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Content for  TS 23.401  Word version:  16.7.0

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J  High Level ISR descriptionWord‑p. 415

J.1  General description of the ISR concept

Idle state Signalling Reduction (ISR) 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 the update signalling between UE and network is reduced. But also network internal signalling is reduced. To some extent the reduction of network internal signalling is also available when ISR is not used or not activated by the network.
UMTS described already RAs containing GERAN and UTRAN cells, which also reduces update signalling between UE and network. The combination of GERAN and UTRAN into the same RAs implies however common scaling, dimensioning and configuration for GERAN and UTRAN (e.g. same RA coverage, same SGSN service area, no GERAN or UTRAN only access control, same physical node for GERAN and UTRAN). As an advantage it does not require special network interface functionality for the purpose of update signalling reduction.
ISR enables signalling reduction with separate SGSN and MME and also with independent TAs and RAs. Thereby the interdependency is drastically minimized compared with the GERAN/UTRAN RAs. This comes however with ISR specific node and interface functionality. SGSN and MME may be implemented together, which reduces some interface functions but results also in some dependencies.
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 and Serving GW) to activate ISR for a UE. For this activation, the MME/SGSN detects whether S-GW supports ISR based on the configuration and activates ISR only if the S-GW supports the ISR. The network can decide for ISR activation individually for each UE. Gn/Gp SGSNs do not support ISR functionality. No specific HSS functionality is required to support ISR.
NOTE. A Release 7 HSS needs additional functionality to support the 'dual registration' of MME and SGSN. Without such an upgrade, at least PS domain MT Location Services and MT Short Messages are liable to fail.
It is inherent functionality of the 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 SGSN or MME indicate during an update procedure no more the activated ISR, 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 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.
When ISR is activated and downlink data arrive, the Serving GW initiates paging processes on both SGSN and MME. In response to paging or for uplink data transfer the UE performs normal Service Request procedures on the currently camped-on RAT without any preceding update signalling (there are however existing scenarios that may require to perform a RAU procedure prior to the Service Request even with ISR is activated when GERAN/UTRAN RAs are used together, as specified in clause 6.13.1.3 of TS 23.060).
The UE and the network run independent periodic update timers for GERAN/UTRAN and for E-UTRAN. When the MME or SGSN do not receive periodic updates MME and SGSN may decide independently for implicit detach, which removes session management (bearer) contexts from the CN node performing the implicit detach and it removes also the related control connection from the Serving GW. Implicit detach by one CN node (either SGSN or MME) deactivates ISR in the network. It is deactivated in the UE when the UE cannot perform periodic updates in time. When ISR is activated and a periodic updating timer expires the UE starts a Deactivate ISR timer. When this timer expires and the UE was not able to perform the required update procedure the UE deactivates ISR.
Part of the ISR functionality is also available when ISR is not activated because the MM contexts are stored in UE, MME and SGSN also when ISR is not active. This results in some reduced network signalling, which is not available for Gn/Gp SGSNs. These SGSNs cannot handle MM and session management contexts separately. Therefore all contexts on Gn/Gp SGSNs are deleted when the UE changes to an MME. The MME can keep their MME contexts in all scenarios.
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J.2  Usage of the TINWord‑p. 416
The UE may have valid MM parameters both from MME and from SGSN. The "Temporary Identity used in Next update" (TIN) is a parameter of the UE's MM context, which identifies the UE identity to be indicated in the next RAU Request or TAU Request message. The TIN also identifies the status of ISR activation in the UE.
The TIN can take one of the three values, "P-TMSI", "GUTI" or "RAT-related TMSI". The UE sets the TIN when receiving an Attach Accept, a TAU Accept or RAU Accept message as specified in table 4.3.5.6-1.
"ISR Activated" indicated by the RAU/TAU Accept message but the UE not setting the TIN to "RAT-related TMSI" is a special situation. Here the UE has deactivated ISR due to special situation handling (see clause J.6). By maintaining the old TIN value the UE remembers to use the RAT TMSI indicated by the TIN when updating with the CN node of the other RAT.
Only if the TIN is set to "RAT-related TMSI" ISR behaviour is enabled for the UE, i.e. the UE can change between all registered areas and RATs without any update signalling and it listens for paging on the RAT it is camped on. If the TIN is set to "RAT-related TMSI", the UE's P-TMSI and RAI as well as its GUTI and TAI(s) remain registered with the network and valid in the UE.
When ISR is not active the TIN is always set to the temporary ID belonging to the currently used RAT. This guarantees that always the most recent context data are used, which means during inter-RAT changes there is always context transfer from the CN node serving the last used RAT. The UE identities, old GUTI IE and additional GUTI IE, indicated in the next TAU Request message, and old P-TMSI IE and additional P-TMSI/RAI IE, indicated in the next RAU Request message depend on the setting of TIN and are specified in table 4.3.5.6-2.
The UE indicates also information elements "additional GUTI" or "additional P-TMSI" in the Attach Request, TAU or RAU Request. These information elements permit the MME/SGSN to find the already existing UE contexts in the new MME or SGSN, when the "old GUTI" or "old P-TMSI" indicate values that are mapped from other identities.
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J.3  ISR activation

