8.6 Relocation Preparation

25.4133GPPRelease 17TSUTRAN Iu interface Radio Access Network Application Part (RANAP) signalling

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

8.6.1 General

The purpose of the Relocation Preparation procedure is to prepare relocation of SRNS either with involving the UE or without involving the UE. The relocation procedure shall be co-ordinated over all Iu signalling connections existing for the UE in order to allow Relocation co-ordination in the target RNC. The procedure uses connection oriented signalling.

The source RNC shall not initiate the Relocation Preparation procedure for an Iu signalling connection if a Prepared Relocation exists in the RNC for that Iu signalling connection or if a Relocation Preparation procedure is ongoing for that Iu signalling connection or in the case of a MOCN configuration if the Rerouting Function is ongoing.

8.6.2 Successful Operation

Figure 5: Relocation Preparation procedure. Successful operation.

The source RNC initiates the procedure by sending a RELOCATION REQUIRED message. The source RNC shall decide whether to initiate an intra-system Relocation or an inter-system handover.

The source RNC shall indicate the appropriate cause value for the Relocation in the Cause IE. Typical cause values are "Time critical Relocation", "Resource optimisation relocation", "Relocation desirable for radio reasons" , "Directed Retry", "Reduce Load in Serving Cell", "Access Restricted Due to Shared Networks", "No Iu CS UP relocation".

The source RNC shall determine whether the relocation of SRNS shall be executed with or without involvement of the UE. The source RNC shall set accordingly the Relocation Type IE to "UE involved in relocation of SRNS" or "UE not involved in relocation of SRNS".

In case of intra-system Relocation, the source RNC:

– shall indicate in the Source ID IE the RNC-ID of the source RNC and in the Target ID IE the RNC-ID of the target RNC only including the RAC if the message is sent towards the PS domain;

– shall in case SRVCC is performed include the SRVCC HO Indication IE in the RELOCATION REQUIRED message. The value of SRVCC HO Indication IE shall be set by the source RNC. In case the source RNC decides to involve at the target side only CS domain, the SRVCC HO Indication IE shall be set to "CS only", to "PS and CS" in case CS and PS domain at the target side shall be involved;

NOTE: The Number of Iu Instances IE within the Source RNC to Target RNC Transparent Container IE shall be set according to the SRVCC HO Indication IE.

– shall include in the RELOCATION REQUIRED message the Source to Target Transparent Container IE. This container shall be encoded according to the Source RNC to Target RNC Transparent Container IE definition. The Source RNC to Target RNC Transparent Container IE shall include:

– the Relocation Type IE and the Number of Iu Instances IE containing the number of Iu signalling connections existing for the UE.

– the Integrity Protection Key IE from the last received domain on which the Security Mode Control procedure has been successfully performed, and the associated Chosen Integrity Protection Algorithm IE that has been selected for this domain.

– the Ciphering Key IE for the signalling data from the last received domain on which the Security Mode Control procedure has been successfully performed if the ciphering has been started, together with the associated Chosen Encryption Algorithm IE that has been selected for this domain. If the ciphering has not been started, the RNC may include the Ciphering Key IE and the Chosen Encryption Algorithm IE if they are available.

– for each domain where the Security Mode Control procedure has been successfully performed in the source RNC, the Chosen Encryption Algorithm IE of CS (PS respectively) user data corresponding to the ciphering alternative that has been selected for this domain. If the Security Mode Control procedure had not been successful or performed for one domain or had proposed no ciphering alternative, the Chosen Encryption Algorithm IE for the user data of this domain shall not be included. When both the CS and the PS user data Chosen Encryption Algorithm IEs are provided, they shall be the same.

– the RRC Container IE. If the Relocation Type IE is set to "UE not involved in relocation of SRNS" and the UE is using DCH(s), DSCH(s), USCH(s), HS-DSCH and/or E-DCH, the Source RNC to Target RNC Transparent Container IE shall:

– for each RAB include the RAB ID, the CN Domain Indicator IE and the mapping between each RAB subflow and transport channel identifier(s) over Iur, i.e. if the RAB is carried on a DCH(s), the DCH ID(s) shall be included, and when it is carried on DSCH(s), USCH(s), HS-DSCH and/or E-DCH, the DSCH ID(s), USCH ID(s), HS-DSCH MAC-d Flow ID and/or E-DCH MAC-d Flow ID respectively shall be included.

