Multi-operator core networks - a highly effective path to the future of railway communications
The standardization of the Future Railway Mobile Communications System (FRMCS) is quickly taking shape in Europe, and it is expected to change how rail companies are approaching communications in a major way in the coming years.
FRMCS is primarily based on using dedicated bands in 900 MHz and 1900 MHz, which in Europe are allocated exclusively for railway use. However, the second edition of FRMCS (V2) will also consider the use of mobile network operator (MNO, also known as communications service providers or CSPs) bands. Some railways are required by regulation to use only the dedicated bands for FRMCS, while other countries will allow usage of MNO bands.
The first edition of FRMCS is expected to be ready by 2024 with pilots taking place the following year, and these pilots will use working FRMCS product components to validate the end-to-end solution. A third edition (V3) of the standard is expected to follow in 2026, applying the lessons learned from the pilots and adding new functionality.
The three methods of utilizing MNO bands
- Use a subscription for data service from an MNO
In this case, the MNO supplies a SIM (Subscriber Identity Module) card which could be embedded in the on-board system. The trackside railway application would be connected directly to the MNO’s data network. Railways are already using this method for Internet of Things (IOT) devices and train control and management system (TCMS) applications, as well as push-to-talk over cellular.
The key disadvantage of this method is the inability to guarantee the required quality of service (QoS) for critical FRMCS applications. Some infrastructure managers (IMs) have sought to achieve the required QoS by leveraging the usage of multiple subscriptions from three MNOs, triplicating the packets between the on-board application and the trackside with packets travelling in parallel over each network.
Surveying the European MNO landscape today, there are very few IMs that could depend on three providers to deliver the required service level and coverage throughout the railway coverage corridors.
- Arrange a roaming agreement
In this case the infrastructure management FRMCS core would be connected to the MNO’s core network, which would be configured to accept access requests from an FRMCS device through the national roaming agreements. In national roaming, the traffic would travel through the MNO’s network and then be routed to the FRMCS core via the interconnection between the MNO core and the IM core network.
Again, there is a key disadvantage - the inability to guarantee the required QoS at the MNO core. A core outage could result in a lengthy communication interruption which would be intolerable for critical FRMCS applications.
Another disadvantage is that there would be a domain change when an on-board system moves from the FRMCS domain to the MNO’s domain. possibly necessitating IP addressing changes as well as re-authentication overhead.
- Establish a RAN (Radio Access Network) sharing agreement with one or more MNOs
RAN sharing agreements utilize a 4G/5G architecture concept known as Multiple Operator Core Networks (MOCNs). This architecture has a dedicated backhaul between each MNO base station covered under the agreements and the FRMCS core. This blog post will consider only this option for analysis, as it has the greatest potential to meet FRMCS requirements.
One of the key benefits of this solution is that MOCN does not require a domain change from the FRMCS domain. It appears as an extension of the FRMCS into additional RAN networks beyond RMR (Rail Mobile Radio, the dedicated railway bands). Since the 5G sessions are set up directly between the on-board user equipment (UE) and the FRMCS core, there is a level of QoS control which can match RMR.
Of these methods, MOCN provides railways with the highest level of security, QoS and control. MOCN is defined in 3GPP TS 23.251 and TS 23.501, while the other methods mentioned above - national roaming and SIM cards - have a key disadvantage as they rely on the availability and resilience of the mobile network operator core and backhaul. The MNO’s core level of availability and resilience for consumer services may be insufficient to meet the high QoS requirements of FRMCS.
Benefits of MOCN
- Reduced costs: MOCN can help to reduce the cost of railway communications by eliminating the need for IMs to build and maintain their own radio sites. This can lead to significant savings, especially for smaller railways or when extending coverage to routes which do not justify the investment in RMR
- Increased throughput: FRMCS applications requiring high throughput on the uplink for critical video as they cannot rely on the throughput provided by the RMR bands on the uplink. Applications such as Grade of Automation level 3 and 4 or remote driving will require the throughputs that only MNOs using a MOCN architecture can provide
- Increased resilience: MOCN can help to increase the resilience of railway networks by providing a backup in case of failure of the dedicated radio access network. This can help to ensure that railway operations continue even in the event of a disaster.
Usage of MOCN today
MOCN has been the architecture of choice when deploying a national public safety network, allowing public safety operators to utilize MNO RAN access. Finland, the United States, South Korea, and France are key examples of this type of public safety MOCN deployment. It is also a proven and deployed network-sharing technique in many commercial mobile networks. In each of these countries, the public safety agencies rely on the MNOs to provide the critical connectivity of the radio access layer. At the same time, these agencies operate their own core networks for higher availability and resilience. In each of these cases the MOCN architecture was chosen for its efficiency, affordability, and faster time to market. MOCN also provides the public safety operator with full control over the QoS of the radio access network.
