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1、Handover1 Intra-E-UTRAN handover1.1 X2-based handover1.1.1 GeneralThese procedures are used to hand over a UE from a source eNodeB to a target eNodeB using the X2 reference point. In these procedures the MME is unchanged. Two procedures are defined depending on whether the Serving GW is unchanged or
2、 is relocated. In addition to the X2 reference point between the source and target eNodeB, the procedures rely on the presence of S1-MME reference point between the MME and the source eNodeB as well as between the MME and the target eNodeB.The handover preparation and execution phases are performed
3、as specified in TS36.3005. If emergency bearer services are ongoing for the UE handover to the target eNodeB is performed independent of the Handover Restriction List. The MME checks, as part of the Tracking Area Update in the execution phase, if the handover is to a restricted area and if so MME re
4、leases the non-emergency bearers as specified in clause5.10.3.If the serving PLMN changes during X2-based handover, the source eNodeB shall indicate to the target eNodeB (in the Handover Restriction List) the PLMN selected to be the new Serving PLMN.When the UE receives the handover command it will
5、remove any EPS bearers for which it did not receive the corresponding EPS radio bearers in the target cell. As part of handover execution, downlink and optionally also uplink packets are forwarded from the source eNodeB to the target eNodeB. When the UE has arrived to the target eNodeB, downlink dat
6、a forwarded from the source eNodeB can be sent to it. Uplink data from the UE can be delivered via the (source) Serving GW to the PDN GW or optionally forwarded from the source eNodeB to the target eNodeB. Only the handover completion phase is affected by a potential change of the Serving GW, the ha
7、ndover preparation and execution phases are identical.If the MME receives a rejection to a NAS procedure (e.g. dedicated bearer establishment/modification/release; location reporting control; NAS message transfer; etc.) from the eNodeB with an indication that an X2 handover is in progress (see TS36.
8、3005), the MME shall reattempt the same NAS procedure either when the handover is complete or the handover is deemed to have failed, except in case of Serving GW relocation. The failure is known by expiry of the timer guarding the NAS procedure.If during the handover procedure the MME detects that t
9、he Serving GW needs be relocated, the MME shall reject any PDN GW initiated EPS bearer(s) request received since handover procedure started and shall include an indication that the request has been temporarily rejected due to handover procedure in progress. The rejection is forwarded by the Serving
10、GW to the PDN GW, with the same indication.Upon reception of a rejection for an EPS bearer(s) PDN GW initiated procedure with an indication that the request has been temporarily rejected due to handover procedure in progress, the PDN GW start a locally configured guard timer. The PDN GW shall re-att
11、empt, up to a pre-configured number of times, when either it detects that the handover is completed or has failed using message reception or at expiry of the guard timer.If the MME receives a rejection to a UE Context Modification Request message with a CS Fallback indicator from the eNodeB with an
12、indication that an X2 handover is in progress, the MME shall resend a UE Context Modification Request message with CS Fallback indicator to the target eNodeB when the handover is complete or to the source eNB when the handover is deemed to have failed.1.1.2X2-based handover without Serving GW reloca
13、tionThis procedure is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged and decides that the Serving GW is also unchanged. The presence of IP connectivity between the Serving GW and the source eNodeB, as well as between the Serving GW and the target eN
14、odeB is assumed.Figure 1.1.2-1: X2-based handover without Serving GW relocationNOTE1:For a PMIP-based S5/S8, procedure steps (A) are defined in TS23.4022.1.The target eNodeB sends a Path Switch Request message to MME to inform that the UE has changed cell, including the TAI+ECGI of the target cell a
15、nd the list of EPS bearers to be switched. If the target cell is a CSG cell , the target eNodeB includes the CSG ID of the target cell in Path Switch Request message. If the target cell is in hybrid mode, it includes the CSG ID of the target cell and CSG Access Mode set to hybrid in the Path Switch
16、Request message. The MME determines the CSG membership based on the CSG ID and CSG Access Mode received from the target eNodeB.The MME updates the User CSG information based on the CSG ID and CSG Access Mode received from the target eNodeB and CSG membership if one of the parameters has changed.NOTE
