call drop analysis

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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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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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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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Predefined report – Call drop rate on the cluster level

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Predefined report – 10 worst performing cells

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Predefined report – cells with high call drop rate

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GIS report – cells with high call drop rate

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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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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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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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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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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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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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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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Drop distribution – cluster level

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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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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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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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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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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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Predefined reporting – problem identification

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AnatomN training– Call Drop Analysis

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call drop analysis

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