on the ETSI Web server) which are, or may be, or may become,
essential to the present document.
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ETSI claims no ownership of these except for any which are indicated as being the property of ETSI, and conveys no
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not constitute an endorsement by ETSI of products, services or organizations associated with those trademarks.
DECT™, PLUGTESTS™, UMTS™ and the ETSI logo are trademarks of ETSI registered for the benefit of its
Members. 3GPP™, LTE™ and 5G™ logo are trademarks of ETSI registered for the benefit of its Members and of the
3GPP Organizational Partners. oneM2M™ logo is a trademark of ETSI registered for the benefit of its Members and of ®
the oneM2M Partners. GSM and the GSM logo are trademarks registered and owned by the GSM Association.
Foreword
This Technical Specification (TS) has been produced by ETSI Technical Committee Railway Telecommunications (RT)
and is now submitted for the combined Public Enquiry and Vote phase of the ETSI Standardisation Request deliverable
Approval Procedure (SRdAP).
The present document is part 1 of a multi-part deliverable covering Building Blocks and Functions for the Future
Railway Mobile Communication System (FRMCS), as identified below:
Part 1: "Transport Stratum";
Part 2: "Service Stratum";
Part 3: "Train On-Board functions and interfaces";
Part 4: "Trackside functions and interfaces";
Part 5: "User Equipment (UE) capabilities".
Modal verbs terminology
In the present document "shall", "shall not", "should", "should not", "may", "need not", "will", "will not", "can" and
"cannot" are to be interpreted as described in clause 3.2 of the ETSI Drafting Rules (Verbal forms for the expression of
provisions).
"must" and "must not" are NOT allowed in ETSI deliverables except when used in direct citation.

ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
6 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
1 Scope
The present document specifies the FRMCS Transport Stratum for the Future Railway Mobile Communications System
(FRMCS). In general, the FRMCS Transport Stratum provides data connectivity and the related services.
The present document covers:
• 3GPP terrestrial access.
• Transport stratum interconnection scenarios in all aspects.
• 3GPP access sharing, mobility within transport stratum.
• QoS and policy control.
• Authentication and authorization for the use of the transport stratum.
• Transport stratum common features.
2 References
2.1 Normative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
Referenced documents which are not found to be publicly available in the expected location might be found in the
ETSI docbox.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents are necessary for the application of the present document.
[1] ETSI TS 123 501 (V18.9.0): "5G; System architecture for the 5G System (5GS) (3GPP TS 23.501
version 18.9.0 Release 18)".
[2] ETSI TS 127 001 (V18.0.0): "Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS); General on Terminal Adaptation
Functions (TAF) for Mobile Stations (MS) (3GPP TS 27.001 version 18.0.0 Release 18)".
[3] ETSI TS 127 007 (V18.8.0): "Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS); LTE; 5G; AT command set for User
Equipment (UE) (3GPP TS 27.007 version 18.8.0 Release 18)".
[4] ETSI TS 138 201 (V18.0.0): "5G; NR; Physical layer; General description (3GPP TS 38.201
version 18.0.0 Release 18)".
[5] ETSI TS 138 300 (V18.5.0): "5G; NR; NR and NG-RAN Overall description; Stage-2 (3GPP
TS 38.300 version 18.5.0 Release 18)".
[6] ETSI TS 103 765-2: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Building Blocks and Functions; Part 2: Service Stratum".
[7] ETSI TS 103 765-3: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Building Blocks and Functions; Part 3: Train On-Board functions and
interfaces".
[8] FRMCS FFFIS-7950: "FRMCS FFFIS - Form Fit Functional Interface Specification", V2.0.0.
ETSI
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[9] ETSI TS 124 501 (V18.10.0): "5G; Non-Access-Stratum (NAS) protocol for 5G System (5GS);
Stage 3 (3GPP TS 24.501 version 18.10.0 Release 18)".
[10] ETSI TS 123 502 (V18.9.0): "5G; Procedures for the 5G System (5GS) (3GPP TS 23.502
version 18.9.0 Release 18)".
[11] ETSI TS 138 211 (V18.6.0): "5G; NR; Physical channels and modulation (3GPP TS 38.211
version 18.6.0 Release 18)".
[12] ETSI TS 138 212 (V18.6.0): "5G; NR; Multiplexing and channel coding (3GPP TS 38.212
version 18.6.0 Release 18)".
