4 General Description

23.2363GPPIntra-domain connection of Radio Access Network (RAN) nodes to multiple Core Network (CN) nodesRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

4.1 Iu Flex Technical Requirements

This provides a (non-exhaustive) set of technical requirements:

1. Iu Flex capable RAN nodes such as the RNC/BSC shall be able to select any CN node such as the SGSN/MSC-Server within a pool area

2. Iu Flex capable RAN nodes and CN nodes shall be able to co-exist with pre Release 5 RAN nodes and pre Release 5 CN nodes.

3. The network shall provide the CN node routing information to the UE and the UE shall store it.

4. The UE shall provide the routing information received from the serving CN node to the RAN node. In some cases, this serving CN node may be an Evolved Packet Core MME.

5. The solution shall enable the reduction of signalling within the core network (e.g. reduction of the HLR signalling traffic).

6. The solution shall enable an improved scaling between radio access nodes and the core network nodes.

4.2 Overview

Editor’s Note: Clarification is required in order to remove RAN nodes and CN node terminology and to capture that this is referring to the control signalling aspects.

The Intra Domain Connection of RAN Nodes to Multiple CN Nodes overcomes the strict hierarchy, which restricts the connection of a RAN node to just one CN node. This restriction results from routing mechanisms in the RAN nodes which differentiate only between information to be sent to the PS or to the CS domain CN nodes and which do not differentiate between multiple CN nodes in each domain. The Intra Domain Connection of RAN Nodes to Multiple CN Nodes introduces a routing mechanism (and other related functionality), which enables the RAN nodes to route information to different CN nodes within the CS or PS domain, respectively.

The Intra Domain Connection of RAN Nodes to Multiple CN Nodes introduces further the concept of "pool-areas" which is enabled by the routing mechanism in the RAN nodes. A pool-area is comparable to an MSC or SGSN service area as a collection of one or more RAN node service areas. In difference to an MSC or SGSN service area a pool-area is served by multiple CN nodes (MSCs or SGSNs) in parallel which share the traffic of this area between each other. Furthermore, pool-areas may overlap which is not possible for MSC or SGSN service areas. From a RAN perspective a pool-area comprises all LA(s)/RA(s) of one or more RNC/BSC that are served by a certain group of CN nodes in parallel. One or more of the CN nodes in this group may in addition serve LAs/RAs outside this pool-area or may also serve other pool-areas. This group of CN nodes is also referred to as MSC pool or SGSN pool respectively.

The Intra Domain Connection of RAN Nodes to Multiple CN Nodes enables a few different application scenarios with certain characteristics. The service provision by multiple CN nodes within a pool-area enlarges the served area compared to the service area of one CN node. This results in reduced inter CN node updates, handovers and relocations and it reduces the HLR update traffic. The configuration of overlapping pool-areas allows to separate the overall traffic into different MS moving pattern, e.g. pool-areas where each covers a separate residential area and all the same city centre. Other advantages of multiple CN nodes in a pool-area are the possibility of capacity upgrades by additional CN nodes in the pool-area or the increased service availability as other CN nodes may provide services in case one CN node in the pool-area fails.

An MS is served by one dedicated CN node of a pool-area as long as it is in radio coverage of the pool-area. Figure 1 shows most of the possible pool-area configurations. It contains CS pool-area 1 (RAN area 1, 2, 5, 6 served by MSCs 1, 2, 3), CS pool-areas 2 (RAN area 2, 3, 6, 7 served by MSCs 4, 5, 6), PS pool-area 1 (RAN area 1, 5 served by SGSNs 1, 2) and PS pool-area 2 (RAN area 2, 3, 6, 7 served by SGSNs 3, 4, 5). In addition the RAN areas 4 and 8 are served by MSC 7 and SGSN 6 without any usage of the Intra Domain Connection of RAN Nodes to Multiple CN Nodes. The possibility to configure overlapping pool-areas is shown by the CS pool-areas 1 and 2. The PS pool-areas 1 and 2 are configured non-overlapping. The pool-areas of the CS and the PS domain may be configured identical as CS pool-area 2 and PS pool-area 2 or they may be configured differently as shown by CS pool-area 1 and PS pool-area 1. The number or capacity of CN nodes is configured independently for each pool-area. The usage of the Intra Domain Connection of RAN Nodes to Multiple CN Nodes may be configured in parts of the network only. It co-exists with other areas not using this feature as shown in the figure with RAN areas 4 and 8 which are served by MSC 7 and SGSN 6.

