Handover 流程

日志

切换

UE从一个eNodeB的覆盖区向另一个eNodeB的覆盖区移动时, 源信号会逐渐减弱, 而目标信号逐渐增强. 当UE处于连接状态时, 当源信号弱到一定强度, 而目标信号逐渐增强到一定程度, 网络 可以在目标网络为用户预先建立承载资源. 这样, UE在源侧连接释放后, 可以很快在目标侧建立连接, 减少在目标侧申请资源等待时间. 这种目标预先准备资源的过程就是所说的切换.

• 切换是由网络侧的 eNodeB 发起的, 只有终端才知道自己当前所在位置各小区信号的好坏
• 终端测量当前小区及邻区的信号强度, 上报给eNodeB
• eNodeB根据终端上报的当前小区, 可供选择的小区的信号强度决定是否发起切换, 向那个小区切换
• eNodeB在RRC连接建立的同时下发测量控制信息(强和弱的上报门限)

X2/S1切换区别
1. 两种切换除切换控制消息路径不同外, UE切换到目标eNodeB后, 源eNodeB侧剩余未转发成功的用户数据面转发通道也不同
2. LTE网络切换过程保持PGW不变, eNodeB改变, MME和SGW可能改变

基于X2接口的切换

X2切换的条件

• eNodeB之间存在 X2接口 (eNodeB配置, 或通过MME/OMC学习)
• 切换过程 MME无需改变 (目标eNodeB和源eNodeB在一个MME下)

X2切换类型

• SGW无改变
• SGW改变

切换前 ECM-CONNECTED状态下 用户面承载 包括:

• UE到源eNodeB的无线承载
• 源eNodeB到源SGW的S1-U承载
• 源SGW到PGW的S5/S8承载

X2-based handover without Serving GW relocation

接入网路径切换

• HO Decision(终端测量当前小区及邻区的信号强度, 上报给eNodeB, eNodeB根据信号强度决定发起切换)
• S-eNodeB->T-eNodeB( Handover Request消息 ), 信息包括:
  
  • S-eNB创建X2转发通道的本端TEID(S-eNB->T-eNB的X2下行隧道)
  • 源SGW的用户面GTP地址和TEID( T-eNB->S-SGW的S1-U上行隧道)
• T-ECGI(告诉要接入哪一个小区)
  
  • 建立无线承载必须的信元(UE-AMBR, 安全能力, KeNB)
• T-eNB->S-eNB(Handover Ack)
  
  • 创建无线承载相关ID(为创建无线承载准备)
  • T-eNB创建X2转发通道的本端TEID(S-eNB->T-eNB的X2上行隧道, X2接口建立完毕)
• 源eNodeB->UE(Handover Command), UE执行切换
  
  • UE通过同步, 信道建立过程接入T-eNB(创建无线承载)
  • UE remove 那些没有到目标cell无线承载的EPS bearer
  • 终端切换完后, downlink data->S-eNodeB->X2->T-eNodeB,

核心网侧路径切换

• T-eNodeB->MME(path Switch Request), 发起核心网侧路径切换
  
  • list of EPS bears to switch(需要切换的承载列表)
  • T-eNodeB的GTP地址和TEID
  • 目标TAI信息(用于构建TAC FADN, 到DNS解析SGW列表, 根据PGW FQDN, "合一优选, 拓扑优先"选择SGW)
  • ECGI(目标小区)
  • CSG Membership status
  • [CSG ID](如果target是CSG cell)[CSG access mode='hybird'][CSG Membership Status IE]
• MME->SGW(Modify bearer request/Modify Access Bearers Request)
  
  • MME根据list of EPS bearer to be switched决定触发没有被T-eNodeB接受的承载的bearer release procedure
  • 如果PDN连接的默认承载没有被接受并且有多个激活的PDN连接, 则触发MME request PDN disconnection procedure
  • 为 接受的承载 传递T-eNodeB的GTP地址和TEID, 每一个默认承载被接受的PDN连接都会有一个消息(S1-U下行隧道, 至此S1-U下行通道建立完成);
  • User location Information IE(if change)
  • UE Time Zone IE(if change)
  • Serving Network IE(if change)
  • CSG Information IE(if change)
• S-GW->PGW(Modify Bearer Request)
  
  • 以上提到关于charging的相关信息
• PGW->SGW(Modify Bearer Response)
• SGW->MME(Create Session Response)
• MME->T-eNodeB(Path Switch Request Ack)
  
