Call drop improvement case analysis
Summary: In country X, Project Y, during the bad cell KPI analysis, we find so many cells have high call drop rate. Customer has given target to improve call drop. RNC level target was <1% for CS and Cell level call drop improvement target was <=1.5% CS and <=5% for PS.
Alarm: before analysis we must check current alarm and alarm history, in my case I don’t find any alarm.
RTWP: RTWP checked and I found few sites having high RTWP but these sites have high traffic, RTWP get poor during the high utilization only.
KPI analysis: - from the below KPI analysis, we can see that in my case mostly 3 counter is failed. 1- Call drop due to UL synchronization
2- Call drop due to Uu No reply 3- Call drop due to SRB reset
Solution: - Cell Level parameters
UL Syn. Solution: To solve this issue, we can change NINSYNCIND and NOUTSYNCIND parameter. Description is given
below:-MML Command to change the Parameter MOD CELL Script For Example
MOD UCELL: CELLID=XXXXX, NINSYNCIND=1, NOUTSYNCIND=100, TRLFAILURE=100;
NInsyncInd: Meaning of this parameter, no of times successive in-sync indications required for the Node-B to trigger the radio link recovery process. Radio link set remains in the initial state until it has received a certain number (the number is defined by NInsyncInd) of successive in-sync indications from L1, and then Node-B triggers the radio link recovery process, which indicates that the radio link set has been synchronized. Once the radio link recovery process is triggered, the radio link set is considered to be in the synchronized state. In the radio link maintenance process, NInsyncInd is used together with the successive out-of-sync indication (NOUTSYNCIND) counter.
Meaning of Parameter: Higher the value is, the stricter the synchronization process becomes, and the more difficult the synchronization occurs. The lower the value is, the easier the synchronization occurs. If the link quality is poor, a simple synchronization requirement leads to waste of the UE power and increase of uplink interference.
Default Value: 5 Optimized Value: 1
NOutsyncInd: When the radio link set is in the synchronized state, the Node-B starts the timer after it receives successive out-of-sync indications of the number defined by NOutsyncInd. The Node-B shall stop and reset the timer after receiving successive in-sync indications of the number defined by NInin-syncInd. When the timer expires, the Node-B triggers the radio link failure process, and indicates which radio link set is out-of-sync.
Meaning of Parameter: If the value is excessively high, the link out-of-sync decision is likely to happen. If the value is excessively low, out-of-sync is not likely to happen. But if the link quality is poor, it may result in a waste of the UE power and increased uplink interference.
Default Value: 50 Optimized Value: 100
TRlFailure: Radio link failure timer duration. When the radio link set is in the synchronized state, the Node-B starts the timer after it receives successive out-of-sync indications of the number defined by NOutsyncInd. The Node-B shall stop and reset the timer after receiving successive in-sync indications of the number defined by NInsyncInd. When the timer expires, the Node-B triggers the radio link failure process, and indicates which radio link set is out-of-sync.
Meaning of parameter: If the value is excessively low, there are few chances for the radio link to get synchronized. If the value is excessively high, the radio link failure process is probably delayed, and the downlink interference increases.
Default Value: 50 Optimized Value: 100
Uu No reply:- This counter failed due to poor coverage, to improve this counter we can increase Max DL Radio Link Power for CS & PS service depend upon requirement and condition of cell. Command to modify DL RL power is given below
For Example CS 12.2 call drop issue
MOD CELLRLPWR: CELLID=XXXXX, CNDOMAINID=CS_DOMAIN, MAXBITRATE=12200, RLMAXDLPWR=30, RLMINDLPWR=-120;
Note:-RL max DL power can’t be smaller than RL min DL power, if value of this parameter is too much high then downlink transmit power on a single radio link is too high that means downlink capacity will be affected. If value of this parameter is set to low value then coverage area of services is decreased that means call drop may increased.
In my network current value was RLMAXDLPWR=0, changed to RLMAXDLPWR=30, after change value, I see the improvement in the failure due to Uu Failed counter.
Solution: - RNC Level parameters
SRB reset: -During the call drop analysis you will find many call drop reason due to SRB reset or TRB reset. Mainly SRB reset happened due to poor coverage or Missing Neighbor.
1) Missing Neighbor: if Neighbor cell not configured, from the IOS trace, we can see that it has the problem of Neighbor cell not configured and drop call occurred easily.
