LTE典型信令过程

Click to edit Master title style,Click to edit Master text styles,Second level,Third level,Fourth level,Fifth level,*,LTE 典型信令过程,Software Architecture,3类NAS信令流程,1. 会话管理相关,2. 位置管理相关,3. 鉴权管理相关,UE,eNB,MME,SGW,PGW,RRC: RRC Connection setup complete,NAS: Attach Request,NAS:PDN connectivity request,S1AP: Initial UE message,Authentication and NAS security procedure,HSS,S6a: Update Location request,S6a: Update Location answer,S11: Create bearer request,S5: Create bearer request,S5: Create bearer response,S11: Create bearer response,S1AP: Initial Context setup request,NAS: Attach accept,NAS: Activate Default EPS bearer request,NAS: Attach Request,NAS:PDN connectivity request,RRC: RRCConnectionReconfiguration,NAS: Attach accept,NAS: Activate Default EPS bearer request,RRC: RRCConReconfigComplete,S1AP: Initial Context setup response,RRC: UL information Transfer,NAS: Attach complete,NAS: Activate Default EPS bearer accept,S1AP: Uplink NAS Transport,NAS: Attach complete,NAS: Activate Default EPS bearer accept,UE initiated Detach Procedure,UE,eNB,MME,SGW,PGW,HSS,RRC: RRC Connection setup complete,NAS: Detach request,S1AP: Initial UE message,NAS: Detach request,Authentication and NAS security,S11: Delete session request,S5: Delete session request,S5: Delete session response,S11: Delete session response,S1AP: Downlink NAS transport,NAS: Detach accept,RRC: DLInformation transfer,NAS: Detach accept,Radio and S1 Release,S6a: TDelay purge timer expires,S6a: Purge UE request,S6a: Purge UE answer,UE Triggered Service Request Procedure,Initial UE Message,Network Triggered Service Request,TAU procedure,Inter MME-TAU-message flow,Handover,Handover Air Interface,TYPES OF HANDOVER,X2 Based Handover,: without SGW change and with SGW change,S1 Based Handover,: with eNB, MME and SGW change,X2 Handover,X2 is the interface between two,eNBs,and X2-AP is the protocols,used for communication over it.,The handover is directly performed between two eNodeBs.,Handover takes place when the eNB detects that UE can no longer,be served by it because of the power constraints.,The MME is only informed at the end of the handover procedure,once the handover is successful,The release of resources at the source side is directly triggered from,the target eNB.,RRC: Connection Reconfiguration Request,RRC: Connection Reconfiguration Complete,UE,S-eNB,T-eNB,MME,SGW,RRC: Measurement Control,X2AP: Handover Request,S11: Modify bearer request,S11: Modify bearer response,X2AP: SN Status Transfer,Sync,UL Allocation + TA for UE,UL Allocation,RRC: Measurement Report,Handover,Decision,X2AP: Handover Response,UE Detach from,old cell and sync,to new cell,S1AP: Path Switch Request,S1AP: Path Switch Response,X2AP: UE Context Release,End Marker,Switch,DL Path,Data Forwarding,Data Forwarding,End Marker,Flush,DL Buffer,S1 Handover,This type of handover takes place when there is no X2 connectivity,between source eNB and target eNB.,Both,eNBs,are served by different,MMEs,.,SGWs,can also be,different.,Source eNB is served by source MME and source SGW while target,eNB is served by target MME and target SGW.,Source and target,MMEs,can communicate over S10 interface.,All the handover decisions are taking place on S1 interface, its,called as S1 based handover.,UE,T-eNB,S-MME,T-MME,T-SGW,S10: