call drop analysis
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Empfohlen wird auf dem Titel der Einsatz eines vollflächigen Hintergrundbildes (Format: 25,4 x 19,05 cm):• Bild auf Master platzieren (JPG, RGB, 144dpi)• Bild in den Hintergrund legen
Empfohlen wird auf dem Titel der Einsatz eines vollflächigen Hintergrundbildes (Format: 25,4 x 19,05 cm):• Bild auf Master platzieren (JPG, RGB, 144dpi)• Bild in den Hintergrund legen Company Confidential / © Siemens Networks 2007
AnatomN – Call Drop Analysis
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Mobile Network EngineeringMarch - 07
Objective
The Call Drop Rate is assumed in many contracts as one of the important criteria to verify a network during its acceptance phase.
The goal of call drop analysis is to identify cells suspected to have excessive call drops problems (cells that overcome a define call drops rate threshold during their busy hour time)
A second target is to identify potential problems analyzing the drop typologies mostly occurring in the cells of the network. This identification is useful to better define the problem that better fit for the further investigations.
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Mobile Network EngineeringMarch - 07
Problem descriptions
High number of call drops
Call Drop distribution
Call Drops because of High Interference in DL
Call Drop because of High Interference in UL
Drops due to TRAU problem
Drops due to A interface problem
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Mobile Network EngineeringMarch - 07
Drops per traffic Erlang overcomes a predefine standard threshold
All the drops due to all the causes are summed and divided by the traffic carried
The standard threshold is defined as:2% drop per traffic Erlang with a standard call holding time of 90 second:
1 Erl means one occupied trunk during one hour assuming MHT = 90 sec - > 1 Erl = 3600sec/90sec=40 calls max CDR = 0.02 [drops/ call] - > max Drops per Erlh = 40calls * CDR = 0.8
Problem identification – High number of call drops
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Mobile Network EngineeringMarch - 07
Predefined report – Call drop rate on the cluster level
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Mobile Network EngineeringMarch - 07
Predefined report – 10 worst performing cells
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Mobile Network EngineeringMarch - 07
Predefined report – cells with high call drop rate
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Mobile Network EngineeringMarch - 07
GIS report – cells with high call drop rate
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Mobile Network EngineeringMarch - 07
Call Drop distribution
4%
50%32%
8%6% DIST_DROPS
DMR_DROPS
HOAF_DROPS
T200_DROPS
MSRFPCI_DROPS
OTHER_DROPS
RLF_DROPS
RTF_DROPS
SER_DROPS
T8_DROPS
T10_DROPS
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Mobile Network EngineeringMarch - 07
Call Drop typologies identification
Release causes, timers and protocol layer relations
L3
T8: determines the time to receive the HO COMPLETE message. T8 is defined as the time that
BSC layer 3 will wait for a HO to complete before releasing the source channel.
T10: determines the time to return the ASSIGNMENT COMPLETE message in case of call setup
and intra-cell HO
T_MSRFPCI: (hardcoded at 20s) at call set-up and HO; expires if no HO CMP or ASS CMP (first L3
message from MS) is received
L2 T200*(N200+1) for LAPDM messages in ACK mode (triggered by missed ACK to SABM, Disconnect or I-
frames).
Error Indication (T200 Expired N200+1 times)
T3105*NY (and T3124 for MS ) for detection of HO Access Failure (triggered by missed SABM or PHYS
INFO)
Connection Failure (HO Access Failure)
SER Unexpected L2 Um message has been received
L1 Connection Failure (Radio Link
Failure)
RLF (S-counter expired)
Connection Failure (Remote Transcoder
Failure)
(TTRAU at call set-up,TSYNC at call on going,
TSYNCDL and TSYNCUL for AMR)
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Mobile Network EngineeringMarch - 07
MS BTSserving BSC BTStarget
CHANNEL ACTIVATION ACK
INTERCELL HCI (DL quality)
HANDOVER COMMAND
CHANNEL ACTIVATION
I(HANDOVER COMMAND)
I(HANDOVER COMPLETE)
FACCH
SABM
FACCHUA
FACCH
HO CMD embedded in ackn. layer 2
“I-frame“
HANDOVER ACCESS
FACCH UI(PHYS INFO)
FACCH
I(HANDOVER COMMAND)
T200 start
T200 expiry
ERR IND after N200 repetitions
T200 expiry
. . .I(HANDOVER COMMAND)
T200 start
T8 start
T200 start
Potential Drop Scenarios during Handover (Drops due to expiry of T200, HOAF, T8, T10)
HANDOVER COMPLETE
T8 stop
ESTABLISH INDICATION
T200 start
T200 expiry
== further transactions with failure
potential
HANDOVER DETECTION
FACCH
FACCH
FACCH
Start TTRAU
Stop TTRAUStart T3124
Stop T3124
Start T3105
Stop T3105
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Mobile Network EngineeringMarch - 07
Drops due to Radio Link Failure
PC_TIMEOUT = 2
RADIO_LINK_TIMEOUT = 8
Radio Link Failure
Radio Link Failure WarningPower Command
not decoded SACCH message
decoded SACCH message
SACCH Frames
RLF Counter S
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Mobile Network EngineeringMarch - 07
Drops due to Remote Transcoder Failure
After expiry of TTRAU
TTRAU is used by the BTS to supervise time-out of TRAU datalink (traffic) at connection establishment or handover.
After receipt of the CHNAV message the BTSE starts the timer TTRAU and starts sending uplink TRAU frames
towards the TRAU. When the BTSE receives the first downlink TRAU frame from the TRAU it stops TTRAU again.
