Steering of Roaming Connected Mode Control Information

Description

Steering of Roaming Connected Mode Control Information (SOR-CMCI) is a control mechanism introduced in 3GPP Release 17 that works in conjunction with Steering of Roaming (SOR) data. It is a parameter delivered by the network to a User Equipment (UE) that is in CONNECTED mode (e.g., RRC_CONNECTED in NG-RAN), providing instructions on when the UE should process and apply newly received SOR information. The SOR-CMCI is included within the SOR transparent container that is sent from the Access and Mobility Management Function (AMF) to the UE via a DOWNLINK NAS TRANSPORT message.

The SOR transparent container, defined in 3GPP TS 24.501, carries the SOR data itself and the associated SOR-CMCI. The SOR-CMCI parameter is a 3-bit field that conveys specific commands to the UE. The primary commands are: 'apply immediately' and 'apply when in idle mode'. When the UE receives SOR information with an 'apply immediately' indication, it must initiate the steering of roaming procedure straight away, even though it is in connected mode. This typically involves the UE leaving connected mode, evaluating the preferred PLMN list from the SOR data, and potentially selecting a new cell/PLMN, which may trigger a registration procedure on a new network.

If the SOR-CMCI indicates 'apply when in idle mode', the UE stores the SOR information and defers its application until the next time it transitions to idle mode (e.g., after the RRC connection is released). This mechanism gives the network precise control over the timing of the steering action. The inclusion of SOR-CMCI addresses a key scenario: without it, a UE in connected mode receiving SOR data might not apply it until an unspecified later time, potentially delaying critical network steering decisions. The secure packet within the SOR data, which is integrity protected and encrypted for the UE, also includes a hash of the SOR-CMCI to ensure its authenticity and prevent tampering.

Purpose & Motivation

SOR-CMCI was introduced to solve the timing ambiguity problem associated with delivering Steering of Roaming information to a UE that is actively in a connected session. In the initial Release 16 SOR procedures, when SOR data was sent to a UE in connected mode, the standard did not explicitly mandate when the UE should execute the steering procedure. This could lead to unpredictable behavior and delays, undermining the operator's ability to dynamically manage roaming traffic in near-real-time. For effective traffic steering, especially in response to changing network conditions or commercial triggers, the home operator needs certainty about when the steering policy will be enacted.

The creation of SOR-CMCI provides the network with explicit control, allowing it to command an immediate steering action (which may interrupt an ongoing session) or to schedule it for a more convenient time (at the next idle period). This enables more sophisticated and reliable roaming management strategies. For instance, an operator might use 'apply immediately' for urgent steering away from a congested partner network, while using 'apply when in idle mode' for less time-critical policy updates, improving both network efficiency and the subscriber experience by avoiding unnecessary session drops where possible.