Description
UE Radio Capability Signalling optimization (RACS) is a network feature designed to minimize the signaling overhead associated with transmitting a User Equipment's (UE) radio capability information. A UE's radio capabilities are a comprehensive set of parameters detailing its supported features, bands, frequency ranges, and technical limits (e.g., maximum number of carriers, supported modulation schemes). Traditionally, this information, contained in the UE Radio Capability ID (UECapabilityInformation), is sent from the UE to the network during initial registration and potentially during each connection setup or inter-RAT mobility, leading to significant signaling load, especially for large-capability containers in advanced UEs.
RACS works by enabling the Core Network (specifically the Access and Mobility Management Function, AMF, in 5GC, or the MME in EPS) to store the UE's radio capability information after the first successful retrieval. The network assigns a UE Radio Capability ID to this stored information. Subsequently, instead of the UE transmitting the full capability set, the network can signal using this much shorter ID. The AMF/MME provides this ID to the Radio Access Network (RAN) node (gNB or eNB). If the RAN node does not have the corresponding capability data cached locally, it can request the full information from the core network using the ID.
The architecture involves coordination between the UE, RAN, and Core Network. Key signaling messages are modified to carry the UE Radio Capability ID. For example, in 5G, the AMF includes the `UE Radio Capability ID` in the `INITIAL CONTEXT SETUP REQUEST` or `UE RADIO CAPABILITY CHECK REQUEST` messages to the gNB. The gNB checks its local storage. If the data is missing or potentially outdated (e.g., after a software update), the gNB can request the AMF to provide the full `UE Radio Capability Information` via a retrieval procedure. The network also manages the lifecycle of this stored data, including mechanisms to detect when capabilities might have changed (e.g., via indication from the UE or a timer-based refresh).
This optimization significantly reduces the size of signaling messages on the radio interface (Uu) and the NG/E1 interfaces. It is particularly beneficial for latency-sensitive procedures like connection resumption from RRC_INACTIVE state and for UEs with extensive capability lists, such as those supporting many NR and LTE bands, carrier aggregation combinations, and features like dual connectivity. By minimizing the amount of data transferred, RACS improves connection setup times, reduces UE battery consumption for signaling, and decreases the processing load on network nodes.
Purpose & Motivation
RACS was introduced to address the growing signaling overhead problem caused by increasingly complex UE radio capabilities. As 3GPP standards evolved through LTE-Advanced and into 5G, the number of supported frequency bands, carrier aggregation combinations, and optional features exploded. Transmitting the full, verbose `UECapabilityInformation` message during every relevant procedure became a significant source of latency and consumed valuable radio resources.
The primary problem RACS solves is the inefficient repetition of large, static data sets. A UE's core radio capabilities change infrequently, typically only after a software update or a major hardware change. Transmitting this multi-kilobyte block repeatedly was wasteful. Prior to RACS, the network had no standardized, efficient way to avoid this repetition, leading to longer call setup times and increased signaling congestion, especially in dense networks.
Its creation was motivated by the need to optimize network performance for massive IoT deployments and enhanced mobile broadband scenarios where fast connection establishment is critical. By shifting to an ID-based model, RACS aligns with general network optimization principles of caching and indirection. It also future-proofs the system for even more complex capability definitions in beyond-5G systems, ensuring that signaling scales efficiently regardless of how detailed UE capabilities become.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (432 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-7, normative work from Rel-15.
In Release 15, the RACS function introduced new procedures to optimize the signalling of large UE radio capabilities, specifically for anticipated EN-DC UEs. This included mechanisms for the network to request UE radio capabilities and for the UE to trigger a Tracking Area Update procedure due to a change in its radio capability. Furthermore, it enabled the retrieval of UE capability information via Downlink NAS Transport signalling.
- UE capability handling for EPC_DC_NR TS 23.401CR3257
- UE Capability for supporting 15 EPS bearers TS 23.401CR3419
- Introducing Early Data Transmission for Control Plane CIoT EPS optimization TS 23.401CR3436
- Support of tracing in 5GS signalling: overview TS 23.501CR0474
- Addition of UE NAS capability for support of dual connectivity with NR TS 24.301CR2892
- Signalling of UE's additional security capabilities TS 24.301CR2954
+ 64 more changes
In Release 16, the RACS function introduced a UE Radio Capability ID to optimize signalling, allowing the network to assign this ID via procedures like GUTI reallocation and the UE to indicate its availability during mobility events like a TAU. This enabled the network to reference a stored capability profile instead of frequently transferring the full UE radio capability information over the air interface. The release also formally introduced the UCMF services and extended support for the feature within the EPS architecture.