[13] ETSI TS 138 213 (V18.6.0): "5G; NR; Physical layer procedures for control (3GPP TS 38.213
version 18.6.0 Release 18)".
[14] ETSI TS 138 214 (V18.6.0): "5G; NR; Physical layer procedures for data (3GPP TS 38.214
version 18.6.0 Release 18)".
[15] ETSI TS 138 321 (V18.5.0): "5G; NR; Medium Access Control (MAC) protocol specification
(3GPP TS 38.321 version 18.5.0 Release 18)".
[16] ETSI TS 138 322 (V18.2.0): "5G; NR; Radio Link Control (RLC) protocol specification (3GPP
TS 38.322 version 18.2.0 Release 18)".
[17] ETSI TS 138 323 (V18.5.0): "5G; NR; Packet Data Convergence Protocol (PDCP) specification
(3GPP TS 38.323 version 18.5.0 Release 18)".
[18] ETSI TS 138 331 (V18.5.1): "5G; NR; Radio Resource Control (RRC); Protocol specification
(3GPP TS 38.331 version 18.5.1 Release 18)".
[19] ETSI TS 129 522 (V18.9.0): "5G; 5G System; Network Exposure Function Northbound APIs;
Stage 3 (3GPP TS 29.522 version 18.9.0 Release 18)".
[20] ETSI TS 133 501 (V18.9.0): "5G; Security architecture and procedures for 5G System (3GPP
TS 33.501 version 18.9.0 Release 18)".
[21] IETF RFC 9113: "HTTP/2".
[22] IETF RFC 8259: "The JavaScript Object Notation (JSON) Data Interchange Format".
[23] ETSI TS 123 289 (V18.10.0): "5G; Mission Critical services over 5G System; Stage 2 (3GPP
TS 23.289 version 18.10.0 Release 18)".
[24] ETSI TS 129 513 (V18.9.0): "5G; 5G System; Policy and Charging Control signalling flows and
QoS parameter mapping; Stage 3 (3GPP TS 29.513 version 18.9.0 Release 18)".
[25] ETSI TS 129 512 (V18.9.0): "5G; 5G System; Session Management Policy Control Service;
Stage 3 (3GPP TS 29.512 version 18.9.0 Release 18)".
[26] ETSI TS 129 511 (V18.3.0): "5G; 5G System; Equipment Identity Register Services; Stage 3
(3GPP TS 29.511 version 18.3.0 Release 18)".
[27] IETF RFC 8684: "TCP Extensions for Multipath Operation with Multiple Addresses".
[28] ETSI TS 124 193 (V18.8.0): "5G; 5G System; Access Traffic Steering, Switching and Splitting
(ATSSS); Stage 3 (3GPP TS 24.193 version 18.8.0 Release 18)".
[29] ETSI TS 129 514 (V18.9.0): "5G; 5G System; Policy Authorization Service; Stage 3 (3GPP
TS 29.514 version 18.9.0 Release 18)".
[30] ETSI TS 129 214 (V18.4.0): "Universal Mobile Telecommunications System (UMTS); LTE; 5G;
Policy and charging control over Rx reference point (3GPP TS 29.214 version 18.4.0 Release 18)".
[31] ETSI TS 138 104 (V18.9.0): "5G; NR; Base Station (BS) radio transmission and reception (3GPP
TS 38.104 version 18.9.0 Release 18)".
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[32] ETSI TS 138 133 (V18.9.0): "5G; NR; Requirements for support of radio resource management
(3GPP TS 38.133 version 18.9.0 Release 18)".
[33] Recommendation ITU-T G.8271.1: "Network limits for time synchronization in packet networks
with full timing support from the network".
[34] Recommendation ITU-T G.8271.2: "Network limits for time synchronization in packet networks
with partial timing support from the network".
[35] ETSI TS 103 793: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Radio Characteristics".
[36] ETSI TS 103 765-5: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Building Blocks and Functions; Part 5: User Equipment (UE) capabilities".
[37] ETSI TS 129 591 (V18.9.0): "5G; 5G System; Network Exposure Function Southbound Services;
Stage 3 (3GPP TS 29.591 version 18.9.0 Release 18)".
[38] ETSI TS 123 122 (V18.10.0): "Digital cellular telecommunications system (Phase 2+) (GSM);
Universal Mobile Telecommunications System (UMTS); LTE; 5G; Non-Access-Stratum (NAS)
functions related to Mobile Station (MS) in idle mode (3GPP TS 23.122 version 18.10.0
Release 18)".