The information flow in Figure J.3-1 shows an example of ISR activation. For explanatory purposes the Figure is simplified to show the MM parts only.
The process starts with an ordinary Attach procedure not requiring any special functionality for support of ISR. The Attach however deletes any existing old ISR state information stored in the UE. With the Attach request message, the UE sets its TIN to "GUTI". After attach with MME, the UE may perform any interactions via E-UTRAN without changing the ISR state. ISR remains deactivated. One or more bearer contexts are activated on MME, Serving GW and PDN GW, which is not shown in the Figure.
The first time the UE reselects GERAN or UTRAN it initiates a Routing Area Update. This represents an occasion to activate ISR. The TIN indicates "GUTI" so the UE indicates a P-TMSI mapped from a GUTI in the RAU Request. The SGSN gets contexts from MME. When the MME sends the context to the SGSN, the MME includes the ISR supported indication only if the involved S-GW supports the ISR. After the ISR activated, both CN nodes keep these contexts because ISR is being activated. The SGSN establishes a control relation with the Serving GW, which is active in parallel to the control connection between MME and Serving GW (not shown in Figure). The RAU Accept indicates ISR activation to the UE. The UE keeps GUTI and P-TMSI as registered, which the UE memorises by setting the TIN to "RAT-related TMSI". The MME and the SGSN are registered in parallel with the HSS.
After ISR activation, the UE may reselect between E-UTRAN and UTRAN/GERAN without any need for updating the network as long as the UE does not move out of the RA/TA(s) registered with the network.
The network is not required to activate ISR during a RAU or TAU. The network may activate ISR at any RAU or TAU that involves the context transfer between an SGSN and an MME. The RAU procedure for this is shown in Figure J.3-1. ISR activation for a UE, which is already attached to GERAN/UTRAN, with a TAU procedure from E-UTRAN works in a very similar way.
Reproduction of 3GPP TS 23.401, Figure J.3-1: ISR Activation example
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J.4  Downlink data transferWord‑p. 417
Figure J.4-1 shows a downlink data transfer to an idle state UE when ISR is activated. The Serving GW receives downlink data. Because of activated ISR, the Serving GW has control connections with both MME and SGSN and sends therefore downlink data notifications to both nodes. MME and SGSN start their paging procedures, which results in paging of the UE in the registered RA and TA(s) in parallel.
In the example illustrated in Figure J.4-1 it is assumed that the UE camps on E-UTRAN. So the UE responds to paging as usual with Service Request. This triggers the MME to setup the user plane connection between eNodeB and Serving GW. The downlink data are transferred to the UE.
When the UE camps on UTRAN/GERAN it performs the paging response as specified for these access systems without any required update or other signalling before. The downlink data are then transferred via UTRAN/GERAN to the UE.
Reproduction of 3GPP TS 23.401, Figure J.4-1: Downlink data transfer with ISR active
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J.5  ISR deactivationWord‑p. 418
Deactivation of ISR for the UE does not require any specific functionality. The status of ISR activation is refreshed in every RAU and TAU Accept message. If there is no explicit indication of ISR Activated in these messages then ISR is deactivated and the UE sets its TIN to "GUTI" or "P-TMSI", as specified in Table 4.3.5.6-1. This causes always ISR deactivation when a UE performs a RAU with a Gn/Gp SGSN of any standards release as these SGSNs never indicate "ISR Activated" to the UE.

J.6  Handling of special situations

Situations may occur that cause unsynchronized state information in the UE, MME and SGSN. Such situations are:
  • Modification of any EPS bearer context or PDP context which was activated before the ISR is activated in the UE;
  • At the time when the UE moves from E-UTRAN to GERAN/UTRAN or moves from GERAN/UTRAN to E-UTRAN, if any EPS bearer context or PDP context activated after the ISR was activated in the UE exists;
  • Missing periodic TA or RA updates, e.g. because the coverage of a RAT is lost or the RAT is no more selected by the UE (this may result also in implicit detach by SGSN or MME);
  • CN node change resulting in context transfer between the same type of CN nodes (SGSN to SGSN or MME to MME);
  • Serving GW change (both with and without UE mobility);
  • Change of the UE specific DRX parameters;
  • Change of the UE Core Network Capabilities;
  • E-UTRAN selection by a UTRAN-connected UE (e.g. when in URA_PCH to release Iu on UTRAN side);
  • E-UTRAN selection from GERAN READY state;
  • GERAN selection by an E-UTRAN-connected UE via Cell Change Order that is not for CS fallback.
There are no ISR specific procedures to handle such situations to avoid additional complexity and error cases. All special situations that cause context in the UE, MME and SGSN to become asynchronous are handled by ISR deactivation. The normal RAU/TAU procedures synchronize contexts in MME and SGSN and activate ISR again when wanted by the network.
Some specific handling is defined to enable combined MME/SGSN. For this the UE signals at UTRAN RRC level always an Intra Domain NAS Node Selector (IDNNS) derived from the ID signalled as P-TMSI (also when mapped from GUTI). At E-UTRAN RRC level the UE indicates the GUMMEI derived from the GUTI that is signalled in the TAU Request message (also when derived from P-TMSI). This handling is performed by the UE independent from the network configuration. It is not visible to the UE whether MME and SGSN are combined.
Given the IP-based architecture of EPS and the IP-based applications such establishment and deactivation of the EPS bearer or PDP context can happen frequently before the UE changes the RAT e.g. a UE asking for delivery of an SMS (over IP) or starting a VoIP over IMS, an entirely new EPS bearer or PDP context may be established for that purpose. Then, after the application/service is finished, the newly established EPS bearer or PDP context gets deactivated. In such particular situation the deactivation of the ISR at the UE and hence performing a RAU or TAU update when the UE changes the RAT is not needed. Preventing the UE from deactivating the ISR in this case ensures an efficient usage of the UE's battery power and reduces the unnecessary signalling load that is seen as the key objective to be achieved by introducing the ISR feature. Thus, UE only locally deactivates ISR when bearer existed at the time of ISR is activated, or when UE changes RAT with bearers which are created after ISR is activated.
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