17、2:X2 handover between HeNBs is possible when the handover is between closed/hybrid access HeNBs having the same CSG ID or when the target HeNB is an open access HeNB.The MME determines that the Serving GW can continue to serve the UE2.The MME sends a Modify Bearer Request (eNodeB address(es) and TEI
18、Ds for downlink user plane for the accepted EPS bearers, ISR Activated) message per PDN connection to the Serving GW for each PDN connection where the default bearer has been accepted by the target eNodeB. If the PDN GW requested UEs location info, the MME also includes the User Location Information
19、 IE in this message. If the UE Time Zone has changed, the MME includes the UE Time Zone IE in this message. If the Serving Network has changed, the MME includes the new Serving Network IE in this message. If ISR was activated before this procedure, MME should maintain ISR. The UE is informed about t
20、he ISR status in the Tracking Area Update procedure. If the Serving GW supports Modify Access Bearers Request procedure and if there is no need for the SGW to send the signalling to the PGW, the MME may send Modify Access Bearers Request (eNodeB address(es) and TEIDs for downlink user plane for the
21、accepted EPS bearers, ISR Activated) per UE to the Serving GW to optimize the signalling.If the PDN GW requested UEs User CSG information (determined from the UE context), the MME includes the User CSG Information IE in this message if the User CSG Information has changed.The MME uses the list of EP
22、S bearers to be switched, received in step1, to determine whether any dedicated EPS bearers in the UE context have not been accepted by the target eNodeB. The MME releases the non-accepted dedicated bearers by triggering the bearer release procedure as specified in clause5.4.4.2. If the Serving GW r
23、eceives a DL packet for a non-accepted bearer, the Serving GW drops the DL packet and does not send a Downlink Data Notification to the MME.If the default bearer of a PDN connection has not been accepted by the target eNodeB and there are multiple PDN connections active, the MME shall consider all b
24、earers of that PDN connection as failed and release that PDN connection by triggering the MME requested PDN disconnection procedure specified in clause5.10.3.If none of the default EPS bearers have been accepted by the target eNodeB or there is a LIPA PDN connection that has not been released, the M
25、ME shall act as specified in step6.3.If the Serving GW has received the User Location Information IE and/or the UE Time Zone IE and/or the Serving Network IE and/or User CSG Information IE from the MME in step2 the Serving GW informs the PDN GW(s) about this information that e.g. can be used for cha
26、rging, by sending the message Modify Bearer Request (Serving GW Address and TEID, User Location Information IE and/or UE Time Zone IE and/or Serving Network IE and/or User CSG Information IE) per PDN connection to the PDN GW(s) concerned. The Serving GW shall return a Modify Bearer Response (Serving
27、 GW address and TEID for uplink traffic) message to the MME as a response to a Modify Bearer Request message, or a Modify Access Bearers Response (Serving GW address and TEID for uplink traffic) as a response to a Modify Access Bearers Request message. If the Serving GW cannot serve the MME Request
28、in the Modify Access Bearers Request message without S5/S8 signalling or without corresponding Gxc signalling when PMIP is used over the S5/S8 interface, it shall respond to the MME with indicating that the modifications are not limited to S1-U bearers, and the MME shall repeat its request using Mod
29、ify Bearer Request message per PDN connection.4.The Serving GW starts sending downlink packets to the target eNodeB using the newly received address and TEIDs. A Modify Bearer Response message is sent back to the MME.5.In order to assist the reordering function in the target eNodeB, the Serving GW s
30、hall send one or more end marker packets on the old path immediately after switching the path as defined in TS36.3005, clause10.1.2.2.6.The MME confirms the Path Switch Request message with the Path Switch Request Ack message. If the UEAMBR is changed, e.g. all the EPS bearers which are associated t
31、o the same APN are rejected in the target eNodeB, the MME shall provide the updated value of UEAMBR to the target eNodeB in the Path Switch Request Ack message.If some EPS bearers have not been switched successfully in the core network, the MME shall indicate in the Path Switch Request Ack message w
32、hich bearers failed to be established (see more detail in TS36.41336) and for dedicated bearers initiate the bearer release procedure as specified in clause5.4.4.2 to release the core network resources of the failed dedicated EPS bearers. The target eNodeB shall delete the corresponding bearer conte
33、xts when it is informed that bearers have not been established in the core network.If none of the default EPS bearers have been switched successfully in the core network or if they have not been accepted by the target eNodeB or the LIPA PDN connection has not been released, the MME shall send a Path