From the IOS Trace, the uplink has been out of synchronization, and from the RRC_MEAS_RPRT, we can see that the downlink of cell XXXX is bad, and the UE has reported the event of 1A, but from the RNC configuration file, we can’t find the neighbor cell XXXX whose PSCRAMBCODE is XXX. Then we need to configure the missing neighbors.
2) Downlink bad for the cell XXXX, it has many times of SRB/TRB reset and UU No reply, we have collected the logs to analyze the reason. There are many call drop procedures as below: After the UE goes to FACH then call drop occurred.
We can see that the UE want to go to the DCH, but From theRRC_MEAS_RPRT we can find the EC/NO is bad now which means t hedownlink is bad, and the UE would not receive the message of RRC_RB_RECFG. Then the Call Drop occurred.
Mostly UL and DL interference create problem and call drop happened. If our RSCP is greater than ±85dBm and Active set EcIo is smaller than ±13 means call drop happened due to down link interference. DL interference may be due to Pilot pollution, when 3 cell’s meet the handover requirement in the coverage area, the active set replaces the best or the best cell change due to fluctuations of signals, when the comprehensive quality of active set is bad (EcIo change around ±10 dB), handover failure usually casus due the SRB reset or TRB reset. In case of handover delay the possibility RSCP might be good but EcIo might be weak but RSCP of EcIo cell in monitor set is good. So we need to make sure there is no pilot pollution in highly call drop cell area. For UL interference, we need to check RTWP if RTWP is 10dBm greater than normal value ±105.9dBm. UL interference increase UE transmit power in connected mode, there over high BLER cause SRB reset or TRB reset. UL interference either internal or external, most of senior UL interference you will find due to external source.
Solution: - To improve call drop due to SRB reset we can modify the parameter NoDiscardMaxDAT value D25 (default) to D40.Meaning of this parameter No-Discard PDU max TX, this parameter is applicable to only AM RLC entity, which is set to NO_DISCARD, means maximum number of PDU transmission before RLC entity is rest, if mode is set to NO_DISCARD, the RLC entity will be directly reset if the number of retransmission of PDU reach the value define by parameter. We can set this parameter at larger value if BLER of transport channel is high. To make this parameter value effective make sure parameter AMRlcDiscardMode is set to NO_DISCARD mode.
Procedure to modify the parameter: command used to change is parameter is
-MOD UTYPSRBRLC: SRBINDEX=1, LOCHINDEX=1, DELAYCLASS=1, TRCHTYPE=TRCH_DCH, OPPOSITETRCHTYPE=TRCH_DCH, RLCMODE=AM, AMRLCCFGPARA=EDIT_SENDER_PARA,
AMRLCDISCARDMODE=NO_DISCARD, NODISCARDMAXDAT=D40, TXWINDOWSIZE=D64, TIMERRST=D300, LASTTXPDUPOLL=TRUE, LASTRETXPDUPOLL=TRUE, TIMERPOLLPROHIBIT=NOTUSED, TIMERPOLL=D200, POLLPDU=NOTUSED, POLLSDU=D1, POLLWINDOW=D50, TIMERPOLLPERIODIC=NOTUSED,
CFGPOLLINGPARA=TRUE, POLLPDUFORENL2=D4, POLLSDUFORENL2=D4; Another method is Call reestablishment Method to improve call
drop:-Call reestablishment is function by which radio links are reestablished when service interruption or access failure occurs due to bad RF environment. Call reestablishment can be initiated by UE or RAN. In case of UE, UE send reestablishment request to network to restore service after detection a downlink RL failure or SRB reset. In case of RAN, RAN informs the UE to start reestablishment procedure. With the help of this feature, user experience will be rich, because RAB successes rate will be better and call drop will get improved. There are mainly 4 reasons for call reestablishment after service get interruption.
1- DL Radio Link failure 2- UL radio Link Failure 3- SRB/TRB reset 4- UE SRB reset.
As soon as RAN detects the service interruption or access failure, before release of Radio links, call reestablishment get started if this feature is activated.
1- DL Radio Link Failure:-
UE continue monitor DL RL quality, if UE receives N313 Number of OUT-SYNC indications, UE start T313 timer and if UE didn’t receive N315 number of IN-SYNC indications within timer T313 So UE will send Cell Update message with cause value “Radio Link Failure” to RNC and get inter in the call reestablishment procedure. Otherwise UE will not send Cell Update message to RNC.
