Call Failure Analysis v3 0301
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Transcript of Call Failure Analysis v3 0301
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8/4/2019 Call Failure Analysis v3 0301
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17/04/2012
AIRCOM International
toOrange
Drive test Call Failure Analysis
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17/04/2012
Presentation Outline and Status
Pilot
Methodology
Net. IssuesServices
Key
Counters Alarms Drive DataNeighbours
Performance
RRC Access
SHO
ISHO
RRC Stp Access
CommonAlarms
CommonFailures
Analysis
CM Service
Test Nos
Script bugs
CC Disc
Power Spikes
HC Failures
6650 ISHO
RRC Active
New
Draft
Complete
Inter RNC HO
TCH/BCH
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Shows how to find network problems from failures observed during adrive test from the collected log files
Failures:
Call Setup failure Different phases being analysed
Dropped calls Main Reasons analysed
This Presentation
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Successful Call Attempt Layer 3 Messages
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Before Call Attempt - 1
Most important factor when a mobile is making an attempt is to ensurethat the signal received is adequate to make that attempt
There should be sufficient 3G coverage (Ec/Io RSCP) in order to make a
successful attempt (not a great Path Loss)
Mobile sends RRC Connection Request message while the reception ofRRC Connection Setup does not take place.
Two possibilities
Node B (cell) doesnt receive RRC Connection Request message
UE does not receive RRC Connection Setup message due toexcessive path loss
The problem here is genuine coverage so it is important to make sureclosest site is transmitting properly or alternatively propose a new one
A cell can only covera certain area
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Before Call Attempt - 2
Ensure that when the call attempt is initiated, it is made to the correctcell
Correct cell reselection in idle mode is crucial when the UE is moving
through the 3G network
Neighbours are important in idle mode as well so it is essential to verify thatthe correct relations have been made
Check previous cell on which UE has camped on and verify with the scannerdata which one the new selected cell should be
Camping to overshooting cells can cause problem
Cell availability must also be checked (with scanner measurements) toensure that closest site is up and running
If the closer cell is up and running thenneighbour relations must be checked (ADJSTable for Nokia RAN)
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Call Attempt Starts Mobile is sending RRC Connection Request message
If the cell/RNC doesnt respond with an RRC Connection Setupmessage then the mobile will wait for a specific amount of time, andmake the request again. The total number of requests and the wait timeis specified by the network.
This usually happens because:
RNC is not able to receive the message due to a possible faultysynchronisation on the Iub (DAC Word/10 Mhz)
Sleeping cell problem Service level counters can provide reasons for such failures (RRC ACC
Fail)
Cell can also respond with RRC Connection Reject Message whereusually the cause is congestion due to loading or high UL noise (AC)
Commissioning file check (MHA setting)
External interference Possible increase in Prx target Autotuning deactivated (hanging resource)
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Call Attempt Starts (examples)
RRC Connection Reject
No Response to RRCConnection Request
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RRC Active State
We are in this state when RRC Connection is Active
After that we are in the RAB setup phase
After MM CM Service request message it is possible to receive a MM CMService reject message this would be a Core Network problem
This is an example for a video call
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RRC Active State (2)
After security procedures RAB Setup should take place
When the hardware processing capacity of the BS is reached at a particular timethen NW will send a CC Disconnectwith reason no circuit/channel available.
This is the most common reason for CC Disconnect
Other reason can be failure to allocate channel on the called number (171,150)
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RRC Active State (2)
Following this and the security procedures is that Radio Bearer Setup:
Radio Bearer Setup message sent from the network takes mobile to RAB accessphase at which
UE might fail to receive or even fail to respond this can be either UE problemor coverage problem (usually this never takes place)
UE responds with Radio Bearer Setup complete which takes the call to RABActive phase
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RAB Active State
When RAB Setup is complete then the messages recorded can be thesame as the ones in the middle of a call until the point when CCAlerting message is received on the DL (sometimes immediately
Connect) this message makes the call setup attempt successful From the moment of the RRC Connection Request message until CC
Alerting message usual mobility management activities and messagessuch as Measurement Report and Active Set update can beexchanged between UE and network.
During this time there can be a drop in the radio link the same way as
it can happen in the middle of the call (dropped call) due to lowcoverage, missing neighbours, failed SHO etc
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Failures due to system/other reasons - 1
Call Setup Failures can also occur due to other reasons:
User not responding
In this case there is a DL CC Release message, happens whencalled mobile fails to return a page (user not responding)
Called mobile in 2G (video calls only)
In this case there is also a DL CC Release message with causeservice option not supported this happens when the called mobilehas made reselection to a 2G site
Resource problem
There can be also a CC Release message in the DL with thefollowing reasons:
Temporary failure/resource unavailable
These are usually Node B/RNC/MSC system problems
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Failures due to system/other reasons (examples)
No user responding
User busy
Called UE in 2G (video)
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Failures due to system/other reasons - 2
While being in the call setup phase there can also be an RRCConnection Release message but usually and if there isnt a higherlayer release here, the cause is unspecified
For these Call Setup Failures it is important to have the traces from theRNC (Iub?) as well because it is not an easy task to identify the reasonof this problem from the DT files
Site issues
It is important that there is a record of the sites that seem to have repeatedproblems. These sites usually have failures on every drive that takes placearound their area. In the drive test trace these can be random drops whilecall is during the setup procedure. It is required most of the times that sitevisit takes place in order to ensure that everything is in place. This happenswhen all other sources of information (please read next slide) do not indicatea problem
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Addition of Neighbour
AdequateRSCP?
