What we’re seeing in 5G SA roaming — and it’s worse than we thought
When 3GPP introduced the Steering of Roaming (SoR) function for 5G standalone (SA) networks in specification 29.550, it represented a step forward. Unlike SS7 and Diameter, where steering was largely managed independently, SoR in 5G SA is a defined function inside the core network itself.
The principle is simple: a preferred PLMN list is created for each country, much like today’s OTA Steering. But instead of delivering the list over SMS via an OTA Server, the list is sent through the signalling channel. When a subscriber registers with a visited network, the UDM queries the SoR function for the correct list. This list is then passed back with the subscriber’s data to the AMF, which in turn is supposed to forward it to the device.
Theoretically, this improves reliability. By embedding the PLMN list delivery within the signalling flow, it avoids the additional failure points that SMS-based delivery introduces. In addition, device-side protections are built in to detect any tampering by the VPLMN, such as modifying or withholding the list, and to automatically trigger a network switch if tampering is detected.
However, experience tells a different story.
Working with an early adopter of 5G SA roaming, Cellusys encountered a series of unexpected challenges. Despite the 3GPP specifications being clear, vendor implementations have lagged significantly behind.
Initially, the UDM our customer deployed did not support the UDM-SoR interface at all. Only after direct work with the UDM vendor was this added. Even now, some core vendors have yet to implement it.
The NRF, essential for enabling NF discovery of the SoR function, also lacked support for registering an SoR NF. Again, this required workarounds — and we suspect this will be a common issue across other vendors.
Most critically, the AMF in the visited network must transmit the received PLMN list to the device. This is not optional; it is a mandatory feature under 3GPP. Yet many core vendors either do not support it or have deferred it as a roadmap item for up to two more years. The result is that the spec-defined SoR method cannot be relied upon.
Faced with these realities, Cellusys has worked closely with our customer to develop steering methods for 5G that more closely resemble the tried-and-tested approaches from SS7 and Diameter, rather than the elegant but currently impractical 5G SA model.
The main takeaway for MNOs is simple: verify everything. Never assume that 5G SA core networks, even from major vendors, fully implement mandatory 3GPP requirements. Every feature needs to be scrutinized and confirmed. Our experience in roaming leads us to believe that these problems may not be limited to SoR, but could apply to other services in the 5G SA core as well.
If you are preparing for 5G SA roaming, a careful review of vendor capabilities isn’t just advisable. It’s essential.
Categorised in: Blog