无线网络-LTE (10) 通信过程之 HO - X2-based handover without Serving GW relocation

This 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 eNodeB is assumed.

这个流程主要是使UE在MME和Serving GW不变的情况下通过 X2接口从源 eNodeB 切换到目标 eNodeB。假定源 eNodeB 和 目标eNodeB同Serving GW的IP连接的连同性是一样的。

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 and the list of EPS bearers
to be switched. The MME determines that the Serving GW can continue to serve the UE

目标eNodeB 发送Path Switch Request(TAI+ECGI 和 EPS bearers 列表) 通知 MME UE 已经更换了小区。MME 决定 Serving GW 是否可以继续为UE服务。

2. The MME sends a Modify Bearer Request (eNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers) message per PDN connection to the Serving
GW. If the PDN GW requested UE's location info, the MME also includes the User Location Information IE in this message. The MME uses the list of EPS bearers to be switched, received in step 1, to determine whether any EPS bearers in the UE context have not
been accepted by the target eNodeB. The MME releases the non-accepted bearers by triggering the bearer release procedure as specified in clause 5.4.4.2. 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. MME 发送 Modify Bearer RequesteNodeB address(es) and TEIDs for downlink user plane for the accepted EPS bearers

通过PDN连接发送到 Serving GW。如果PDN GW 请求手机的位置信息，MME应在消息中包含User Location Information IE。MME使用第一步收到的EPS bearers可选的承载列表，来检查是否在目标eNodeB上，UE上下文中存在未被接受的EPS bearers。MME通过触发bearer release过程来释放这些未被接受的承载。如果Serving GW通过未接受的承载收到了下行的数据包Serving GW会删除它，并且不向MME发送Downlink Data Notification。

3. If the Serving GW has received the User Location Information IE from the MME in step 2 the Serving GW informs the PDN GW(s) about this information that e.g. can be used
for charging, by sending the message Modify Bearer Request (Serving GW Address and TEID, User Location Information IE) per PDN connection to the

PDN GW(s) concerned. A Modify Bearer Response message is sent back to the Serving GW.

如果Serving GW 收到了MME在Step 2中发送的User Location Information IE。Serving GW 会向 PDN GW 发送消息: Modify Bearer Request (Serving GW Address and TEID, User Location Information IE) 通知这一信息。PDN GW会向Serving GW 返回Modify Bearer Response消息。

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.

Serving GW开始使用新收到的地址和TEIDs，发送下行数据包到目标eNodeB。Modify Bearer Response会返回到MME。

5. In order to assist the reordering function in the target eNB, the Serving GW shall send one or more "end marker" packets on the old path immediately after switching
the path as defined in TS 36.300 [5], clause 10.1.2.2.

为了辅助目标eNB的记录功能，Serving GW会在老的路径切换到新的时，马上发送一个或多个"end marker"包

6. The MME confirms the Path Switch Request message with the Path Switch Request Ack message. If the UE AMBR is changed, e.g. all the EPS bearers which are associated
to the same APN are rejected in the target eNodeB, the MME shall provide the updated value of UE AMBR 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 which bearers failed to be established (see more detail in TS 36.413 [36]) and initiate the bearer release procedure
as specified in clause 5.4.4.2 to release the core network resources of the failed EPS bearers. The target eNodeB shall delete the 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, the MME shall send a Path Switch Request Failure message (see more detail in TS 36.413 [36]) to the target eNodeB. The target eNB does not update bearer contexts with
the UE. The MME immediately performs explicit detach of the UE as described in the MME initiated detach procedure of clause 5.3.8.3.

MME 通过 Path Switch Request Ack 来响应 Path Switch Request消息。如果手机 AMBR改变了，例如：所有的与相同APN相关的EPS都被目标eNodeB拒绝了，MME 会更新UE的AMBR，并通过Path Switch Request Ack消息发给目标eNodeB。

如果在核心网EPS并没有切换成功，MME应该通过Path Switch Request Ack消息反应出承载建立失败，并且开始承载释放过程，来

释放核心网资源。目标eNodeB应该删除相应的建立失败的承载的承载上下文。

如果在核心网没有默认的EPS承载切换成功，MME应该发送Path Switch Request Failure 到目标eNodeB。目标eNodeB则不会更新

手机的承载上下文。MME会立即执行显示的去附着(detach)过程。

7. By sending Release Resource the target eNodeB informs success of the handover to source eNodeB and triggers the release of resources. This step is specified in TS 36.300
[5].

目标eNodeB通过发送Release Resource到源eNodeB来表明切换已经成功了，并且触发释放过程。

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 MME should maintain ISR by indicating ISR Activated in the Tracking Area Update Accept message.

当在clause 中有"Triggers for tracking area update"时，UE会触发Tracking Area Update过程。当MME收到Tracking Area Update Request手机的ISR是处于激活状态时，MME 应该在Tracking Area Update Accept消息中维护ISR Activated。