Call Drop 3G

7/30/2019 Call Drop 3G

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RANT Dropped and Blocked Calls Classification Summary

([Only registered and activated users can see links and Download

Documents ][Only registered and activated users can see links and Download

Documents ] MTN South Africa)

1 Introduction 2

2 Dropped Calls Classification 32.1 Missing Neighbours 3

2.2 Poor Coverage 42.3 Bad Radio Environment 5

2.4 Congestion 6

2.5 Not Radio 72.6 Equipment Fault 8

3 Blocked Calls Classification 9

3.1 Missing Neighbour 103.2 Poor RF (Bad Coverage and Radio Environment) 113.3 Equipment Fault (UE) 12

3.3.1 Security and Authentication Mode Failure 12

3.3.2 UE Freeze 13

3.4 Not Radio 143.4.1 Disconnect on RAB Setup 14

3.4.2 Resources Unavailable (Congestion) 153.4.3 UE Sensitivity Fault 163.5 Abnormal blocked calls 17

3.5.1 Unclassified - Unanswered RRC requests 17

3.5.2 Faulty Block Recording - Barred [Only registered and activated users can seelinks and Download Documents ] 183.5.3 Call Initialisation during L.U. signalling 19

1 IntroductionThis document describes some of the typical dropped/blocked call classifications

encountered during several RAN Tuning Projects for [Only registered and activatedusers can see links and Download Documents ] networks. The purpose of thedocument is for RAN Consultants to understand some of the problems to make the UEanalysis and classification easier. The analysis has been based on limited data (for example

no [Only registered and activated users can see links and DownloadDocuments ] data or data based on similar tool, no usage of RBS and RNC tracers) and forthis result there could be a deviation in the results. Some of these classifications are only a

guideline and should be viewed merely as documenting the typical problem until the correct

reason behind the dropped call is established.All these dropped/blocked calls categories are adapted to the radio environment ev[Only

registered and activated users can see links and Download Documents ]ation

thresholds set up for the MTN 3G RAN Tuning project in the post-processing tool [Only

registered and activated users can see links and Download Documents ] Deskcat

5.6.In the table below are listed the main causes that must be considered during the analysis,

with the explanation for each cause.

CLASSIFICATION CRITERIA

Poor Coverage The Drop occurs in regions where conditions[Only registered andactivated users can see links and Download Documents ] RSCP and/or[Onlyregistered and activated users can see links and Download Documents ] Ec/No are


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measured in critical v[Only registered and activated users can see links andDownload Documents ]es not suitable for a proper connection.

Bad Radio Environment Every drop that occurs when Best Server is Missing (Mostly in goodCPICH RSCP conditions). The UE active set update cannot follow the quick coverage

changes. In this case Pilot Pollution situations are included as well (3 cells in AS and morethan 1 strong SCs is interfering the connection within a range of 5 dB - Ec/No basis

evaluation).Congestion Every drop that occurs when there are no more available radio resources for the

connection. The network sends an RRC Connection Release when the RBS reaches themaximum available Power in DL.

Not Radio Every drop that occurs when the radio conditions are good, the logging

equipment is working properly and the RRC connection release cause (marked usually as"Unspecified") could be attributed to a RBS/Network fault, (including UL UE power going to

maximum even if the CPICH RSCP is measured at good values, crossed feeders causing

false missing neighbours, crossed UL-DL feeders, wrong parameter settings that can affectaccessibility/SHOs in the cell).Equipment Fault Every drop that occurs when [Only registered and activated users can

see links and Download Documents ]Investigation/UE are Blocking or Freezing and/or

SW is crashing, so that it's not possible to maintain the connection.

Missing Neighbours Every drop that occurs when there are poor RSCP and/or Ec/Nolevels/quality on the Best Server/AS, with the contemporary possibility for the UE to

perform a SHO on a better cell that is not declared as a Neighbour for the AS cellsthemselves.2 Dropped Calls Classification

The main cases and dropped call causes are listed below.

2.1 Missing NeighboursEvery drop that occurs when there are poor RSCP and/or Ec/No levels/quality on the BestServer/AS, with the contemporary possibility for the UE to perform a SHO on a better cell

that is not declared as a Neighbour for the AS cells themselves. An example of dropped callfor missing neighbour problem is shown in figure below.

Problem Description: The Active Set best server is cell 61888 (SC = 205). During the callsector 62096 (SC = 293) becomes the strongest sector but is not added to the active set,as the two cells are not defined neighbours. This can be seen in the Serving/Active Setwindow in TEMS. The cell 62096 act as an increasing interferer until eventually the call is

released. The release cause is classified as unspecified.2.2 Poor CoverageThe Drop occurs in regions where conditions CPICH RSCP and/or CPICH Ec/No are measured

in critical values not suitable for a proper connection. In the table below an example is

shown.