[39] IETF RFC 8200: "Internet Protocol, Version 6 (IPv6) Specification".
2.2 Informative references
References are either specific (identified by date of publication and/or edition number or version number) or
non-specific. For specific references, only the cited version applies. For non-specific references, the latest version of the
referenced document (including any amendments) applies.
NOTE: While any hyperlinks included in this clause were valid at the time of publication, ETSI cannot guarantee
their long-term validity.
The following referenced documents may be useful in implementing an ETSI deliverable or add to the reader's
understanding, but are not required for conformance to the present document.
[i.1] ETSI TR 103 791: "Rail Telecommunications (RT); Future Railway Mobile Communication
System (FRMCS); Terminology for FRMCS specifications".
[i.2] IETF RFC 5905: "Network Time Protocol Version 4: Protocol and Algorithms Specification".
[i.3] IETF RFC 8915: "Network Time Security for the Network Time Protocol".
[i.4] ETSI TS 138 413 (V18.5.0): "5G; NG-RAN; NG Application Protocol (NGAP) (3GPP TS 38.413
version 18.5.0 Release 18)".
[i.5] ETSI TS 138 423 (V18.5.0): "5G; NG-RAN; Xn Application Protocol (XnAP) (3GPP TS 38.423
version 18.5.0 Release 18)".
[i.6] ETSI TS 138 463 (V18.0.0): "5G; NG-RAN; E1 Application Protocol (E1AP) (3GPP TS 38.463
version 18.0.0 Release 18)".
[i.7] ETSI TS 138 473 (V18.5.0): "5G; NG-RAN; F1 Application Protocol (F1AP) (3GPP TS 38.473
version 18.5.0 Release 18)".
[i.8] ETSI TS 136 423 (V18.4.0): "LTE; Evolved Universal Terrestrial Radio Access Network
(E-UTRAN); X2 Application Protocol (X2AP) (3GPP TS 36.423 version 18.4.0 Release 18)".
ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
9 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
3 Definition of terms, symbols and abbreviations
3.1 Terms
For the purposes of the present document, the terms given in ETSI TR 103 791 [i.1] and the following apply:
gNodeB: base station that provides NR radio access for 5G networks
multipath dataflow: application flow of IP packets processed by the FRMCS Multipath Function, characterized by its
application-related attributes and identified via IP-5-tuples
multipath data path: network route within a Transport Domain
multipath policy: set of rules assessed by the FRMCS Multipath Function to determine the distribution of a Multipath
Dataflow to one or more Multipath Datapaths
multipath protocol: protocol operating over an IP transport network supporting the use of multiple network interfaces
(such as MP-TCP, MP-QUIC, etc.)
3.2 Symbols
Void.
3.3 Abbreviations
For the purposes of the present document, the following abbreviations apply:
5G NR 5G New Radio
5GC 5G Core Network
5G-EIR 5G-Equipment Identity Register
5GS 5G System
5QI 5G QoS Identifier
AF Application Function
AKA Authentication and Key Agreement
AMF Access and Mobility Function
API Application Programming Interface
APTS Assisted Partial Timing Support
ARP Allocation and Retention Priority
AUSF Authentication Server Function
AVP Attribute Value Pair
BSF Binding Support Function
CP Control Plane
CSC Communication Session Category
CSCF Call Server Control Function
DL Downlink
DN Data Network
DNS Domain Name Server
DRB Dara Radio Bearer
DSCP Differentiated Service Code Point
EF Expedited Forwarding
ESP Encapsulating Security Payload
FFFIS Form Fit Functional Interface Specification
FQDN Fully Qualified Domain Name
FRMCS Future Railway Mobile Communication System
GBR Guaranteed Bit Rate
gNodeB Next generation Node B
GNSS Global Navigation Satellite System
GPSI Generic Public Subscription Identifier
GSM-R Global System for Mobile Communications - Railway
ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
10 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
GW Gateway
HPLMN Home PLMN
HR Home Routed
HTTP Hyper Text Transfer Protocol
ID Identity
IKEv2 Internet Key Exchange version 2
IM Infrastructure Manager
IMS IP Multimedia Subsystem
IMSI International Mobile Subscriber Identifier
IP Internet Protocol
IPSec Internet Protocol Security
IPv6 IP version 6
I-UPF Intermediate UPF
JSON JavaScript Object Notation
LBO Local Breakout
LIF Lawful Intercept Function
