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Use case Call Drop Analysis

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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 UC 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 Use Cases that better fit for the further investigations.

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Problem descriptions

High number of call drops

Call Drop typologies identification

Call Drops because of High Interference in DL

Call Drop because of High Interference in UL

Too high RTF Drops figures

Too high T_MSRFPCI Drops figures

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Drops per traffic Erlang overcomes a predefine standard threshold

All the drops due to all the causes are summed and a ratio to the number of used traffic channels is made

The standard threshold is defined as:2% drop per traffic Erlang

with a standard call holding time of 90 second.

Problem identification – High number of call drops

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Problem identification –Call Drop typologies identification

Call drop typologies identification simply provide a set of KPI for the classification of the different causes of dropping, useful for the further call drop analysis:

RLF_DROPS RTF_DROPS SER_DROPS T200_DROPS T3105_HOAF_DROPS TOO_HIGH_DIST_DROPS T_MSRFPCI_DROPS T8_INTERCELL_HO_DROP T10_INTRACELL_HO_DROPS

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Call Drop typologies identification

Release causes, timers and protocol layer relations

L3T8: 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. Therefore a HO Failure message is triggered by missed HOCMP by expiration of this timer.

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 T3105*NY (and T3124 for ms ) for detection of HO Access Failure (triggered by missed PHYS INFO)Connection Failure (HO Access Failure)

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)

SER Unexpected L2 Um message has been received

L1 Connection Failure (Radio Link Failure)RLF (S-counter expired)

Connection Failure Link (Remote Transcoder Failure) (TTRAU at call set-up,TSYNC at call on going, TSYNCDL and TSYNCUL for AMR)

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Successfull HO Call Flow

T200

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Problem identification – Call Drop because of High Interference in DL

High number of drops due to T200 timer expiration (over the 40% of the total call drop figures) could mean an high interference in DL (ASSCMD or HOCMD are not received at MS)

High number of drops because T200 and S-COUNTER (RLF) expirations whereT200 is more than 40% of the sum of the two causes

OR

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Problem identification – Call Drop because of High Interference in UL or excessive distance

Preconditions: TTRAU < T8 and TTRAU < T10 then RTF

drops are due to TSYNC (TSYNCDL,TSYNCUL) timer expiration.

T200 call drops are under a certain threshold (20% of the total drops)

If RTF overcome a defined threshold (30% of the total drops) or RLF and RTF drops overcome a defined threshold (45% of the total drops) a problem in UL (level or interference) is suspected

It is suggested to verify with other UC the state of the UL interference and the performance of the HO procedures in the suspected cells

All thresholds are configurable by operator

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Problem identification – Too High RTF Drops figures

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%)

With the given timers configuration such a drop cause points out a problem on the terrestrial interfaces

Past experience that in most of the cases such drops are due to HW/SW problems in the TRAU NE

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Problem identification – Too High T_MSRFPCI Drops figures

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 20%)

With the given timers configuration such a drop cause points out a possible bottleneck on the terrestrial resources.

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Predefined reporting

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Analysis in the GIS

COV_DL