– only in the case the active SRBs in SRNC are not all mapped onto the same DCH, include the SRB TrCH Mapping IE containing for each SRB the SRB ID and the associated transport channel identifier over Iur, i.e. if the SRB is carried on a DCH, the DCH ID shall be included, and when it is carried on DSCH, USCH, HS-DSCH and/or E-DCH, the DSCH ID, USCH ID,HS-DSCH MAC-d Flow ID and/or E-DCH MAC-d Flow ID respectively shall be included.

– the d-RNTI IE, if the Relocation Type IE is set to "UE not involved in relocation of SRNS".

– the Target Cell ID IE, if the Relocation Type IE is set to "UE involved in relocation of SRNS".

– in the PS RAB To Be Replaced IE the RAB ID of the voice RAB which is relocated from the PS to the CS CN domain, in case SRVCC is performed.

– the d-RNTI for No IuCS UP IE, if the source RNC doesn’t have an Iu-CS user plane connection, the Relocation Type IE is set to "UE involved in relocation of SRNS" and the reason for the relocation is the source RNC cannot support CS service. The Cause IE shall be set as "No Iu CS UP relocation" in the RELOCATION REQUIRED message.

– the MBMS Linking Information IE, if available, in case the UE has been linked to at least one Multicast Service.

– the UE History Information IE and the source RNC shall add the stored information to the Last Visited Cell List IE, in case the source RNC is configured to collect UE history information.

– the Subscriber Profile ID for RAT/Frequency priority IE if available in the source RNC.

– the Management Based MDT Allowed IE only or the Management Based MDT Allowed IE and the Management Based MDT PLMN List IE, if this has been provided to the RNC and the serving PLMN of the relocation target is included in the Management Based MDT PLMN List.

– the Last E-UTRAN PLMN Identity IE if available in the source RNC.

– may in case a Trace Recording Session is active in the Source RNC due to a Signalling Based Activation (see TS 32.421 [37]), include the Trace Recording Session Information IE containing information identifying the Trace Record being generated in the Source RNC to Target RNC Transparent Container IE.

– may in case a QoE function is active in the Source RNC (see TS 25.300 [72]), include the UE Application layer measurement configuration for relocation IE.

In case of inter-system handover to GSM CS domain, the RNC:

– the source RNC shall indicate in the Source ID IE the Service Area Identifier and in the Target ID IE the cell global identity of the cell in the target system;

– shall include the MS Classmark 2 and MS Classmark 3 IEs received from the UE in the RELOCATION REQUIRED message to the CN;

– shall include the Old BSS to New BSS Information IE within the RELOCATION REQUIRED message only if the information is available. This information shall include, if available, the current traffic load in the source cell, i.e. prior to the inter-system handover attempt. This information shall also include the source cell identifier the included traffic load values correspond to. In the case the UE is using, prior to the inter-system handover attempt, radio resources of more than one cell, it is implementation specific for which cell the source RNC should report the current traffic load and the cell identifier.

– shall in case SRVCC is performed include the SRVCC HO Indication IE in the RELOCATION REQUIRED message. The value of SRVCC HO Indication IE shall be set to "CS only" by the source RNC;

In case of inter-system handover to GSM PS domain, the RNC:

– shall indicate in the Source ID IE the Service Area Identifier, in the Target ID IE the cell global identity of the cell in the target system and shall also indicate routing area code for the relevant cell in the target system;

– shall include the Source BSS to Target BSS Transparent Container IE within the RELOCATION REQUIRED message to the CN. It may indicate in this container whether it requests to receive the SI/PSI container from the external inter-system handover target in the RELOCATION COMMAND message.