MAC Medium Access Control
MC Mission Critical
MCC Mobile Country Code
MCPTT Mission Critical Push To Talk
MCS Mission Critical Services
MCX Mission Critical Services
ME Mobile Equipment (in the 3GPP context)
MIMO Multi Input Multi Output
MNC Mobile Network Code
MNO Mobile Network Operator
MO Mobile Originated
MOCN Multi Operator Core Network
MP Multipath Protocol
MPF Multipath Function
MPLS Multiprotocol Label Switching
MPM Multipath Management
MP-QUIC Multipath QUIC
MP-TCP Multipath TCP
MPU Multipath User plane
MT Mobile Termination (in the 3GPP context)
N9HR N9 Home Routing
NAI Network Access Identifier
NAS Non-Access Stratum
NEF Network Exposure Function
NF Network Function
NR SA NR Stand-Alone
NR New Radio (In the context of 5G)
NRF Network Repository Function
NTP Network Time Protocol
O&M Operation & Management
OB On-Board
OTDOA Observed Time Difference of Arrival
PCC Policy and Charging Control
PCF Policy Control Function
P-CSCF Proxy Call Service Control Function
PDR Packet Detection Rule
PDU Protocol Data Unit
PFD Packet Flow Description
PLMN Public Land Mobile Network
PRTC-A Primary Reference Time Clock - Class A
PTP Precision Time Protocol
QFI QoS Flow identifier
QoS Quality of Service
QUIC Quick UDP Internet Connection
RAN Radio Access Network
RAT Radio Access Technology
ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
11 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
RF Radio Frequency
RLC Radio Link Control
RMR Railway Mobile Radio
RU Railway Undertaking
SBA NF Service-Based Architecture Network Function
SCP Service Communication Proxy
SDI Session Description Information
SEG Security Gateway
SEPP Security Edge Protection Proxy
SIB System Information Block
SIM Subscriber Identity/Identification Module
SIP Session Initiation Protocol
SLA Service Level Agreement
SM Session Management
SMF Session Management Function
SUCI Subscriber Concealed Identifier
SUPI Subscriber Permanent Identifier
TAP Transport Access Point
TCMS Train Control & Monitoring System
TCP Transport Control Protocol
TDD Time Division Duplex
TE Terminal Equipment (in the 3GPP context)
UAC Unified Access Control
UDM Unified Data Management
UDR Unified Data Repository
UE User Equipment (in the 3GPP context)
UICC Universal Integrated Circuit Card
UL Uplink
UPF User Plane Function
USIM Universal Subscriber Identity Module
VAS Voice Application Subsystem
VPLMN Visited PLMN
4 Principles and concept
4.1 General
Clause 4 describes the system architecture of the FRMCS Transport Stratum to support packet switched data
connectivity and service enabling deployments. The system architecture applicable for FRMCS is the 3GPP 5G System
(5GS) and the related 5GS architecture defined in ETSI TS 123 501 [1].
4.2 Architecture
4.2.1 Common Architecture Principles
The Transport Stratum consists of the access domain and the 5G core network domain (see Figure 4.2.2-1).
The transport stratum User Equipment (UE) is part of the transport stratum and requires appropriate credentials
necessary to make use of the transport stratum. FRMCS Transport Stratum identities are described in clause 10.
The FRMCS Transport Stratum utilizes terrestrial wireless 3GPP access based on 5G NR RAT and shall be
implemented as described in ETSI TS 138 201 [4], ETSI TS 138 300 [5] and ETSI TS 123 501 [1].
For 5GC, the UE supports N1-reference point for Non-Access Stratum (NAS) signalling for the types of access.
ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
12 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
The 5G core domain used for FRMCS Transport Stratum shall provide the convergence between the types of access for
the control plane N2-reference point and for the user plane N3 - reference point. In the case of devices (e.g. VAS
controller) integrated in IM/RU internal IT/LAN structures, FRMCS transport access control is granted using best
practice methods, and FRMCS service access control is granted based on FRMCS SIP Core and MCX procedures. QoS
within the underlying trackside IP network (for both gNodeB - UPF connection or outside of 5G) is based on
implementations that map FRMCS QoS towards QoS mechanisms in the IP network. FRMCS SIP Core is as specified
in ETSI TS 103 765-2 [6].