Figure 1: Pool-area configuration example

4.3 Pool-Area and Network Resource Identification

A pool-area is an area within which an MS may roam without a need to change the serving CN node. A pool-area is served by one or more CN nodes in parallel. The complete service area of a RAN node (RNC or BSC) belongs to the same one or more pool-area(s). A RAN node service area may belong to multiple pool-areas, which is the case when multiple overlapping pool-areas include this RAN node service area. The pool-areas of the CS and of the PS domain are configured independently with the granularity of RAN node service areas. Therefore, all uniqueness statements below apply to each of the domains (CS/PS) separately. If LAs or RAs span over multiple RAN node service areas then all these RAN node service areas have to belong to the same pool-area.

The Network Resource Identifier (NRI) identifies uniquely an individual CN node out of all CN nodes, which serve in parallel a pool-area. The length of the NRI shall be the same in all nodes of a domain in one pool-area. In areas where pool-areas overlap the NRI identifies uniquely a CN node out of all CN nodes, which serve all these overlapping pool-areas, i.e. an NRI identifies uniquely a CN node within a RAN node. In case of overlapping pool-areas the NRI length shall be configured to be the same in all the nodes of a specific domain serving these pool-areas. Note again, that the NRIs of the CS and the PS domain are independent of each other as the PS and the CS domain CN nodes are addressed independently. More than one NRI may be assigned to a CN node.

The NRI is part of the temporary identity TMSI (CS domain) or P-TMSI (PS domain), which is assigned by the serving CN node to the MS. Each CN node which supports the "Intra Domain Connection of RAN Nodes to Multiple CN Nodes" is configured with its specific one or more NRI(s). The (P-)TMSI allocation mechanism in the CN node generates (P-)TMSIs which contain a configured NRI in the relevant bit positions. The NRI has a flexible length between 10 and 0 bits (0 bits means the NRI is not used and the feature is not applied).

In Iu mode the MS provides an Intra Domain NAS Node Selector (IDNNS) TS 25.331 [5] in the AS part of the RRC-Initial-direct-transfer message to the RAN node (RNC or BSC). The IDNNS contains a routing parameter with a fixed length of 10 bits. This routing parameter transports the NRI value. In addition the IDNNS contains an indication from which identity (TMSI, IMSI, IMEI, …) the routing parameter is derived. The RAN node masks the significant bits out of the routing parameter part of the IDNNS to determine the NRI which is relevant to identify the CN node. The most significant bit of the NRI shall correspond with the most significant bit of the routing parameter in the IDNNS. When the IDNNS is derived from the IMSI, the IDNNS has a value (V) from the range 0 to 999 as defined in TS 25.331 [5]. The RAN node shall be configured to use the value (V) to select a CN node. Each value (V) corresponds a single CN node. Typically many values of (V) may point to the same CN node. In A/Gb mode.

In A/Gb-mode for the A interface the RAN node derives the NRI from any initial NAS signalling message. The RAN node masks the significant bits out of the TMSI to determine the NRI, which identifies the CN node. In A/Gb-mode for the Gb interface the RAN node derives the NRI from the TLLI. The RAN node masks the significant bits out of the TLLI to determine the NRI, which identifies the CN node.

