Actix Analyzer GPRS and GSM Analysis Guide

GPRS/GSM Analysis Guide

August 2009

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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 2009. All rights reserved. All trademarks are hereby acknowledged.

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Contents 1 INTRODUCTION ..........................................................................................4 COMBINING DATA FILES ............................................................................................ 5

2 USING GPRS ANALYSES ...............................................................................7 ABOUT ANALYZER GPRS APPLICATIONS ...................................................................... 10 DT GPRS MM/SM Procedures and IP Data Summary .......................................... 10 DT GPRS Radio Link Performance Analysis ....................................................... 10 MM/SM AND IP REPORTS ....................................................................................... 11 MM Attach Detach report ............................................................................... 11 Other MM Procedures report .......................................................................... 12 PDP Context report ....................................................................................... 13 AA PDP Context report .................................................................................. 15 Other SM Info report ..................................................................................... 17 TCP data summary report .............................................................................. 19 DT RADIO LINK REPORTS........................................................................................ 20 Coding Schemes and Throughput per TS report ................................................ 20 Timeslot allocation statistics report ................................................................. 21 Level and Quality report ................................................................................ 22 RLC and LLC Throughput Stats report.............................................................. 24 RLC Throughput and Coding Scheme report ..................................................... 26 RLC Throughput and Level report ................................................................... 27 RLC Throughput and Quality report ................................................................. 28 RLC and LLC Throughput per TBF report .......................................................... 29 BLER and Retransmissions report ................................................................... 31

3 USING GSM ANALYSES ............................................................................... 32 DT Design Validation and Quick Analysis.......................................................... 32 Design Validation report ................................................................................ 33 Neighbors within 5 dB from server report ........................................................ 34 Handover settings and power control .............................................................. 35 HO Quality report ......................................................................................... 35 HO Level report ............................................................................................ 36 HO Interval report ........................................................................................ 36 HO Classification report ................................................................................. 37 MS Tx Power Distribution report ..................................................................... 38 MS Tx Power Step distribution ........................................................................ 39 MS Tx Power Step Interval Distribution report .................................................. 39

4 COMBINED ANALYSIS ................................................................................. 40 USER APPLICATIONS, IP PROTOCOLS AND RADIO LINK ...................................................... 40 RADIO NETWORK AND IP/APPLICATION LAYERS............................................................... 42 Example 1 ................................................................................................... 43 Example 2 ................................................................................................... 47

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Actix Analyzer GPRS/GSM Analysis Guide August 2009

Introduction 4

1 Introduction This document describes methods for analyzing GSM and GPRS log file data. It assumes that you have already installed, licensed and configured your Analyzer system as described in the documents Actix Software Installation Guide and the Getting Started Guide. Analyzer supports the following performance data sources for a wide variety of test equipment vendors: Mobile Stations (Test and Commercial-Um interface) Scanners (Um interface) Call Trace devices for GSM IP Sniffers (supporting the WinPCAP architecture)

IP and Application Layer performance data are supported for IP sniffers: powerful data collection software for recording messaging, events and statistics from IP links, as well as for vendor-specific data sets, typically integrated with drive-test hardware solutions. Please check the release notes for information on the latest file formats supported, new analyses and functionality.

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Combining data files Data from IP sniffer files can be combined with drive test data collected at the same time. This allows a combined analysis of the attributes referring to different layers (from the radio link to the TCP/UDP), using tables, charts, queries and reports. Moreover, the GPS coordinates in the drive test data can be matched to the IP sniffer information, so the IP information can then be plotted on a map. This process of combining data from different streams is called superstreaming. The drive test and IP sniffer files are collected on the same portable computer, so the time alignment should match exactly. However, sometimes there is a fixed time shift (for example, 1 or 2 hours), depending on the time references of the two applications. This may also occur if the Time offset setting under Tools/Preferences is not zero. To quantify the time difference, it is sufficient to see the time of the first messages in the two files. If the two files are in time alignment, use the Merge Method (see below) Use timestamps, otherwise select Time offsets and enter the time mismatch in seconds between the two files.

To superstream the data 1

Open the IP Sniffer file and the Um drive test file to be superstreamed together.

2

From the Tools menu, select Create Superstream to open the superstreaming dialog.