In general, a UE in transport stratum is considered to be a logical block is subdivided into several domains separated by
reference points. It may consist of one or multiple physical units for different types of access. A UE is subdivided into
the UICC and the Mobile Equipment (ME), which can be further subdivided into Mobile Termination (MT) and
Terminal Equipment (TE) in accordance with - ETSI TS 127 001 [2] and ETSI TS 127 007 [3]. In a broader sense, the
UE allows a service client to obtain transport services for its communication. The FRMCS Transport Stratum shall
support IPv6 IP addressing as specified in IETF RFC 8200 [39].
Accordingly, two Transport Access Points (TAP) shall be provided for interaction between the 5G transport stratum and
the service stratum:
• TAP Service Client forms the transition to the potential service clients.
• TAP Service Agent is located at the edge of the 5G core domain. N5/Rx-reference point or N33-reference
point (if NEF is deployed in 5GC) shall be used for the control plane and N6-reference point shall be used for
the user plane in accordance with ETSI TS 123 501 [1].
4.2.2 Architecture for a Standalone FRMCS Transport Domain
The Standalone FRMCS Architecture represents a FRMCS network without external connections, neither to foreign
FRMCS Domain, nor to a GSM-R system or a Mobile Network Operator's 5G Core domain.
FRMCS On-Board FRMCS Trackside
TAP
TAP
Service
Service
Agents
FRMCS FRMCS
User Equipment MNO Access Domain
Clients
5GC Domain
MC System
3GPP capable and
NG-RAN
non-3GPP via WAN
Radio Access
N2
Network
Serving
N3
AUSF N5/Rx
N14
(See NOTE 2)
MC Service
NR Uu
N13
Server)
N12
PCF
AMF UDM
Service Agents
Radio Module
IP
FRMCS Access Domain N10
N11
Board
N2
N16
N7
IP
NR Uu
SMF
IP
N4
NG-RAN
Data Network
N6
Radio Access
UPF
N3
Network
IP
IP
Service
Service
Clients Clients
Transport Stratum
Figure 4.2.2-1:FRMCS Standalone Transport Domain Architecture (non-roaming)
NOTE 1: N2 and N3 reference points are protected by ESP and IKEv2 certificates-based authentication as per
clauses 9.2 and 9.3 in ETSI TS 133 501 [20]. IPsec is mandatory to implement on the gNodeB. On the
core network side, a SEG may be used to terminate the IPsec tunnel.
NOTE 2: Use of MOCN is subject to subject to FRMCS operator policy.
ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
13 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
The minimum 5G system (5GS) for a standalone FRMCS Domain Architecture shall be based on the non-roaming
reference architecture as outlined in clauses 4.2.2 (Network Functions and entities) and 4.2.3 (Non-roaming reference
architecture) of ETSI TS 123 501 [1] and shall contain the logical following building blocks:
• Authentication Server Function (AUSF).
• Access and Mobility Management Function (AMF).
• Data Network (DN), e.g. operator services, Internet access or 3rd party services.
• Policy Control Function (PCF).
• Session Management Function (SMF).
• Unified Data Management (UDM).
• Unified Data Repository (UDR).
• User Plane Function (UPF).
• User Equipment (UE).
• (Radio) Access Network ((R)AN).
4.2.3 Architecture for a FRMCS Transport Domain in interaction with other
FRMCS Domains
4.2.3.0 General
A FRMCS Transport Domain in interaction with other FRMCS Domains shall implement a home routed interconnected
domain architecture depicted in Figure 4.2.3.0-1, which enables the UE (from the own FRMCS Transport Domain or
from another FRMCS Transport Domain) in a visited FRMCS network to communicate with the service stratum of its
home network.