For all three cases, Iu, A interface and Gb mode, it is configured in the RAN node which bits out of the information elements provided by the MS are significant for the NRI The NRI is coded in bits 23 to 14 of TMSI or P-TMSI. Regardless of the NRI length the most significant bit of the NRI is always in bit 23 of TMSI or P-TMSI(examples of NRI position are given in annex A.2), see also TS 23.003 [18].

The whole network may be configured as one pool-area, a network may configure multiple pool-areas and the configuration of pool-areas may be combined with MSC or SGSN service areas which are not belonging to pool-areas. The change of a pool-area is not visible to the MS. In general there is no need to detect a pool-area change. It may be advantageous for load balancing purposes to detect pool-area changes in the network to distribute MSs entering a pool-area to CN nodes with an appropriate load status. MSs changing a pool-area may be detected by configuration of different NRI values for adjacent pool-areas. The pool-area change information potentially provided in the IDNNS by an MS in Iu mode is ignored by the network.

4.4 NAS Node Selection Function

This function is used in RAN nodes and potentially in CN nodes. In the RAN node the function selects the specific CN node (i.e. MSC or SGSN) to which initial NAS signalling messages or LLC frames are routed. The NRI identifies the specific CN node. If the NAS Node Selection Function has a CN node address configured for the NRI derived from the initial NAS signalling message or from the LLC frame then this message or frame is routed to this address. If no CN node address is configured for the derived NRI or if no NRI can be derived (e.g. the MS indicated an identity which contains no NRI) then the NAS Node Selection Function selects an available CN node (e.g. according to load balancing) and routes the message or LLC frame to the selected CN node.

The pool-area has no influence on the decisions of the NAS Node Selection Function as pool-areas may overlap. The NAS Node Selection Function in the RAN node derives the NRI from the IDNNS when the MS is supported in Iu mode. When the MS is supported in Gb mode the NRI is derived from the TLLI and for A interface mode the NRI is derived from the TMSI.

NOTE: A routing-area update after SRNS relocation is not an initial NAS signalling message, thus it is routed along the existing Iu-connection to the SGSN.

In A/Gb mode in case a MSC/VLR sends a paging-request/paging with IMSI (i.e. the paging message does not contain a TMSI), the NAS node selection function in the BSC shall upon reception temporarily store the Global-CN- ID of the node that issued the paging-request/paging message. If the NAS node selection function in A/Gb mode receives a paging-response with an IMSI then it should check the temporarily stored Global-CN-ID on entries matching this IMSI and forward the paging-response to the node identified by this Global-CN-ID.

In Iu mode in case a MSC/VLR sends a paging-request/paging with IMSI (i.e. the paging message does not contain a TMSI), the NAS node selection function in the BSC/RNC may upon reception temporarily store the Global-CN-ID of the node that issued the paging-request/paging message. If the NAS node selection function in Iu mode receives an Initial Direct Transfer message with an IDNNS derived from IMSI as a result of IMSI paging:

– and if BSC/RNC has temporarily stored the Global-CN-ID then it should check the temporarily stored Global-CN-ID on entries matching this IDNNS and forward the paging-response to the node identified by this Global-CN-ID or

– the BSC/RNC shall use the IDNNS derived from IMSI to select a CN node. In this case the IDNNS has a value (V) from the range 0 to 999 as defined in TS 25.331 [5]. The RAN node shall be configured to use the value (V) to select a CN node. Each value (V) corresponds a single CN node. Typically many values of (V) may point to the same CN node.

In UMTS, an MS answering a paging with IMSI includes in its response an IDNNS derived from its TMSI, if the MS has a valid TMSI. Temporarily storing the IMSI in the RNC increases the success rate to reach the MS that have both lost their TMSI and are paged with IMSI. In GSM, an MS paged with IMSI always answers with IMSI.

If the MSC/VLR initiates the paging procedure via Gs-interface the SGSN has to add the MSC/VLR-identity to the paging-request/paging message.