Procedure:-Now UE will send Cell Update Message with cause value “Radio Link Failure” to RNC for call
reestablishment. Meanwhile UE will start timer T314 for CS service and T315 for PS service which is maximum allowed time for UE to try to send Cell Update Message. UE can resend Cell Update Message to network every T302 time and for maximum number N302 times if UE didn’t get reply from the Network. If Faulty Radio Links are restored before T314 (for CS)/T315 (for PS) timer gets expired, the call
reestablishment is successful, in this case services get restored or UE access succeeds. If Faulty Radio Links are not restored before T314/T315 timer get expired means service are released or UE access fails.
2- UL Radio Link
Failure:-Node B monitors RL link quality and receives
NOUTSYNCIND number of “out –sync” indications. Node-B start the TRIFailure timer if Node-B didn’t receive
NINSYNCIND “in-sync” indication within timer TRIFailure, Node-B stops and reset the timer then RNC activate the synchronization timer. If Node-B does not detect UL RL Synchronization before Synchronization timer expires then UL RL call reestablishment triggered.
Procedure: - RNC send Radio Link Activation command to Node-B with Information Element “deactivate”, Node-B stop downlink power transmission for UE’s Radio Link set. Now RNC start timer for call reestablishment RSVDPARA2 timer for CS service and RSVDPARA3 timer for PS service. If RNC receives Cell Update Message from UE before timer RSVDPARA2/3 get expired, then RNC stop the timer and start call reestablishment and IF RNC receives Cell Update message after timer get expired then call drop and UE access failure occur.
Implementation:- After Node stop downlink power transmission for a UE’s Radio links, UE will receive N313 number of Out-sync Indications, UE will start timer T313 and If didn’t receive N315 number IN-SYNC indication within the timer get expire then UE will transmit the Cell
Update Message with cause Value “Radio Link Failure” to RNC for call reestablishment. Meantime UE start T314/315 timer which is maximum allowed time for UE to send the Cell Update Message to RNC. UE can resend Cell Update Message to RNC if UE didn’t get the reply from Network (RNC) every T302 time for Maximum number for N302 times. If Faulty Radio links get restored before timer get expired T314/315 then Call reestablishment is successful otherwise service get released or UE access fails.
3- SRB rest: - as we Know in Huawei system, AMRLCDISCARDMODE
parameter is set to NO_DISCARD, RNC send a signaling message in AM( Ack Mode) to UE. Now RNC does not receive the response from UE before the number of SRB RLC PDU retransmission reaches the value of NODISCARDMAXDAT
parameter. RNC triggers an SRB reset and sends the reset PDU message to UE to reestablish RLC entities on both sides. SRB reset call reestablishment procedure and implementation is same as UL RL failure.
4- UE SRB
Reset:-RNC does not send Radio Link Activation command message containing the Information Element (IE) “deactivation” to the Node-B. Now Node-B does not stop downlink power transmission for the UE’s RL reset. Instead, RNC start call
reestablishment after receiving cell update message from UE. Process and implementation is same as UL RL failure.
Configuration for Call reestablishment:-Switch
parameters:-Call reestablishment is controlled by service switch, Process switch and cause switch parameters. First we have to enable service switch before enabling the process switch or cause switch.
Parameter Name Huawei default value
Recommende d value
Remark
Service switch RSVDBIT1_BIT24 Disabled Enabled Switch that controls call reestablishment for CS service (also for combine CS+PS service)
RSVDBIT1_BIT25 Disabled Enabled Switch that controls call reestablishment for PS service Cause switch RSVDBIT1_BIT22 Disabled Enabled Switch that controls call reestablishment triggered by SRB reset
RSVDBIT1_BIT23 Disabled Enabled Switch that controls call reestablishment triggered by RL failures.