FRMCS On-Board FRMCS Trackside
Foreign FRMCS
User Equipment Foreign FRMCS
3GPP capable and Access Domain 5GC domain
non-3GPP via WAN
TAP N15
AMF
Service N11
N2
Clients
V-SMF V-PCF
N4
V-UPF
Home FRMCS
NG-RAN
N12 N16
Radio Access N3 N8 N24 MC System
NR Uu TAP
Network
Service
V-SEPP V-NRF
N27 Agents
Serving
N9
N32
FS MC Service
NNI
Server)
H-SEPP
H-NRF AUSF
Radio Module
IP N27
N13
Board
Service agents
N12
N8
UDM
N24
N16 N10
N5/Rx
IP
H-SMF
H-UPF N4
N7
IP
H-PCF
Data Network
N6
IP
Home FRMCS 5GC Domain
Service
Clients
IP
Transport Stratum
Service
Clients
Figure 4.2.3.0-1: Architecture for a FRMCS Transport Domain in interaction with
a foreign FRMCS Domain, MC services provided by the Primary (Home) FRMCS MC system
ETSI
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14 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
NOTE: N2 and N3 reference points are protected by ESP and IKEv2 certificates-based authentication as per
clauses 9.2 and 9.3 in ETSI TS 133 501 [20]. IPsec is mandatory to implement on the gNodeB. On the
core network side, a SEG may be used to terminate the IPsec tunnel.
The minimum 5G system (5GS) for a FRMCS Transport Domain in interaction with other FRMCS Domains shall be
based on the roaming reference architecture as outlined in clauses 4.2.2 (Network Functions and entities) and 4.2.4
(Roaming reference architecture) of ETSI TS 123 501 [1] and shall comprise of the building blocks listed in
clause 4.2.2 of the present document and the following additional logical building blocks for security reasons, mostly:
• Network Repository Function (NRF).
• Security Edge Protection Proxy (SEPP).
FS in Figure 4.2.3.0-1 refers to the Network-to-Network Interconnection (NNI), connecting different FRMCS
NNI
Domains.
4.2.3.1 FS reference point (at FRMCS Transport Domain level)
NNI
The FS reference point is the reference point between FRMCS Domains. At FRMCS Transport Domain level, it
NNI
enables mobility of FRMCS Users between FRMCS Domains.
The FS reference point is composed of the 5GC reference points listed in Table 4.2.3.1-1 for Local Breakout (LBO),
NNI
Home Routing (HR) or both.
Table 4.2.3.1-1: FS basic reference points
NNI
Reference point Between Purpose LBO/HR
N8 AMF (VPLMN) ↔ UDM (HPLMN) Subscription retrieval Both
N9 UPF (VPLMN) ↔ UPF (HPLMN) User-plane traffic forwarding HR
N10 SMF (VPLMN) ↔ UDM (HPLMN) Session-related subscription data LBO
N12 AMF (VPLMN) ↔ AUSF (HPLMN) Authentication Both
N16 SMF (VPLMN) ↔ SMF (HPLMN) Inter-PLMN session management Both
N24 PCF (VPLMN) ↔ PCF (HPLMN) Mobility-related policies Both
N32 SEPP (VPLMN) ↔ SEPP (HPLMN) Secure inter-PLMN control-plane communication Both

NOTE: N14 can be implemented to enable smooth mobility and network selection across different PLMNs in 5G,
by allowing AMFs to exchange all necessary information to maintain UE service continuity.
A FRMCS Transport Domain shall support the reference points listed in Table 4.2.3.1-1.
A FRMCS Transport Domain should implement the N27 reference point to enable SBA NF discovery (under
communication model B and C as described in ETSI TS 123 501 [1], clause E.1).
4.2.4 FRMCS National Roaming Architecture (Home Routed)
National roaming allows the use of Mobile Network Operator spectrum in a National Roaming architecture. This
architecture uses the FRMCS Access Domain radio module, which is equipped to handle Mobile Network Operator
(MNO) spectrum as well, or a separate MNO radio module in the On-Board FRMCS, which is configured to roam on
the MNO network. The FRMCS Multipath Function, specified in clause 5.3, determines which On-Board FRMCS
Radio Module is used for the Multipath Datapath of the application.
In the home routed national roaming architecture (depicted in Figure 4.2.4-1), FRMCS On-Board Service Clients can
use FRMCS On-Board UE that is attached to MNO roaming partner's radio access. Home routed FRMCS bearers
traverse both the VPLMN MNO's 5G Core and the Home FRMCS 5G core. Serving MC System's FRMCS Service
Agents and wireline Service Clients are connected to the Home FRMCS Operator's domain.
ETSI
oSIST-TS ETSI/prTS 103 765-1 V1.0.0:2025
15 Draft ETSI TS 103 765-1 V1.0.0 (2025-07)
On-Board Trackside
MNO roaming partner
User Equipment VPLMN
VPLMN (MNO’s 5G Core Domain)
3GPP capable and (MNO
...