An MS will return an Attach Request containing the IMSI parameter as a response to a PS IMSI paging. Also, a PS IMSI paging is not time supervised from the SGSN sending the message. Therefore the RAN node receiving such a paging request does not have to buffer the associated SGSN identity. This again means that the NAS Node Selection Function in the RAN node selects an available SGSN (e.g. according to load balancing) when it receives an Attach Request containing the IMSI parameter.

4.5 Load Balancing

Preferably, the NAS Node Selection Function in the RAN node balances the load between the available CN nodes. This is performed by an appropriate selection of the CN node for an MS which was not yet assigned to a CN node, i.e. when there is no CN node configured for the NRI indicated by the MS, when a ‘random TLLI’ is received (Gb-mode BSC), when no NRI can be derived or in exceptional cases, e.g. when the CN node corresponding to an NRI cannot be reached.

The load-balancing algorithm is implementation specific.

When the NAS Node Selection Function in the RAN (or, if using Network Mode of Operation I, the SGSN) receives an indication that the MS is configured for low access priority (e.g. because it is used for Machine Type Communication), the NAS Node Selection Function may take this indication into account when selecting the CN node (e.g. to select an MSC that has a particularly large VLR capacity or an SGSN that is optimized to handle UEs configured for low access priority).

In case of handover/relocation into a pool-area a load balancing between all the target CN nodes serving this pool-area is gained by configuration. Source CN nodes which support Intra Domain Connection of RAN Nodes to Multiple CN Nodes may be configured with all possible target CN nodes for each handover/relocation target. Source CN nodes which do not support the Intra Domain Connection of RAN Nodes to Multiple CN Nodes can configure only one target CN node per handover/relocation target. In this case each of source CN nodes which handover/relocate to the same pool-area may be configured with another target CN node out of all target CN nodes serving the same handover/relocation target. The mechanism for distribution of the traffic between the handover/relocation target CN nodes is implementation specific. This load balancing is complemented by the NAS Node selection Function in the RAN, which distributes MSs between the CN nodes when these MSs enter the pool-area in idle mode.

As more than one SGSN may send downlink data at the same time for a cell or a BVCI the total possible downlink traffic has to shared between the SGSNs as described in clause 5.3.2.

When dedicated core networks (DCNs) are used, load balancing by RNC/BSC is only performed between SGSNs that belong to the same DCN within the same SGSN pool area. Load balancing may also be performed per DCN in the case of an SGSN serving multiple DCNs when one DCN is supported by multiple SGSNs. The DCN Load Balancing functionality permits UEs that are entering into a pool area and being re-directed to an appropriate DCN to be distributed in a manner that achieves load balancing between the core network nodes of the same DCN within the same SGSN pool area. An example of NRI configuration that can achieve this is when DCNs are used is provided in clause A.3.

In case of CS domain subsequent handover/relocation, if the controlling MSC supports Intra Domain Connection of RAN Nodes to Multiple CN Nodes then it should check the target indicated by the serving MSC. If it is in its own pool area then the handover/relocation is handled as one back to the controlling MSC (the target MSC indicated by the serving MSC is not taken into account). If the target location is not in its own pool area then the target MSC indicated by the serving MSC is taken into account as normal (handover/relocation to a third MSC).

If the controlling (anchor) MSC is not in the same pool as target third MSC and if the controlling MSC does not support Intra Domain Connection of RAN Nodes to Multiple CN nodes, because the serving MSC can select any third MSC in the target pool area, the operator should configure the addresses of all possible third MSCs within the controlling MSC.

4.5a Load Re-Distribution

4.5a.1 General

There are situations where a network operator will wish to remove load from one CN node in an orderly manner (e.g. to perform scheduled maintenance, or, to perform load re-distribution to avoid overload) with minimal impact to end users and/or additional load on other entities. The re-distribution procedure does not require any new functionality in the terminal, that is, all terminals can be moved.

