Section: 5.5.2.4.2 Preparation phase
Content: Figure 5.5.2.4.2-1: GERAN A/Gb mode to E-UTRAN inter RAT HO, preparation phase 1. The source access system, Source BSS, decides to initiate an Inter-RAT Handover to the E-UTRAN. At this point both uplink and downlink user data is transmitted via the following: Bearers between UE and Source BSS, BSSGP PFC tunnel(s) between source BSS and source SGSN, GTP tunnel(s) between Source SGSN, Serving GW and PDN GW. NOTE 1: The process leading to the handover decision is outside of the scope of this specification. 2. The source BSS sends the message PS handover Required (TLLI, Cause, Source Cell Identifier, Target eNodeB Identifier, Source eNodeB to Target eNodeB Transparent Container and active PFCs list) to Source SGSN to request the CN to establish resources in the Target eNodeB, Target MME and the Serving GW. NOTE 2: In contrast to most inter-RAT handover preparation phases, this Source to Target Transparent Container does not contain the UE's target RAT radio capabilities. 3. The Source SGSN determines from the 'Target eNodeB Identifier' IE that the type of handover is IRAT PS Handover to E-UTRAN. The Source SGSN selects the Target MME as described in clause 4.3.8.3 on "MME Selection Function". The Source SGSN initiates the Handover resource allocation procedure by sending message Forward Relocation Request (IMSI, Target Identification, MM Context, PDN Connections, SGSN Tunnel Endpoint Identifier for Control Plane, SGSN Address for Control plane, Source to Target Transparent Container, RAN Cause, Packet Flow ID, SNDCP XID parameters, LLC XID parameters, MS Info Change Reporting Action (if available), CSG Information Reporting Action (if available), UE Time Zone, ISR Supported, Serving Network) to the target MME. When ISR is activated the message should be sent to the MME that maintains ISR for the UE when this MME is serving the target identified by the Target Identification. If indicated, the information ISR Supported indicates that the source SGSN and associated Serving GW are capable to activate ISR for the UE. This message includes all PDN Connections active in the source system and for each PDN Connection includes the associated APN, the address and the uplink tunnel endpoint parameters of the Serving GW for control plane, and a list of EPS Bearer Contexts established in the source system. The EPS Bearer Contexts indicate the PFIs and the XID parameters related to those EPS Bearer Contexts, and the uplink Tunnel endpoint parameters of the Serving GW. The old Serving Network is sent to target MME to support the target MME to resolve if Serving Network is changed. In network sharing scenarios Serving Network denotes the serving core network. The RAN Cause includes the value from the Cause IE received from the source BSS. Source to Target Transparent Container includes the value from the Source eNodeB to Target eNodeB Transparent Container received from the source BSS. The MM context includes information on the EPS Bearer context(s). If none of the UE's EPS Bearers can be supported by the selected target MME, the source SGSN rejects the handover attempt by sending a PS Handover Required Negative Acknowledge (Cause) message to the Source BSS. NOTE 3: If the handover is successful, the source SGSN will signal to the SGW and/or SCEF to release any non-included EPS Bearers after step 6 of the Execution procedure. The non-included bearers are locally released by the UE following the Bearer Context Status synchronisation that occurs during the Tracking Area Update at step 12 of the Execution procedure. Prioritization of EPS Bearer Contexts is performed by the target core network node. The MME establishes the EPS bearer(s) in the prioritized order. The MME deactivates, as provided in step 8 of the execution phase, the EPS bearers which cannot be established. The MM context contains security related information, e.g. supported ciphering algorithms as described in TS 29.274[ 3GPP Evolved Packet System (EPS); Evolved General Packet Radio Service (GPRS) Tunnelling Protocol for Control plane (GTPv2-C); Stage 3 ]  [43]. Handling of security keys is described in TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [41]. For the EPS Bearer Context with traffic class equals 'Background', the source SGSN shall indicate via the Activity Status Indicator IE that radio bearers shall be established on the target side. The target MME shall determine the Maximum APN restriction based on the APN Restriction of each bearer context received in the Forward Relocation Request, and shall subsequently store the new Maximum APN restriction value. 4. The target MME determines if the Serving GW is to be relocated, e.g. due to PLMN change. If the Serving GW is to be relocated, the target MME selects the target Serving GW as described under clause 4.3.8.2 on "Serving GW selection function". The target MME sends a Create Session Request message (IMSI, MME Tunnel Endpoint Identifier for Control Plane, MME Address for Control plane, PDN GW address(es) for user plane, PDN GW UL TEID(s) for user plane, PDN GW address for control plane, and PDN GW TEID(s) for control plane, the Protocol Type over S5/S8, Serving Network) per PDN connection to the target Serving GW. The Protocol Type over S5/S8 is provided to Serving GW which protocol should be used over S5/S8 interface. 