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Idle Mode Signalling Reduction on the S4-SGSN
How ISR Works ▀
SGSN Administration Guide, StarOS Release 18 ▄
How ISR Works
ISR requires special functionality in both the UE and the network (i.e. in the SGSN, MME, SGW and HSS) to activate
ISR for a UE. The network can decide for ISR activation individually for each UE. ISR support is mandatory for E-
UTRAN UEs that support GERAN and/or UTRAN and optional for the network. Note that the Gn/Gp SGSN does not
support ISR functionality.
ISR for a UE. The network can decide for ISR activation individually for each UE. ISR support is mandatory for E-
UTRAN UEs that support GERAN and/or UTRAN and optional for the network. Note that the Gn/Gp SGSN does not
support ISR functionality.
ISR is not activated on Attach. ISR can only be activated when a UE first registers in a RA on an SGSN and then
registers in a TA on an MME or vice-versa. It is an inherent functionality of the mobility management (MM) procedures
to enable ISR activation only when the UE is able to register via E-UTRAN and via GERAN/UTRAN. For example,
when there is no E-UTRAN coverage there will be also no ISR activation. Once ISR is activated it remains active until
one of the criteria for deactivation in the UE occurs, or until the SGSN or the MME indicate ISR is no longer activated
during an update procedure, i.e. the ISR status of the UE has to be refreshed with every update.
registers in a TA on an MME or vice-versa. It is an inherent functionality of the mobility management (MM) procedures
to enable ISR activation only when the UE is able to register via E-UTRAN and via GERAN/UTRAN. For example,
when there is no E-UTRAN coverage there will be also no ISR activation. Once ISR is activated it remains active until
one of the criteria for deactivation in the UE occurs, or until the SGSN or the MME indicate ISR is no longer activated
during an update procedure, i.e. the ISR status of the UE has to be refreshed with every update.
When ISR is activated this means the UE is registered with both the MME and the SGSN. Both the SGSN and the MME
have a control connection with the SGW. The MME and the SGSN are both registered at the HSS. The UE stores
mobility management parameters from the SGSN (for example, P-TMSI and RA) and from the MME (for example,
GUTI and TAs). The UE stores session management (bearer) contexts that are common for E-UTRAN and
GERAN/UTRAN accesses. In an 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. the SGSN and MME
store each other's address when ISR is activated.
have a control connection with the SGW. The MME and the SGSN are both registered at the HSS. The UE stores
mobility management parameters from the SGSN (for example, P-TMSI and RA) and from the MME (for example,
GUTI and TAs). The UE stores session management (bearer) contexts that are common for E-UTRAN and
GERAN/UTRAN accesses. In an 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. the SGSN and MME
store each other's address when ISR is activated.
The S4 SGSN supports the following scenarios for 2G ISR:
ISR activation by SGSN on new SGSN RAU from MME
ISR activation on SGSN in old SGSN RAU to MME
Ready to standby state transition triggered Release Access Bearer Request to SGW
Downlink data notification from SGW:
Downlink data notification UE responds to SGSN
Downlink data notification no response from UE
Stop paging indication
UE initiated detach for ISR activated subscriber under GERAN
UE initiated detach under EUTRAN/MME initiated detach or Detach notification from MME
SGSN initiated detach for ISR activated subscriber
HSS/HLR initiated detach for ISR activated subscriber
ISR deactivation due to delete bearer request with ISR deactivation cause
ISR deactivation due to last PDN connection deletion (SGSN/UE/PGW/HSS/HLR-initiated)
ISR deactivation due to SGW change
ISR-deactivation due to context transfer between same Node types(S4 SGSN to and from S4 SGSN)
Intra-RAU without SGW change for ISR-activated subscriber
Inter-GPRS service RAU without SGW change for ISR-activated subscriber
Intra-SGSN inter-system handover from 2G to 3G without SGW change for ISR activated subscriber
Intra-SGSN inter-system handover from 3G to 2G without SGW change for ISR activated subscriber