Re-distribution of UEs is initiated via an O&M command in the CN node, which needs to be off-loaded. In a first phase (a couple of Periodic LU/RAU periods long), UEs doing LU/RAU or Attach are moved to other CN nodes in the pool. When the CN node receives the Location Update, Routing Area Update or Attach request, it returns a new TMSI/P-TMSI with a null-NRI, and a non-broadcast LAI/RAI in the accept message.

In CS domain the non-broadcast LAI will cause the terminal to immediately send a new Location Update, which the RAN node then will route to a new MSC due to the null-NRI. In the PS domain, a new Routing Area Update is triggered by setting the periodic routing area update timer to a sufficiently low value (recommended value is 4 seconds) in the accept message. The UE will shortly after send a new Routing Area Update that the RAN node then will route to a new SGSN due to the presence of a null-NRI.

In a second phase (PS domain specific), the SGSN requests all UEs trying to set up PDP Contexts to detach & reattach. When they reattach, the SGSN moves them as in the first phase described above.

A third phase includes scanning through remaining UEs and initiating a move of them to other CN nodes. In the PS domain UEs are requested to detach and reattach, which will cause them to be moved. In case of CS domain a new TMSI is allocated to these UEs using the TMSI re-allocation procedure (with null-NRI and non-broadcast LAI) so that a Location Update is triggered when the ongoing CM transaction ends, which will cause them to be moved.

UEs being moved from one CN node are stopped from registering to the same CN node again by an O&M command in BSCs and RNCs connected to the pool. UEs moving into a pool area may also be stopped from registering into a CN node being off-loaded in the same manner.

In network configurations using MOCN network sharing, re-distribution is always done between CN nodes within the same CN Operator. This is ensured by each CN Operator using his own unique null-NRI. The RAN node is preconfigured with the null-NRIs for the different CN Operators, and it uses the null-NRI to select a CN node within the same CN Operator.

When dedicated core networks (DCNs) are used, re-distribution of UEs is always done to core network nodes of the same DCN as the node being off-loaded. The details are as follows:

– If null-NRI is used to identify the group of SGSNs that belong to a DCN within a PLMN (see TS 23.401 [22], clause 4.3.25), then the procedure stated in the clause above may be used. The null-NRI corresponds to the DCN of the node being off-loaded.

– If either "SGSN group ID" or null-NRI is used to identify the group of SGSNs that belong to a DCN within a PLMN, then the following procedure may instead be used:

– To off-load UEs in PMM-IDLE/STANDBY state that perform RAU Updates or Attaches, the SGSN uses the NAS Message Redirection Procedure (see TS 23.401 [22], clause 5.19.1) to redirect the UE to another SGSN within the same DCN. To off-load UEs in PMM-IDLE/STANDBY state without waiting for the UE to perform a RAU procedure, the SGSN first pages the UE to bring it to PMM-CONNECTED/READY state and then proceeds as below.

– To off-load UEs in PMM-CONNECTED/READY state, the SGSN first performs P-TMSI reallocation procedure, includes the UEs unchanged P-TMSI and a non-broadcast RAI, and sets the periodic routing area update timer to a sufficiently low value (recommended value is 4 seconds) and forces the UE to change state to PMM-IDLE/STANDBY state. The UE shortly performs a RAU procedure, during which the SGSN uses the NAS Message Redirection Procedure (see TS 23.401 [22] clause 5.19.1) to redirect the UE to another SGSN within the same dedicated CN.

A CN node should ensure that move operations does not overload the network. BSCs and RNCs shall be able to handle situations where several CN nodes are off-loaded simultaneously.

4.5a.2 Network Mode of Operation = 1

If an operator is using Network Mode of Operation = 1 (i.e. using combined MM and GMM procedures and the Gs interface), then redistribution of MSC load needs to be handled in a special way.