4a. The target Serving GW allocates its local resources and returns them in a Create Session Response (Serving GW address(es) for user plane, Serving GW UL TEID(s) for user plane, Serving GW Address for control plane, Serving GW TEID for control plane) message to the target MME. 5. The Target MME will request the Target eNodeB to establish the Bearer(s) by sending the message Handover Request (UE Identifier, S1AP Cause, Integrity protection information (i.e. IK and allowed Integrity Protection algorithms), Encryption information (i.e. CK and allowed Ciphering algorithms), EPS Bearers to be setup list, Source to Target Transparent Container, Handover Restriction List). The Target MME ignores any Activity Status Indicator within an EPS Bearer Context and requests the eNodeB to allocate resources for all EPS Bearer Contexts received from the source side. The S1AP Cause includes the value from the RAN Cause IE received from the source SGSN. The target eNodeB shall ignore it if the number of radio bearers in the Source to Target Transparent container does not comply with the number of bearers requested by the MME and allocate bearers as requested by the MME. Handover Restriction List is sent if it is available in the Target MME; it is described in clause 4.3.5.7. For each EPS bearer requested to be established, 'EPS Bearers To Be Setup' IE shall contain information such as ID, bearer parameters, Transport Layer Address, "Data forwarding not possible" indication, and S1 Transport Association. The Transport Layer Address is the Serving GW Address for user data, and the S1 Transport Association corresponds to the uplink Tunnel Endpoint Identifier Data. "Data forwarding not possible" indication shall be included if the target MME decides the corresponding bearer will not be subject to data forwarding. The ciphering and integrity protection keys will be sent transparently from the target eNodeB to the UE in the Target to Source Transparent Container, and in the message PS Handover Command from source BSS to the UE. This will then allow data transfer to continue in the new RAT/mode target cell without requiring a new AKA (Authentication and Key Agreement) procedure. More details are described in TS 33.401[ 3GPP System Architecture Evolution (SAE); Security architecture ]  [41]. 5a. The Target eNodeB allocates the request resources and returns the applicable parameters to the Target MME in the message Handover Request Acknowledge (Target to Source Transparent Container, S1AP Cause, EPS Bearers setup list, EPS Bearers failed to setup list). Upon sending the Handover Request Acknowledge message the target eNodeB shall be prepared to receive downlink GTP PDUs from the Serving GW for the accepted EPS bearers. 6. If 'Indirect Forwarding' and relocation of Serving GW apply, the target MME sends a Create Indirect Data Forwarding Tunnel Request message (Target eNodeB Address(es) and TEID(s) for DL data forwarding) to the Serving GW. Indirect forwarding may be performed via a Serving GW which is different from the Serving GW used as the anchor point for the UE. 6a. The Serving GW returns a Create Indirect Data Forwarding Tunnel Response (Cause, Serving GW Address(es) and DL TEID(s) for data forwarding) message to the target MME. 7. The Target MME sends the message Forward Relocation Response (Cause, List of Set Up PFCs, MME Tunnel Endpoint Identifier for Control Plane, RAN Cause, MME Address for control plane, Target to Source Transparent Container, Address(es) and TEID(s) for Data Forwarding, Serving GW change indication) to the Source SGSN. Serving GW change indication indicates whether a new Serving GW has been selected. The RAN Cause includes the value from the S1AP Cause IE received from the target eNodeB. The Target to Source Transparent Container includes the value from the Target to Source Transparent Container received from the target eNodeB. If 'Direct Forwarding' applies or if 'Indirect Forwarding' but no relocation of Serving GW applies, then the IEs 'Address(es) and TEID(s) for Data Forwarding' contain the DL GTP-U tunnel endpoint parameters to the eNodeB received in step 5a. If 'Indirect Forwarding' and relocation of Serving GW apply the IEs 'Address(es) and TEID(s) for Data Forwarding' contain the DL GTP-U tunnel endpoint parameters to the Serving GW received in step 6a. 8. If 'Indirect Forwarding' applies, the source SGSN shall send the message Create Indirect Data Forwarding Tunnel Request (Address(es) and TEID(s) for Data Forwarding (received in step 7)) to the Serving GW used for indirect packet forwarding. Indirect forwarding may be performed via a Serving GW which is different from the Serving GW used as the anchor point for the UE. 8a. The Serving GW returns the forwarding user plane parameters by sending the message Create Indirect Data Forwarding Tunnel Response (Cause, Serving GW Address(es) and TEID(s) for Data Forwarding). If the Serving GW doesn't support data forwarding, an appropriate cause value shall be returned and the Serving GW Address(es) and TEID(s) will not be included in the message.