Problem Description: In this case the RF environment as reported by the UE is very poor

before the call is dropped. The best server RSCP=-127dBm and Ec/No=-32dB. The scanneralso reports poor radio conditions for the same SC at the same instant i.e. RSCP = -

11.97dBm and Ec/Io = -10.69dB. The last message sent is the UL Active Set Completemessage sent by the UE. The UE then goes into idle mode.

2.3 Bad Radio Environment

Every drop that occurs when Best Server is Missing (Mostly in good CPICH RSCP conditions).

The UE active set update cannot follow the quick coverage changes. In this case PilotPollution situations are included as well (3 cells in AS and more than 1 strong SCs isinterfering the connection within a range of 5 dB - Ec/No basis evaluation).


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Problem Description: The Active Set best servers are cells with SCs 49 and 303, and all MN

(SCs 304, 179 and 180) are close to them on a RSCP basis (from 70dBm to 79 dBm).

During the call UE is performing a continuous number of SHOs, affecting both the servicecontinuity and the quality of the connection (low values of Ec/No on each cell). So, the

Radio Environment is not suitable for a good connection and therefore the dropped calloccurs. Sometimes the TEMS Inv. doesnt show the event on the chart, like in this case, so

its advisable to check the Events and Layer 3 messages: In the L3 table the RRCConnection Release DL corresponds (into the Event table) to an RRC Connection Abnormal

Release with the specification Unspecified.2.4 Congestion

Every drop that occurs when there are no more available radio resources for the connection.

The network sends an RRC Connection Release when the RBS reaches the maximumavailable Power in DL. In the corresponding DL Layer 3 message RRC Connection Release

when the drop happens, the release cause is clearly marked as Congestion. An example is

shown in figure below.

Problem Description: In this case the radio environment doesnt show any critical issue: the

AS is full, and there is a Best Server cell (SC 352) with two more cells that are carrying the

service. Also the MN set is good, and the Layer 3 messages sequence is regular.

The radio resource unavailability pops up suddenly after a certain number of fast SHOs, andan RRC Connection Release message from the network comes to interrupt the call. In the

TEMS L3 messages window details the release cause is clearly marked as Congestion.

2.5 Not Radio

Every drop that occurs when the radio conditions are good, the logging equipment is

working properly and the RRC connection release cause (marked usually as "Unspecified")could be attributed to a RBS/Network fault, (including UL UE power going to maximum evenif the CPICH RSCP is measured at good values, crossed feeders causing false missing

neighbours, crossed UL-DL feeders, wrong parameter settings that can affectaccessibility/SHOs in the cell).This case also includes the UE Sensitivity drop, as shown in figure below.

Problem Description: The radio environment as reported by the UE is very poor just before

the call dropped i.e. Best server RSCP=-127dBm and Ec/No=-27dB. The scanner reportsmuch better radio conditions for the same SC at the same instant i.e. RSCP =-93.57dBmand Ec/Io = -8.66dB. Although the drop is as a result of low RSCP as measured by the UE it

should not be classified, as a poor coverage drop since the scanner indicates the radio

conditions should be ok to maintain the call.2.6 Equipment FaultEvery drop that occurs when TEMS Investigation/UE are Blocking or Freezing and/or SW is

crashing, so that it's not possible to maintain the connection.An example of UE freezing dropped call is shown in figure below.

Problem description: In the example the network sends a DL Measurement Control message

and then the UE freezes. There are no further massages sent between the network and theUE. The RF environment was good at the time of the drop as can be seen from the scanner

information i.e. RSCP =-82.72dBm and Ec/Io = -5.44dB.

3 Blocked Calls ClassificationThe main cases and blocked call causes are listed below. In this situation the cases ofBlocked Calls are pretty equal to the corresponding ones in the dropped calls list. Its

advisable to consider that it is intended as blocked call a connection interrupted for the


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same problems shown in section 2 (with some obvious exceptions shown in the following

paragraphs) and according the Classification Criteria table shown in section 1. Its crucial to

consider that the blocked call event always happens during the call setup phase.Due to these differences, the classification for the blocked calls is different for the dropped

calls one.

