Service Request procedures 分为2种， UE Trigger 和 Network Trigger
1) UE Trigger
Step 1
The UE sends NAS message Service Request towards the MME encapsulated in an RRC message to the eNodeB. The RRC message(s) that can be used to carry the S-TMSI and this NAS message are described in TS 36.300 [5].
手机向eNodeB发送NAS：Service Request，这个消息被包含在RRC消息中。RRC消息可以用来封装S-TMSI和 NAS消息。
Step 2
The eNodeB forwards NAS message to MME. NAS message is encapsulated in an S1-AP: Initial UE Message (NAS message, TAI+ECGI of the serving cell, S-TMSI). Details of this step are described in TS 36.300 [5]. If the MME can't handle the Service Request it will reject it.
eNodeB向MME转送NAS消息。NAS 消息被封装在S1-AP: Initial UE Message中。
Step 3
NAS authentication/security procedures as defined in clause 5.3.10 on "Security function" may be performed.
Step 4
The MME sends S1-AP Initial Context Setup Request (Serving GW address, S1-TEID(s) (UL), EPS Bearer QoS(s), Security Context, MME Signalling Connection Id, Handover Restriction List) message to the eNodeB. This step activates the radio and S1 bearers for all the active EPS Bearers. The eNodeB stores the Security Context, MME Signalling Connection Id, EPS Bearer QoS(s) and S1-TEID(s) in the UE RAN context. The step is described in detail in TS 36.300 [5]. Handover Restriction List is described in clause 4.3.5.7 "Mobility Restrictions".
MME发送S1-AP Initial Context Setup Request到eNodeB。这步为活动的EPS承载激活了无线和S1的承载。
Step 5
The eNodeB performs the radio bearer establishment procedure. The user plane security is established at this step, which is described in detail in TS 36.300 [5]. When the user plane radio bearers are setup the Service Request is completed and bearer state is synchronized between the UE and the network, i.e. the UE should remove the EPS bearer for which no radio bearers are setup.
eNodeB 开始处理无线承载的建立过程，并且user plane的安全也是在步建立的。当user plane的无线承载被建立起来了，Service Request也就结束了EPS 承载的状态将会在手机和网络间进行同步。如果没有无线的承载被建立起来，手机应该删除EPS 承载。
Step 6
The uplink data from the UE can now be forwarded by eNodeB to the Serving GW. The eNodeB sends the uplink data to the Serving GW address and TEID provided in the step 4. The Serving GW forwards the uplink data to the PDN GW.
现在手机发送的上行数据可以通过eNodeB被转发到Serving GW。eNodeB通过Serving GW地址和TEID来转发数据。Serving GW继续转发数据到Serving GW。
Step 7
The eNodeB sends an S1-AP message Initial Context Setup Complete (eNodeB address, List of accepted EPS bearers, List of rejected EPS bearers, S1 TEID(s) (DL)) to the MME. This step is described in detail in TS 36.300 [5].
Step 8
The MME sends a Modify Bearer Request message (eNodeB address, S1 TEID(s) (DL) for the accepted EPS bearers, Delay Downlink Packet Notification Request, RAT Type) per PDN connection to the Serving GW. The Serving GW is now able to transmit downlink data towards the UE. The usage of the Delay Downlink Packet Notification Request Information Element is specified in clause 5.3.4.2 below. If the PDN GW requested UE's location info and the UE's location info has changed, the MME also includes the User Location Information IE in this message.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通过每个PDN连接发送Modify Bearer Request到Serving GW。这时Serving GW可以发送下行数据到手机。如果PDN GW请求手机的位置信息，并且手机的位置信息也被改变了，MME也会在该消息中包含User Location Information。
Step 9
If the RAT Type has changed compared to the last reported RAT Type or if the UE's Location Info IE is present in step 8, the Serving GW shall send the Modify Bearer Request message (RAT Type) per PDN connection to the PDN GW. User Location Information IE is also included if it is present in step 8.
如果RAT Type和上次使用的不一致，或者手机的位置信息被包含在消息中。Serving GW应该向PDN GW发送Modify Bearer Request。
Step 10
If dynamic PCC is deployed, the PDN GW interacts with the PCRF to get the PCC rule(s) according to the RAT Type by means of a PCEF initiated IP CAN Session Modification procedure as defined in TS 23.203 [6]. If dynamic PCC is not deployed, the PDN GW may apply local QoS policy.
如果网络中动态的部署了PCC，PDN GW可以根据RAT Type通过IP CAN Session Modification过程来获取PCC规则。如果没有部署PCC，PDN GW可以使用本地的QoS策略。
Step 11
The PDN GW sends the Modify Bearer Response to the Serving GW.
Step 12
The Serving GW sends a Modify Bearer Response to the MME.
