LTE Access Issue Troubleshooting
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Content
 Poor Accessibility Reasons
 Checklist of Required Action for Access Problem
 Access Performance Evaluation
 Analysis of Tracing
 Case Study
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Page 2
Poor Accessibility Reasons
Common causes
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Network Planning and Optimization
Incorrect ZC sequence
Improper TAC/TAL planning
Weak coverage
Coverage overlap
External interference
Failure of resource admission on the air interface
CPU overload
CAPS overcapacity
Insufficient license capacity
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Poor Accessibility Reasons
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Parameter, RF channel, Interference and Transmission
Improper parameter setting
Use of non-optimal parameters
Improper license sharing policy
RF channel fault
GPS clock lost and ineffective subframe offset configuration(TDD)
Intermodulation interference (FDD)
Incorrect transmission IP address
SCTP link intermittence
Packet loss on or long delay of a transmission link
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Devices
UE’s are faulty
Bad access performance of the UE
ENodeB fault(Fault alarms, VSWR alarm, board alarm)
Sleeping cell
EPC fault(abnormal operations, message response timeout, NAS procedure exception)
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Content
 Poor Accessibility Reasons
 Checklist of Required Action for Access Problem
 Access Performance Evaluation
 Analysis of Tracing
 Case Study
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Page 5
Checklist Of Required Actions For Access Problems
Action
Prerequisite
Purpose
1.Determine whether the problem is a top cell problem or
network-wide problem.
Action 1: performing
scope identification, KPI
trend analysis, and cause
resolution
None
2.Analyze the major causes of access failures and come up with
priority actions.
3.Submit a sleeping cell problem to R&D for analysis.
Action 2: checking
operation logs, device
faults, alarms, and
external events
None. Pay attention to
this action in the
event of sudden
deterioration of KPIs.
1.Check operation logs to identify parameter modification and
operations that may cause problems. Then, find out why
parameters are modified and take measures to restore modified
parameters.
2.Check whether there are device faults that result in access
problems or whether access-related alarms are reported.
3.Check whether EPC upgrades or network cutover are
performed, or the transmission networking changes.
Action 3: checking
parameters
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1.Analyze whether the core access parameters are proper on or
consistent between the eNodeB and EPC.
None
2.Check whether parameter settings are consistent before and
after parameter mapping is finished in a network swapping
scenario.
Page 6
Checklist Of Required Actions For Access Problems
Action
Prerequisite
Purpose
Perform proper actions based on site conditions:
Action 4: checking version
differences and known issues
None
1.Check for version difference on the top sites.
2.Check for known issues.
1.Check whether the access problem is caused by improper configurations
of parameters such as TAC, TAL, and subframe configuration.
Action 5: checking network
planning and optimization
Perform this action based on the
analysis results provided by action
1.
2.Check whether an access problem is caused directly by resource
congestions or indirectly by improper parameter configurations. For details
how to analyze a congestion problem caused by the system CAPS overcapacity, see LTE performance-Basic Network Planning and Optimization.
3.Check whether an access failure is caused by weak coverage.
For top cell problems, perform
this action.
Action 6: checking RF channels
Perform this action for
interference check based on the
analysis results provided by action
1.
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1.Check whether RF channels are normal.
2.Check for uplink interference
Page 7
Checklist Of Required Actions For Access Problems
Action
Action 7: checking top UE types and
top users
Prerequisite
Purpose
For top-cell problems, check top users.
1.Check whether access-related KPI deterioration happens on individual user(s).
For network-wide problems, check top
UE types.
2.Check whether the access problem is caused by a certain type of UEs.
1.Check whether an access failure is caused by the EPC faults.
Action 8: checking the EPC
Perform this action based on the
analysis results provided by action 1.
Action 9: checking transmission
Perform this action based on cause
resolution results.
Action 10: repeating a complained
problem
Perform this action for analyzing a
complained problem.
For other problems, perform this action
when problems needs to be repeated.
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2.Check whether an access problem is caused by operations on the EPC (for example
upgrades and configuration changes).
Check whether the access failure is caused by transmission faults.
1.Provide the general principles for analyzing a complained problem.
2.Repeat the problems and collect the related logs.
