ISR or Idle mode signalling reduction is a feature that reduces the mobility signaling and improves the battery life of handset. As most of the deployment of LTE are in the initial stage and coverage is limited so inter-RAT changes will be frequent and ISR plays a very important role.
Suppose UE is in a border area which supports LTE on one side and UMTS on other in this case UE will see a keep continuing to do the cell reselection i.e. UE can get into a loop where it sends RAU to UMTS/de-registering from LTE and vice versa by continuously doing cell reselection. To avoid this ISR was introduced as an annexure in 23.401
Assume that UE is registered to the EUTRAN. Now UE re-selects a GERAN/UTRAN network and initiates a Routing Area Update. SGSN will send context request to MME asking for the UE context. Here MME may reply with context response along with ISR activate indication. Network shall switch on the ISR once it realizes the UE context is being pulled from a different RAT. Once the context response is received, RAU accept along with ISR active indication is sent to UE. Once UE receives the RAU accept along with ISR active, it shall store the GUTI from EUTRAN and also the newly received P-TMSI from GERAN/UTRAN. MME will also keep the UE context. SGW is informed about ISR activation by SGSN in Modify bearer request (ISR flag set). At this point UE is registered in both EUTRAN and GERAN/UTRAN, has both the network identifiers (P-TMSI and GUTI) and UE may not initiate a TAU or RAU (until the UE has identified a new TA/RA that is not in its list).
When data for UE arrives from network, SGW will send downlink data notification to both SGSN and MME. Both MME and SGSN will initiate paging. UE may respond to which ever paging message it has received and initiate a normal service request procedure later.
Thus the ping pong effect of TAU and RAU is minimized at the expense of paging the both networks.
Ref: http://wired-n-wireless.blogspot.in/2010/06/idle-mode-signaling-reduction.html