In case of inter-system handover towards the GSM CS domain and GSM PS domain in parallel, the source RNC:

– shall include in the Target ID IE the same cell global identity of the cell in the target system for CS domain and PS domain and set the appropriate information about the nature of the CS/PS inter-system handover (see TS 43.055 [47]) in the Old BSS to New BSS Information IE and Source BSS to Target BSS Transparent Container IE accordingly.

– shall in case SRVCC is performed include the SRVCC HO Indication IE in the RELOCATION REQUIRED message. The value of SRVCC HO Indication IE shall be set to "PS and CS" by the source RNC;

In case of inter-system handover to E-UTRAN (as specified in TS 23.401 [48]), the RNC:

– shall indicate in the Source ID IE the RNC-ID of the source RNC and in the Target ID IE either the eNB-ID or the Corresponding RNC-ID of the target eNodeB in the target system (see subclause 9.2.1.25);

– shall include the Source to Target Transparent Container IE within the RELOCATION REQUIRED message. The information in the container shall be encoded according to the Source eNB to Target eNB Transparent Container IE definition as specified in TS 36.413 [49].

– shall, in case rSRVCC is performed, include the rSRVCC HO Indication IE in the RELOCATION REQUIRED message. The value of rSRVCC HO Indication IE shall be set to "PS only" by the source RNC.

When the source RNC sends the RELOCATION REQUIRED message, it shall start the timer T_RELOCprep.

When the preparation including resource allocation in the target system is ready and the CN has decided to continue the relocation of SRNS, the CN shall send a RELOCATION COMMAND message to the source RNC and the CN shall start the timer T_{RELOCcomplete}.

If the CSG Id IE and no Cell Access Mode IE are received in the RELOCATION REQUIRED message, the CN shall perform the access control according to the CSG Subscription Data of that UE and if the access control is successful, or if one of the RABs has some particular ARP values (see TS 23.060 [21]), it shall continue the relocation and propagate the target CSG Id IE to the target side. If the access control is unsuccessful but at least one of the RABs has some particular ARP values (see TS 23.060 [21]) the CN shall also provide the CSG Membership Status IE set to "non-member" to the target side.

If the CSG Id IE and the Cell Access Mode IE set to "hybrid" are received in the RELOCATION REQUIRED message, the CN shall provide the CSG Membership Status IE of the UE and the target CSG Id to the target side.

If the Target To Source Transparent Container IE or the L3 information IE or the Target BSS to Source BSS Transparent Container IE is received by the CN from the relocation target, it shall be included in the RELOCATION COMMAND message.

The RELOCATION COMMAND message may also contain the Inter-System Information Transparent Container IE.

In case of SRVCC operation, when the target system is GERAN the RELOCATION COMMAND message shall contain the SRVCC Information IE.

In case of rSRVCC operation, the RELOCATION COMMAND message shall contain the rSRVCC Information IE.

If the Target BSS to Source BSS Transparent Container IE is received in the RELOCATION COMMAND message, only the value part of the UE related containers received shall be sent to the UE.

For each RAB successfully established in the target system and originating from the PS domain, the RELOCATION COMMAND message shall contain at least one pair of Iu transport address and Iu transport association to be used for the forwarding of the DL N-PDU duplicates towards the relocation target. If more than one pair of Iu transport address and Iu transport association is included, the source RNC shall select one of the pairs to be used for the forwarding of the DL N-PDU duplicates towards the relocation target. Upon reception of the RELOCATION COMMAND message from the PS domain, the source RNC shall start the timer T_DATAfwd.

The Relocation Preparation procedure is terminated in the CN by transmission of the RELOCATION COMMAND message.

If the target system (including target CN) does not support all existing RABs, the RELOCATION COMMAND message shall contain a list of RABs indicating all the RABs that are not supported by the target system. This list may include information on RABs from the PS domain not existing in the source RNC which shall be ignored by the source RNC. This list is contained in the RABs to Be Released IE. The source RNC shall use this list to avoid transferring associated contexts where applicable and may use this list e.g. to decide if to cancel the relocation or not. The resources associated with not supported RABs shall not be released until the relocation is completed. This is in order to make a return to the old configuration possible in case of a failed or cancelled relocation.

