Advanced Handover Analysis

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March 2008

Advanced Handover Analysis

Guide

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The content of this manual is provided for information only, is subject to change without notice, and should not be construed as a commitment by Actix. Actix assumes no responsibility or liability for any errors or inaccuracies that appear in this documentation.

Copyright © Actix 2008. All rights reserved. All trademarks are hereby acknowledged.

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Contents 1 INTRODUCTION.........................................................................................4

2 INTRACELL HANDOVERS ...............................................................................5

3 REPEATED HANDOVERS ...............................................................................7

4 DRAGGED HANDOVERS................................................................................9

5 PING-PONG HANDOVERS ...........................................................................12

6 ANALYSIS PACK STATEFORMS......................................................................16

7 SUMMARY .............................................................................................17

8 ANNEX - ADVANCED HANDOVER ANALYSIS PACK ...............................................18

Actix Analyzer Advanced Handover Analysis Guide March 2008 Introduction 4

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1 Introduction Handovers fulfill a number of roles in any GSM network. Not only do they allow for load-balancing of resources, thus maximizing the capacity of a cell at any one time; but they allow the fundamental feature of being able to sustain a call whilst on the move. All resources that are assigned to a subscriber, whether it is on the Um interface or the Abis/A circuit further back into the switch are controlled by various network elements. Therefore to change any of these resources, whether it is due to the physical movement of the subscriber, degrading signal strength and/or quality, or load balancing, involves communication between these network elements.

All this highlights a key network performance optimization area to offer subscribers the ability to seamlessly move around inside each location area, and one that cannot be described by just one traditional KPI such as “Handover Success Rate”.

As such, Actix Analyzer includes 4 types of handover analysis:

• Intracell handovers

• Repeated handovers

• Dragged handovers

• PingPong handovers

These are explained in detail in this paper below. The scenario is explained, with the Actix implementation fully documented. Examples are included where relevant, and finally the StateForms allowing detailed synchronization are explained. The Annex gives examples of the KPI reporting layer available with this Advanced Handover Analysis Pack in Actix Analyzer.

Actix Analyzer Advanced Handover Analysis Guide March 2008 Intracell handovers 5

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2 Intracell handovers Intracell handovers refer to the mobile being transferred from one BTS resource to another. The main reason for this is for load-balancing on a cell. The resource can be either the current timeslot or the current traffic channel that the mobile is using to communicate with the BTS. Therefore, the options for an intracell handover can be “timeslot only”, “traffic channel only” or “both timeslot and traffic channel”.

5 new attributes have been added to the Workspace Explorer:

Message Attribute

Intracell handover command Event_Intra_HO_Command

Successful intracell handover Event_Intra_HO_OK

Failed timeslot intracell handover Event_Intra_TS_HO_Fail

Failed traffic channel intracell handover Event_Intra_TCH_HO_Fail

Failed combined timeslot/traffic channel intracell handover

Event_Intra_TCH_TS_HO_Fail

The intracell handover command is triggered by an RR Assignment Command Layer 3 message and can occur while the handset is in dedicated mode or in-call. At this point, a new attribute Event_Intra_HO_Command is set depending on whether the command is to change either the Timeslot only, the TCH only or both the TCH and timeslot using the following algorithm and event diagram:

If current TCH is equal to previous TCH then Set attribute to “TS only” command Else If current TS is equal to previous TS then Set attribute to “TCH only” command Else Set attribute to “TCH and TS” command End if End if

The format of the RR Assignment Command (hopping / non-hopping) is as follows:

L3 DL RR Assignment Command Channel Type: TCH/F + FACCH/F and SACCH/F TimeSlot: 5 TrainingSeq: 4 Hopping: Yes, MAIO: 1 HoppingSeq (HSN): 0 MsTxPower Ordered: (2 GSM) CA List: 9 13 19 71 (Bit Map 0) Channel Mode: Speech full rate or half rate version 1 MA: 1 2 3 4

or

L3 DL RR Assignment Command Channel Type: TCH/F + FACCH/F and SACCH/F TimeSlot: 4 TrainingSeq: 2 Hopping: No, ARFCN: 88 MsTxPower Ordered: (5 GSM) Channel Mode: Speech full rate or half rate version 1

Actix Analyzer Advanced Handover Analysis Guide March 2008 Intracell handovers 6

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The Intracell Handover event diagram at RR Assignment Command message is as follows:

If the handset mode is unknown at the beginning of the drivetest file, the event diagram waits in the “Unknown @ start of file / idle” state.