Forward Relocation,Request,S11: Create Session Request,S11: Create Session Response,S1AP: eNB Status Transfer,S1AP: Handover Required,S1AP: Handover Request,UE Detach from old cell,and sync to new cell,S11: Modify Bearer Request,S1AP: UE Context Release Command,S-eNB,S-SGW,PGW,Handover Decision,S1AP: Handover Request Ack,S10: Forward Relocation,Response,S1AP: Handover Command,RRC: Connection Reconfiguration,RRC: Connection Reconfiguration Complete,S1AP: Handover Notify,S11: Modify Bearer Response,S5: Modify Bearer,Request,S1AP: UE Context Release Complete,S11: Delete Session Request,S11: Delete Session Response,S1AP: MME Status Transfer,S10: Forward SRNS,Context Notification,S10: Forward SRNS,Context Ack,S10: Forward Relocation,Complete,S10: Forward Relocation,Complete Ack,S5: Modify Bearer,Response,开始,Before the UE can do anything, a Radio Resource Control connection must be established. The RRC is a logical connection between the UE and E-UTRAN. There are two different RRC states:,RRC IDLE,RRC CONNECTED,RRC IDLE,This state indicates that there is no signaling radio bearer is established i.e. no RRC connection is established.,The UE is considered RRC_IDLE once it obtains the center frequency, reads the timing information, syncs to the eNodeB, and is ready to receive system broadcast information.,UE monitors a paging channel to detect incoming calls.,UE can performs neighboring cell measurements and measurement reporting.,RRC CONNECTED,This state indicates that there is signaling radio bearer established i.e. RRC connection is established.,Transfer of unicast data to/from an UE.,The UE Monitors control channels associated with the shared data channel to determine if data is scheduled for it.,The UE Provides channel quality and feedback information.,The UE Perform neighboring cell measurements and measurement reporting.,Radio Resource Control (RRC) protocol states,RRC_IDLE,state,vs.,RRC_CONNECTED,state,:,UE,does not have context,in eNB and only Signalling Radio Bearer 0 (SRB0) is available for signalling between eNB and UE.,UE,has context,in eNB and communications between the UE and the network occurs on Signalling Radio Bearer 1 (SRB1) and Signalling Radio Bearer 2 (SRB2),UE generally has an id unique in the,tracking area,(,S-TMSI,= 40 bits long);,UE generally has an id unique in the,eNB area,(,C-RNTI,= 40 bits long,),UE may perform,cell selection/reselection,without involvement from eNB,;,UE can perform,handove,r (inter-eNB, intra-eNB, inter-RAT),under eNB control,;,UE can,request connection,establishment or re-establishment; eNB processes these requests,UE and eNBcan,transmit and/or receive data,to/from each other;,UE listens for,pages,at specific times (,DRX, configured by,MME); on request from the MME, eNB sends pages to the UE during UEs DRX up wake time,At PDCP/RLC/MAC level:,UE monitors L1/L2,control signalling channel to see if any transmission over the shared data channel has been allocated to the UE,UE reports channel quality,and feedback information to eNB,DRX/DTX period,can be,configured,according to UE activity level for UE power saving and efficient resource utilization. This is under control of eNB (RRC).,DRX = Discontinuous Reception; DTX = Discontinuous Transmission,Control