Timer and counter
parameters:-Parameter name HUAWEI
default Value
Recommende d value
Remark
NOUTSYNCIND 50 8 Times of successive out-sync indications required for starting the radio link failure timer
NINSYNCIND 5 8 Times of successive in-sync indications required for the Node-B to trigger the radio link recovery process
TRIFailure 5 sec 3 sec Node-B starts the timer after it receives successive out-of-sync indications of the number defined by NOutsyncInd. The Node-B shall stop and reset the timer after receiving successive in-sync indications of the number defined by NInsyncInd
rvsdpara1:RSVDBIT1_BIT2 8
1 0 Switch that controls Synchronization Timer for CS reestablishment triggered, 1:RlRstrTmr,0:T313
RIRstr Tmr 11000msec 11000msec A timer to RNC wait for radio link restoration indication in the radio link procedure
Rvsdpara2 0msec 15000msec CS Timer for Enhanced Call Reestablishment, start after RNC send the RL Active CMD , stop after RNC receives cell update message
Rvsdpara3 0msec 30000msec PS Timer for Enhanced Call Reestablishment, , start after RNC send the RL Active CMD , stop after RNC receives cell update
message
N313 50 20 Maximum number of successive "out of sync" indications received from L1
N315 1 1 Maximum number of successive "in sync" indications received from L1 when T313 is activated
T313 3 sec 3 sec T313 is started after the UE detects consecutive N313 "out of sync“indications from L1. T313 is stopped after the UE detects consecutive N315 "in sync" indications from L1
T314 0sec 12sec T314 is started when the criteria for CS radio link failure are fulfilled and only radio bearers associated with T314 exist.T314 is stopped after the Cell Update procedure has been completed. (must be greater than T302*N302)
T315 0sec 30sec T315 is started when the criteria for PS radio link failure are fulfilled, and only radio bearer associated with T315 exists.T315 is stopped after the Cell Update procedure has been completed (must be greater than T302*N302)
N302 3 3 Maximum number of retransmissions of CELL UPDATE/URA
UPDATE
T302 2000msec 1400msec CELL UPDATE/URA UPDATE will be resent upon expiry of the timer if V302 less than or equal to N302
Note: -
1- If Call reestablishment feature already enabled in network then don’t change value NOUTSYNCIND, NINSYNCIND & TRIfailure value as I have suggested at Page 1 & 2 for cell level call drop improvement.
2- After Call reestablishment feature enabled in the network you will see more signaling traffic than before the call reestablishment feature enabled, reason behind is number of cell update message will get increase for the due to call reestablishment feature activation.
Suggestion:- I suggest before implementing call reestablishment feature in the network, first we have to make sure that access failure or Call drop is really happening due to RL failure or not. For this purpose need to analysis one or two days CHR/MR data through OMSTAR and you will get to know how many Drop call or access failure happening due to UL Radio Link Failure, if this value is much then suggest to Implement Call reestablishment feature otherwise No need.
Method of analysis call drop reason by
OMSTAR:-For call drop analysis through OMSATR tool we need 3 kinds of data. 1- Engineering parameter, 2- Configuration data, 3 –CHR/MR data. You have to put latest configuration data and CHR/MR data, you can take 1 day or 2 day CHR/MR data for call drop analysis.
After analysis of CHR logs, you will get result like
below:-
DropCall_DueTo_ASU_TimeOut:-For call drop improvement due to ASU (Active set Update) time out, we can change timer value as per given below-MML command Parameter name Default value Recommended Value Remark
SET USTATETIMER HoAsuTmr 5000 10000 Improve the CDR due
to Drop call_Due to_ASU_TimeOut Parameter Understanding: HOAsuTmr, HO active set update response timer, A timer to RNC wait for response to active set update in soft handover procedure. If in your network CDR Due to _ASU_TimeOut is more than you can increase value of this parameter as per suggested.
DropCall_DueTo_Ul_Radiolink_Failure:- for this case we can implement Call reestablishment feature to Improve drop call rate, complete study and procedure is given in the previous slides.
DropCall_DueTo_PHY_CH_RECFG(RLCC)_TimeOut, DropCall_DueTo_PHY_CH_RECFG(HHO)_TimeOut
:-To improve the call drop due to Physical channel reconfiguration timer out we can modify the parameter HoPhychRecfgTmr as per given below
MML command Parameter name Default value Recommended Value Remark SET USTATETIMER
HoPhychRecfgTmr 5000 10000 Improve the CDR due to
DropCall_DueTo_PHY_CH_RECFG(RLCC)_TimeOut
Parameter understanding: HoPhychRecfgTmr, HO Physical Channel reconfiguration timer, A timer to RNC wait for response to Physical channel reconfiguration in hard handover procedure. If you found DropCall_DueTo_PHY_CH_RECFG(RLCC)_TimeOut is high then you can increase this timer value from 5000 to 10000 and monitor the result if result is +ve then keep the value same otherwise revert it back.