Once a dedicated channel has been assigned to the handset, the event diagram moves into the “Voice/data channel assigned” state. Any following RR Assignment Command (before the handset returns to idle mode) results in either a timeslot, traffic channel or both re-allocation attempt. At this point, the event diagram moves into the “Intracell HO” state.

From the “Intracell HO” state, it is possible to drop/complete the call, but a typical result would be for the RR Assignment Complete or RR Assignment Failure message to be found.

If an RR Assignment Complete message is found, the Event_Intra_HO_OK event is set.

If an RR Assignment Failure message is detected, depending on the type of intracell handover command, it sets the corresponding failure event: Event_Intra_TS_HO_Fail / Event_Intra_TCH_HO_Fail / Event_Intra_TCH_TS_HO_Fail.

Actix Analyzer Advanced Handover Analysis Guide March 2008 Repeated Handovers 7

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3 Repeated Handovers Repeated handovers are situations where the network instructs the MS handover to a new cell. This handover fails, but the network continues to request that the MS hands over into the same cell. Actix triggers the Repeated Handover commands, and also any associated failures, until either the MS succeeds in its attempt, the call is terminated, or the network requests the MS to handover to a different cell.

At the first message, the handset moves into the “Normal” state. Only when it finds the first handover failure does it move into the “HO_Window” state. If the handover succeeds or a handover command to a new cell is detected, the MS returns to the “Normal” state.

The MS also moves back to the Normal state at a dropped call, a channel release or a call setup failure. From the “HO_Window” state, any further handover commands to the same cell as the previous command that has just failed, result in the Event_Rpt_HO_Commands event being triggered. If another Handover Failure is found, it can only have resulted from a repeated handover command, so it sets the Event_Rpt_HO_Failures event.

In this way, it will identify the following scenarios:

HO Commands

HO Fails

Rpt HO Commands

Rpt HO Fails

1 0 0 0

1 1 1 0

1 1 2 1

2 1 1 1

Actix Analyzer Advanced Handover Analysis Guide March 2008 Repeated Handovers 8

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If the final Handover Command to the new cell fails, the process is repeated, with the new handover command being the initial command of the new sequence.

This can be clearly seen when synchronizing the Layer 3 Message Browser with the table view (plotting Target BCIC/BCCH and the Layer 3 message), where multiple RR Handover Failures are triggered, all being requested in the same target BCCH/BSIC at the previous RR Handover Command.

Actix Analyzer Advanced Handover Analysis Guide March 2008 Dragged Handovers 9

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4 Dragged Handovers This situation exists when the MS loses its dominance, but for whatever reason the network either doesn’t instruct it to handover, or it fails to handover successfully. If this loss of dominance situation persists for longer than a specified time window, and either a Handover Failure or even a Dropped Call occurs, then these failures/drops are also triggered as two new events: EventDragHOFail or EventDragDrop.

And in more detail at the loss of dominance:

Therefore, any dropped call or Handover Failure that occurs within the dark blue “Dragged Handover Region” will be triggered as a Dragged Drop, or a Dragged Handover Failure.

This scenario is useful to investigate as the symptoms to the problem could occur in a different location to the root cause of the problem. The lack of successful handover during the initial loss of dominance should be investigated, instead of the causes of the dropped call, and depending on the speed of the MS at the time, these 2 situations could occur in different areas. Therefore, this type of analysis allows engineers to target the problem area correctly with any resulting handover parameter changes being made to the correct cell.

The loss of dominance in this scenario is defined as:

Serving_RxLevel_Sub < (strongest_neighbor_RxLevel hysteresis)


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This is to ensure this event is not triggered unnecessarily, and is this hysteresis is defined inside the Tools > Display Thresholds. The time window after which a handover can be describe as “dragged” is also controlled from the thresholds settings. Default values of 5dB for the hysteresis and 10000 milliseconds are suggested, but can be modified before a file is loaded.