plane: Radio-interface Related,PHY,MAC,RLC,PDCP,NAS,RRC,PHY,MAC,RLC,PDCP,RRC,NAS,UE,eNB,MME,L1,L2,IP,SCTP,S1-AP,L1,L2,IP,SCTP,S1-AP,eNB,MME,S1-MME,L1,L2,IP,SCTP,X2-AP,L1,L2,IP,SCTP,X2-AP,eNB,eNB,X2-C,Non-Access Stratum (NAS),: The key control interface between MME and UE,Radio Resource Control (RRC),: The main control interface between eNB and UE,Packet Data Convergence Protocol (PDCP),: Duplicate detection, ROHC,Radio Link Control (RLC),: Segmentation/re-assmebly, ARQ, acknowledge mode (AM)/ un-acknowledged mode (UAM), etc,Medium Access Control (MAC),: Access the channel,Physical Layer (PHY),: Radio layer, eg modulation etc.,S1-AP,: S1-Application Protocol,X2-AP,: X2-Application Protocol,SCTP,: Stream Control Transmission Protocol,状态转换图,RRC连接建立过程,UE,eNB,1. RRC连接建立准备阶段,RRC,RRC,RRC,RRC,RRC,RRC,2.CCCH:RRC连接请求,3.CCCH:RRC连接建立,4.DCCH:RRC连接建立完成,RRC连接建立过程示意图,1.准备阶段,(1)UE用默认的物理信道参数来配置物理信道,(2)UE为调度配置默认的半静态调度参数,(3)UE配置默认的MAC主配置参数,(4)UE配置CCCH参数,(5)UE设置上行对齐定时器参数,2.发RRC连接请求阶段,(1)UE通过接入过程接入E-UTRAN,获得在UL_SCH信道上传数据的许可,(2)设置UE标识(ue-Identity):S-TMSI或在02,40,-1之间任选一个数,(3)根据高层提供的RRC建立原因填写原因(establishmentCause信元),(4)通过PUSCH/UL_SCH/CCCH向E-UTRAN发RRC连接请求消息,消息中携带UE标识和建立原因,3.接收响应阶段,E-UTRAN通过PDSCH/DL_SCH/CCCH想UE发RRC连接建立消息。UE收到RRC连接建立消息后，根据消息中的信元进行以下操作：,（1）根据radioResourceConfigDedicated信元进行无线资源配置过程,（2）填写RRC连接建立完成消息,4.设置完成阶段,UE通过PUCH/UL_SCH/DCCH向E-UTRAN发RRC连接建立完成消息,进入RRC连接状态,结束本次RRC连接建立过程,An example of a possible connection flow,UE,eNB,MME,Power ON, measurements, cell selection,BCCH/BCH:,MasterInformationBlock,BCCH/BCH:,SystemInformation,(,SIB-1),BCCH/BCH:,SystemInformation,(,SIB-2),RACH: preamble_id,PDCCH: UL grant,CCH: SRB0:,RRCConnectionRequest,(S-TMSI),CCH: SRB0:,RRCConnectionSetup,(S-TMSI, C-RNTI),DCCH: SRB1:,RRCConnectionSetupComplete,(C-RNTI, NAS_info),S1: INITIAL UE MESSAGE,S1: INITIAL CONTEXT,SETUP REQUEST (UE cap),DCCH: SRB1:,Security Mode Command,DCCH: SRB1:,Security Mode Complete,DCCH: SRB1:,RRCReconfigurationRequest,DCCH: SRB1:,RRCReconfigurationComplete,Random Access Procedure Contention Based (Initial Access Example),The four steps of the RACH procedures are:,1)(MSG. 1) Random Access Preamble on RACH in uplink:,6 bits to carry: a 5 bit random ID, and 1 bit to indicate information on size of message 3 limited by radio conditions.,2)(MSG. 2) Random Access Response in downlink:,semi-synchronous (within a flexible window) with message 1;,No HARQ;,Sent on DL-SCH, and intended for one or multiple UEs,3)(MSG. 3) RRC Connection Request in uplink:,Uses HARQ;,RLC TM: no segmentation;,If the size of the message allows it, the initial NAS message (or something allowing to build the initial NAS message in eNB) can be included;,Conveys UE identity,at least NAS UE identifier (initial access), or C-RNTI (other cases),4)(MSG. 4) RRC Contention Resolution on DL-SCH:,Not synchronous with message 3.,HARQ is applied;,UE identity, as provided in message 3 is echoed in RRC Contention Resolution message,HARQ feedback is transmitted only by the UE which detects its own UE identity,RRC连接释放过程,由E-UTRAN发起,要求一个处于RRC连接状态的UE释放RRC连接,包括释放已建立的无线承载(RB)和所有无线资源,RRC连接重配置过程,E-UTRAN通过向UE发RRC连接重配置消息,发起RRC连接重配置过程。当要求建立，修改，释放RB，或进行切换，或建立，修改，释放测量时，使用RRC连接重配置过程来修改RRC连接。RRC连接重配置消息中包括测量配置，移动控制，无线资源配置（包括RB，MAC主配置和物理信道配置）等消息以及所有相关的专用NAS信息和安全配置信息。 