The Superstreaming dialog

3

Select the appropriate data streams.

4

Select the appropriate Merge Method settings for synchronizing the data streams.

5

Click OK to begin the superstreaming process.

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Introduction 6

When the process is complete, the superstream is displayed in the Workspace Explorer, and may now be examined using the usual Analyzer data displays (for example, maps, charts, tables, analysis reports).

Right-click options for the new superstream

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2 Using GPRS analyses Analyzer provides a top-down approach that allows you to start from high level reports (focused on the content of the drive test and on the high level problems, if any) and to drill down only if and where it is necessary (time saving for the operators).

Data view

Technique

High level

Statistics shown in the Cell Explorer

Medium level

The reports included in the modules

Drill-down capability

Queries, charts, customized reports, maps, etc.

The starting point for the Quality of Service Verification and Troubleshooting tasks are the modules: DT GPRS MM/SM Procedures and IP Data Summary DT GPRS Radio Link Performance Analysis These are available under the Analysis menu. As with any of the analyses created by Actix, these consist of a complete set of reports that combine the different key performance indicators, allowing you to navigate to not only the problems but also the causes and possible solutions. To improve the processing time, close the Statistical Explorer before running the reports. The summary view for an analysis—called the Cell Explorer—shows a statistical summary of the data and an appropriate set of reports. The data can be broken down by a key dimension, such as "cell", displaying a number of pertinent statistics for each value of the key dimension that appears in the data, allowing you to focus quickly on only those portions of the data that are of interest (for example the cells with the highest BLER, as shown below).

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The Cell Explorer, showing the upper filtering and data display, and the lower report selection panel

The dimension (cells) can be sorted by ascending or descending values of a parameter (for example DL RLC throughput), by clicking on the title of the column.

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By selecting a subset of the rows in the Cell Explorer before choosing to run a report, you can filter the data in the reports to include only data for the selected values of the key dimension (as shown below).

Example of report based on the selected cells

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About Analyzer GPRS Applications DT GPRS MM/SM Procedures and IP Data Summary This application allows analysis of the procedures between the Mobile Station and the SGSN (with success rates and causes for failures), and provides a summary of packet data traffic. The mobile contains these reports: MM Attach Detach Other MM Procedures PDP Context AA PDP Context Other SM Info TCP data summary (must be run on a superstream consisting of an IP sniffer file and the relevant drive test file).

DT GPRS Radio Link Performance Analysis This application allows analysis of the radio link performances, and contains these reports: Coding Schemes and Throughput per TS Timeslot allocation statistics Level and Quality RLC and LLC Throughput Stats RLC Throughput and Coding Scheme RLC Throughput and Level RLC Throughput and Quality RLC and LLC Throughput per TBF BLER and Retransmissions From the metrics in the Cell Explorer, you can identify any cells that need specific analysis. Then by going through the reports on the selected cells, you can investigate the possible causes. For example, if the BLER is showing a bad behavior, it is possible to analyze the signal level and the quality or of the interference, then see the impact on the throughput, the relation with coding scheme usage, and so on.

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MM/SM and IP reports MM Attach Detach report In this report, attach and detach procedures are analyzed. The success rate is provided. It is important to note the difference between "Attach Reject" and "Lost". The first is a failure message received from the network; the second takes into account the requests that receive no response from the network (so they are lost). Only after four unsuccessful retries, the procedure is aborted (see the note in the report). The same apply to the detach procedure. For the detach, no network response is expected in the case of mobile powering off. The cause breakdown table highlights the causes of reject messages, helping you to find the reasons for the failures. The Information Element is not mandatory in these messages, but if present, the table also shows the attach accept and detach request causes. The attach type and attach results are shown here:

First two tables of example MM Attach Detach report www.actix.com

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Second half of example MM Attach Detach report

Other MM Procedures report The following procedures are shown (giving an indication of the success rate): Routing area update P-TMSI reallocation (if requested by the network) Authentication and Cyphering The causes for failures are presented, as well as the RA update types.