Page 8
Content
 Poor Accessibility Reasons
 Checklist of Required Action for Access Problem

Access Performance Evaluation
 Analysis of Tracing
 Case Study
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Access Performance Evaluation
• From performance management
› Access successful ratio = RRC setup successful ratio x ERAB setup successful ratio
› Random access and NAS failures are not considered in eNodeB performance management,
but these factors also impact user experience
• From drive test tools (Nemo)
› It can identify access failure caused by random access failure and NAS failure
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Accessibility- RRC Setup Success Rate
KPI Name
RRC Setup Success Ratio
Measurement Scope
Network Level
Measurement Period
Network Busy Hour
Formula
L.RRC.ConnReq.Succ/L.RRC.ConnReq.Att*100%
Associated
Counters
A: L.RRC.ConnReq.Att
C: L.RRC.ConnReq.Succ
Unit
Percentage (%)
Target
99%
Remark
KPI 2.03
RRC Overall
RRC Setup
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Accessibility- RRC Setup Success Rate (Service)
KPI Name
E UTRAN RRC Setup Success Ratio
(Service)
Measurement Scope
Network Level
Measurement Period
Network Busy Hour
Unit
(L.RRC.ConnReq.Succ.Emc+L.RRC.ConnReq.Succ.Hig
hPri+L.RRC.ConnReq.Succ.Mt+L.RRC.ConnReq.Succ.
MoData+L.RRC.ConnReq.Att.DelayTol)/(L.RRC.ConnRe
q.Att.Emc+L.RRC.ConnReq.Att.HighPri+L.RRC.ConnRe
q.Att.Mt+L.RRC.ConnReq.Att.MoData+L.RRC.ConnReq.
Succ.DelayTol)*100%
A:
(L.RRC.ConnReq.Att.Emc+L.RRC.ConnReq.Att.HighPri
+L.RRC.ConnReq.Att.Mt+L.RRC.ConnReq.Att.MoData+
L.RRC.ConnReq.Att.DelayTol))
C:
(L.RRC.ConnReq.Succ.Emc+L.RRC.ConnReq.Succ.Hig
hPri+L.RRC.ConnReq.Succ.Mt+L.RRC.ConnReq.Succ.
MoData+L.RRC.ConnReq.Succ.DelayTol)
Percentage (%)
Target
Monitoring
Remark
KPI 2.031
RRC Service
Formula
Associated
Counters
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RRC Setup
Page 12
Accessibility- RRC Setup Success Rate (Signaling)
KPI Name
E UTRAN RRC Setup Success Ratio
(Signaling)
Measurement Scope
Network Level
Measurement Period
Network Busy Hour
Formula
L.RRC.ConnReq.Succ.MoSig/L.RRC.ConnReq.Att.M
oSig*100%
Associated
Counters
A: L.RRC.ConnReq.Att
C: L.RRC.ConnReq.Succ Mosig
Unit
Percentage (%)
Target
Monitoring
Remark
KPI 2.032
RRC Signaling
RRC Setup
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Accessibility- S1 SIG Setup Success Rate
KPI Name
S1 SIG Setup Success Rate
Measurement Scope
Network Level
Measurement Period
Network Busy Hour
Formula
L.S1Sig.ConnEst.Succ / L.S1Sig.ConnEst.Att *100%
Associated
Counters
A: L.S1Sig.ConnEst.Att
B: L.S1Sig.ConnEst.Succ
Unit
Percentage (%)
Target
98.5%
Remark
KPI 2.04
S1 Setup
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Accessibility- eRAB Setup Success Rate
KPI Name
eRAB Setup Success Rate
Measurement Scope
Network Level
Measurement Period
Network Busy Hour
Formula
L.E-RAB.SuccEst/L.E-RAB.AttEst *100%
Associated
Counters
A: L.E-RAB.AttEst
B: L.E-RAB.SuccEst
Unit
Percentage (%)
Target
99%
Remark
KPI 2.05
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Initial Context Setup
Page 15
eRAB Setup
Accessibility- eRAB Setup Success Rate (QCI=i)
KPI Name
eRAB Setup Success Rate
(QCI=i)
Measurement Scope Network Level
Measurement
Period
Network Busy Hour
Formula
L.E-RAB.SuccEst.QCI.i/L.ERAB.AttEst.QCI.i*100%
A: L.E-RAB.AttEst.QCI.i
B: L.E-RAB.SuccEst.QCI.i
(i=1,2,3,…,9)
Associated
Counters
Unit
Percentage (%)
Target
Monitoring
Remark
KPI 2.051
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Initial Context Setup
Page 16
eRAB Setup
General Process
• Determine the scale of problem
› From the performance statistic, we can determine the scale of the problem, is it global
problem, or cell level problem or just some individual UE problem. Then we can select
bottom N cell as optimization target
› Customer complain is an effective way to locate the individual UE problem
• For large scale access failure
› Check eNodeB alarm and basic configuration
› Check RF channel problem
» Once RF channel is abnormal, such as high VSWR, low RSSI Too Low or RSSI is
unbalanced, then the access performance should be significantly affected.