RRC连接重配置消息以及RRC连接重配置完成消息使用SRB1且承载在DCCH逻辑信道上,E-UTRAN 附着,UE,eNB,MME,SGW,PGW,1.RRC: RRC Connection setup complete,NAS: Attach Request,NAS:PDN connectivity request,2.S1AP: Initial UE message,3.Authentication and NAS security procedure,HSS,4.S6a: Update Location request,5.S6a: Update Location answer,6.S11: Create bearer request,7.S5: Create bearer request,8.S5: Create bearer response,9.S11: Create bearer response,10.S1AP: Initial Context setup request,NAS: Attach accept,NAS: Activate Default EPS bearer request,NAS: Attach Request,NAS:PDN connectivity request,11.RRC: RRCConnectionReconfiguration,NAS: Attach accept,NAS: Activate Default EPS bearer request,12.RRC: RRCConReconfigComplete,13.S1AP: Initial Context setup response,14.RRC: UL information Transfer,NAS: Attach complete,NAS: Activate Default EPS bearer accept,15.S1AP: Uplink NAS Transport,NAS: Attach complete,NAS: Activate Default EPS bearer accept,UE附着到E-UTRAN中时，要建立一个默认的EPS承载，该承载使UE获得一个“总是在线”的IP连接,1. UE发起附着过程，向eNodeB发一个RR消息（RRC连接配置完成消息），消息中携带高层的附着请求消息。包括以下参数：,UE的IMSI，旧的GUTI以及附加GUTI：旧GUTI可以由P-TMSI和,RAI(routing area identity),导出。如果UE没有一个有效的GUTI或P-TMSI，则必须提供IMSI；如果IMSI也没有，则必须提供ME的设备标识（IMEI）,上一次访问的TAI：如果UE中保存有上一次访问的TAI，则应该把该参数提供给MME,UE核心网能力：描述UE的能力，包括支持的NAS和AS安全算法,UE专用DRX参数：在接入过程中MAC为UE分配的DRX参数,PDN类型：表示请求的IP地址类型（IPv4,IPv6,IPv4/IPv6),协议配置选项PCO：用于在UE和P-GW之间传输参数，比如UE希望的地址分配方式,加密选项传输标志位,附着类型：可以设为“切换”，“附着”，“紧急附着”,KSI,ASME,，NAS序列号，NAS-MAC,此外，还携带两个RRC参数：被选网络，旧GUMMI,2. eNodeB根据收到附着请求消息中的RRC参数进行配置,并向MME转发附着请求消息(通过S1AP消息:初始UE消息),3.身份ID请求和响应,NAS鉴权和安全功能激活,ME标识校验请求和确认,向HSS发一个更新位置请求消息,告诉HSS目前在该MME中UE(由IMSI标识)正在发起附着,通过更新位置确认消息向MME提供该UE的签约数据(包括一个或多个PDN签约上下文)。MME正式确认UE驻留在某个TA中，并为该UE生成上下文,向S-GW发生成默认承载请求消息,要求S-GW为该UE生成一个默认承载,在其EPS承载列表中建立一个新的条目,并向P-GW发送生成默认承载请求消息,要求P-GW为该UE生成一个默认承载,在其EPS承载列表中建立一个新的条目,开始在外部PDN网络和S-GW之间转发用户面PDU,并生成一个新的计费ID,开始计费。P-GW向S-GW发送默认承载建立响应消息，其中包含给UE分配的PDN地址,向MME发默认承载建立响应消息,通知MME默认承载已经建立,通知eNodeB(通过S1AP的初始上下问建立请求消息)UE的附着请求已经被接受,并要求建立一个初始上下文,通知UE附着请求已经被接受(通过RRC连接重配置消息),进行RRC连接重配置,完成后,向eNodeB回复一个RRC连接重配置完成消息,向MME回复一个初始上下文建立响应消息,通过RRC的上行信息传输消息向eNodeB发一个NAS消息:附着完成消息,这标志着UE侧和附着相关的操作及设置已经完成,通过S1AP的上行NAS传输消息向MME转发UE的附着完成消息。本步后,一旦UE获得PDN地址,UE就想eNodeB发上行分组数据,这些分组数据可以通过隧道传到S-GW和P-GW,更新承载请求及响应在MME S-GW P-GW之间完成后,EPS承载的E-UTRAN部分和EPC部分就连起来了,这时候下行分组数据也开始传送了,会话管理过程,UE和网络都能发起服务请求过程.UE发起的主呼,UE被叫以及网络发起ECM空闲态,UE的去附着都属于服务请求,UE Triggered Service Request Procedure,Initial UE Message,UE触发的服务请求过程,(1)UE向MME发送服务请求消息(为NAS消息,封装在给eNodeB的RRC消息中,如上行信息传输消息或RRC连接建立完成消息),(2)eNodeB向MME转发该NAS消息(封装在S1AP的初始UE消息中,初试UE消息携带了服务小区的TAI和ECGI),如果MME不能处理服务请求,则会拒绝服务请求,(3)NAS 鉴权,(4)MME通过S1AP向eNodeB发初始上下文建立请求消息,包含上行到S-GW的隧道端口号和地址,要求激活的所有EPS的S1承载和无线承载.,(5)eNodeB发起无线承载建立过程,(6)UE的上行链路数据通过eNode转发给S-GW,(7)eNodeB通过S1AP向MME发初始上下文建立完成消息,其中携带下行到eNodeB的S1-U隧道端口号和地址,(8)MME向S-GW发更新承载请求消息,告之下行的S1-U TEID和基站地址,以建立下行S1承载.MME从这一步开始能给UE传下行数据,(9)如果RAT类型改变,S-GW还要向P-GW发更新承载请求消息,(10)如果配置了PCC,P-GW要和PCRF交互获得不同的RAT的PCC规则,如果没有配置动态PCC,P-GW可以使用本地QoS策略,(11)P-GW向S-GW回复更新承载响应消息,(12)S-GW向MME回复更新承载响应消息,Network Triggered Service Request,网络触发的服务请求,当MME需要给在ECM空闲态的UE发消息,如进行MME/HSS发起的去附着过程,或有下行数据到达,要求S-GW生成或修改专用承载,在MME会发起网络触发的服务请求过程,(1)当S-GW收到一个给UE的下行分组数据包,但S-GW的UE上下文中又没有下行用户面的TEID,表示当前没有和UE建立用户面连接,则S-GW缓存下行分组数据包,并查询目前服务UE的是哪个MME(SGSN),(2a)S-GW向MME(SGSN)发下行数据到达通知消息,请求用户面连接,(2b)MME(SGSN)向S-GW发下行数据到达通知确认消息,(3)如果UE注册到MME,则MME向UE注册TA的所有eNodeB发寻呼消息,(4)如果eNodeB收到MME的寻呼消息,则开始寻呼UE,(5)一旦收到E-UTRAN的寻呼消息,UE发起” UE触发的服务请求过程”,(6)如果ISR是激活的,当E-UTRAN收到寻呼响应,S-GW向MME发停止寻呼消息,切换过程,RRC连接状态的UE在E-UTRAN中进行移动的一般原则如下,在E-UTRAN中,网络控制UE的移动,网络决定UE该进入哪个小区,哪个频点或哪个RAT。