8.5 Iu Release

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

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

8.5.1 General

The purpose of the Iu Release procedure is to enable the CN to release an Iu connection for a particular UE and all UTRAN resources related only to that Iu connection. The procedure uses connection oriented signalling.

The Iu Release procedure can be initiated for at least the following reasons:

– Completion of transaction between the UE and the CN.

– UTRAN-generated reasons, e.g. reception of an IU RELEASE REQUEST message.

– Completion of successful relocation of SRNS.

– Cancellation of relocation after successful completion of a Relocation Resource Allocation procedure.

– Detection of two Iu connections in the same domain toward one UE.

The Iu release procedure should also be initiated when there is a period of Iu signalling inactivity with no existing RAB.

8.5.2 Successful Operation

Figure 4: Iu Release procedure. Successful operation.

The CN initiates the procedure by sending an IU RELEASE COMMAND message to the UTRAN.

After the IU RELEASE COMMAND message has been sent, the CN shall not send further RANAP connection-oriented messages on this particular connection.

The IU RELEASE COMMAND message shall include a Cause IE indicating the reason for the release (e.g. "Successful Relocation", "Normal Release", "Release due to UTRAN Generated Reason", "Relocation Cancelled", "No Remaining RAB").

When the RNC receives the IU RELEASE COMMAND message:

1. Clearing of the related UTRAN resources is initiated. However, the UTRAN shall not clear resources related to other Iu signalling connections the UE might have. The Iu transport bearers for RABs subject to data forwarding and other UTRAN resources used for the GTP-PDU forwarding process, are released by the RNC only when the timer T_DATAfwd expires.

2. The RNC returns any assigned Iu user plane resources to idle i.e. neither uplink user data nor downlink user data can be transferred over the Iu interface anymore. Then the RNC sends an IU RELEASE COMPLETE message to the CN. (The RNC does not need to wait for the release of UTRAN radio resources or for the transport network layer signalling to be completed before returning the IU RELEASE COMPLETE message.) When an IU RELEASE COMPLETE message is sent, the procedure is terminated in the UTRAN.

If the IU RELEASE COMMAND message included the End Of CSFB IE, the RNC may use the indication to determine which of the existing mechanisms that should be used to move the UE to E UTRAN.

If the IU RELEASE COMMAND message includes the Out Of UTRAN IE, the RNC may use the indication to determine whether to explicitly release the RRC connection.

If the IU RELEASE COMMAND message included the Last E-UTRAN PLMN Identity IE, the RNC may take this information into account when selecting the target cell or frequency and then act as defined in TS 23.272 [67].

In case the UE has been linked to Multicast Service(s) in UTRAN and the RNC receives the IU RELEASE COMMAND message from PS domain or from CS domain when no Iu signalling connection exists towards the other domain the RNC shall perform UE de-linking as described in TS 25.346 [42].

The IU RELEASE COMPLETE message shall include within the RABs Data Volume Report List IE for each RAB towards the PS domain successfully addressed and for which data volume reporting was requested during RAB establishment, the total amount of unsuccessfully transmitted DL data for the RAB since its establishment.

If the release was initiated by the UTRAN, for each RAB towards the PS domain for which the DL GTP-PDU Sequence Number IE and/or the UL GTP-PDU Sequence Number IE are (is) available, the RNC shall include the available sequence number(s) in the RABs Released Item IE (within the RAB Released List IE) in the IU RELEASE COMPLETE message.

The RAB Release Item IE shall not be present if there is no sequence number to be reported for that RAB.

Reception of an IU RELEASE COMPLETE message terminates the procedure in the CN.

Interaction with Trace:

In case of simultaneous Iu signalling connections for both CS and PS domains, if a trace session was activated by both domains, the successful release of one of the connections should not close this trace session. If the trace session was activated by only one domain and the Iu connection for this domain is successfully released, this trace session shall be stopped in UTRAN.

8.5.3 Abnormal Conditions

If the Iu Release procedure is not initiated towards the source RNC from the CN before the expiry of timer T_RELOCoverall, the source RNC should initiate the Iu Release Request procedure towards the CN with a cause value "T_RELOCoverall expiry".