Section: D.3.8.2 Preparation phase
Content: Figure D.3.8.2-1: GERAN A/Gb mode to E-UTRAN inter RAT HO, preparation phase 1. The source BSS decides to initiate a PS handover. At this point both uplink and downlink user data is transmitted via the following: TBFs between MS and source BSS, BSSGP PFCs tunnel(s) between source BSS and old SGSN, GTP tunnel(s) between old SGSN and GGSN. NOTE 1: The UE acts as MS, and the PDN GW acts as the GGSN. 2. The source BSS sends the message PS handover Required (TLLI, Cause, Source Cell Identifier, Target eNodeB Identifier, Source to Target Transparent Container (RN part), 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: The Source SGSN acts as the Old SGSN. NOTE 3: As an implementations option for supporting introduction scenarios with pre-Rel8 SGSNs the source BSS may be configured to use RNC IDs instead of eNodeB IDs to identify a target eNodeB. The Cause is relayed transparently by the SGSN to the MME and the MME maps the BSSGP cause code to an S1AP cause code. Source to Target Transparent Container carries information for the target eNodeB. This container is relayed transparently by the SGSN. 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 initiates the Handover resource allocation procedure by sending message Forward Relocation Request (IMSI, Target Identification, MM Context, PDP Context, SGSN Tunnel Endpoint Identifier for Control Plane, SGSN Address for Control plane, Source to Target Transparent Container (RN part), Packet Flow ID, SNDCP XID parameters, LLC XID parameters) to the target MME. This message includes all PDP contexts that are established in the source system indicating the PFIs and the XID parameters related to those PDP Contexts, and the uplink Tunnel endpoint parameters of the Serving GW. The PDP Contexts shall be sent in a prioritized order, i.e. the most important PDP Context first. The prioritization method is implementation dependent, but should be based on the current activity. NOTE 3: Assigning the highest priority to the PDP context without TFT could be done to get service continuity for all ongoing services regardless of the number of supported EPS bearers in the UE and network. The target MME maps the PDP contexts to the EPS bearers 1-to-1 and maps the Release 99 QoS parameter values of a PDP context to the EPS Bearer QoS parameter values of an EPS bearer as defined in Annex E. The MME establishes the EPS bearer(s) in the indicated order. The MME deactivates the EPS bearers which cannot be established. The MM context contains security related information, e.g. supported ciphering algorithms as described in TS 29.060[ General Packet Radio Service (GPRS); GPRS Tunnelling Protocol (GTP) across the Gn and Gp interface ]  [14]. For the PDP Context with traffic class equals 'Background', the source SGSN shall indicate via the Activity Status Indicator IE that EPS bearers shall be established on the target side. NOTE 4: The Source SGSN acts as the old SGSN. 4. The target MME selects the 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, APN-AMBR, Serving Network) per PDN connection to the Serving GW. The Protocol Type over S5/S8 is provided to Serving GW which protocol should be used over S5/S8 interface. For relocation from Gn/Gp SGSN, the target MME provides the APN-AMBR if not received explicitly from the Gn/Gp SGSN based on the mapping from MBR (as specified in Annex E) to the Serving GW 4a. The 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). The Target MME shall not request resources for which the Activity Status Indicator within a PDP Context indicates that no active bearer exists on the source side for that PDP Context. 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 target MME shall not request the target eNodeB to establish EPS GBR bearers with maximum bitrate set to 0 and those EPS bearers should not be included in the EPS Bearers to be setup list and should be deactivated by the MME. For the remaining EPS Bearer Contexts the MME ignores any Activity Status Indicator within an EPS Bearer Context and requests the target eNodeB to allocate resources for all the remaining EPS Bearer Contexts. 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. The MME shall compute the UE-AMBR, as per clause 4.7.3, based on explicit APN-AMBR values received from the Gn/Gp SGSN. If explicit APN-AMBR values are not received by the MME, a local UE-AMBR shall be included in the 'EPS Bearers be setup list ' IE. The local UE-AMBR is described in Annex E. 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, 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. 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. 6. If 'Indirect Forwarding' applies, the target MME sends a Create Indirect Data Forwarding Tunnel Request message (Cause, Target eNodeB Address(es), TEID(s) for DL user plane) to the Serving GW. Cause indicates that the bearer(s) are subject to data forwarding. 6a. The Serving GW returns a Create Indirect Data Forwarding Tunnel Response (Cause, Serving GW Address(es) and TEID(s) for Data Forwarding) message to the target MME. 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. 7. The Target MME sends the message Forward Relocation Response (Cause, List of Set Up PFCs, MME Tunnel Endpoint Identifier for Control Plane, BSSGP cause, MME Address for control plane, Target to Source Transparent Container, Address(es) and TEID(s) for Data Forwarding) to the Source SGSN. If 'Direct Forwarding' is applicable, then the IEs 'Address(es) and TEID(s) for Data Forwarding' contains the DL GTP-U tunnel endpoint parameters to the eNodeB. If 'Indirect Forwarding' applies the IEs 'Address(es) and TEID(s) for Data Forwarding' contain the DL GTP-U tunnel endpoint parameters to the Serving GW. NOTE 5: The Source SGSN acts as the old SGSN.