DropCall_DueTo_RB_RECFG(DCCC)_TimeOut, DropCall_DueTo_RBrecfg(Periodic DRD)_TimeOut, DropCall_DueTo_RBREL
MML command Parameter name Default value Recommended Value Remark
SET USTATETIMER RbSetupRspTmr 5000 10000 Improve the CDR
DueTo_RBsetup_TimeOut
SET USTATETIMER RbRecfgRspTmr 5000 10000 Improve the CDR
DueTo_RBrecfg_TimeOut
SET USTATETIMER RbRelRspTmr 5000 10000 Improve the CDR
DueTo_RBREL_TimeOut Parameter understanding: 1- RbSetupRspTmr, Wait RB setup response timer, this is A timer to RNC wait for the RB setup response from UE in the RB procedure, 2- RbRecfgRspTmr, Wait RB reconfiguration response timer, this is A timer used to wait for the RB reconfiguration response from UE in the RB procedure, 3- RbRelRspTmr, Wait RB release response timer, this is A timer to RNC wait for the RB release response from UE in the RB procedure. If you found call drop due to these 3 reasons in your network you can change parameter value from 5000 to 10000, hopefully call drop will get improve.
DropCall_Dueto_MeasCtrl_No_Reply:- drop call due to measurement control_No_reply is under the case of poor coverage. UE sends measurement to RNC but didn’t get reply from the RNC. This case I have already discussed in the previous slides, we can change the Max RL DL power to improve the drop call.
Some other parameter or switch’s to improve the drop call rate.
CfgSwitch:-Channel Configuration Strategy Switch, CFG_DL_BLIND_DETECTION_SWITCH, when the switch is on, the DL blind transport format detection function is used for single SRB and AMR+SRB bearers. Note that the UE is only required to support the blind transport format stipulated. But you may find some UE which will not support DL BLIND detection. But still we can make switch enable to improve the drop call.
MML command Parameter name Default
value Recommend ed Value Remark SET UCORRMALGOSWITCH CFG_DL_BLIND_DETECTIO N_SWITCH
ON Some UE may not support DL blind detection
To improve these 3 counter we can modify the RB setup and reconfiguration timers, as per detail given below
Some parameter for PS call drop:-
Need to check below PS inactive timers and optimize as per the suggested value
MML command Parameter name Default value Recommended Value Remark SET UPSINACTTIMER PSINACTTMRFORCO
N
20 15
SET UPSINACTTIMER PSINACTTMRFORSTR 20 15 SET UPSINACTTIMER PSINACTTMRFORINT 20 15 SET UPSINACTTIMER PSINACTTMRFORBAC 20 15 Parameter
Understanding:-1- PsInactTmrForCon:- When detecting that the Ps' Conversational User had no data to transfer for a long time which longer than this timer, the PDCP layer would request the RRC layer to release this RAB. If this parameter value set to very small value then RNC will release the UE RAB that stop processing service this will saves network resources. But this will lead to frequent RAB release and RAB assignment request, it will result as number of signaling message increase, CPU loading increase. If parameter value set to larger value, RNC will hold Radio Links for that UE till timer get expire. This will lead to waste network resources.
2- PsInactTmrForStr:- When detecting that the Ps' Streaming User had no data to transfer for a long time which longer than this timer, the PDCP layer would request the RRC layer to release this Radio Access Bear. If this parameter value set to very small value then RNC will release the UE RAB that stop processing service this will saves network resources. But this will lead to frequent RAB release and RAB assignment request, it will result as number of signaling message increase, CPU loading increase. If parameter value set to larger value, RNC will hold Radio Links for that UE till timer get expire. This will lead to waste network resources.
3- PsInactTmrForInt:- When detecting that the Ps' Interactive User had no data to transfer for a long time which longer than this timer, the PDCP layer would request the RRC layer to release this Radio Access Bear. . If this parameter value set to very small value then RNC will release the UE RAB that stop processing service this will saves network resources. But this will lead to frequent RAB release and RAB assignment request, it will result as number of signaling message increase, CPU loading increase. If parameter value set to larger value, RNC will hold Radio Links for that UE till timer get expire. This will lead to waste network resources.
4- PsInactTmrForBac:- When detecting that the Ps' Background User had no data to transfer for a long time which longer
than this timer, the PDCP layer would request the RRC layer to release this Radio Access Bear. If this parameter value set to very small value then RNC will release the UE RAB that stop processing service this will saves network resources. But this will lead to frequent RAB release and RAB assignment request, it will result as number of signaling message increase, CPU loading increase. If parameter value set to larger value, RNC will hold Radio Links for that UE till timer get expire. This will lead to waste network resources.
Suggestion:- I have tried to describe the all the case which I faced during my project work. I hope this document will help you to solve the drop call issue. I suggest , before change the parameter , make proper Change request and take approval from Technical director , supervisor , customer etc. also make sure parameter change doesn’t no impact at live network if have impact at live network then change only in night time during wee hours only.