- Adds UE Radio Capability ID in signalling procedures TS 23.401CR3503
- EPS architecture supporting RACS TS 23.401CR3510
- UPF Selection influenced by the indication of the identity/identities of 5G AN N3 User Plane capability TS 23.501CR0862
- Introduction of RACS: UCMF services TS 23.501CR1037
- Introduction of Radio Capabilities Signalling Optimisation feature TS 23.501CR1071
- Introducing 5GS UP optimization TS 23.501CR1186
+ 143 more changes
In Release 17, the RACS function was enhanced to enable the detection of RACS support at the target network during handover procedures, specifically for S1, N2, and X2 handovers. This allows the network to optimize UE radio capability signalling by avoiding unnecessary retransmission of capabilities when both source and target nodes support RACS. Additionally, the release introduced clarifications on the handling of UE radio capabilities for paging when the MME changes.
- Introduction of MUSIM capability exchange TS 23.401CR3630
- Alignment of NWDAF discovery of data exposure capability in TS 23.501. TS 23.501CR2759
- Introduction of MUSIM capability exchange TS 23.501CR2927
- Resolving the Editor's note related to supporting paging timing collision control as a capability for MUSIM in EPS TS 24.301CR3568
- Using Service Request procedure for removing paging restrictions in EPS for MUSIM UE that uses the control plane CIoT EPS optimization TS 24.301CR3564
- Clarification of E-UTRA capability handling TS 24.301CR3545
+ 78 more changes
In Release 18, the RACS function was enhanced to support the network-assigned UE radio capability ID for equivalent SNPNs, ensuring its applicability and storage alongside other subscriber parameters. This optimization allows for more efficient radio signalling procedures by enabling the reuse of a previously assigned capability identifier when a UE registers with an equivalent SNPN. The update integrates this capability management within the broader framework for subscriber identity and parameter handling defined for 5G systems.
- TS 23.501 Enhancing External Exposure of Network Capability TS 23.501CR3715
- Considering ML model management capability during ADRF discovery and selection TS 23.501CR3929
- Optimizations for the support of time vality policies for a network slice and graceful network slice PDU sessions release. TS 23.501CR4004
- CN based MT communication capability indication TS 23.501CR4081
- Extension of NWDAF registration information to reflect new accuracy checking capability TS 23.501CR3764
- Optimization consideration for satellite backhaul QoS monitoring TS 23.501CR4246
+ 73 more changes
In Release 19, the RACS function introduced new analytics for signalling storm mitigation and enhanced capability handling for functionalities like MPQUIC-IP and MPQUIC-E steering. It also brought clarifications and updates for specific UE radio capability procedures, including the handling of UE capability updates and the conditions for indicating S&F capability. Furthermore, the release specified corrections for control plane CIoT EPS optimization, such as the S&F wait time duration, and defined rules for re-enabling or not disabling E-UTRA capability in scenarios like eCall inactivity.
- New analytics for signalling storm mitigation (TS 23.501) TS 23.501CR5661
- Handling of UE capability for MPQUIC-IP and MPQUIC-E steering functionalities TS 23.501CR5844
- NAS signalling connection release after EMM cause 83 TS 24.301CR4402
- Capability negotiation for CP CIoT EPS optimization with overhead reduction TS 24.301CR4491
- Enhancement of 5G ProSe capability for multi-hop relays TS 24.501CR6552
- Update 5GMM capability for 5G ProSe multi-hop relays TS 24.501CR6692
+ 44 more changes
Explore further
Broader topics and technologies where RACS plays a role.
Defining Specifications
3GPP specifications that define or reference RACS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.003 vj50 | Numbering, addressing and identification in 3GPP | Rel-19 |
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 23.417 v1700 | IMS Core Component for NGN Architecture | Rel-7 |
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 23.517 v1800 | IMS Core Component for NGN Architecture | Rel-8 |
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.523 vj00 | NGCN-NGN Interconnection Scenarios | Rel-19 |
| TS 24.524 vj00 | Hosted Enterprise Services Architecture | Rel-19 |
| TS 29.421 v810 | IMS Interworking with External IP Networks | Rel-8 |
| TS 29.675 vj10 | UE Radio Capability Provisioning Service | Rel-19 |
| TS 36.413 vj10 | S1 Application Protocol (S1AP) | Rel-19 |