Wednesday, November 11, 2009

Idle Mode Signaling Reduction - ISR

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 reselecting 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 singalling between E-UTRAN & UTRAN/GERAN.

The dependancy 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. Simillarly 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.


Rahul Kumar Bhadani said...

Can you please tell, what 3GPP specifications you referred to , to write this article?

马俊超 said...

I think it's TS23.401 and TS24.301

马俊超 said...

I have some problems about ISR function... From 3GPP specifications, the ISR function activate and deavtivate are controled by 3G/LTE periodic update timers, i want to know whether the both timers be resetted when the ISR is activated. Also, when an incoming call arrives, whether the ISR state will reset. IF you have the answers,please email me, thank you~ ps: my email is