Idle Mode Signaling Reduction

For the call flows refer Annex J in TS 23.401. Also refer this spec for an introduction to some of the terminologies used here.

What is ISR?

ISR means Idle Mode Signaling Reduction. This is a feature that allows an EPS subscriber to roam between E-UTRAN and UTRAN/GERAN coverage while in IDLE state without updating Routing Area / Tracking Area to MME / SGSN.

ISR is mandatory to be supported in an EPS capable UE but can be optionally configured in the EPC core network (S4 SGSN and MME)

Why is it needed?

The deployment of E-UTRAN network will co-exist with existing UTRAN and GERAN networks. So it is possible that within the same are a subscriber moves between E-UTRAN and UTRAN/GERAN coverage. This should not generate unnecessary signalling with the core network when the UE is in IDLE state. So what is IDLE state? It means that the UE is not having a signaling connection towards the core network (SGSN / MME). When in IDLE state,

packet transfer will not happen.

When is ISR activated?

ISR is activated ONLY on the following cases:

CASE 1

1. Subscriber is already registered in a MME (EMM-REGISTERED state at MME)
2. Subscriber is in ECM-IDLE state in the UE.
3. Subscriber does a RAU into UTRAN / GERAN. RAU request carries P-TMSI mapped from GUTI. SGSN checks if ISR is activated in the config. If yes, it returns the ISR active flag in RAU accept to UE
4. UE sees that its current TIN is GUTI and hence sets its TIN to "RAT Related TMSI"
5. SGSN sends Update Location to HSS / HLR. HLR / HSS does not send Cancel Location to MME. A release 8 HLR / HSS keeps an EPS subscriber registered at both MME and SGSN at the same time.

CASE 2

1. Subscriber is already attached in a SGSN
2. Subscriber is in PMM-IDLE state in the UE.
3. Subscriber does a TAU into E-UTRAN. TAU request carries GUTI mapped from P-TMSI. MME checks if ISR is activated in the config. If yes, it returns the ISR active flag in TAU accept to UE
4. UE sees that its current TIN is P-TMSI and hence sets its TIN to "RAT Related TMSI"
5. MME sends Update Location to HSS / HLR. HLR / HSS does not send Cancel Location to MME.

How does the UE know that ISR is activated and hence it need not do RAU / TAU while roaming within the TAI list / RAI in IDLE mode?

If TIN is set to "RAT Related TMSI" then UE knows that ISR is activated.

When does ISR get deactivated in UE?

ISR is deactivated in following scenarios

1. TIN = "RAT Related TMSI" and RAU / TAU into a new RA / TA outside of the stored TAI list / RAI. Upon successful completion of the new RAU, TIN will be set to P-TMSI. Upon successful completion of new TAU, TIN will be set to GUTI.
2. TIN = "RAT Related TMSI", periodic RAU timer expires (probably lost radio coverage in UTRAN), starts deactivate ISR timer and the deactivate ISR timer also expires. TIN will be set to GUTI
3. TIN = "RAT Related TMSI", periodic TAU timer expires (probably lost radio coverage in E-UTRAN), starts deactivate ISR timer and the deactivate ISR timer also expires. TIN will be set to P-TMSI

Will ISR be activated if current TIN is same as the temporary identifier of the RAT to which UE is doing an update procedure?

No. For example if TIN = P-TMSI, and UE does a RAU into a new RA, the SGSN returns a new P-TMSI. The context transfer is between two RAs in the same radio technology. So even if the SGSN includes the "ISR Activated" flag in RAU accept, UE should set its TIN to P-TMSI only.

How is a downlink packet delivered to the UE when it is registered with both MME and SGSN and the UE is in IDLE state?

S-GW sees that it has an active GTP-C v2 tunnel with an S4-SGSN and MME and hence it sends downlink data indication to both of them. Both SGSN and MME initiate paging. The UE responds to one of them with a service request.

Once the radio bearer establishment is completed and service request procedure is completed, the S-GW sends a stop paging request to the other RAN from which service request was not received. For example if the UE latched onto E-UTRAN,

stop paging will be sent to SGSN.

Is ISR done when roaming from MME to a Gn/Gp SGSN?

No. ISR is applicable only between MME and S4 SGSN.

Updates:

Found the book

"SAE and the Evolved Packet Core: Driving the Mobile Broadband Revolution"

covering this topic in good detail. Preview of it is available in Google books. The ISR chapter, luckily is there for preview