Radio Access Network rules

Description

RAN rules are a set of rules and procedures defined by 3GPP to enable the radio access network (RAN) to assist and control the selection of access networks and the steering of traffic between cellular networks (E-UTRAN or UTRAN) and Wireless Local Area Networks (WLAN). These rules are specified in 3GPP TS 36.304, TS 25.304, and TS 36.331, and are referenced in core network specifications such as TS 24.302 and TS 29.212. The primary purpose of RAN rules is to allow the network to offload traffic to WLAN when it is beneficial, or to keep traffic on cellular when WLAN is not suitable, thereby improving user experience and optimizing network resource usage.

The RAN rules consist of two main components: RAN assistance parameters and RAN steering commands. RAN assistance parameters are provided by the RAN to the UE via broadcast or dedicated signaling. These parameters include thresholds and conditions related to signal strength, load, and other criteria that the UE uses to evaluate whether to steer traffic to WLAN. RAN steering commands are explicit instructions from the RAN to the UE to steer specific traffic to or from WLAN, often based on network policies or real-time conditions. The UE applies these rules according to the procedures defined in the access stratum specifications.

The operation of RAN rules involves interaction between the UE, the RAN, and the core network. The RAN provides assistance parameters to the UE, which then evaluates them against its current situation (e.g., signal quality, WLAN availability). If the conditions are met, the UE may initiate traffic steering, such as moving a PDN connection or a specific flow to WLAN. In RAN-controlled LTE-WLAN interworking (LWI), the RAN can also send steering commands to the UE to force traffic steering. The core network may be involved in aspects such as authentication, authorization, and policy control, but the RAN rules themselves are primarily executed by the UE and the RAN.

RAN rules are crucial for enabling seamless integration of cellular and WLAN networks, a key feature in heterogeneous networks. They allow operators to leverage WLAN to offload traffic, especially in congested scenarios, without sacrificing service quality. The rules are designed to be flexible and extensible, supporting different deployment scenarios and operator policies. Over successive 3GPP releases, the definition of RAN rules has been refined to include RAN steering commands and to clarify the scope of interworking, reflecting the evolution of LTE-WLAN interworking features.

In summary, RAN rules provide a standardized framework for access network selection and traffic steering between cellular and WLAN, enabling operators to optimize network performance and enhance user experience. They are a fundamental component of RAN-level interworking and are essential for the efficient operation of multi-access networks.

Purpose & Motivation

RAN rules were introduced to address the growing need for efficient traffic management in heterogeneous networks where cellular and WLAN technologies coexist. As mobile data traffic increased, operators sought ways to offload traffic to WLAN to alleviate congestion in cellular networks. However, simple offloading based on static policies often led to suboptimal user experience because WLAN conditions vary. RAN rules provide a dynamic, network-controlled mechanism to steer traffic based on real-time RAN assistance parameters and steering commands, ensuring that offloading decisions are made intelligently.

The motivation for RAN rules stems from the limitations of earlier approaches that relied solely on UE-based decisions or core network policies without RAN involvement. Without RAN assistance, the UE might select WLAN even when cellular provides better service, or vice versa. RAN rules enable the network to influence UE decisions by providing thresholds and conditions that reflect current network conditions, such as load and signal quality. This allows for better load balancing and improved quality of experience.

Furthermore, RAN rules support both RAN-assisted and RAN-controlled interworking, offering flexibility for operators to choose the level of RAN involvement. This evolution from Rel-12 to later releases reflects the increasing sophistication of LTE-WLAN interworking, including the introduction of RAN steering commands in Rel-13 to enable more direct control. Overall, RAN rules solve the problem of inefficient traffic steering by integrating RAN intelligence into the decision process.