• For individual UE problem, signaling analysis can be used to locate the detailed cause
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Handle Enodeb Generic Alarm
• Tools: M2000 or WebLMT
• Procedure
› Check the generic hardware fault alarm, such as hardware fault, board unusable , or any
other alarms which are related with hardware failure or link failure.
» Solution: Replace the faulty component.
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Check DL Power Configuration
• To ensure the coverage, the DL power parameters should be properly set
› RS power = Maximum RRU power/channel – 10lg(total subcarriers ) + 10lg(Pb+1)
› PDSCH power = PA + RS power
» PA = -10lg(Pb+1)
• Related command:
› LST PDSCHCFG: check Pb and RS power setting
› LST CELLDLPCPDSCHPA: check PA setting
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Check CRPI Rate
• If the bandwidth is 10MHz or above, the CPRI rate should be equal or more than 2.5Gbps, otherwise,
the cell can’t be setup
• If the rate is not correct, change the CPRI module
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Check S1 Interface Status
• If SCTP link is faulty, check the SCTP
configuration, such as peer IP address, SCTP
port number
• If S1intrface state is faulty, check the PLMN
and operator ID.
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Check Cell Barred Status
• If cell is barred, use the following command to recover it
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Check RF Channel Problem
• Due to the bad feeder quality, incorrect feeder connection or strong interference, it
might affect RF channel performance which has impact on UE access.
• Relevant problem:
› High VSWR: ALM-26529 RF Unit VSWR Threshold Crossed
› Low RSSI: ALM-26521 RF Unit RX Channel RSSI Too Low
› RSSI unbalance: ALM-26522 RF Unit RX Channel RSSI Unbalanced
» RSSI of the main RX channel and the RSSI of the diversity RX channel exceeds 10 dB.
› High RSSI: Caused by interference
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Cause Of High VSWR
• VSWR: Voltage Standard Wave Ratio, indicate the if feeder impedance is match or
not. The normal VSWR range is 1~1.5. If current VSWR is more than a specified
threshold, then eNodeB will generate relevant alarm.
• Alarm generation:
› Cell is activated
› RF power is more than 34dBm/channel
› VSWR is more than a specified threshold
• Possible cause:
› Incorrect VSWR alarm threshold
› RF unit hardware fault
› RRU/RFU wrong connection
› Bad feeder quality or nonstandard feeder installation
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Solution For High VSWR
•
Step 1: Check if the VSWR alarm threshold is correct (default is 2dB).
›
Note: RRU TX channel will automatic closed if VSWR is extremely high (more than post- processing threshold)
Threshold for TX shutdown
Threshold VSWR alarm generation
•
•
Step 2: If alarm threshold is correct, then check if relevant feeder installation and RRU connections are meet the
standard.
Step3: After the feeder tuning, if TX channel is closed, then activate TX channel again
›
MOD TXBRANCH
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Cause Of Low RSSI
• Alarm generation: If RSSI is less than a specified value
• Incorrect RX attenuation setting
› If no TMA, the attenuation should be 0
› If 12dB TMA is used, the attenuation should be 4 to 11dB
› If 24dB TMA is used, the attenuation should be 11 to 22dB
• Feeder problem
› Due to bad feeder quality, causing additional loss
› RRU fault
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Solutions For Low RSSI
• Step 1: Check if RRU RX attenuation is correct
• Step2: If attenuation is correct, then check the feeder installation and
connection
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Cause Of Imbalance Of RSSI
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Alarm generation: RSSI of the main RX channel and the RSSI of the diversity RX
channel exceeds 10 dB.
• Possible causes
› High interference
› RRU cross connections
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Solution of Imbalance of RSSI
• UL interference check
› From web-LMT: Perform spectrum detection to evaluate UL interference
› From M2000 client: Perform interference detect monitoring
› Find out interference source
• Check RRU connections, avoid cross connections, show as below
A
N
T
1
A
N
T
2
RRU1
RRU2
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Classification Of Interference
• Passive inter-module interference:
› Due to non-linearity of radio component ,the high phase harmonic wave might be generated
and inter-modulated by RX signal, thus the new inter-module signal will arrive the receiver
causing the interference.
