Description
Service Gap Control (SGC) is a Non-Access Stratum (NAS) functionality introduced in 5G and evolved in later releases to enable User Equipment (UE) to temporarily interrupt an active data session for measurement purposes. It operates by defining a 'service gap'—a configured time window during which the network refrains from scheduling user data, allowing the UE to tune its radio away from the serving cell to scan other frequencies or radio access technologies (RATs). The SGC parameters, including gap duration, periodicity, and validity, are negotiated between the UE and the core network via NAS signaling, specifically defined in specifications 24.301 (EPS NAS) and 24.501 (5GS NAS).
Architecturally, SGC involves coordination between the UE and the Access and Mobility Management Function (AMF) in 5GC, or the MME in EPS. The UE requests a service gap pattern based on its capabilities and needs, such as inter-frequency or inter-RAT measurements for cell reselection or handover preparation. The network evaluates the request against policy and resource availability, then approves or modifies the pattern via a NAS message like the SERVICE GAP CONTROL message. During active gaps, the UE's RRC connection may be maintained, but user plane data transmission is halted, ensuring the session is preserved.
How it works: Once a service gap is activated, the UE uses the gap intervals to perform measurements on target cells, which could be on different NR bands, LTE carriers, or even non-3GPP networks. The network suspends downlink data scheduling and buffers any incoming packets, resuming transmission after the gap ends. This mechanism is distinct from traditional measurement gaps in RRC, as SGC operates at the NAS layer, providing more flexibility and longer durations suitable for background tasks like network scanning for edge computing or power saving.
Key components include the SERVICE GAP TIMER, which defines the gap length, and the SERVICE GAP PERIOD, which sets the recurrence interval. The UE reports gap utilization and measurement results to the network, enabling optimized mobility decisions. SGC enhances efficiency by allowing measurements without establishing new RRC connections or causing session drops, crucial for always-on services and battery-constrained devices.
Purpose & Motivation
SGC was created to address limitations in existing measurement gap mechanisms, which were primarily RRC-controlled and often insufficient for extensive inter-RAT or background scanning. In earlier releases, UEs relied on configured measurement gaps that were short and frequent, potentially disrupting latency-sensitive services and lacking NAS-level coordination. SGC solves this by introducing a NAS-based, negotiable service interruption, allowing for longer, tailored gaps that align with UE capabilities and network policies.
Historically, as networks evolved toward 5G and multi-RAT deployments (e.g., NR-LTE coexistence), UEs needed efficient ways to discover and measure alternative cells without degrading the user experience. SGC, introduced in Release 15 with 5G, provides this by enabling scheduled pauses in service, facilitating smooth mobility preparation and network discovery. It is particularly useful for power-saving modes and edge computing scenarios where UEs may need to periodically scan for local services.
The motivation stems from the need to balance service continuity with measurement requirements. By allowing controlled gaps, SGC reduces signaling overhead and battery consumption compared to frequent RRC reconfigurations. It supports advanced features like network slicing and non-terrestrial networks, where measurement intervals may vary significantly. Ultimately, SGC enhances overall system performance by enabling proactive mobility and resource optimization.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (241 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the Service Gap Control (SGC) function was newly introduced as a NAS-level congestion control mechanism. It allows the network to impose a "service gap" timer on a UE, during which the UE's mobility management and session management requests, such as for attach or PDN connectivity, can be rejected or restricted. The release also defined specific UE behaviours for when this timer is running, including handling for non-supporting UEs and procedures for attach without a PDN connection.
- Introduction of Service Gap Control; basics and feature negotiation TS 24.301CR2982
- Service Gap Control feature; non supporting UEs TS 24.301CR2983
- Service Gap Control; UE behaviour service gap timer is running TS 24.301CR2984
- Service Gap Control feature cleanup and corrections TS 24.301CR3010
- Service Gap Control, attach without PDN connection for supporting UEs TS 24.301CR3036
- Network control for always-on PDU sessions TS 24.501CR0107
+ 26 more changes
In Release 16, the Service Gap Control (SGC) function was introduced for the 5GS, providing the network with a mechanism to enforce a time-based restriction on a UE's access for specific services. This was specified through new general description and activation procedures, including the enforcement of SGC in both the UE and the AMF. The release also defined a new method for updating the service gap time value via the UE Configuration Update (UCU) procedure and specified the rejection of UL NAS TRANSPORT messages as part of SGC enforcement.
- NAS security mode control handling in case of RLOS access TS 24.301CR3218
- Small data rate control parameters received in EPS TS 24.301CR3245
- SINE_5G: Back-off control and retry restriction mechanisms in 5GS TS 24.501CR0730
- Small data rate control, general description TS 24.501CR0970
- Small data rate control, activation TS 24.501CR0971
- Serving PLMN rate control, general description TS 24.501CR0972
+ 73 more changes
In Release 17, the SGC function was enhanced to introduce paging timing collision control as a defined capability for MUSIM UEs in both EPS and 5GS. This specifically assists UEs using control plane CIoT EPS optimization, where procedures like the Service Request can be used to remove paging restrictions. Furthermore, clarifications were added regarding the behaviors of the UE and network when supporting Network Slice Admission Control.
- 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
- Uplink control during EPS UUAA-SM TS 24.301CR3615
- Introducion of Network Slice Admission Control TS 24.501CR3111
- Clarificaiton on behaviors of the UE and the network supporting Network Slice Admission Control TS 24.501CR3112
- Using Service Request procedure for removing paging restrictions in 5GS for MUSIM UE that uses the control plane CIoT 5GS optimization TS 24.501CR3439
+ 19 more changes
In Release 18, the Service Gap Control function was enhanced with the introduction of UE unavailability period reporting to override mobility management congestion control during periods of enhanced discontinuous coverage. Furthermore, support for network slice usage control was added, including UE storage of related control information and enforcement during registration and UE configuration update procedures.
- UE unavailability period reporting for enhanced discontinuous coverage overrides mobility management congestion control - EPS TS 24.301CR3939
- Equivalent SNPNs usage for congestion control TS 24.501CR4838
- CH controlled prioritized list of preferred SNPNs and GINs for access for localized services in SNPN TS 24.501CR5036
- Capability indication to support of network slice usage control TS 24.501CR5329
- General introduction on support of network slice usage control TS 24.501CR5328
- CH controlled prioritized list of preferred SNPNs and GINs for access for localized services in SNPN TS 24.501CR5203
+ 26 more changes
In Release 19, the SGC function was enhanced with new RAT utilization control capabilities for both EPS and 5GS, including support for satellite E-UTRAN and NG-RAN accesses. The release introduced procedures for storing this control information in non-volatile memory and for handling it during procedures like periodic tracking area updating and GUTI reallocation. It also expanded the control's applicability to equivalent PLMNs and provided clarifications on its implementation.
- Control of UE RAT utilization by EPS TS 24.301CR4077
- Addition of satellite E-UTRAN and satellite NG-RAN in RAT utilization control TS 24.301CR4107
- Storing RAT utilization control information in non-volatile-memory TS 24.301CR4119
- RAT utilization control information for equivalent PLMNs TS 24.301CR4111
- Control of UE RAT utilization in EPS TS 24.301CR4138
- The clarification of the applicability of RAT utilization control TS 24.501CR6350
+ 67 more changes
Explore further
Broader topics and technologies where SGC plays a role.
Defining Specifications
3GPP specifications that define or reference SGC, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |