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5/21/2018 Advanced RANOP Chapter 4 - Drop Call Ratio
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Chapter 4: Drop Call Ratio
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Objectives
Objectives: This section explains the DCR8 Drop Call Rate formula and why it is better that the DCR 3G. It also
explains how the new DCR BREAKDOWN OBSERVATION can be used as an effective tool to
troubleshoot drop calls. There is also a brief description about AMR drops and RLT with AMR.
References: Michael Ledesma
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Drop Call Rate Formula
The common formula is DCR3G and it is composed of the following counters :tch_abis_fail_call
tch_abis_fail_old
tch_lapd_fail
tch_bts_fail
tch_a_if_fail_call
tch_a_if_fail_oldtch_act_fail_call
tch_tr_fail
tch_tr_fail_old
tch_user_act
tch_bcsu_reset
tch_netw_act
tch_radio_failtch_rf_old_ho
The new drop call counter 1202 counts customer perceived drops. These are TCHfailures that happens between ASSIGNMENT COMPLETE and DISCONNECT.
This new counter has a smaller supervision period than the DCR3G countersthus
lesser drop call counts but measures the exact user experience The succeeding slides shows the difference between DCR3G and the new formula
DCR8E and why is it better to use
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1202 (DCR8E) Counter Supervision Time
DCR3G
Start Point : Channel ActivationChannel Activation command from the BSC. Voice Callhas not yet been successfully established
End Point : RF Channel Release Ack
RF Channel Rel Ack received from the BTS
1202 (DCR8E)
Start Point : Assignment CompleteThis implies that the voice or data call has beensuccessfully established.
End Point : Disconnect
BSS and MS are aware of the disconnection. Callrelease continues.
SupervisionPeriod of DCR3G
Supervision Period
of DCR8E
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Advantages
The 1202 counter is much closer to customer experience when it
comes to drop calls
The counter has been verified and mapped together with the DCRBREAKDOWN OBSERVATION measurement (S11). This will giveyou the ability to drill down on the DCR investigation.
Better drop call details in S11.5 with the enhanced DCRBREAKDOWN OBSERVATION
This drop call counter was developed to match the drop call countersof other GSM vendors
The counter is already available in S10.5 and S11 as 57044 and 1202respectively
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Drop Call Breakdown Observation in S11
Detail Information per drop call
This observation can be activated in the BSC with noconstraints of overload.
It can be activated and available across all the network 24/7.
In S11, this observation provides:
TRX Information Phase In and Cause In Phase Out and Cause Out
The information above is already available and helps to bettertroubleshoot network incidents.
This observation currently is not integrated in OSS, therefore it isrequired to FTP the BSCto access this information.
Length : 25
Type : 29
Tracing Status : 0
Tracing Reference (hex) : 0000Header Version : 13
Compatibility Version : 1
BTS ID : 20
Release time : 2005-05-13
10:48:50,63
Phase out : 2
Cause out : 368
TRX ID : 2
Phase In : 3
Cause in : 43
Drop Call Observation Example
One drop call event, c1202 = 1
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Example of DCR Breakdown Observation
Length : 25
Type : 29
Tracing Status : 0
Tracing Reference (hex) : 0000
Header Version : 13
Compatibility Version : 1
BTS ID : 20
Release time : 2005-05-13 10:48:50,63
Phase out : 15
Cause out : 317
TRX ID : 2
Phase In : 3
Cause in : 43
In S11 you can have the time, Phase
out, Cause Out, Phase in, Cause inand TRX ID of the drop call
Using the Call Related DX Causes in
BSCdocument you can narrow down
the cause of the drop call
This gives you better information onfinding a solution to improve your drop
call rate.
Drop occurred during conversation
The Abis interface receives a CONNECTION FAILUREmessage and its cause is radio link failure
Counter 1013 from the traffic table is pegged
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S11.5 DCR Breakdown ObservationEnhancements
The purpose of Breakdown of counter 1202 is to get more accurate data on
dropped calls for BSC counter 'Drop After TCHAssignment. With the data providedby new observation, TCH drops happening after TCH is assigned can be analysed inTRX level with the actual reason for the drop. When the observation is activated inBSC, it will collect data from the whole BTSsunder BSC.
SUDDEN DROPADJ CELL PLMN ID 1
ADJ CELL LAC 1
ADJ CELL ID 1
RX LEVEL ADJ 1
.
.
.
ADJ CELL PLMN ID 6
ADJ CELL LAC 6ADJ CELL ID 6
RX LEVEL ADJ 6
BTS ID
RELEASE TIME
PHASE OUT
CAUSE OUT
PHASE IN
CAUSE IN
TIMING ADVANCE
DL RX LEVEL
UL RX LEVEL
DL RX QUALITY
UL RX QUALITY
DL FER
UL FER
DL LAST USED BIT RATE
UL LAST USED BIT RATE
Drop
call
DetailInformationper Drop
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5 Minute DCR BD OBS S11.5 Exercise
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Activity
Attached is a DCR BD OBS file in txt format
Using the Call Related DX Causes in BSCdocument. Try to figure outwhat caused the call to drop.
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AMR and Drop Calls
The introduction of the AMR codec gives us the MS the ability to carry
a conversation in C/I conditions where EFR cannot.
This improvement in the speech codices bring it closer to the robustperformance of the SACCH and at the worst conditions evenoutperform the SACCH.
This can result in more radio related drops as the customer can carrythe conversation but the signaling is lost (measurement reports)
It is imperative then that we consider how we use AMR and theparameterization that it requires to operate without sacrificing the user
perceived gains and still maintaining reasonable KPIs. More details about AMR will be discussed in the succeeding chapters.
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AMR and Radio Link Time Out
Recommendations:
EFR 20 SACCH periods
AMR FR 32 to 44 SACCH periods
AMR HR 20 to 32 SACCH periods
AMR HR is not as robust as AMR FR. It is generally used in good C/Iconditions. At RXQUAL 4, AMR HR is already giving poor voiceperformance that an extremely long RLT will artificially improve yourDCR as the subscriber will terminate the call due to bad voice quality.