ISR stands for Idle state Signalling Reduction and is used -
1. To reduce the frequency of signalling between UE and Network and also Context transfer between networks
2. Prevent Ping-Pong effect
Requirements for ISR
SGW should support ISR
ISR Mandatory for 4G UEs that support GERAN/UTRAN and whereas for network it's optional
Release 8 SGSN and above
Procedure
Lets assume UE is in 4G coverage area and UE attaches to E-UTRAN and sets TIN to GUTI
Now if UE has moved to an area where it receives a stronger 3G signal and reselects GERAN/UTRAN, it will initiate RAU. This is when ISR is activated. As TIN is set to GUTI,UE maps P-TMSI from GUTI and sends the same in RAU Request
After the SGSN receives the context from MME, both CN nodes keep the context depending on if SGW would have indicated ISR supported to MME.
RAU accept indicates ISR activation as a result UE stores both GUTI, received during the attach and P-TMSI,received during 3G RAU as registered and sets TIN as 'RAT related TMSI'
MME and SGSN are both registered to HSS and both have connection to SGW
Once this is done,there is no need for UE to update the network [E-UTRAN or UTRAN ] as long as it is within the TA/RA s which it has registered to.
Any context transfer that involves RAU/TAU can be the point where this ISR activation may take place.
If UE is camped on to UTRAN,TAU initiated ISR [First attach to 3G and then moving to 4G] works in similar way as RAU initiated ISR.
Downlink Data
Downlink data is sent to SGW.If ISR is activated,the SGW has connection to both MME and SGSN and sends notification to both nodes.MME and SGSN page in their respective TA/RA
Depending on from where UE responds,the connection is established and data transfer begins
ISR deactivation
Depending on indication in RAU or TAU Accept,the ISR will be deactivated. If there is no explicit indication of ISR activated,then it means it is deactivated
UE sets TIN to GUTI or P-TMSI in this case depending on whether it is connected to U-TRAN or E-UTRAN
If UE performs a RAU with a Gn/Gp SGSN of any standards release as these SGSNs never indicates "ISR Activated" to the UE.
Credit: http://all-u-wanted-2-know-about-networking.blogspot.in/2011/10/isr-idle-state-signalling-reduction.html