Redistribution of UEs is initiated by O&M command in the SGSN providing the Gs interface to the MSC to be off-loaded. The corresponding NRI distribution or IMSI Hash table is reconfigured to reflect the redistribution. If the SGSNs are also configured in a pool, this is repeated for any SGSN connected to that MSC. The NRI distribution or IMSI Hash table shall have a consistent configuration in all SGSNs in the pool (to ensure that a redistribution of SGSN load doesn’t affect the MSC registration of UEs).

The redistribution is done in two phases. During the first phase, the UEs that are performing combined RA/LA updates are moved to a new MSC. When the SGSN receives a Routing Area Update Request (combined RA/LA updating), it checks if the particular UE shall be moved (i.e. it has a Gs association with the MSC being off-loaded). If the UE shall be moved, the SGSN invokes the MSC selection function (NRI distribution or IMSI Hash) to decide where the UE should be distributed. SGSN sends the (BSSAP+) Location-Update-Request (IMSI attach) to the new selected MSC where the UE is registered. Stationary UEs (i.e. UEs not performing RA/LA updates) are not moved during this first phase.

During the second phase, the SGSN scans its Gs associations to find out which UEs shall be moved. For each UE with an association to the MSC being off-loaded, the SGSN sends a Detach Request (indicating IMSI detach). The UE is forced to re-attach to non-GPRS service (note that there is no impact on PDP contexts in this case). The UE sends a RAU request (combined RA/LA updating with IMSI Attach). SGSN checks if the UE shall be moved. If the UE shall be moved, the SGSN invokes the MSC selection function (NRI distribution or IMSI Hash) to select another MSC. SGSN sends the (BSSAP+) Location-Update-Request (IMSI attach) to the new MSC where the UE is registered.

During the redistribution, incoming IMSI Detach messages are (as during normal operation) routed to respective existing associated MSC. That is, the reconfigured NRI distribution or IMSI Hash doesn’t affect the routing of IMSI Detach messages.

4.6 Mobility Management

An MS performs LA or RA Updates and Attachments, which may result in a change of the serving CN node. In these procedures the new CN node requests from the old CN node MS specific parameters. If multiple CN nodes are configured in the new CN node for the old RA or LA indicated by the MS then the new CN node derives the NRI from the old (P-)TMSI indicated by the MS. The new CN node uses the old RA or LA together with the NRI to derive the signalling address of the old CN node from its configuration data. If the network contains nodes that cannot derive the old CN node from LAI/RAI and NRI a default CN node for each RA or LA (as described below) shall be used to resolve the ambiguity of the multiple CN nodes serving the same area.

4.7 Default CN node and Backwards Compatibility

CN nodes that can only derive one CN node from the LAI or RAI (e.g. because they do not support the Intra Domain Connection of RAN Nodes to Multiple CN Nodes, or no detailed knowledge of the NRIs is configured) are not aware, that multiple CN nodes may serve a LA or RA. These nodes can therefore contact only one CN node per LA or RA, respectively. This node will further on be referred to as default node.

A default node resolves the ambiguity of the multiple CN nodes per LA or RA by deriving the NRI from the TMSI and P-TMSI. The default node relays the signalling between the new CN node and the old CN node.

Note that the default node is configured per LA or RA. So different CN nodes in a network might have configured different default nodes for a LA or RA. With this approach more than one of the CN nodes that serve a pool-area can be used as default-node, so load concentration on one node and a single point of failure can be avoided.

Note further, that it may be required to keep information on ongoing MAP/GTP dialogues in the default nodes.

The handover/relocation from CN nodes which do support the Intra Domain Connection of RAN Nodes to Multiple CN Nodes to CN nodes not supporting this features does not need a NAS Node Selection Function in the originating CN node as there is only one target CN node. The originating CN node discovers from its configuration data, that there is only one target CN node for the requested handover/relocation target ID. See clause 4.5 for handling of subsequent handover/relocation in the controlling CN node.