  • UE-AMBR
  • verified CSG membership status
  • indicate which bearers failed to be established, 释放失败的EPS bearers的核心网资源(bearer release procedure)
  • 如果没有一个默认承载切换成功, 则->T-eNodeB(Path Switch Request Failure message), 并且显示的 detach UE
  • T-eNodeB收到消息后删除对应的bearer上下文
• T-eNodeB通知S-eNodeB切换成功, S-eNodeB触发资源释放
• 进行位置更新(新的TAU是否在TAL中)

X2-based handover with Serving GW relocation

接入网路径切换

• HO Decision(终端测量当前小区及邻区的信号强度, 上报给eNodeB, eNodeB根据信号强度决定发起切换)
• source eNodeB->target eNodeB( Handover Request消息 ), 信息包括:
  
  • S-eNB创建X2转发通道的本端TEID(S-eNB->T-eNB的X2下行隧道)
  • 源SGW的用户面GTP地址和TEID( T-eNB->S-SGW的S1-U上行隧道, 由于SGW发生了改变, 所以此地址无用)
  • ECGI(告诉要接入哪一个小区)
  • 建立无线承载必须的信元(UE-AMBR, 安全能力, KeNB)
• T-eNB->S-eNB(Handover Ack)
  
  • 创建无线承载相关ID(为创建无线承载准备)
  • T-eNB创建X2转发通道的本端TEID(S-eNB->T-eNB的X2上行隧道, X2接口建立完毕)
• 源eNodeB->UE(Handover Command)
  
  • UE通过同步, 信道建立过程接入T-eNB
  • UE remove 那些没有到目标cell无线承载的EPS bearer
  • 终端切换完后, downlink data->S-eNodeB->X2->T-eNodeB

核心网侧路径切换

• (1)T-eNodeB->MME(path Switch Request), 发起核心网侧路径切换
  
  • list of EPS bears to switch(需要切换的承载列表)
  • T-eNodeB的S1-U的GTP地址和TEID
  • 目标TAI信息(用于构建TAC FADN, 到DNS解析SGW列表, 根据PGW FQDN, "合一优选, 拓扑优先"选择SGW)
  • ECGI(目标小区)
  • [CSG ID](如果target是CSG cell)[CSG access mode='hybird'][CSG Membership Status IE]
• (2)MME->SGW(create session request), 传递T-eNodeB的GTP地址和TEID(S1-U下行隧道)
  
  • MME根据list of EPS bearer to be switched决定触发没有被T-eNodeB接受的承载的bearer release procedure
  • 如果PDN连接的默认承载没有被接受并且有多个激活的PDN连接, 则触发MME request PDN disconnection procedure
  • 为 接受的承载 传递T-eNodeB的GTP地址和TEID, 每一个默认承载被接受的PDN连接都会有一个消息(S1-U下行隧道, 至此S1-U下行通道建立完成);
  • User location Information IE(if change)
  • UE Time Zone IE(if change)
  • Serving Network IE(if change)
  • CSG Information IE(if change)
• (3)目标S-GW->PGW(Modify Bearer Request), 目标SGW的GTP地址和TEID(下行S5接口隧道)
  
  • 包括以上有关charging的内容
• (3b)PGW->目标SGW(Modify Bearer Response), PGW的GTP地址和TEID(上行S5接口隧道)
• (4)目标SGW->MME(Create Session Response), 传递SGW的GTP地址和TEID
• (5)MME->T-eNodeB(Path Switch Request Ack)
  
  • SGW的GTP地址和TEID(S1-U上行隧道) #
  • UE-AMBR
  • verified CSG membership status
  • indicate which bearers failed to be established, 释放失败的EPS bearers的核心网资源(bearer release procedure)
  • 如果没有一个默认承载切换成功, 则->T-eNodeB(Path Switch Request Failure message), 并且显示的 detach UE
  • T-eNodeB收到消息后删除对应的bearer上下文
• (6)T-eNodeB->S-eNodeB, 删除源eNodeB资源(Release resource)
• (7)MME->S-SGW, 删除源SGW Session资源(Delete session request)
  
  • cause
  • Operation Indication, 这个flag is not set, 表示SGW不会向PGW发起delete procedure
• (8)进行位置更新(新的TAU是否在TAL中) #