Upon reception of the RELOCATION COMMAND message the source RNC shall stop the timer T_RELOCprep, start the timer T_RELOCOveralland terminate the Relocation Preparation procedure. The source RNC is then defined to have a Prepared Relocation for that Iu signalling connection.

When the Relocation Preparation procedure is successfully terminated and when the source RNC is ready, the source RNC should trigger the execution of relocation of SRNS.

Interactions with the SRVCC Preparation procedure:

In case of SRVCC operation, the Source RNC to Target RNC Transparent Container IE shall include the Integrity Protection Key IE, the Ciphering Key IE for the signalling data and the SRVCC Information as received during the SRVCC Preparation procedure.

Interactions with the rSRVCC Preparation procedure:

In case of rSRVCC operation, the RNC shall only initate the Relocation Preparation procedure on the Iu-CS signalling connections existing for the UE.

Interactions with other procedures:

If, after a RELOCATION REQUIRED message is sent and before the Relocation Preparation procedure is terminated, the source RNC receives a RANAP message initiating another connection oriented RANAP class 1 or class 3 procedure (except IU RELEASE COMMAND message, which shall be handled normally) via the same Iu signalling connection, the source RNC shall either:

1. cancel the Relocation Preparation procedure i.e. execute the Relocation Cancel procedure with an appropriate value for the Cause IE, e.g. "Interaction with other procedure", and after successful completion of the Relocation Cancel procedure, the source RNC shall continue the initiated RANAP procedure;

2. terminate the initiated RANAP procedure without any changes in UTRAN by sending the appropriate response message with the cause value "Relocation Triggered" to the CN. The source RNC shall then continue the relocation of SRNS.

If during the Relocation Preparation procedure the source RNC receives a DIRECT TRANSFER message it shall be handled normally up to the anticipated limit according to section 14.12.4.2 TS 25.331 [10].

If during the Relocation Preparation procedure the source RNC receives connection oriented RANAP class 2 messages (with the exception of DIRECT TRANSFER message) it shall decide to either execute the procedure immediately or suspend it. In case the relocation is cancelled, the RNC shall resume any suspended procedures (if any).

After the Relocation Preparation procedure is successfully terminated, all RANAP messages (except IU RELEASE COMMAND message, which shall be handled normally) received via the same Iu signalling bearer shall be ignored by the source RNC.

8.6.2.1 Successful Operation for GERAN Iu-mode

The relocation between UTRAN and GERAN Iu-mode shall be considered in the Relocation Preparation procedure as intra-system relocation from RANAP point of view.

For GERAN Iu-mode and to support Relocation towards a GERAN BSC in Iu mode the following shall apply in addition for the successful operation of the Relocation Preparation procedure:

– In case of a Relocation to GERAN Iu-mode (only for CS), the RNC shall include, if available, the GERAN Classmark IE within the RELOCATION REQUIRED message in those cases where the transmission of the GERAN Classmark IE is required, as defined in TS 43.051 [27].

8.6.3 Unsuccessful Operation

Figure 6: Relocation Preparation procedure. Unsuccessful operation.

If the CN or target system is not able to even partially accept the relocation of SRNS, or a failure occurs during the Relocation Preparation procedure in the CN, or the CN decides not to continue the relocation of SRNS, the CN shall send a RELOCATION PREPARATION FAILURE message to the source RNC.

The RELOCATION PREPARATION FAILURE message shall contain the appropriate value for the Cause IE, e.g. "T_RELOCallocexpiry", "Relocation Failure in Target CN/RNC or Target System", "Relocation not supported in Target RNC or Target System", "Relocation Target not allowed", "No Radio Resources Available in Target Cell" or "Traffic Load In The Target Cell Higher Than In The Source Cell".

Transmission of the RELOCATION PREPARATION FAILURE message terminates the procedure in the CN. Reception of the RELOCATION PREPARATION FAILURE message terminates the procedure in UTRAN.