根据无线条件或负载情况等，网络会触发切换过程。,在给UE发送切换消息之前，源eNodeB为切换准备一个或多个目标eNodeB。目标eNodeB生成用于进行切换的消息，消息中包括目标小区的接如层详细配置信息，源eNodeB把该消息透穿给UE。等合适的时候，源eNodeB发起DRB的转发。接到切换消息后，UE在第一个可用的RACH时刻就开始尝试接入到目标小区，也就是说，切换是异步的。因此，如果目标小区为这个随机接入分配了一个专用前缀（Preamble)，应该确保其在UE的第一个可能的RACH时刻就可用。一旦完成切换，UE给目标eNodeB发一个切换完成确认消息。源eNodeB中会保留UE的上下文一段时间，以防UE切换失败返回。检测到切换失败后，UE尝试在源小区或另一个也做好准备的小区使用RRC连接重建立过程，重新发起RRC连接。,RRC: Connection Reconfiguration Request,RRC: Connection Reconfiguration Complete,UE,S-eNB,T-eNB,MME,SGW,RRC: Measurement Control,X2AP: Handover Request,S11: Modify bearer request,S11: Modify bearer response,X2AP: SN Status Transfer,Sync,UL Allocation + TA for UE,UL Allocation,RRC: Measurement Report,Handover,Decision,X2AP: Handover Response,UE Detach from,old cell and sync,to new cell,S1AP: Path Switch Request,S1AP: Path Switch Response,X2AP: UE Context Release,End Marker,Switch,DL Path,Data Forwarding,Data Forwarding,End Marker,Flush,DL Buffer,Source eNB keeps asking UE for the power measurements in a RRC measurement control message.,UE responds with the measurements in RRC Measurement Report. The details of these messages can be found RRC protocol specification.,Based on the measurement report source eNB takes the decision to handover or not. If the source eNB realizes that other eNB can serve better, then it shall start the handover procedure.,If the handover decision has been taken, then source eNB sends an X2-AP message Handover Request to the target eNB.,The Handover Request may consist of UE Context IE and E-RABs(E-UTRANRadio Access Bearer) to be setup. UE context IE typically consists of MME information, UE security capabilities etc. E-RABs to be setup has ERAB ID, QoS, RRC contexts etc. Multiple E-RABs pertaining to multiple EPS bearers can be sent in this message.,If send the Handover Request Ack to source eNB. The Ack includes the ERABs that have been accepted by the target eNB.,Upon reception of Handover Request Ack from target eNB, source eNB sends RRC Connection Reconfiguration message to UE, to indicate that handover should take place. Source eNB sends SN Status Transfer message to target eNB to indicate PDCP and HFN status. At this moment, UE detaches from source eNB and syncs with target eNB.,UE completes the RRC reconfiguration by sending RRC Connection Reconfiguration Complete message to target eNB. And also UE starts transmitting the data towards target eNB. This data is buffered at target eNB until the EPS Bearers have been modified.,Next, target eNB sends an S1-AP patch Switch Request message to MME indicating that handover has taken place. This message includes the ERABs that have been accepted by the eNB.,MME now has to modify the EPS bearers, corresponding to ERABs accepted. MME sends Modify bearer request to SGW, which includes all EPS bearers that are to be modified and also the target eNBs FTEID. SGW, if request is accepted, sends the response along with the S1-U SGW FTEID, if they are modified.,MME upon reception of Modify Bearer Response sends Path Switch Request Ack to target eNB indicating the successful modification of EPS bearers. Target eNB now asks the source