Figure 1 below shows the proposed MOCN architecture for FRMCS. The pink network elements represent the FRMCS core, and the blue elements represent the MNO core.
MOCN provides resources which can be independently and directly controlled by the FRMCS core including radio, QoS and transport resources.
In the MOCN configuration, as shown in the figure above, each shared gNodeB will be connected to two backhauls. One is connected to the FRMCS core and the other to the MNO core.
How MOCN works
The MOCN architecture involves adding MNO RAN sites to the FRMCS domain using a dedicated backhaul between the FRMCS core and the MNO MOCN RAN site. In the MOCN configuration, the FRMCS public land mobile network (PLMN) ID is broadcast over the air together with the MNO PLMN ID from the MNO trackside radio.
The RMR trackside radio broadcasts the FRMCS PLMN ID exclusively. An FRMCS on-board unit would receive the PLMN ID broadcast from both the nearby RMR trackside radio or the nearby MOCN trackside radio. Based on the broadcast PLMN ID, the on-board unit UE would create neighbour lists for handoff, comprising both the RMR and MOCN cells. The on-board unit would select the cell among both the RMR and MOCN with the best radio frequency (RF) conditions for connection. In case of RMR radio access failure in either the RMR radio, the RMR broadband or the RMR backhaul, the MOCN radio network would provide the redundancy needed to maintain the needed level of critical availability.
Based on 3GPP RAN sharing configuration guidelines, RMR and MOCN bands are fully interoperable. MNO access can be configured as a complementary access or for use as fallback only.
Chipsets planned for FRMCS will come with a sub-6 GHz band capability. The chipset vendors will add the N100 and N101 band availability using special licensing to the IMs or the railway undertaking. The other sub-6 MNO bands will be available by default
On-board rooftop antennas are now available with a broad range of frequencies supported up to 3.5 GHz. On-board user equipment using the same antenna connection could utilize RMR bands or any MNO band up to 3.7 GHz.
Since MOCN access QoS is managed in the FRMCS 5G core by the same policy control and session management functions as the RMR bands, there is no degradation of QoS when using MOCN access. The infrastructure manager can select between two QoS mechanisms:
- 5QI flow prioritization on the MOCN bands in line with the prioritization configured in the RMR bands. This would guarantee prioritization of FRMCS traffic over mobile broadband traffic using the MOCN shared radio resources.
- Partitioning the MOCN band using network slicing so that the FRMCS traffic is isolated for mobile broadband traffic in usage of radio resource blocks. This would guarantee that FRMCS traffic would have exclusive use of radio resources in the MOCN band. In addition, the 5QI flows can be configured in the FRMCS core to apply only on the FRMCS partition of the share radio access.
Contractual, regulatory and liability considerations
In terms of service level agreements (SLAs), the MNO must be obligated contractually to provide the same level of RF coverage in the MOCN band as the IM has planned for the RMR bands. In many cases, due to passenger connectivity obligations, the MNO already provides RF coverage close to this target railway corridor. However, the MOCN radio sites will need to be hardened with the same level of hardening and power supply backup as is designed for RMR radio sites. An extra investment by the MNOs will be required in this regard.
Regulations in the Technical Specification for Interworking (TSI) currently prohibit the use of any bands for European Train Control System (ETCS) or railway emergency calls, except for RMR bands. Future versions of the TSI would need to amend this clause once MNO RAN sharing has been fully validated in trials. Some countries have also mandated dedicated bands exclusively for railway critical communications. National safety rules would need to be reviewed and amended to allow MNO MOCN access for FRMCS.
Liability issues would also need to be reviewed; however, since the critical traffic does not leave the FRMCS domain in a MOCN configuration, the legal liabilities would remain exclusively with the infrastructure manager. The MNO would be liable to maintain the contractual SLA, but a major outage due to radio failure is unlikely, since the FRMCS has its own dedicated backhaul to each MOCN GNodeB.
Maturity of RAN Sharing
RAN sharing is mature technology that has been used by mobile network operators to share radio assets between them for more than 20 years, and RAN sharing was available in 3G.
Critical communications also typically utilize RAN sharing today. US FirstNet, Finland Erilisverkot and France’s “Radio Network of the Future” all utilize MOCN for critical public safety communications. There are several cybersecurity guidelines available for MOCN deployments.
