无线网络-LTE (05) 通信过程之 S1 Release procedure

This procedure is used to release the logical S1-AP signalling connection (over S1-MME) and all S1 bearers (in S1-U) for a UE. The procedure will move the UE from ECM-CONNECTED to ECM-IDLE in both the UE and MME, and all UE related context information is deleted in the eNodeB.The initiation of S1 Release procedure is either:

- eNodeB-initiated with cause e.g. O&M Intervention, Unspecified Failure, User Inactivity, Repeated RRC signalling Integrity Check Failure, Release due to UE generated signalling connection release, etc.;
- MME-initiated with cause e.g. authentication failure, detach, etc.In case of eNodeB Failure, MME may based on operator policy either preserve all bearers or initiate the Dedicated Bearer Deactivation procedure for GBR bearers. See clause 5.4.4.2.


此过程是用于释放S1-AP逻辑连接和S1和手机间的承载。它将会使手机的自身的状态和在MME上的状态从ECM-CONNECTED转移到ECM-IDLE在eNodeB上所有手机相关的上下文信息会被删除。
初始化 S1 Release 过程也可以由不同的网元触发：

-   eNodeB-initiated 可能原因： O&M Intervention, Unspecified Failure, User Inactivity, Repeated RRC signalling Integrity Check Failure, Release due to UE generated signalling connection release, etc.

- MME-initiated 可能原因：cause e.g. authentication failure, detach。

Step 1

If the eNodeB detects a need to release the UE's signalling connection and all radio bearers for the UE, the eNodeB sends an S1 UE Context Release Request (Cause) message to the MME. Cause indicates the reason for the release (e.g. O&M intervention, unspecified failure, user inactivity, repeated integrity checking failure, or release due to UE generated signalling connection release).
NOTE: Step 1 is only performed when the eNodeB-initiated S1 release procedure is considered. Step 1 is not performed and the 
procedure starts with Step 2 when the MME-initiated S1 release procedure is considered.


如果eNodeB检测发现需要与手机断开信令连接和所有的无线承载，eNode会向MME发送S1 UE Context Release Request，并且包含释放原因。注意：第一步只是考虑eNodeB初始化S1释放的过程。如果是MME初始化S1 release过程，则不需要考虑这步。

Step 2

The MME sends a Release Access Bearers Request message to the S GW that requests the release of all S1-U bearers for the UE. This message is triggered either by an S1 Release Request message from the eNodeB, or by another MME event.


MME发送Release Access Bearers Request到S GW，请求释放所有和手机相关的S1-U承载。此消息既可以由从eNodeB发来的S1 Release Request消息触发也可以由MME的其他事件触发。

Step 3

The S GW releases all eNodeB related information (address and TEIDs) for the UE and responds with a Release Access Bearers Response message to the MME. Other elements of the UE's S GW context are not affected. The S GW retains the S1-U configuration that the S GW allocated for the UE's bearers. The S GW starts buffering downlink packets received for the UE and initiating the "Network Triggered Service Request" procedure, described in clause 5.3.4.3, if downlink packets arrive for the UE.


S GW释放所有eNodeB相关于该手机的信息，并且向MME返回Release Access Bearers Response，手机的S GW上下文其他的元素则不受影响。S GW保留对该手机承载配置的相关信息。S GW开始缓存到手机的数据包，并且初始化"Network Triggered Service Request" 过程。

Step 4

The MME releases S1 by sending the S1 UE Context Release Command (Cause) message to the eNodeB.



Step 5
If the RRC connection is not already released, the eNodeB sends a RRC Connection Release message to the UE in Acknowledged Mode. Once the message is acknowledged by the UE, the eNodeB deletes the UE's context.


如果RRC连接已经释放了，eNodeB会发送RRC Connection Release 消息 Acknowledged Mode到手机。



Step 6
The eNodeB confirms the S1 Release by returning an S1 UE Context Release Complete message to the MME. With this, the signalling connection between the MME and the eNodeB for that UE is released. This step shall be performed promptly after step 4, e.g. it shall not be delayed in situations where the UE does not acknowledge the RRC Connection Release.

The MME deletes any eNodeB related information (address and TEIDs) from the UE's MME context, but, retains the rest of the UE's MME context including the S GW's S1-U configuration information (address and TEIDs). All non-GBR EPS bearers established for the UE are preserved in the MME and in the Serving GW. If the cause of S1 release is different from User inactivity, e.g. "Radio Connection With UE Lost", the MME shall trigger the MME Initiated Dedicated Bearer Deactivation procedure (clause 5.4.4.2) for the GBR bearer(s) of the UE after the S1 Release procedure is completed.If the cause of S1 release is because of UE inactivity, the MME shall preserve the GBR bearers.

NOTE: EPC does not support the GPRS preservation feature with setting the MBR for GBR bearers to zero.

eNodeB通过向MME发送S1 UE Context Release Complete消息确认S1 Release。通过此消息该手机在MME和eNodeB间的信令连接被断开。MME删除了所有eNodeB的从手机MME上下文中得来的信息，但保留了其他的手机的MME上下文关于S GW的 S1-U配置信息。