X2接口切换抓包验证

试验目的：将一个UE从基站A移动到基站B, 查看切换过程的码流

切换流程

• 启动eNodeB网元

# 本文使用aricent公司的SCTP协议, 配置两个基站和10个UE资源
sudo ./sim_enodeb -H 3.3.0.1 -r 7.7.7.7 -a 1 -i br1 -n 2 -u 5

• 启动两个基站

# 命令帮助:
start <enb id 1> <enb id 2>:      start enb from id1 to id2
start <enb id> :                  close and restart the sctp connection, send
                                  the s1-setup to MME

start 0 1

• 启动一个UE进行验证

# 命令帮助:
r U:                              start ALL UEs for all the registered enodeBs
r U <enb_id>:                     start ALL UEs within the enodeB
r U <enb id 1> <enb id 2>:        start ALL UEs from enb_id1 to enb_id2
r u <enb id>:                     start one UE within the enodeB
r u <enb id 1> <enb id 2>:        start one UE from enb_id1 to enb_id2

r u 0

• 将UE[0]从eNodeB Id 0移动到eNodeB Id 1

# 只有X2切换才会发送, PathSwitch Request, 一下命令用于X2接口切换
# p s(path switch)
p s <enb id1> <enb id2>:          move one active-ue from enodeb id1 to enodeb
                                  id2,  send  path switch request  to mme
p r <enb id1> <rmt ip> <enb id2>: move one active-ue from enodeb id1 to REMOTE
                                  enodeb id2, send path switch request to mme

# 移动一个UE
p s 0 1

UE从eNodeB Id 0 移动到了eNodeB Id 1

错误提示:
旧的ENB为确保清理干净，多删了一次UE， 重复删除时就会有这个提示

基于S1接口的切换

• 当X2切换不可用是执行S1切换
• MME不应该重定位除非UE离开MME POOL的服务区
• MME根据SGW FQDN判断是否重定位SGW
• S-eNodeB决定哪些EPS承载需要将下行或者上行(可选)数据从S-eNodeB转发到T-eNodeB
  
  • 可以是eNodeB的X2接口直接转发(由S-eNodeB决定并且(indiation)->MME)
  • 也可以是S-eNodeB->S/T-SGW->T-eNodeB的间接转发(S-eNodeB->(indication)->S-MME->(indiation)->T-MME)
• 如果MME从eNodeB收到一个S1接口流程拒绝, 指示一个S1 handover正在进行, 则
  
  • 如果MME没有改变, 继续S1接口相关流程
  • 如果MME改变, 终止S1接口相关流程

• (1)S-eNodeB决定发起S1-base handover
  
  • 与T-eNodeB无X2连接
  • X2切换失败
  • 根据S-eNodeB学到的dynamic information决定
• (2)S-eNodeB->S-MME(HO Required, 不是Request)
  
  • target TAI, 用于选择T-MME
  • target eNodeB
  • Direct Forwarding Path Availability, 直接转发(X2)是否可用
  • S-eNodeB指示哪些Bearer需要进行数据转发(list of Bearers)
  • 其他(CSG ID/CSG access mode/S1AP cause/Source to Target transport container)
• (3)S-MME->T-MME(Forward Relocation Request)
  
  • MME UE context(IMSI/ME Identity/UE security context/UE network capability/AMBR/selected CN operator ID/ANN restriction/SGW address and TEID for control signalling/EPS Bearer context(PGW address and TEIDs/APN/SGW address and TEIDs/TI))
  • source to target transparent container, RAN Cause, target enodeB identity, CSG ID, CSG membership indication, target TAI, MS info change reporting action, CSG infomation reporting action, UE time zone, Direct forwarding Flag(指示是否X2转发), Serving Network, local Home Network ID
• (4)T-MME->T-SGW(create session request)
  
  • bearer contexts with PGW address and TEID(T-SGW->PGW的S5-U的上行通道)
  • Serving Network/UE Time Zone
• (4a)T-SGW->T-MME(create session response)
  
  • T-SGW addresses and TEIDs per bearer
• (5)T-MME->T-eNodeB(Handover Request)
  
  • EPS bearers to setup(帮助T-eNodeB建立承载)/AMBR/S1 AP Cause/source to target transport container/CSG ID/CSG Membership Indication/Handover Restriction List
  • 在T-eNodeB创建UE上下文(bearer information(SGW address and TEID/EPS Bearer QoS)/security context/Data forwarding not possible indication(指示还没有间接转发通道))T-eNodeB->T-SGW的S1-U上行通道
  • UE-AMBR(if change)
• (5a)T-eNodeB->T-MME(Handover request Acknowledge)
  