If TTRAU expires, the BTSE reports a CONFL IND with cause RTF' to the BSC and the connection is released.
- TTRAU > T8 and TTRAU > T10
After expiry of TSYNC
TSYNC is used by the TRAU to supervise time-out of TRAU frame handling.
The TRAU starts this timer if 3 uplink TRAU frames have not been correctly received from the BTSE and it is reset if
a correct frame is received again (It is only used if a BTS-TRAU traffic connection is established).
If it expires, BTS sends a CONNECTION FAILURE INDICATION with cause 'Remote Transcoder Failure' to the
BSC.
In case of AMR TSYNCDL/TSYNCUL is used instead
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Mobile Network EngineeringMarch - 07
Drops due to other reason
Drops due to DIST
Call is dropped because the MS overcame the threshold of the max distance allowed in that cell set in the database.
Drops due to DMR
This kind of drops is due to unexpected sequences on L2 LAPD messages. In high interference cases a L2 message of interfering mobile is interpreted as
message of the victim MS and the call of the victim MS is dropped. L2 messages of different MS are mixed up (high interferences in the network with limited
number of RF for BCCH and no ciphering is active).
Drops due to SER
This kind of drops is due to unexpected sequences on L2 LAPD messages. In high interference cases a L2 message of interfering mobile is interpreted as
message of the victim MS and the call of the victim MS is dropped. L2 messages of different MS are mixed up (high interferences in the network with limited
number of RF for BCCH and no ciphering is active).
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Mobile Network EngineeringMarch - 07
Drops due to other reason
Drops due to MSRFPCI
timer MS radio Freq Power Capability Indication – hardcoded at 20s at call set-up and HO; expires if no HO CMP or ASS CMP (first L3 message from MS) is
received – should be an event that never appear – depends on the setting of T8 and T10; If timer T_MSRFPCI is bigger than T8 and T10 then drops due to
T_MSRFPCI timer expiration could indicate possible troubles on the signaling between BSC and target BTS or possible BSC congestion or A interface problem.
Drops due to Other
This drop occurs if the T8 expires in the BSC during an outgoing Inter-BSC handover procedure.
T8 is started when the BSC sends the HO CMD to the MS during an outgoing HO procedure. This timer waits for the receipt of a HO COMP (for outgoing Intra-BSC
HO), a CL CMD with cause 'handover successful' (for outgoing Inter-BSC HO) or (in the unsuccessful case) for the HO FAILURE message. If this timer expires, the
call has most probably dropped during the HO procedure.
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Mobile Network EngineeringMarch - 07
Drop distribution – cluster level
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Mobile Network EngineeringMarch - 07
Drop distribution – cell level
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Mobile Network EngineeringMarch - 07
Timers related to call drops
T200 format:
sdcchSAPI0 [5ms] – facchTCHF [5ms] – facchTCHH [5ms] - sacchTCHSAPI0[10ms] – sacchSDCCH[10ms] - sdcchSAPI3[5ms] -sacchTCHSAPI3 [10ms]
N200 - restricts the total number of repetitions of the messages controlled by T200 it is a fixed value for the BTS side specified by GSM which depends on the control channel types: N200(SDCCH)=23, N200(SACCH)=5, N200(FACCH/FR)=34, N200(FACCH/HR)=29
T3105 - MS10-10 means T3105 =10 [10ms]= 100ms
TTRAU [10ms] -> TTRAU = 1400*10ms = 14s S0 = 4 + 4 RDLNKTO
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Mobile Network EngineeringMarch - 07
Problem identification – Call Drop because of High Interference in DL
•Drops due to T200 >0.5
•Drops due to T200 and RLF > 0.6
•Drops due to T200 >0.3
All thresholds are configurable by operator
OR
AND
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Mobile Network EngineeringMarch - 07
Problem identification – Call Drop because of High Interference in UL (tgt cell)
•Drops due to T200 <0.25•HOAF > 0.3
•Drops due to T8, T10, HOAF and RLF > 0.6 •Drops due to T200 <0.25
All thresholds are configurable by operator
OR
AND
AND
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Mobile Network EngineeringMarch - 07
Problem identification – Drops due to TRAU problem
Preconditions:
TTRAU >T8 and TTRAU >T10 then RTF drops are due to TSYNC (TSYNCDL,TSYNCUL) timer
expiration.
Percentage of drops due to RTF cause is over a given threshold definable by operator (default
is 10%)
• many RTF for a specific BTS only - > the Abis resp. PCMB link is suspect and must be checked for wrongly
configured DXCs
• many RTF for the whole BSC the following checks have to be made:
- the Asub resp. PCMS link is suspect and must be checked for configured DXCs
- possibly some TRAC boards do not operate correctly. As the TRAC boards are responsible to generate the
downlink TRAU frames faulty TRAC boards could cause call drops in the worst case of malfunction.
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Mobile Network EngineeringMarch - 07
Problem identification – Drops due to problem on A interface
Preconditions: T_MSRFPCI >T8 and T_MSRFPCI>T10
In this case the expiration of this timer can happen only after the completion of all the procedure “on-air”.
Percentage of drops due to T_MSRFPCI cause is over a given threshold definable by operator (default provided is 10%)
With the given timers configuration such a drop cause points out a possible bottleneck on the terrestrial resources.
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Mobile Network EngineeringMarch - 07
Predefined reporting – problem identification
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Mobile Network EngineeringMarch - 07
AnatomN training– Call Drop Analysis
Thank You for Your AttentionThank You for Your Attention
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