Idle Mode Signaling Reduction - ISR

ISR이란 UE가 IDLE 모드에서 Sinalling하는 빈도를 줄위기 위한 기술이다.

이를 왜 하냐고 하면, 휴대용 기기의 최대 과제인 배터리의 효율을 높이기 위한 하나의 방법론이라고 이야기 할 수 있을 것 같다.

일반적으로 구글을 검색해보면, 아래와 같은 문구로 정의를 내려놓고 있다.

Reducing the frequency of TAU and RAU procedures caused by UEs reselecting between E-UTRAN and GERAN/UTRAN which are operated together.

이를 다시 이야기 하면, 4G와 3G/2G 사이에서 일어나는 TAU 또는 RAU 등의 빈도를 줄여서 오가는 메시지를 줄이고,

이로 인해 배터리와 기지국의 처리량을 줄이겠다는게 목표로 보인다.

아래 내용은 아래 링크에서 발춰한 내용으로 내용 정리가 잘 되어 있다.

http://lte-epc.blogspot.kr/2009/11/idle-mode-signaling-reduction-isr.html

Idle mode signalling reduction is a feature that allows the UE to roam between LTE & 2G/3G. Basically it aims at reducing the frequency of TAU and RAU procedures caused by UEs re-selecting between E-UTRAN and GERAN/UTRAN which are operated together. Especially it not only reduces the signalling between UE and network, but also reduces the signalling between E-UTRAN & UTRAN/GERAN.

The dependency between 2G/3G and EPC is minimized at the cost of ISE-specific node and interface functionality. The idea behind ISR feature is that UE can be registered in UTRAN/GERAN RA at the same time it is registered in  an E-UTRAN TA or list of TAs. The UE keeps the two registrations in parallel and run periodic timers for both registrations independently. Similarly UE keeps the two registrations the two registrations in parallel and it also ensures that UE can be paged in both the RA and the TAs it is registered in.

ISR support is mandatory for E-UTRAN UEs that support GERAN and/or UTRAN and optional for the network. ISR requires special functionality in both the UE and the network (i.e. in the SGSN, MME, Serving GW and HSS) to activate ISR for a UE. The network can decide for ISR activation individually for each UE. Gn/Gp SGSNs do not support ISR functionality.

When ISR is activated this means the UE is registered with both MME and SGSN. Both the SGSN and the MME have a control connection with the Serving GW. MME and SGSN are both registered at HSS. The UE stores MM parameters from SGSN (e.g. P-TMSI and RA) and from MME (e.g. GUTI and TA(s)) and the UE stores session management (bearer) contexts that are common for E-UTRAN and GERAN/UTRAN accesses. In idle state the UE can reselect between E-UTRAN and GERAN/UTRAN (within the registered RA and TAs) without any need to perform TAU or RAU procedures with the network. SGSN and MME store each other's address when ISR is activated.

Implicit detach by one CN node (either SGSN or MME) deactivates ISR in the network. ISR is deactivated in the UE when the UE cannot perform periodic updates in time.