8.22 Initial UE Message

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

8.22.1 General

The purpose of the Initial UE Message procedure is to establish an Iu signalling connection between a CN domain and the RNC and to transfer the initial NAS-PDU to the CN node as determined by the NAS Node Selection Function – if this function is active, or otherwise to the default CN node- or by the Rerouting Function (see TS 25.410 [2]) in case of MOCN configuration. The procedure uses connection oriented signalling.

8.22.2 Successful Operation

Figure 24: Initial UE Message procedure. Successful operation.

When the RNC has received from radio interface a NAS message (see TS 24.008 [8]) to be forwarded to a CN domain to which no Iu signalling connection for the UE exists, the RNC shall initiate the Initial UE Message procedure and send the INITIAL UE MESSAGE message to the CN. If NNSF is active, the selection of the CN node is made according to TS 23.236 [26].

In addition to the received NAS-PDU, the RNC shall add the following information to the INITIAL UE MESSAGE message:

– CN domain indicator, indicating the CN domain towards which this message is sent.

– For CS domain, the LAI which is the last LAI indicated to the UE by the UTRAN via the current RRC connection, or if the UTRAN has not yet indicated any LAI to the UE via the current RRC connection, then the LAI of the cell via which the current RRC connection was established.

– For PS domain, the LAI+RAC which are the last LAI+RAC indicated to the UE by UTRAN via the current RRC connection, or if the UTRAN has not yet indicated any LAI+RAC to the UE via the current RRC connection, then the LAI+RAC of the cell via which the current RRC connection was established.

– Service Area corresponding to at least one of the cells from which the UE is consuming radio resources.

– Iu signalling connection identifier.

– Global RNC identifier.

– Selected PLMN Identity, if received from radio interface by a network sharing supporting UE in shared networks.

– Redirect Attempt Flag, in MOCN configuration for a network sharing non supporting UE in order to indicate that the CN shall respond with a Redirection Indication IE or a Redirection Completed IE.

The Iu Signalling Connection Identifier IE contains an Iu signalling connection identifier which is allocated by the RNC. The value for the Iu Signalling Connection Identifier IE shall be allocated so as to uniquely identify an Iu signalling connection for the RNC. The CN should store and remember this identifier for the duration of the Iu connection.

Whereas several processing entities within the CN (e.g. charging, interception, etc.) may make use of the location information given in the SAI IE and the LAI (and RAC for PS domain) IE, the mobility management within the CN shall rely on the information given within the LAI IE (respectively LAI and RAC IEs for PS domain) only.

If the establishment of the Iu signalling connection towards the CN is performed due to an RRC connection establishment originating from a CSG cell and if the UE is CSG capable, the CSG Id IE shall be included in the INITIAL UE MESSAGE message.

If the establishment of the Iu signalling connection towards the CN is performed due to an RRC connection establishment originating from a Hybrid cell and if the UE is CSG capable, the CSG Id IE and Cell Access Mode IE shall be included in the INITIAL UE MESSAGE message.

If the RNC has a co-located L-GW for LIPA operation, it shall include the L-GW Transport Layer Address IE in the INITIAL UE MESSAGE message.

If the SIPTO L-GW Transport Layer Address IE is received in the INITIAL UE MESSAGE message, the CN shall, if supported, use it for SIPTO@LN operation as specified in TS 23.060 [21].

If the LHN ID IE is included in the INITIAL UE MESSAGE message, the CN shall, if supported, use it as specified in TS 23.060 [21].

If the Higher bitrates than 16 Mbps flag IE is included in the INITIAL UE MESSAGE message then the CN shall, if supported, use the IE as described in TS 23.060 [21].

If the Tunnel Information for BBF IE is received in the INITIAL UE MESSAGE message, the CN shall, if supported, use the IE as described in TS 23.139 [65].

If the SGSN Group Identity IE is included in the INITIAL UE MESSAGE message this indicates that the message is rerouted for DCN, and the SGSN shall, if supported, use the IE as described in TS 23.060 [21].

If the UE Usage Type IE is included in the INITIAL UE MESSAGE message, then the selected SGSN node in the DCN shall if supported, use it as defined in TS 23.401 [48].

If the DCN ID IE is included in the INITIAL UE MESSAGE message, the SGSN shall, if supported, use it as defined in TS 23.401 [48].

If the UE indicates the support for UE Application Layer Measurement, the RNC shall if supported include the UE Application Layer Measurement Capability IE in the INITIAL UE MESSAGE message.

Interaction with Direct Transfer procedure

In MOCN configuration, if the RNC receives the Redirection Indication IE in the DIRECT TRANSFER message from a CN node which is not the last attempted, it shall initiate the Initial UE Message procedure towards another CN operator when possible (or possibly to the same CN in case when CS/PS coordination is required), with the following additional information in the INITIAL UE MESSAGE message:

– NAS Sequence Number IE, if received from previously attempted CN operator;

– Permanent NAS UE Identity IE, if received from one of previously attempted CN operators.

In MOCN configurations, if the RNC receives the Redirection Indication IE containing the Additional CS/PS coordination information IE in the DIRECT TRANSFER message from a CN node and if the RNC cannot identify one operator from this information as described in TS 23.251 [39], then the RNC shall if supported initiate the UE Registration Query procedure towards the CN nodes of the other domain before initiating the Initial UE Message procedure towards another CN operator.

8.22.2.1 Successful Operation for GERAN Iu-mode

For GERAN Iu-mode, the following shall apply in addition for the successful operation of the Initial UE Message procedure:

– In case of establishment of a signalling connection towards the CS domain in GERAN Iu-mode, the INITIAL UE MESSAGE message shall contain the GERAN Classmark IE in order to provide the CN with GERAN-specific information (see TS 43.051 [27]).