• External interference:
› Interference from the other system, such as 2G, 3G, CDMA, radio, radar eg.
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How To Located Inter-modulated Interference
• This kind of interference increases with eNodeB transmit power, so we can simulate the DL load,
and check RSSI, if it obviously increases with the DL load incensement, then we can determine the
interference
• Load simulation:
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How To Located External Interference
• If we exclude inter-modulated interference, then it should be external interference
› Lower the TX of RRU/RFU, if RSSI is still high, then we can conclude this is external
interference
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Analysis Of Radio Network Fault
• Possible cause
› UU message process failure due to UE capability (UE doesn’t support relevant service)
› eNodeB failure during ERAB setup
• Solutions:
› Check UE capability
› Check eNodeB status
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Analysis Of No Radio Resource
• Possible cause
› Radio admission failure for ERAB
• Solution:
› Check if admission parameters is correct
› Consider to enable load balance algorithm
› Consider capacity extension
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Analysis Of Security Mode Failure
• Possible cause:
› UE security capability is not identical with eNodeB security capability
• Solution:
› Modify the eNB security algorithm to match it.
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Analysis Of EPC Fault
• Cause：
› S1 message validity check failure, including
» Error exist in S1 message
» Duplicated ERAB ID
» Conflict with other procedures
• Solution:
› Contact EPC engineer to locate the problem
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Analysis Of Transport Fault
• Possible causes:
› Transport admission failure
› S1 link failure during S1 message exchange
› Incorrect IP path configuration
› Wrong QoS configuration in EPC
• Solutions:
› Check transport admission algorithm, consider extend S1 bandwidth
› Check S1 interface
› Check IP path configuration
› Check relevant QoS configuration, AMBR should be more than 0, and for initial ERAB setup,
the QCI must be 6~9
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Content
Poor Accessibility Reasons
Checklist of Required Action for Access Problem
Access Performance Evaluation
Analysis of Tracing
Case Study
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Analysis Of Tracing
• Driver test tracing analysis
› We can get all UU interface messages, internal UE message as well as RSRP value
• eNdoeB tracing analysis
› We can get UU interface message and S1 interface message
• Form the above tracing analysis, we can locate the problem with the following
causes:
› Random access failure
› RRC setup failure
› NAS procedure failure
› ERAB setup failure
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Random Access Failure
• Generic scenario: There is no “RRC connect request ”message
• Possible cause:
› UE doesn’t support the band
» Scenario: UE doesn't receive any system message
» Solution: Check if UE band is identical with eNdoeB configuration
› Poor coverage
» Scenario :Due to poor DL or UL coverage, UE or eNodeB miss receiving the relevant
random access message
» Solution: Perform RF tuning. Use TMA on uplink
› Cell is barred
» Check the cell status, should be not barred.
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RRC Connection Failure
eNodeB
UE
RRC_CONN_REQ
RRC_CONN_REJECT
• Scenario 1: eNodeB directly send reject message
› Possible cause: No radio resource due to no enough resource for PUCCH and SRS
› Solution: Check the cell load status, if it is very high, consider capacity extension
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RRC Connection Failure (Cont.)
eNodeB
UE
RRC_CONN_REQ
RRC_CONN_SETUP
RRC_CONN_REQ
• Scenario 2: UE doesn’t receive “RRC connection setup” message, and repeat sending
request message until timer is out
• Possible cause:
› Poor DL coverage, check RSRP and RSRQ
• Solution:
› Perform RF tuning
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RRC Connection Failure (Cont.)