RAN Sharing underlying 3GPP standards
The feature is based on the following standards:
- 3GPP TS 38.331: NR; Radio Resource Control (RRC); Protocol speciﬁcation
- 3GPP TS 38.413: NG-RAN; NG Application Protocol (NGAP)
- 3GPP TS 38.423: NG-RAN; In Application Protocol (XnAP)
- 3GPP TS 38.463: NG-RAN; E1 Application Protocol (E1AP)
- 3GPP TS 38.473: NG-RAN; F1 Application Protocol (F1AP)
- 3GPP TS 36.423: Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP)
- 3GPP TS 38.300: NR; NR and NG-RAN Overall description; Stage-2
- 3GPP TS 23.501: System architecture for the 5G System (5GS)
- 3GPP TS 23.251: Network sharing; Architecture and functional description
- 3GPP TS 36.300: Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio
- Access Network (E-UTRAN); Overall description; Stage 2
Further benefits of MOCN architecture for FRMCS
There are several key benefits to MOCN architecture. Primarily, it reduces costs. By sharing a common radio network, IMs can avoid the need to build and maintain their own networks. This can save railways a significant amount of money, especially in rural areas where the cost of building a network is high. Using the MOCN approach the Infrastructure Managers can save the following significant expenses:
- Dedicated site acquisition fees
- Dedicated spectrum license fees
- Civil works expense
- Passive site equipment (shelter, power, tower, feeder cables, antenna) expenses
- Active site equipment (radio units, broadband units) expenses
Second, MOCN architecture can improve efficiency. By sharing a common radio network, railways and MNOs can better coordinate their resources and improve the overall performance of their networks. This can lead to better service for FRMCS and reduced congestion on the network. Many FRMCS applications requiring high uplink throughput, (such as Grade of Automation Levels 3 and 4, remote driving and real-time passenger surveillance) are possible only using MNO spectrum, which is capable of providing the needed throughput.
Third, MOCN architecture can make it easier and faster for railways to implement FRMCS. By sharing a common radio network, railways can utilize the 5G corridors already planned for many major intercity routes in Europe. The European Union has adopted a 5G Strategic Deployment Agenda for 5G corridors along the 45,000 km of the Trans-European Transport Network (TEN-T) core network as well as the 30,000km of the TEN-T complementary network.
The Strategic Deployment Agenda is intended to help achieve the desired ‘shift to rail’: the ‘Gigabit Train’ (reliable and high-performance connectivity for passengers), which enhances the customer experience; and digital rail operations (communication services necessary for train movement and rail operation enhancement), which improve capacity and regularity of railways (manifested as FRMCS). It refers specifically to the possible use of MNO RAN sharing:
“Sharing active network elements (e.g., 5G slicing) is not per se excluded. Depending on national policy, public services might be used e.g., for regional lines, performance applications or as a backup. However, the use of public networks for rail operation services is subject to national regulatory, liability and legal constraints. The feasibility of 5G active network sharing is challenging and has to be proven. Critical aspects such as interoperability, QoS, service level agreements, regulations, legal issues and liability requirements must be scrutinized.” (source: STRATEGIC DEPLOYMENT AGENDA “5G CONNECTIVITY AND SPECTRUM FOR RAIL” – EIM)
European Union (EU) member states have already introduced licensing rules for 5G frequencies which mandate minimal coverage for railway corridors. A good example is Germany, where 100 Mbit/s to the train is mandated by licensing regulation for each of the four MNOs. Another example is Finland where over 97% of the railway corridors are already covered by MNOs to the QoS required by FRMCS applications.
Spain’s infrastructure manager, ADIF, has also taken advantage of this capability and is planning 5G corridors under its ownership, utilizing MNO spectrum for many major intercity routes. The first application of these 5G corridors is improving passenger connectivity and will provide a foundation for a rapid FRMCS deployment once standards are ready for tendering.
In addition to the benefits mentioned above, MOCN architecture can also help to improve the resilience of FRMCS networks. In case of failure of the dedicated (900 MHz and/or 1900 MHz) radio access networks, the MNO shared access network will provide a backup. In addition, many railway routes have insufficient traffic to justify dedicated radio investment. In these cases, the MOCN radio can extend coverage to these less utilized railway corridors economically MOCN architecture is also a more environmentally friendly approach to building and managing mobile networks. Railways can reduce the amount of energy that is used to power their networks by sharing a radio network. This can help to reduce greenhouse gas emissions and contribute to a more sustainable future.
Overall, MOCN architecture is a promising innovative approach to building and managing FRMCS mobile networks as a complement or an alternative to dedicated 900 and 1900 MHz radios. It has the potential to improve efficiency, reduce costs, improve security, and make FRMCS networks more resilient and environmentally friendly. As MOCN architecture continues to develop, it is likely to play an increasingly significant role in FRMCS networks.
Feb 20, 2023
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