Steering of Roaming (SoR)

Description

Steering of Roaming (SoR) is a technique used in mobile networks to influence a roaming UE to select a preferred Visited Public Land Mobile Network (VPLMN) when multiple networks are available. The Home Public Land Mobile Network (HPLMN) sends steering information to the UE, which then uses this information to prioritize the preferred VPLMN during network selection. This is particularly important in international roaming scenarios, where the HPLMN may have preferential agreements with certain VPLMNs due to lower costs, better service quality, or specific features.

The SoR mechanism operates during the UE's roaming network selection process. When a UE is roaming, it typically selects a VPLMN based on signal strength and other criteria. With SoR, the HPLMN can provide a list of preferred VPLMNs, along with priority information, to the UE. This information is conveyed using protocols such as the Steering of Roaming Application (SoR) in the SIM/USIM, or via Over-the-Air (OTA) mechanisms. The UE then uses this list to select the preferred VPLMN if it is available. If the preferred VPLMN is not available, the UE may fall back to other networks.

In 3GPP specifications, SoR is defined in TS 21.905 (Vocabulary for 3GPP Specifications). The technique has evolved over releases, with enhancements to support more dynamic and secure steering. For example, in later releases, SoR can be combined with other mechanisms such as Preferred PLMN (PPLMN) lists and Equivalent PLMN (EPLMN) lists. The HPLMN can also use SoR to steer UEs away from networks that are not preferred, for example, due to high roaming charges or poor performance.

The implementation of SoR involves several network elements. The HPLMN's Home Subscriber Server (HSS) stores the steering information for each subscriber. When the UE registers with a VPLMN, the VPLMN's Mobility Management Entity (MME) or Visitor Location Register (VLR) may request steering information from the HSS. The HSS then provides the steering information, which is sent to the UE. Alternatively, the UE may receive steering information via SMS or other OTA methods. The UE's SIM/USIM card may also contain a list of preferred VPLMNs.

SoR is important for both operators and subscribers. For operators, it allows them to optimize roaming costs and ensure that subscribers use preferred networks, which may offer better rates or services. For subscribers, it can lead to better roaming experience, with lower charges and improved service quality. However, SoR must be implemented carefully to avoid negative user experience, such as forcing the UE to a network with poor coverage. Therefore, 3GPP specifications include requirements for SoR to ensure that the UE can still access emergency services and that the steering is not overly aggressive.

In summary, Steering of Roaming is a key mobility management feature that enables HPLMNs to influence the roaming network selection of UEs. It is defined in 3GPP TS 21.905 and has been supported since Release 7, with ongoing enhancements in subsequent releases.

Purpose & Motivation

Steering of Roaming exists to allow Home Public Land Mobile Network (HPLMN) operators to influence which Visited Public Land Mobile Network (VPLMN) a roaming subscriber uses. Before SoR, roaming UEs would typically select a VPLMN based solely on signal strength or random order, which could result in the UE connecting to a network that is not preferred by the HPLMN. This could lead to higher roaming costs for the operator and the subscriber, as well as potentially poorer service quality. SoR solves this problem by providing the UE with a list of preferred VPLMNs and encouraging it to select one of those networks.

Historically, the need for SoR arose as international roaming became more prevalent and operators negotiated complex roaming agreements. Operators wanted to steer their subscribers to networks with which they had favorable terms, such as lower wholesale rates or better coverage. The initial approach was to use Preferred PLMN lists stored on the SIM card, but these were static and difficult to update. SoR introduced a more dynamic mechanism, allowing the HPLMN to update steering information remotely. This was particularly important as roaming agreements changed frequently.

SoR also addresses the limitation of Equivalent PLMN (EPLMN) lists, which treat multiple networks as equivalent. While EPLMN lists allow the UE to select any equivalent network, they do not provide a priority order. SoR adds priority, enabling the HPLMN to specify which VPLMN is most preferred. This gives operators greater control over roaming traffic and helps them manage costs and quality of service. In summary, SoR was created to provide dynamic, prioritized steering of roaming UEs, benefiting both operators and subscribers.