eNB to release all contexts related to the UE.,If PMIP based S5/S8 interface is considered, then there will be no message over that same interface for this handover scenario.,UE,T-eNB,S-MME,T-MME,T-SGW,S10: Forward Relocation,Request,S11: Create Session Request,S11: Create Session Response,S1AP: eNB Status Transfer,S1AP: Handover Required,S1AP: Handover Request,UE Detach from old cell,and sync to new cell,S11: Modify Bearer Request,S1AP: UE Context Release Command,S-eNB,S-SGW,PGW,Handover Decision,S1AP: Handover Request Ack,S10: Forward Relocation,Response,S1AP: Handover Command,RRC: Connection Reconfiguration,RRC: Connection Reconfiguration Complete,S1AP: Handover Notify,S11: Modify Bearer Response,S5: Modify Bearer,Request,S1AP: UE Context Release Complete,S11: Delete Session Request,S11: Delete Session Response,S1AP: MME Status Transfer,S10: Forward SRNS,Context Notification,S10: Forward SRNS,Context Ack,S10: Forward Relocation,Complete,S10: Forward Relocation,Complete Ack,S5: Modify Bearer,Response,Source eNB takes a decision to handover the UE to target eNB. I realizes that, it doesnt have a connectivity with the target eNB, so S1 based handover is chosen for this purpose.,Source eNB sends S1AP message Handover Required to source MME, indicated S1 based handover and the cause for the handover. This message includes MME UE S1 AP and eNB UE S1 AP ID. These IDs are used to identify the UE uniquely in a given eNB and MME.,Source MME sends GTP message Forward Relocation Request to target MME over S10 interface indicating S1 based handover. This message includes all the details that were sent when the UE was attached to the EUTRAN. This message also includes all the EPS bearer contexts that were established for UE.,As the same SGW is not serving both source and target MMEs, target MME sends a GTP message Create Session Request to target SGW. This message includes all the EPS bearer contexts established and also all the information that was send during initial attach. (QoS, APN, PDN type etc). Target SGW, if accepted, sends a Create Session Response to target MME indicating success.,Target MME then sends S1AP message Handover Request to target eNB. This message includes the E-RAB contexts that are to be established along with new MME UE S1AP ID. The handover type is set to S1 based handover. Target MME responds to this message Handover Request Ack. This ack includes the ERAB contexts admitted.,In the next step, target MME sends GTP message Forward Relocation Response to source MME indicating that handover request has been accepted. This response also includes all the EPS bearers that have been admitted.,Once the above response is received by source MME, it sends S1AP message Handover Command to source eNB asking it to handover the UE to target eNB. Source eNB forwards the same to UE. After this UE breaks from old cell and attaches to new cell. Target eNB informs that UE has attached to it by sending Handover Notify to target MME over S1 interface.,Target MME sends GTP message Modify Bearer Request to target SGW indicating the new eNB FTEID. Target accepts the same.,Source MME asks source eNB to delete all the UE contexts. Also it sends delete session request to source SGW asking it to delete all the EPS bearers f