2) Network Trigger
Step 1
When the Serving GW receives a downlink data packet for a UE known as not user plane connected (i.e. the S GW context data indicates no downlink user plane TEID), it buffers the downlink data packet. and identifies which MME or SGSN is serving that UE.If that MME has requested the S GW to delay sending the Downlink Data Notification (see clause 5.3.4.2 on "Handling of abnormal conditions in UE triggered Service Request"), the Serving GW buffers the downlink data and waits until the timer expires before continuing with step 2. If the DL-TEID and eNodeB address for that UE is received before the expiry of the timer, the timer shall be cancelled and the Network triggered Service Request procedure is finished without executing the steps below, i.e. DL data are sent to the UE.If the Serving GW receives additional downlink data packets for this UE before the expiry of the timer, the Serving GW does not restart this timer.
Serving GW收到下行的数据包，但目的手机并没有建立起user plane连接时，它将该数据包缓存，然后找到为该手机服务的MME 或 SGSN。如果MME 要求 S GW 延迟发送Downlink Data Notification，Serving GW会继续缓存该数据直到计时器超时。如果在计时器结束前手机收到了DL-TEID 和 eNodeB 的地址，计时器将被取消，Network triggered Service Request结束。
Step 2
The Serving GW sends a Downlink Data Notification message to the MME and SGSN nodes for which it has control plane connectivity for the given UE. The MME and SGSN respond to the S GW with a Downlink Data Notification Ack message. If the Serving GW receives additional downlink data packets for this UE, the Serving GW buffers these downlink data packets and the Serving GW does not send a new Downlink
Data Notification.
Serving GW发送Downlink Data Notification 与手机有control plane连接的 MME 和 SGSN。MME 和 SGSN 返回Downlink Data Notification Ack给S GW。如果Serving GW继续收到了到手机的数据，它会继续缓存这些数据，但不会发送新的Downlink Data Notification。
Step 3a
If the UE is registered in the MME, the MME sends a Paging message (NAS ID for paging, TAI(s), UE identity based DRX index, Paging DRX length) to each eNodeB belonging to the tracking area(s) in which the UE is registered. The step is described in detail in TS 36.300 [5] and TS 36.413 [36]. Steps 3-4 are omitted if the MME already has a signalling connection over S1-MME towards the UE.
如果手机在MME上进行了注册，MME会发送Paging消息到属于tracking area中的每一个该手机注册的eNodeB上。
Step 3b
If the UE is registered in the SGSN, the SGSN sends paging messages to RNC/BSS, which is described in detail in TS 23.060 [7].
Step 4a
If eNodeBs receive paging messages from the MME, the UE is paged by the eNodeBs. The step is described in detail in TS 36.300 [5] and TS 36.304 [34].
Step 4b
If RNC/BSS nodes receive paging messages from the SGSN the UE is paged by the RNSC/BSS, which is described in detail in TS 23.060 [7].
Step 5
When UE is in the ECM-IDLE state, upon reception of paging indication in E-UTRAN access, the UE initiates the UE triggered Service Request procedure (clause 5.3.4.1). If the MME already has a signalling connection over S1-MME towards the UE, then the messages sequence performed start from the step when MME establishes the bearer(s). Upon reception of paging indication in UTRAN or GERAN access, the MS shall respond in respective access as specified TS 24.008 [47] and the SGSN shall notify the S GW. The MME and/or SGSN supervises the paging procedure with a timer. If the MME and/or SGSN receives no response from the UE to the Paging Request message, it may repeat the paging. The repetition strategy is operator dependent. If the MME and/or SGSN receives no response from the UE after this paging repetition procedure, it shall use the Downlink Data Notification Reject message to notify the Serving GW about the paging failure. In that case, if ISR is not activated, the Serving GW deletes the buffered packet(s). If ISR is activated and the Serving GW receives paging failure from both SGSN and MME, the Serving GW deletes the buffered packet(s) or rejects the control signalling which triggers the Service Request procedure.
当手机在ECM-IDLE状态，如果收到了E-UTRAN 接入的paging消息，UE会发起Service Request过程。如果MME和手机间已经通过S1-MME建立起了连接，消息将会通过承载进行发送。根据来自UTRAN or GERAN接入网络的paging，手机应该回应相应的接入网，并且SGSN需要通知S GW。MME 或 SGSN 通过一个Timer来监管Paging 过程。如果MME 或 SGSN 没有收到手机对Paging Request的响应，他们可以重复发送Paging Request消息。如果通过重复Paging过程，MME 或 SGSN 仍然没有收到手机响应，他们会通过发送Downlink Data Notification Reject到Serving GW。
Step 6a
If ISR is activated and paging response is received in E UTRAN access the Serving GW sends a "Stop Paging" message to the SGSN.
Step 6b
If ISR is activated and paging response is received in UTRAN or GERAN access the Serving GW sends a "Stop Paging" message to the MME.