When the Relocation Preparation procedure is unsuccessfully terminated, the existing Iu signalling connection can be used normally.

If the Relocation Preparation procedure is unsuccessfully terminated, the CN shall release the possibly existing Iu signalling connection for the same UE and related to the same relocation of SRNS towards the target RNC by initiating the Iu Release procedure towards the target RNC with an appropriate value for the Cause IE, e.g. "Relocation Cancelled".

The RELOCATION PREPARATION FAILURE message may contain the Inter-System Information Transparent Container IE.

If the CSG Id IE and no Cell Access Mode IE are received in the RELOCATION REQUIRED message and the access control is unsuccessful and if none of the RABs has some particular ARP values (see TS 23.060 [21]), the CN shall send the RELOCATION PREPARATION FAILURE message with an appropriate cause value to the source RNC. Upon reception, the source RNC may decide to prevent relocation for that UE towards CSG cells with corresponding CSG Id.

Interactions with Relocation Cancel procedure:

If there is no response from the CN to the RELOCATION REQUIRED message before timer T_RELOCprep expires in the source RNC, the source RNC shall cancel the Relocation Preparation procedure by initiating the Relocation Cancel procedure with the appropriate value for the Cause IE, e.g. "T_RELOCprep expiry".

8.6.4 Abnormal Conditions

If the target RNC indicated in the RELOCATION REQUIRED message is not known to the CN:

1. The CN shall reject the relocation of SRNS by sending a RELOCATION PREPARATION FAILURE message to the source RNC with Cause IE set to "Unknown target RNC".

2. The CN shall continue to use the existing Iu connection towards the source RNC.

NOTE: In case two CN domains are involved in the SRNS Relocation Preparation procedure and the Source RNC receives the Target RNC to Source RNC Transparent Container IE via two CN domains, it may check whether the content of the two Target RNC to Source RNC Transparent Container IE is the same. In case the Source RNC receives two different Target RNC to Source RNC Transparent Container IEs, the RNC behaviour is left implementation-specific.

NOTE: In case two CN domains are involved in the SRNS Relocation Preparation procedure due to the inter-system handover towards the GSM CS domain and GSM PS domain in parallel and the Source RNC receives the L3 Information IE from CS domain and the Target BSS to Source BSS Transparent Container IE from PS domain, it may check whether the content of the L3 Information IE and the content of the Target BSS to Source BSS Transparent Container IE is the same. In case the Source RNC receives two IEs with different contents, the RNC behaviour is left implementation-specific.

8.6.5 Co-ordination of Two Iu Signalling Connections

If the RNC decides to initiate the Relocation Preparation procedure for a UTRAN to UTRAN relocation, the RNC shall initiate simultaneously a Relocation Preparation procedure on all Iu signalling connections existing for the UE. The source RNC shall also include the same Source RNC to Target RNC Transparent Container IE, Relocation Type IE, Source ID IE and Cause IE in the RELOCATION REQUIRED message towards the two domains.

For intersystem handover to GSM, the Relocation Preparation procedure shall be initiated either only towards the circuit-switched CN or only towards the packet-switched CN, if the inter-system handover towards the GSM CS domain and GSM PS domain in parallel is not supported. Otherwise the Relocation Preparation procedure shall be simultaneously initiated towards both the circuit-switched CN and the packet-swithched CN.

The source RNC shall not trigger the execution of relocation of SRNS unless it has received a RELOCATION COMMAND message from all Iu signalling connections for which the Relocation Preparation procedure has been initiated, except for the case where there is at least one of the RABs that has a particular ARP value (see TS 23.060 [21]).

If the source RNC receives a RELOCATION PREPARATION FAILURE message from the CN, the RNC shall initiate the Relocation Cancel procedure on the other Iu signalling connection for the UE if the other Iu signalling connection exists and if the Relocation Preparation procedure is still ongoing or the procedure has terminated successfully in that Iu signalling connection, except for the case where there is at least one of the RABs that has a particular ARP value (see TS 23.060 [21]) in the other domain.