The goal of roaming is to keep mobile users seamlessly connected to a network, wherever they are. This has been the case for each generation of wireless communication technology, but as operators launch their commercial 5G services, roaming is about to get more complex.
Earlier adopters have launched commercial 5G services by leveraging their existing 4G network infrastructure. The 5G new radio (NR) air interface offers much greater speeds and capacity, but it utilises the LTE network infrastructure as the control plane. This approach is sometimes called non-standalone (NSA) 5G and accounts for the vast majority of the world’s 220 million 5G subscribers in December 2020.
This steppingstone approach to launching 5G will soon be followed by standalone (SA) 5G, which includes a new core (NC) network. With SA 5G, operators can offer new use cases that require either ultra-low latency, very high reliability, or massive Internet of Things (IoT) connectivity. The Global Mobile Suppliers Association (GSA) forecasts that 5G subscriptions will comprise nearly 30% of the global mobile market by the end of 2025, just over 3 billion subscriptions.
Mobile operators need to start doing the groundwork for both individual users and connected objects to roam effectively on NSA and SA 5G networks.
As NSA 5G uses the 4G evolved packet core (EPC), roaming is straightforward. So long as the home network, visiting network, and all international networks in-between are LTE-enabled and EPC-based, roaming can work perfectly well on these existing infrastructures. Roaming signalling is exchanged between 4G cores on Diameter/IP exchange (IPX), while legacy signalling SS7 (2G/3G) is still carried over IPX. Retail and wholesale operators now need to focus on upgrading IPX bandwidth to support 5G’s increase in roaming data usage.
This architecture cannot support all 5G use cases, however. It is suitable for Enhanced Mobile Broadband (eMBB) where speeds faster than 4G are required, such as video streaming or online gaming. ultra-reliable and low latency communications (uRLLC) use cases, like connected cars or telesurgery, require real-time connectivity and exceptional reliability levels. Massive machine-type communications (mMTC) will support enormous volumes of lower-speed connections, thereby proving an alternative to networks like LoRa. As SA 5G is deployed to enable these innovative use cases, traditional voice and data roaming will be impacted.
Both retail and wholesale operators will be impacted by 5G roaming. SA 5G requires a brand new core network that relies on new technologies like software-defined networking (SDN), network function virtualisation (NFV), network slicing, HTTPS proxies, regional P-Gateways and more. Signalling is based on service communication proxy (SCP), binding support function (BSF), and security edge protection proxy (SEPP) functions. 5G will also require HTTP as the transport protocol. This necessary network evolution is underway, but the industry is still in the process of defining standards. Orange is working closely with 3GPP and the GSMA to develop these standards and roll them out as soon as possible. Orange is also ready to work with early SA 5G adopters on trials and proofs of concept for 5G roaming.
All operators must sign new 5G bilateral agreements and establish interconnections with peers. This can be bilateral, but, like today, the complicated management and rollout of roaming agreements will be simplified using IPX and roaming hub providers. Signalling interworking will require a SEPP, which ensures end-to-end confidentiality and integrity between source and destination networks. All signalling traffic across operator networks will transit via these security proxies. Authentication between operators’ SEPP is required to prevent unauthorised communication between networks. Operators will benefit from connecting to a 5G-compliant IPX hub as it offers adapted levels of security from all the other operators connected to the hub.
VoLTE will become an essential service. Because 5G does not allow for circuit-switched fallback, reallocating 3G frequencies requires prior implementation of VoLTE, even for 4G roaming. At this time, however, few operators have implemented VoLTE roaming agreements, according to the GSMA. This is a key element in full SA 5G deployments with optimised frequency bands to support multiple use cases while optimising costs. It is something the GSMA is urging all operators to progress.
uRLLC and mMTC roaming use cases will probably take a few years longer to become fully available. This is due to the 3GPP only having published the first key 5G specifications for uRLLC and mMTC relatively recently, with Release 16 being published in July 2020. The standard is being developed further and will be updated in 3GPP Release 17, which is scheduled for publishing in June 2021.
Furthermore, local breakout architectures are yet to be defined, and roamers’ access is currently via their home networks. This means traffic must be carried back to that home network before it can access the internet, which makes sense in terms of continuity of service and appropriate billing, but isn’t suited to ultra-low latency 5G use cases like connected cars or AR/VR. Orange is planning to use our worldwide IPX network to propose regional hubs with data breakout features through regional P-gateways.
5G network slicing will enable use cases that require specific resources and QoS levels. But operators will have to manage slices and QoS levels across their respective networks, and there is no formal standard in place yet: trials are ongoing.
Major wholesale providers are investing in slicing tests and trials. In 2020, Orange carried out a proof of concept with Deutsche Telekom using blockchain technology to offer enhanced IPX services for critical IoT. This would enable operators to offer QoS and improve network performance forecasting and incident management.
Roaming must be addressed
As 5G deployments ramp up, the industry has no choice but to address the roaming issue. It is hoped that most interconnections will be in NSA mode, which is already available on existing IPX networks and architecture. But migration to full VoLTE is a prerequisite for the next stage, which would be 5G SA roaming and 2G/3G sunsets. International carriers should use this time to formulate new value proposals with upgraded IPX bandwidth, and support of new signalling protocols.
As of March 2021, according to the GSA, only seven operators have launched public 5G SA networks, among them China Mobile, T-Mobile in the USA using spectrum at 600MHz across the country, and RAIN in parts of Cape Town, South Africa to support 5G FWA.
Telstra in Australia has announced that is has deployed a 5G core network, and reports it will launch its SA 5G network when there are enough devices in circulation in Australia to justify doing so.
The GSA reports that in March 2021, “sixty-eight operators in thirty-eight countries/territories are investing in 5G standalone for public networks in the form of trials, plans, paying for licences, deploying or operating networks.”