eNodeB
UE
RRC_CONN_REQ
RRC_CONN_SETUP
Waiting “RRC
Compelet”time out
• Scenario 3: eNodeB send RRC setup message, but doesn’t get complete message
• Possible cause
› If UE doesn’t reply RRC complete, it should be UE problem
› If UE already send RRC complete, then it should be poor UL coverage
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NAS Procedure Failure
MME
eNodeB
UE
S1AP_INITIAL_UE_MSG
S1AP_DL_INFO_TRANSF
RRC_DL_INFO_TRANSF
RRC_UL_INFO_TRANS
S1AP_UL_INFO_TRANSF
S1AP_UE_CONTEXT_REL_CMD
S1AP_UE_CONTEXT_REL_CMP
• Note: NAS failure is not counted in performance statistic
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eRAB Setup Failure
eNodeB
MME
UE
Uu:RRC/NAS
RRC_SECUR_MODE_CMD
S1-MME:S1AP
S1AP_INITIAL_CONTEXT_SETUP_REQ
RRC_SECUR_MODE_CMP
RRC_UE_CAP_ENQUIRY
RRC_UE_CAP_INFO
RRC_UE_CAP_INDICATION
RRC_CONN_RECFG
RRC_CONN_RECFG_CMP
S1AP_INITIAL_CONTEXT_SETUP_RSP
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eRAB Setup Failure Cause
• If any failure occurs during ERAB setup, we can directly get the failure cause form
performance statistic, the relevant counters are:
› L.E-RAB.FailEst.NoReply (No response from UE)
› L.E-RAB.FailEst.MME (EPC fault)
› L.E-RAB.FailEst.TNL (Transport fault)
› L.E-RAB.FailEst.RNL (Radio network fault)
› L.E-RAB.FailEst.NoRadioRes (No radio resource)
› L.E-RAB.FailEst.SecurModeFail (Security mode setup fault)
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Faulty Signaling of No UE Response
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Fault Signaling of Other Cause
• Scenario: eNodeB receives “initial context setup request” or “E-RAB setup request”
message, after that, if failures happen, then eNB replies response message with
failure cause
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Content
Access Performance Evaluation
Poor Accessibility Reasons
Checklist of Required Action for Access Problem
Typical Cases Analysis
Case Study
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Case 1 – IP Path Configuration Leads To Low Access
Ratio
• Description: In one live network, ERAB setup ratio is very low, but it’s not zero.
• Alarm info: None
• Analysis:
› Check the performance statistic of ERAB release, we found that most of failures are
caused by transport resource not available
› From the message tracing, we can also confirm this result
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Case 1 –IP Path Configuration Leads To Low Access Ratio
(Cont.)
• Now we can locate that the problem is due to IP path configuration. We check eNodeB
configuration and find a valid IP path, the connection is valid. And it is also weird that this
problem occurs with a probability
• Then we analyze the message again, and find that in the “UE context setup request”
message delivers variable GTPU address, and these address is not identical with eNodeB
configuration.
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Case 1 –IP Path Configuration Leads To Low Access Ratio
(Cont.)
• Conclusion :As the SGW deliveries multiple GTUP address to eNodeB, but we only configure one
IP path to SGW, so it causes the partly ERAB setup failure
• Solutions: We confirm this with EPC engineer, SGW pool function is used, so there are multiple IP
addresses for SGW. Then we make up the IP path for all address, the problem is cleared.
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Case 2 – Multi-mode UE Attach Failure
S1 trace
UU trace
• Description: During the commissioning of one live network, we found that UE attach fails.
• Alarm info: None
• Message Tracing: From the message tracing, we can see that most of access procedures are
normal, after ERAB setup, MME release the connection with the cause “normal release”
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Case 2 – Multi-mode UE Attach Failure (Cont.)
• Analysis
• Since the most of procedures are normal, and eNodeB consider that it’s a normal release, so
we locate that the problem is due to NAS failure.
• So we continue to analyze the previous NAS message, and in the DL NAS message, we can
see that MME response the attach with the cause “MSC-temporatily-not-reachable”, as show
above. now we can confirm that this is root cause of attach failure
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Case 2 – Multi-mode UE Attach Failure (Cont.)
• Analysis (Cont.)
› Why should UE need attach to MSC ? We check the UE model, it ‘s Huawei E398, with
multi-modes(GSM/UMTS/LTE). So we assume that this UE performs combined attach.
From the attach request message we verify our assumption.
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Case 2 – Multi-mode UE Attach Failure (Cont.)
• Conclusion:
› We check that there is no CS domain configuration in current EPS network, so MME only replies PS attach
accept, and also inform that MSC can’t reachable
• Temporary solution:
› Change UE attach mode with PS only
› Add CS domain configuration in EPC
• Final solution:
› Update MME to compatible with combined attach even there is no CS domain
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Case 3: eRAB Setup Low due to RRC License Limited
• In Thailand True LTE Network, several Clusters cannot pass KPI E-RAB Setup Success Rate;
• Checked cell level KPI and found the failures, several sites have much high number of eRAB
Setup failures, which cause the KPI not pass;
• The major faiure is due to “No Resource”;
• Checked the number of RRC user, which reached the licensed number;
• After expanded license, the KPI became normal.
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Thank You
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