Description
Idle mode Signalling Reduction (ISR) is a core network feature defined in the 3GPP Evolved Packet System (EPS) architecture. It optimizes mobility management for User Equipment (UE) capable of operating in both the Evolved UTRAN (E-UTRAN, i.e., LTE) and the legacy UTRAN or GERAN (3G/2G) access networks. The fundamental principle of ISR is to allow a UE to be registered in two different packet core domains simultaneously: the MME (Mobility Management Entity) for the E-UTRAN and the SGSN (Serving GPRS Support Node) for the UTRAN/GERAN. When ISR is activated, the UE is assigned two independent but concurrently valid temporary identities: a Globally Unique Temporary Identity (GUTI) from the MME and a Packet-Temporary Mobile Subscriber Identity (P-TMSI) from the SGSN. The core network nodes (MME and SGSN) are aware of each other's context for that UE and establish an association, often via the S3 interface between them.
How ISR works revolves around the UE's behavior during cell reselection in idle mode. Without ISR, moving from an LTE cell to a 2G/3G cell (or vice versa) would trigger a full Routing Area Update (RAU) or Tracking Area Update (TAU) procedure, respectively. These procedures involve signaling with the core network to update the UE's location and potentially re-establish packet data network (PDN) connections, consuming radio resources, network signaling load, and UE battery power. With ISR activated, the UE, when reselecting to a cell within the registered routing area (RA) or tracking area (TA) of the *other* access technology, does not need to perform an update procedure immediately. It can simply camp on the new cell using the existing registration from that core network domain. The UE only needs to perform a TAU or RAU when it moves outside its currently registered TA or RA for the respective system, or when a periodic update timer expires.
The role of ISR in the network is to significantly reduce the signaling load on the core network interfaces (S3, S6a, Gr) and the radio interface, especially in deployment scenarios with tight inter-RAT (Radio Access Technology) coverage overlap or frequent cell reselection by mobile users. It enhances user experience by providing seamless idle mode mobility with minimal service interruption and conserves UE battery life by reducing the frequency of power-intensive signaling procedures. Network operators benefit from reduced operational costs due to lower signaling traffic and improved scalability. ISR is a key enabler for smooth network migration from 2G/3G to 4G LTE, allowing operators to leverage existing infrastructure while rolling out new technology.
Purpose & Motivation
ISR was introduced to solve the problem of excessive and inefficient signaling generated by dual-mode LTE/2G-3G devices frequently moving between different radio access technologies while in idle mode. As LTE networks were deployed, they often provided islands of coverage within a wider 2G/3G sea. A mobile device moving in and out of LTE coverage would trigger a TAU upon entering LTE and a RAU upon falling back to 2G/3G, creating a signaling storm at the boundaries. This wasted valuable radio and core network resources, increased latency for the user when re-entering active state, and drained device batteries.
The motivation for its creation in 3GPP Release 8 was directly tied to the introduction of the EPS and the need for efficient interworking with existing GSM/GPRS and UMTS networks. Previous approaches in pre-Release 8 systems did not allow for simultaneous registrations; a device was attached to only one packet core domain at a time. This forced an update procedure at every RAT change. ISR addressed this limitation by introducing the concept of dual registration, effectively 'hiding' RAT changes from the core network's mobility management as long as the UE remained within its known location areas. It provided a pragmatic solution that balanced complexity (maintaining two contexts) with the significant benefit of signaling reduction, which was critical for the commercial success and performance of early LTE deployments.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (41 CRs across 5 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-8, normative work from Rel-15.
In Release 15, the ISR function was updated to introduce the E-UTRAN Deactivate ISR timer and the GERAN/UTRAN Deactivate ISR timer, which a UE starts upon expiry of its periodic update timer when out of coverage or camped on the opposite RAT, respectively. Furthermore, the specification clarified that ISR shall not be activated if the UE is allowed to use SIPTO at the local network and its tracking area list covers both local and macro networks. These changes provide more precise control over ISR activation and deactivation in specific network and mobility scenarios.
In Release 16, the ISR function was enhanced with the introduction of the E-UTRAN Deactivate ISR timer and the GERAN/UTRAN Deactivate ISR timer to more efficiently handle periodic update timer expiries when the UE is out of coverage. Specifically, these timers prevent immediate ISR deactivation and unnecessary signalling by allowing the UE to remain in a state where ISR is temporarily maintained until it returns to coverage and can perform a successful update procedure. Furthermore, clarifications were made regarding the interaction of ISR with features like SIPTO at the local network, where ISR activation is prohibited if a tracking area list spans both local and macro networks.
- Adds UE Radio Capability ID in signalling procedures TS 23.401CR3503
- Signalling of UE support for RACS and of UE radio capability ID TS 24.301CR3242
- Avoiding UE indicating RLOS access in RRC Signalling TS 23.401CR3502
- Corrections of PLMN assigned Capability signalling TS 23.401CR3520
- PSM,ISR and extended idle mode DRX handling in RLOS TS 23.401CR3553
- Correcting active flag and signalling active flag wording TS 24.301CR3314
+ 4 more changes
In Release 17, the ISR function was updated with specific procedures for MUSIM UEs and emergency services in EPS. The enhancements include rules for a UE to release its NAS signalling connection and indicate paging restriction during a mobility Tracking Area Update (TAU) when moving to a TA outside its current list, but only if no emergency service is ongoing. Furthermore, a TAU with the "signalling active flag" is now triggered if initiated during a CPSR procedure, and NAS signalling connection maintenance was addressed for PLMN selection.
- Releasing NAS signalling connection and Paging restriction during mobility TAU in a TA outside the current Tracking Area List for MUSIM UE in EPS TS 24.301CR3645
- Trigger TAU with signalling active flag if initiated during CPSR procedure TS 24.301CR3648
- UE to release NAS signalling connection and indicate Paging restriction during mobility TAU only if no emergency service is ongoing in EPS TS 24.301CR3677
- NAS signalling Connection maintenance for PLMN selection TS 24.301CR3791
In Release 18, the updates to the Idle mode Signalling Reduction (ISR) function clarified the conditions for releasing a NAS signalling connection established from the EMM-IDLE state. Specifically, it was updated to state that this NAS connection is released only when the start of user plane activity is not provided. Furthermore, the release conditions for the NAS signalling connection were refined based on network unavailability information.
In Release 19, the ISR function was updated with new procedures and corrections specifically for Control Plane CIoT EPS Optimization with Overhead Reduction, including clarifications for the service request procedure and the handling of the T3448 timer. The release also introduced corrections for the network procedures to negotiate this optimization and clarified the usage of EPS services when this feature is active. Furthermore, it provided specific guidance on the NAS signalling connection release and the handling of the Deactivate ISR timers during periodic update procedures.
- NAS signalling connection release after EMM cause 83 TS 24.301CR4402
- Capability negotiation for CP CIoT EPS optimization with overhead reduction TS 24.301CR4491
- Control plane CIoT with overhead reduction TS 23.401CR3940
- NAS overhead reduction for CP CIoT data transport, RRC establishment cause TS 24.301CR4465
- Miscellaneous corrections for NAS overhead reduction TS 24.301CR4490
- NAS overhead reduction for CP CIoT data transport, corrections TS 24.301CR4461
+ 15 more changes
Explore further
Broader topics and technologies where ISR plays a role.
Defining Specifications
3GPP specifications that define or reference ISR, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 24.301 vj60 | NAS protocol for Evolved Packet System | Rel-19 |
| TS 24.801 v810 | CT1 SAE NAS Aspects for EPC | Rel-8 |
| TS 33.401 vj10 | EPS Security Architecture | Rel-19 |