AdequateEc/Io?
Stronger
cell fromScanner?
CSF
Physical antenna
reconfiguration, Addition ofnew site
RRC CxSetup
received?
RB Setup
received ?
Admission
Control?
Check failurecause (Not radio
related)
Call dropduring setup?
Dropped callprocess
Yes
Yes
No
No
No
Yes
No
Yes
No
No
Yes
Yes
Yes
No
CSF Analysis Summary
Check Node B (Configuration,Iub/10 MHz, DAC Word, CellResource counters, Alarms)
Issue TT, Furtherinvestigation
Check noiselevels,
Autotuning,
ChannelAvailability
RRC CxReject
received?
AdmissionControl?
No
Yes Yes
No
Check noiselevels, Autotuning
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Coverage Related Dropped Calls
Pilot RSCP
Node Bs cannot cover indefinitely
Excessive path loss results in poor Radio coverage levels
UE will lose synchronisation and will not be able to sustain the call
These issues are resolved by:
Antenna tilt/relocation (higher mast or closer to rooftop edge)
New Site
On the call trace these appear usually as either sharp transitions to idlemode or after a high number of measurement reports (requesting weakcells)
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Coverage Related Dropped Calls
Pilot Ec/Io
In order for an adequate connection to take place at least one radio linkneeds to have good Signal to Noise Ratio this is expressed by the Pilot
Ec/Io There are cases where a certain Pilot quality is not achieved due to the
nature of WCDMA technology
Overshooting sites (Pilot Pollution)
Uneven Node B deployment (Pilot dominance)
No clear dominant cell in the areadue to inter-site distance resulting
in poor Ec/Io
Below -16.0 (0)
>= -16.0 to < -14.0
>= -14.0 to < -12.0
>= -12.0 to < -10.0
>= -10.0 to < -8.0 (
>= -8.0 to < -6.0 (5
Above -6.0 (17651
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Handover related drop calls
Missing neighbours can result in a failed connection
This is the dominant cause of dropped calls in a well covered region
Missing neighbours are easy to identify from DT log files but scannermeasurements need to be collected.
In Ec/Io measurements from UE level appears to be decreasing verylow until dropped call takes place.
Immediately after dropped call Ec/Io becomes much stronger due to cellreselection
Dropped call occurshere for 3G mode UE
Missing neighbours
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Handover related drop calls (system)
It is possible for SHO to fail even when neighbour relations are present
This can happen due to:
Site issues (Channel elements, AC, Alarms, transmission) Iur problems (can be quite common)
UE measured Ec/Io is appearing the same as in the case of a missingneighbour
Dropped call occurshere
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Handover related drop calls (Dual mode)
When mobile is in dual mode it is possible to handover to 2G system
First mobile will enter compressed mode (Physical ChannelReconfiguration) so that it can measure the 2G cells
In order to find missing 2G neighbours then the Measurement Controlmessage, after entering compressed mode has to be examined for thecorrect BCCH ARFCNs
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Check inter-system issues,BCCH/TCH clash
Compressedmode?
Adequate2G
Coverage?
DC
Physical antennareconfiguration, Addition of
new site (2G)
No
YesNo
Yes
Dropped Call Analysis Summary
2G NeighbourList Correct?
Yes
No
Add missing neighbours
Add missing neighbours
Strongercell fromScanner?
Physical antennareconfiguration, Addition of
new site
Yes
No
Yes
Active SetAdequate
RSCP-Ec/Io?
Yes
No
Strongercell in
MonitoredSet?
No
Measurement Report
Sent?
No
Check UE
Site ondifferent
RNC?
Yes
Check Iur
No
Check Node B (Configuration,Iub/10 MHz, DAC Word, Cell
counters, Alarms)
Yes
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Information needed during Analysis
When performing call setup - or any kind of Drive test log fileanalysis the following information should be at the disposal of theengineer. It is apparent that this information must be updated as often
as possible to ensure analysis accuracy:1. Site Availability Status
2. RNC dump/RNC Browser Access
3. Counter information
4. Site configuration (MHA, WSP, ATM, DAC, etc)
5. Reference database (ex. sites and neighbours)6. Trouble ticket and maintenance Status
7. Integration plan
8. Drive test team report, comments and observations