34、 Switch Request Failure message (see more detail in TS36.41336) to the target eNodeB. The MME performs explicit detach of the UE as described in the MME initiated detach procedure of clause5.3.8.3.7.By sending Release Resource the target eNodeB informs success of the handover to source eNodeB and tr
35、iggers the release of resources. This step is specified in TS36.3005.8.The UE initiates a Tracking Area Update procedure when one of the conditions listed in clause Triggers for tracking area update applies. If ISR is activated for the UE when the MME receives the Tracking Area Update Request, the M
36、ME should maintain ISR by indicating ISR Activated in the Tracking Area Update Accept message.NOTE3:It is only a subset of the TA update procedure that is performed by the MME, since the UE is in ECMCONNECTED state and the MME is not changed.1.1.3X2-based handover with Serving GW relocationThis proc
37、edure is used to hand over a UE from a source eNodeB to a target eNodeB using X2 when the MME is unchanged and the MME decides that the Serving GW is to be relocated. The presence of IP connectivity between the source Serving GW and the source eNodeB, between the source Serving GW and the target eNo
38、deB, and between the target Serving GW and target eNodeB is assumed. (If there is no IP connectivity between target eNodeB and source Serving GW, it is assumed that the S1-based handover procedure in clause 1.2 shall be used instead.)Figure 1.1.3-1: X2-based handover with Serving GW relocationNOTE1:
39、For a PMIP-based S5/S8, procedure steps (A) and (B) are defined in TS23.4022.1.The target eNodeB sends a Path Switch Request message to MME to inform that the UE has changed cell, including the ECGI of the target cell and the list of EPS bearers to be switched. If the target cell is a CSG cell , the
40、 target eNodeB includes the CSG ID of the target cell in Path Switch Request message. If the target cell is in hybrid mode, it includes the CSG ID of the target cell and CSG Access Mode set to hybrid in the Path Switch Request message. The MME determines the CSG membership based on the CSG ID and CS
41、G Access Mode received from the target eNodeB.The MME updates the User CSG information based on the CSG ID and CSG Access Mode received from the target eNodeB and CSG membership if one of the parameters has changed.NOTE2:X2 handover between HeNBs is possible when the handover is between closed/hybri
42、d access HeNBs having the same CSG ID or when the target HeNB is an open access HeNB.The MME determines that the Serving GW is relocated and selects a new Serving GW according to clause 4.3.8.2 on Serving GW Selection Function.NOTE3:The MME knows the SGW Service Area with a TA granularity.2.The MME
43、sends a Create Session Request (bearer context(s) with PDN GW addresses and TEIDs (for GTP-based S5/S8) or GRE keys (for PMIP-based S5/S8) at the PDN GW(s) for uplink traffic, eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers, the Protocol Type over S5/S8, Serving Net
44、work, UE Time Zone) message per PDN connection to the target Serving GW for each PDN connection where the default bearer has been accepted by the target eNodeB. The target Serving GW allocates the SGW addresses and TEIDs for the uplink traffic on S1_U reference point (one TEID per bearer). The Proto
45、col Type over S5/S8 is provided to Serving GW which protocol should be used over S5/S8 interface. If the PDN GW requested UEs location info, the MME also includes the User Location Information IE in this message. If the PDN GW requested UEs User CSG information (determined from the UE context), the
46、MME includes the User CSG Information IE in this message if the User CSG Information has changed.The MME uses the list of EPS bearers to be switched, received in step1, to determine whether any dedicated EPS bearers in the UE context have not been accepted by the target eNodeB. The MME releases the
47、non-accepted dedicated bearers by triggering the bearer release procedure as specified in clause5.4.4.2 via target Serving GW. If the Serving GW receives a DL packet for a non-accepted bearer, the Serving GW drops the DL packet and does not send a Downlink Data Notification to the MME.If the default
48、 bearer of a PDN connection has not been accepted by the target eNodeB and there are multiple PDN connections active, the MME shall consider all bearers of that PDN connection as failed and release that PDN connection by triggering the MME requested PDN disconnection procedure specified in clause5.1
49、0.3 via source Serving GW.If none of the default EPS bearers have been accepted by the target eNodeB or there is a LIPA PDN connection that has not been released, the MME shall act as specified in step5.3.The target Serving GW assigns addresses and TEIDs (one per bearer) for downlink traffic from the PDN GW. The Serving GW allocates DL TEIDs on S5/S8 even for non-accepted bearers. It sends a Modif