The event definition below shows how the MS moves from the “Normal” state to the “Poor Dominance” state when the server is weaker than the strongest neighbor (minus the hysteresis). From here, if a handover is successful, the call completes, the call is dropped, or the server regains dominance, the MS returns to the “Normal” state.

If the window expires while the MS is still in the “Poor Dominance” state, it moves into the “Should have handed over” state.

Any RR Handover Failure message will trigger the EventDragHOFail, and any dropped call detected will trigger the EventDragDrop. However, if a handover is successful, the call completes, or the server regains dominance, the MS returns to the “Normal” state.


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To illustrate this situation, the following Dragged Handover will be analyzed. Here it is possible to see the previous Handover Failure which occurred while the MS was still in the “Poor Dominance” state, but once the window expired, the next failure triggered the EventDragHOFail event.

Actix Analyzer Advanced Handover Analysis Guide March 2008 Ping-Pong Handovers 12

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5 Ping-Pong Handovers Ping-Pong handovers are situations where the MS is handed over from one cell to another, but is handed back to that original cell within a certain time window. This causes unnecessary signaling on the Um, Abis and A interface, and can give a clear indication of either incorrect handover parameter settings or a dominance problem in the area. This window size whereby a handover is considered as a “ping-pong” handover varies between operators, and the default window size in Actix Analyzer is 6 seconds.

So why should this type of handover be analyzed individually? This type of handover will not be visible from a simple “handover success rate” KPI. To analyze the distribution of handover frequency would give an indication towards whether Ping-Pong handovers are occurring, but Actix new EventPingPongHandover event allows engineers to configure the window size, investigate the original and intermediate cells, inter-handover time, and report on trends of cells where this occurred.

The event definition is as follows:


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When a successful handover occurs from cell A to cell B, the MS moves from the “Normal” state to the “Ping” state.

If the window times-out, or the MS returns to idle (through either a call completion or a dropped call) the MS moves back into the “Normal” state.

If a successful handover occurs before the window times-out, the MS moves to the “Pong” state. From here, the same conditions apply.

This can be explained using the following algorithm:

MS_state = “Normal” If HandoverOK from cell A to cell B then MS_state = “Ping” If timeout window expires OR MS returns to Idle mode then MS_state = “Normal” Else if HandoverOK within timeout window then If target cell = new cell C then EventPingPongHandover is not triggered MS_state = “Pong” Else if target cell = cell A EventPingPongHandover is triggered MS_state = “Pong” End if End if End if

The following screenshot shows two Ping Pong Handovers, the first from cell 12424 to cell 12368 and back; and the second from cell 12424 to cell 12366 and back. The route shows the ServCI parameter with underneath the route denoting the new CI at a handover:


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This can also be clearly visualized when the lines to cells are enabled for all datapoints, and colored using the ServCI parameter:

When synchronizing the second Ping Pong Handover with the stateform, the handover back to BCCH/BSIC: 79/13 can be analyzed in more detail. It occurred 5 seconds after the previous (within the 6 second window), and the actual handover itself was executed in 130 milliseconds. However, the dominance of the strongest neighbor at that point was only 1dB.


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The main cause for this handover was the poor dominance of cell 12366 in its main lobe area, which can be seen by plotting the histogram of ServRxLevSub NborRxLev[0], and applying a regional filter to this area. The average dominance was -3.1dB compared to a side-lobe of cell 12424 (seen by writing a statistical query on the same expression).

There was a handover back to cell 12366 after the second Ping Pong Handover, but this was outside of the defined time window, and the change in dominance after the handover would be 9dB a much healthier handover.

Actix Analyzer Advanced Handover Analysis Guide March 2008 Analysis Pack StateForms 16

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6 Analysis Pack StateForms Four StateForms are provided for individual analysis, accessible from the View menu, or by right-clicking on a device. Chart views can be zoomed using the drop-down selection at the bottom right of the panel. Mouse hover-over tips change, giving useful summaries e.g.: timestamps or serving & neighbor levels.

The top-left Dragged form shows two traces (Serving Level and strongest neighbor Level), colored according to the serving dominance (x). Red: x<-10dB, Yellow: -10<=x< -5dB, Orange: -5<=x<0dB, Green: 0<=x<5dB, Cyan: 5<=x<10dB, Blue: x >=10dB. The panels describe the RR and CC cause codes for handover failures and drops, the distance to the server and strongest neighbor, dominance of server and Radio Link Timeout value.

