Optimisation Mode supported

Description

Optimisation Mode supported (OM) is a parameter or capability flag within the 3GPP specifications that indicates whether a User Equipment (UE) supports specific optimization procedures for mobility and handover between different radio access technologies (RATs). It is part of the UE's radio access capabilities exchanged with the network. The primary context for OM is in the inter-RAT mobility between GSM/EDGE Radio Access Network (GERAN) and UMTS Terrestrial Radio Access Network (UTRAN), particularly for handover and cell reselection procedures.

Architecturally, the OM indicator is stored in the UE and reported to the network. In circuit-switched (CS) domains, this occurs during location updating, IMSI attach, or routing area updating procedures. The Serving GPRS Support Node (SGSN) or Mobile Switching Center (MSC) receives this capability. The network uses this information to decide whether to invoke optimized handover procedures. When OM is supported, the network may use specific mechanisms that can reduce handover interruption time and improve success rates. For example, it might enable "Network Assisted Cell Change" (NACC) from GERAN to UTRAN or vice-versa, where the network provides the UE with system information of the target cell before the handover, speeding up the access process.

How it works is procedural. The UE includes the OM indicator in its capability information message to the core network. The network element (SGSN/MSC) stores this information. Later, when a handover to another RAT is being prepared, the network checks if the UE supports OM. If supported, and if the target RAT and network also support the corresponding optimization features, the network may choose an optimized handover path. This could involve different signaling sequences or the pre-provisioning of information. The key components are the UE's protocol stack that generates the capability, the radio access network, and the core network nodes that store and utilize it. Its role is to facilitate more efficient inter-system mobility, which is crucial for seamless user experience in multi-RAT deployments.

Purpose & Motivation

The Optimisation Mode supported capability was introduced in 3GPP Release 5 to address the challenges of handover and mobility management between 2G (GERAN) and 3G (UTRAN) networks. Early inter-RAT handovers were often "blind" or less assisted, meaning the UE had to acquire all necessary system information of the target cell after arriving there, leading to longer service interruption times and a higher risk of handover failure. This degraded the user experience during technology transitions.

The problem was the lack of a standardized mechanism for the network to know if a UE could handle more advanced, network-assisted handover procedures. The creation of the OM indicator solved this by providing a simple signaling flag. It allowed the network to identify UEs capable of supporting optimized inter-RAT procedures, such as those defined in TS 43.129 and TS 25.331. This enabled the network to tailor its handover strategy: for OM-capable UEs, it could use faster, more reliable methods, while for legacy UEs, it would fall back to basic procedures. The motivation was to improve the seamlessness of mobility in evolving networks where 2G and 3G coexisted, enhancing call drop rates and handover performance as operators deployed UMTS alongside existing GSM networks.

Key Features

• UE capability indicator for inter-RAT optimization
• Signaled from UE to core network (SGSN/MSC)
• Enables network to use optimized handover procedures
• Aims to reduce handover interruption time
• Applicable for GERAN-UTRAN mobility
• Stored in the network for future handover decisions

Evolution Across Releases

Defining Specifications