3.1 Missing Neighbour

Problem Description: An example of a blocked call caused by a missing neighbour is shownin figure above. The call is being set up on cell 49046 (SC = 387). During the set up sector

48007 (SC = 451) becomes the strongest sector but is not added to the active set as the

two cells are not defined neighbours. This can be seen in the Serving/Active Set window inTEMS. The cell 48007 act as an increasing interferer until eventually the call itself is

released. The release cause is classified as unspecified.

3.2 Poor RF (Bad Coverage and Radio Environment)

Problem Description: In this case the RF environment as reported by the UE is very poor

when the UE attempts to initiate a call. The attempt is indicated by the event Call

Initialization in TEMS Investigation, which is triggered at the start of a command sequence.The best server RSCP=-121dBm and Ec/No=-21dB. The scanner also reports poor radio

conditions for the same SC at the same instant i.e. RSCP =-114.17dBm and Ec/Io = -11.29dB.

3.3 Equipment Fault (UE)

3.3.1 Security and Authentication Mode Failure

Problem Description: In this case there is a problem with the security and Authentication

procedure, which causes the connection to be dropped and result in a blocked call. As canbe seen in figure above the radio environment at the time of the blocked call is good i.e.Best server RSCP=-71dBm and Ec/No=-2dB.

The call is released normally during call set up and 6 seconds after the network sends thesecurity command. There is a timer for the correct response to this security command,which is set to 6 seconds. This indicates a problem in the security and authenticationresponse by the UE.

3.3.2 UE FreezeProblem description: This type of dropped call classification is caused by the UE freezingwhich can be seen in figure below. In the example the UE sends a UL Active Set Update

Complete message and then the UE freezes. There are no further massages sent between

the network and the UE before the next call attempt, which is indicated by the Callinitialisation event in TEMS Investigation. The RF environment was good at the time of thedrop as can be seen from the scanner information i.e. RSCP =-94.17dBm and Ec/Io = -

5.13dB.3.4 Not Radio

3.4.1 Disconnect on RAB SetupProblem description: In this case the radio environment is good as shown in figure above

(i.e. Best server RSCP= -94dBm and Ec/No= -3dB). During call set up, after the DL CallProceeding message the network sends a Disconnect message. This can be seen in the

Layer 3 messages window below. The cause value is (127) Interworking, unspecified as

shown in L3 message details window. The call is then released.This case can include both DC Config Failure and RAB setup failure.3.4.2 Resources Unavailable (Congestion)

Problem description: In this example the network sends a Disconnect message with the


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following message: cause value (47) Resources Unavailable, Unspecified. This can be seen

in the Layer 3 messages window in figure above. The Disconnect message occurs after the

Radio Bearer is set up. The radio environment is good at the time of the Disconnectmessage as seen in figure above.

Sometimes this example of blocked call can be also referred to Congestion problems in themeasured cell.

3.4.3 UE Sensitivity FaultProblem Description: In figure above, an example of a drop classified as UE sensitivity fault

is shown. The radio environment as reported by the UE is very poor when the UE attemptsto initiate a call (i.e. Best server RSCP=-122dBm and Ec/No=-18dB). The scanner reports

much better radio conditions for the same SC at the same instant i.e. RSCP =-89.06dBm

and Ec/Io = -2.65dB. Although the blocked call is as a result of low RSCP as measured bythe UE it should not be classified, as a poor RF block since the scanner indicates the radio

conditions should be sufficient to set up a call.

3.5 Abnormal blocked calls3.5.1 Unclassified - Unanswered RRC requestsProblem description: In this example an RRC request is sent but the network never

responds with an RRC connection setup message. It is not clear whether or not the network

receives the RRC request. The radio environment in the downlink as seen by TEMS is good

i.e. good RSPC, low CPICH Ec/No.

3.5.2 Faulty Block Recording - Barred NetworkProblem description: This occurs when the UE attempts to initiate a call on a network otherthan the measured one (in our case MTN). In the serving/Active Set window in TEMS in

figure above it can be seen that the DL UARFCN for the network the UE is camped on in idle

mode is 10564. The example shows the case that happened for the [Only registered andactivated users can see links and Download Documents ] network (DL UARFCN10712). DeskCat considers this a blocked call but since the blocked call does do not occur

on the [Only registered and activated users can see links and DownloadDocuments ] network, it has to be excluded from our analysis.3.5.3 Call Initialisation during L.U. signalling

Problem description: In this case the UE is involved in Location Update signalling. As seen inthe RRC Connection Request message the establishment cause is registration and theLocation Update request message is for Normal Location Updating. During the L.U. signallinga new call attempt is triggered by the command sequence in TEMS Investigation. This can

be seen in the events window in figure above.

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