The top-right Intracell form shows the CI, site and sector names, BCCH, BSIC, TCH, TS and intracell handover type. The colored bar shows the CI and the columns show quality. Yellow = intracell attempt, grey = TS+TCH failure, cyan = TCH failure, magenta = TS failure.

The bottom-left PingPong form shows Timing Advance and CI, with successful (green) and PingPong handovers (red). Panels show previous BCCH/BSIC and current BCCH/BSIC, time from previous handover, time to complete handover, handset mode and server dominance.

The bottom-right Repeated form shows CI, Serving and Target BCCH/BSIC, RR Cause at handover failure and a chart showing Serving Level, Quality and Timing Advance, along with yellow Repeated Commands, and red Repeated Failures.

Actix Analyzer Advanced Handover Analysis Guide March 2008 Summary 17

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7 Summary While traditional handover KPIs have sufficed for many GSM operators to tune their network to the current state, a more detailed approach should be undertaken when the network reaches a mature state. For this reason, Actix Analyzer offers the Advanced Handover Analysis Pack, to allow diagnosis and troubleshooting of four types of handover that are commonly overlooked: Intracell, Repeated, Dragged and Ping-Pong.

Through the Actix reporting layer, problems can be identified in individual files, with enough engineering data to allow the root cause of the problem to be identified. The individual data file can then be analyzed, using the available StateForms synchronized with any map, chart, table or Layer 3 Message Browser view to give the engineer full troubleshooting capability.

Actix Analyzer Advanced Handover Analysis Guide March 2008 Annex - Advanced Handover Analysis Pack 18

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8 Annex - Advanced Handover Analysis Pack In order to allow trends to be studied on larger volumes of data, reports can be run from the Advanced HO Analysis pack. These reports can be run on singles files, or superstreams of merged files. Examples of these reports are shown below.


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The Ping Pong Handover Statistics report shows a histogram of the time between ping-pong handovers within the defined window, and also the change in serving level either side of the handover.


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The Ping Pong Cell Breakdown report allows top-level KPIs to be reported and when used with a superstream, which file they occurred in, timestamp, and BCCH & BSIC either side of the handover.


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The Repeated HO Target Cells report shows KPIs for repeated handovers, and the BCCH and BSIC distribution for the file (height of column = count of occurrences of commands).


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The Repeated HO Quality report shows the distribution of serving cell quality during a 5 second window before the Repeated Handover Command. The second histogram shows the distribution of level and quality against Timing Advance, showing trends of how far away from the serving cell these commands occurred.


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The Intracell Type report shows the distribution of the Intracell Handover attempts throughout the file(s) and also the count of failures for each type. The table below shows the individual file KPIs broken down by “Timeslot Only”, “TCH Only” and “Combined timeslot & TCH”.


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The Intracell Attempts report shows KPIs for the Intracell handover health, and also the breakdown of timeslot and traffic channel for the target cells. Clicking the Show Excel Report button allow the engineer to load the report into Excel and modify the pivot table to identify new trends.


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The Intracell Fail Analysis report shows the breakdown of failed intracell handovers, and the actual TS or TCH which was requested for each failure.


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The Dragged Time Delay report shows the KPIs for Dragged Handovers and Dragged Dropped Calls, and the time distribution between the handover failure or dropped call and the initial loss of dominance.


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The Dragged Dominance report allows the engineer to view the dominance of the server over the strongest neighbor cell (filtered for only valid Measurement Reports). The second (composite) histogram shows the distributions of serving level of the server and strongest neighbor overlaid on the same chart. Finally, the list of Cell IDs and their dominance throughout the data file is given (highlighted red if negative), with the count of measurements allowing statistical weighting to be applied to the actual results.


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The Individual Drag Analysis report allows the engineer to analyze each individual dragged handover or dropped call in detail, showing the filename, call ID, CI, target BCCH/BSIC, distance to the serving cell, dominance, time to failure, timestamp and Radio Link Timeout value (not shown in screenshot).


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Documents

Advanced Handover Analysis