4.8 Support of combined mobility management procedures

4.8.1 Attach

In case of ‘combined GPRS/IMSI attach’ or ‘GPRS attach when already IMSI attached’, the SGSN sends the Location Update Request message to the MSC/VLR. The SGSN selects an MSC/VLR from the available MSC/VLRs which serve the current LA of the MS. The selection is based on the TMSI based NRI as provided by the UE or a hash value derived from the IMSI. It is configured in the SGSN which NRI values and range of the hash values relates to which MSC/VLR. This selection mechanism avoids a random change of the MSC/VLR for MSs using combined procedures when an SGSN fails. The new SGSN will select the same MSC/VLR.

In some networks, the SGSN may be configured to select the MSC/VLR for "MSs configured for low access priority" with a different load balance to that used for MSC/VLR selection for other UEs. In this case the SGSN maintains a separate NRI and hash value function for "MSs configured for low access priority".

4.8.2 Routing area update

The CN node changes in the following considerations result from pool-area changes (when pool-areas are configured) or from CN node service area changes (when no pool-areas are configured). For each domain (PS or CS) it is configured independently whether pool-areas are used or not.

When neither the MSC nor the SGSN are changed, the association for an MS between both CN nodes will also not change.

When the MSC changes but the SGSN does not change, the SGSN selects a new MSC because the new LA is not served by the old MSC/VLR. The selection mechanism is as described for the attach above.

When the SGSN changes but the MSC does not change, the new SGSN selects the old MSC to establish a Gs association because the new SGSN uses the same selection mechanism as described above for the attach with the same parameters as configured in the old SGSN.

When both the MSC and the SGSN change, the new SGSN selects a new MSC to establish a Gs association. The selection mechanism is as described for the attach above.

4.9 VGCS/VBS Compatibility Issues

Voice Group Call Services, TS 43.068 [19], as well as Voice Broadcast Service, TS 43.069 [20], are specified for A-interface only. Architectural enhancements and procedures needed to support these services in an area where Intra Domain Connection of RAN Nodes to Multiple CN Nodes is applied on the A-interface are specified in TS 43.068 [19] and TS 43.069 [20].

4.10 Interactions with E-UTRAN

The MS has separate core network temporary identities for GERAN/UTRAN (the RAI plus P‑TMSI in the packet domain) and E‑UTRAN (the GUTI).

Following movement between GERAN/UTRAN and E‑UTRAN the MS will have both a P‑TMSI and a GUTI. Independent of whether the Idle mode Signalling Reduction (ISR) feature is active, one or both of the P‑TMSI and GUTI may be valid. If the MS is E UTRAN capable, then TS 23.401 [22], TS 23.060 [2] and TS 23.003 [18] define rules as to how the MS shall select and encode the identity to place in the P‑TMSI/TLLI parameters used in the Routing Area Update procedure.

NOTE: ISR is specified and described in TS 23.401 [22].

In some network deployments the MME and SGSN may be combined in the same physical platform (while in other deployments they may be separated).

TS 23.003 [18] specifies a mapping from the E‑UTRAN core network temporary identity (the GUTI) to the TLLI that facilitates the BSC to route an RA update caused by movement from E‑UTRAN to GERAN to an SGSN that is on the same physical platform as the MME.

TS 23.003 [18] and TS 25.331 [5] specify a mapping from the E‑UTRAN core network temporary identity (the GUTI) to the IDNNS that facilitates the RNC to route an RA update caused by movement from E‑UTRAN to UTRAN to an SGSN that is on the same physical platform as the MME.

TS 23.003 [18] also specifies a mapping from the RAI and P‑TMSI to the GUTI that facilitates the E‑UTRAN to route to an MME that is on the same physical platform as the SGSN. For this mapping function to work correctly when using combined MME/SGSNs, the effective maximum length of the NRI is reduced from 10 bits to 8 bits.