LTE 切换信令流程.docx

1、LTE 切换信令流程 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 u

2、nchanged or 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

3、 performed 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 i

4、f so MME releases 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 comma

5、nd it will 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, d

6、ownlink data 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

7、 GW, the handover 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

8、 (see TS36.3005), 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 det

9、ects that the 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 t

10、he Serving 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 s

11、hall re-attempt, 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 eNod

12、eB with an 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.1.1.2 X2-based handover without Servi

13、ng GW relocationThis 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 t

14、he target eNodeB 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

15、 target cell and 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 th

16、e Path Switch 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 ha

17、s changed.NOTE2: 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 ad

18、dress(es) and TEIDs 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 Loc

19、ation Information 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

20、 informed about the 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 us

21、er plane for the 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

22、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 non-accepted dedicated bearers by triggering the bearer release procedure as specified in clause5.4.4.2.

23、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 bearer of a PDN connection has not been accepted by the target eNodeB and there are multiple PDN connections active, the MME

24、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 clause5.10.3. If none of the default EPS bearers have been accepted by the target eNodeB or there is a LIPA PDN connection that has not

25、 been released, the MME 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.

26、g. can be used for charging, 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 Be

27、arer Response (Serving 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

28、serve the MME Request 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

29、 its request using Modify 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

30、eNodeB, the Serving GW shall 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 beare

31、rs which are associated to 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 Sw

32、itch Request Ack message which 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

33、corresponding bearer contexts 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,