9.2.5 Preservation Procedures

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By sending a RAB Release Request or Iu Release Request message to the SGSN, an Iu mode RAN initiates the release of one or more RABs. The preservation procedure allows the active PDP contexts associated with the released RABs to be preservedin the CN, and the RABs can then be re-established at a later stage, see clause 9.2.5.2 and clause 9.2.3.5.

An Iu mode RAN uses the Iu Release Request to request release of all RABs of an MS, and the RAB Release Request in other cases.

9.2.5.1 Release of RABs Triggered by an Iu mode RAN

9.2.5.1.1 RAB Release Procedure

An Iu mode RAN initiates a RAB release procedure to release one or several RABs. The RAB Release procedure is described in clause 12.7.2.a.

9.2.5.1.2 Iu Release Procedure

An Iu mode RAN initiates an Iu release procedure to release all RABs of an MS and the Iu connection. The Iu Release procedure is described in clause 12.7.3.

9.2.5.2 Re-establishment of RABs

The procedure for re-establishment of RABs allows the SGSN to re-establish RABs for active PDP contexts that don't have an associated RAB. The MS initiates the re-establishment of RABs by using the Service Request (Service Type = Data) message. This is described in the sub-clause "MS Initiated Service Request Procedure". SGSN shall not establish RABs for PDP contexts with maximum bit rate for uplink and downlink of 0 kbit/s. For these PDP contexts, the MS shall perform a MS-initiated PDP Context Modification or Deactivation procedure. When RABs for an MS that has no RRC connection needs to be re-established, the CN must first page the MS. The clause "Network Initiated Service Request Procedure" describes this.

通过发送RAB release请求或Iu release请求消息给SGSN，RNC可以发起释放一个或多个RAB。保持流程允许将释放的RAB相关联的激活的PDP上下文在核心网中保持下来（即不被去激活，并且RAB可以在后续阶段被重新建立，参考章节9.2.5.2和9.2.3.5）

RAN使用Iu release请求来请求释放和MS相关的所有RAB，使用RAB释放请求消息用于其他的释放RAB的场景。

9.2.5.1 由RNC来触发释放RAB

9.2.5.1.1 RAB释放流程

RNC发起RAB释放流程来释放一个或多个RAB。RAB释放流程在章节12.7.2a中定义。

9.2.5.1.2 Iu释放流程

RNC发起Iu释放流程来释放MS和所有RAB以及Iu连接。Iu释放流程在章节12.7.3中定义。

9.2.5.2 RAB的重建

重建RAB将允许SGSN为一个active但没有相关联的RAB的PDP上下文重建相关联的RAB。MS通过发送service request消息（业务类型=数据）给SGSN来发起对RAB的重建。这在章节“MS Initiated Service Request Procedure“中描述。SGSN不应为上下行MBR都为0的PDP上下文来重建RAB。对于这些PDP上下文，MS应执行一个PDP上下文修改或去激活流程。当一个没有RRC连接的RAB需要重建的话，那么核心网将首先寻呼MS（目的就是为了建立RRC连接）。这在章节“Network Initiated Service Request Procedure“中有对应的描述。

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