  • EPC Bearers setup list(建立了哪些承载, T-eNodeB addresses and TEIDs for downlink trffic on S1-U reference point接受下行数据/T-eNodeB addresses and TEIDs for receing forwarded data用于间接转发数据)
  • EPC Bearers failed to setup list(哪些承载建立失败)
  • target to souce transparent container
• (6)T-MME->T-SGW(Create Indirect Data Forwarding Tunnel Request)用于建立间接转发参数
  
  • Target eNodeB addresses and TEIDS(for forwarding), T-SGW->T-eNodeB的forwarding单向通道
• (6a)T-SGW->T-MME(Create Indirect Data Forwarding Tunnel Response)
  
  • T-SGW addresses and TEIDs(for forwarding)
• (7)T-MME->S-MME(Forwarding Relocation Reponse)
  
  • cause/target to source transparent container/SGW change indication(指示SGW是否发生改变)/EPS Bearer setup list
  • T-SGW addresses and TEIDs(for forwarding)
• (8)S-MME->S-SGW(Create Indirect Data Forwarding Tunnel Request)
  
  • T-SGW addresses and TEIDs(for forwarding), S-SGW->T-SGW的forwarding单向通道
• (8a)S-SGW->S-MME(Create Indirect Data Forwarding Tunnel Response)
  
  • S-SGW addresses and TEIDs(for forwarding)
• (9)S-MME->S-eNodeB(Handover Command)
  
  • target to source transparent container
  • Bearers subject to forwarding(S-SGW addresses and TEIDs for forwarding), S-eNodeb->S-SGW的单向forwarding通道
  • bearers to release(指示哪些承载需要释放)
• (9a)S-eNodeB->UE(Handover Command)
  
  • Targert to source transparent container
  • UE删除在T-eNodeB上无相应radio bearer的EPS Bearer
• (10)S-eNodeB->T-eNodeB(eNB Status Transfer), via MME
  
  • 需要保留使用的PDCP和HFN status
• data forwarding
  
  • S-eNodeB->T-eNodeB
  • S-eNodeB->S-MME->T-MME->T-eNodeB
• (12)T-eNodeB->UE(Handover Confirm)
  
  • downlink data: T-eNodeB->UE
  • UPlink data: UE->T-eNodeB->T-SGW->PGW(上面已经建立)
• (13)T-eNodeB->T-MME(Handover Notify)
  
  • TAI/ECGI/Local Home Network ID
• (14)T-MME->S-MME(Forwarding Relocation Complete Notification)
  
  • S-MME设置一个timer, 监视UE和SGW上的资源
  • S-MME删除UE不包括在step3中的EPS bearers, 就是删除不需要切换的承载
• (14b)S-MME->T-MME(Forwarding Relocation Complete Acknowledge)
  
  • T-MME设置定时器监视indirect forwarding分配的资源
• (15)T-MME->T-SGW(Modify Bearer Request)
  
  • eNodeB address and TEID(T-SGW->T-eNodeB的S1-U单向通道, 对应(5))
  • PDN connection that need to be released
  • 其他(user location information IE/CSG information IE/UE time zone IE/Serving Network IE)
  • MME释放non-accepted dedicated bearers
  • 如果default bearer of a PDN connection不被接受, 出发MME requested PDN disconnection
• (16)T-SGW->PGW(Modify Bearer Request)
  
  • T-SGW addresses and TEIDs for S5-U downlink traffic(PGW->T-SGW的S5-U下行通道, 对应(4))
  • Serving Network/PDN Charging Pause Support Indication
  • User Location information IE/UE Time Zone IE/User CSG Information IE/Serving Network IE
• (16a)PGW->T-SGW(Modify Bearer Response)
  
  • Charging Id, MSISDN, PDN Charging Pause Enabled Indication (if PDN GW has chosen to enable the function)
• (17)T-SGW->T-MME(Modify Bearer Response)
  
  • Serving GW address and TEID for uplink traffic
• (18)TAU procedure
• (19a)UE Context Release Command, timer expires
  
  • S-eNodeB释放与UE相关的resource
• (19b)UE Context Release Complete
• (19c)Delete Session Request, timer expires
  
  • delete EPS bearer 资源
• (19d)Delete Session Response
• (20a)Delete Indirect Data Forwarding Tunnel Request
  
  • release the temporary resources used for indirect forwarding that were allocated at step 8
• (20b)Delete Indirect Data Forwarding Tunnel Response
• (21a)Delete Indirect Data Forwarding Tunnel Request
  
  • release temporary resources used for indirect forwarding that were allocated at step 6
• (21b)Delete Indirect Data Forwarding Tunnel Response