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Example Other MM Procedures report

PDP Context report The following procedures are shown (giving an indication of the success rate): PdP Context Activation Network originated PdP Context Activation (if network-originated data calls are enabled, preliminary messages precede PdP Context Activation) PdP Context Deactivation PdP Context Modification (especially with more advanced features for QoS support, this procedure will be

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used to modify the QoS provisioned to the subscribers and negotiated between the MS and the SGSN) For the meaning of "loss", refer to the first report and to the notes below the tables.

Example top half of PDP Context report

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Example bottom half of PDP Context report

AA PDP Context report AA stands for Anonymous Access. The procedures of the previous report are presented in case Anonymous Access is implemented.

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Example AA PDP Context report

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Other SM Info report This report presents the accessed APNs and the parameters relevant for the QoS.

Example Other SM Info report, top tables

In particular, the NSAPI and SAPI (requested and negotiated, used by the SNDCP and LLC layers to manage the QoS) and the QoS classes (requested by the MS and obtained after negotiation with the SGSN) are shown. www.actix.com

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When QoS management is implemented, these statistics will be key in verifying the correct dimensioning of the network to support the subscribed QoS profiles (the QoS modification will be particularly interesting). The same parameters for the radio interface (Radio Priority) are presented.

Example Other SM Info report, middle tables

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Example Other SM Info report, bottom tables.

TCP data summary report This report applies to the super stream of drive test and IP sniffer files, and shows a recap of the data sessions during the drive test. These indicators are quite close to the application behavior because signaling packets are excluded by the computation (the same applies to the Uplink and Downlink throughput calculations). The three main indicators are: The duration of the TCP transfers in milliseconds (the single sessions and the total) The quantity of data sent and received in Kbytes The mean and maximum end-to-end round trip time (calculated as the time between the transmission of an Uplink TCP packet and the time of the reception of the acknowledge) in milliseconds.

Example of TCP data summary report

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DT Radio Link reports Coding Schemes and Throughput per TS report In this report, the value of the throughput (orange line) and the coding scheme usage (CS1 and CS2 columns) is presented for every possible number of allocated timeslots in downlink (from 1 to 4, for the present mobiles).

Example DT GPRS Radio Link Performance Analysis report

It is important to note that coding scheme 1 is used for signaling, while coding scheme 2 is preferably used by the allocation algorithms to give higher throughput values, especially in good radio conditions. Some vendors use dynamic algorithms to move from CS1 to CS2 depending on the radio conditions, while others have a fixed CS2 allocation. In this last case, CS1 is only given by the signaling traffic.

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Timeslot allocation statistics report This report shows the percentage usage for the different number of allocated timeslots, which can be combined with the results of the previous report.

Example Timeslot allocation statistics report

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Level and Quality report This report gives an overview of the radio conditions on the cells tested during the drive. Three graphs are shown: The distribution (vertical bars) and cumulative distribution function or total (dotted line) of the samples of C value (signal strength) The distribution (vertical bars) and CDF (dotted line) of the samples of the received quality The interference analysis (obtained combining level and quality measurements) The last graph highlights the possible presence of interference, when bad quality is shown in combination of good signal strength. The information about C value and Quality are taken from the acknowledge messages. For any given signal strength interval, the number of occurrences (to weight the importance) and the mean received level is presented.

C Value Distribution and Statistics from example Level and Quality report

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RxQual distribution from example Level and Quality report

Interference Analysis from example Level and Quality report

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RLC and LLC Throughput Stats report This report provides statistics for the throughput for the RLC/MAC and LLC layers, in both Uplink and Downlink directions. The first two graphs give the minimum, mean and maximum values for the bit rate, and are useful as general references for radio link performance.

Downlink RLC and LLC Throughput from an example RLC and LLC Throughput Stats report

Uplink RLC and LLC Throughput from an example RLC and LLC Throughput Stats report

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The distributions of the throughput values—the bars indicate the number of occurrences— are useful for studying the nature of the traffic supported. FTP traffic will have a profile similar to that shown below with a throughput concentrated in the higher intervals, while the profile generated by web browsing will be more scattered on the different values, given the less regular pattern of the application level. Accordingly, the Uplink throughput values of an FTP download will be concentrated on low intervals, given by the acknowledge messages.

Downlink LLC and RLC Throughput Distribution from an example RLC and LLC Throughput Stats report

Uplink LLC and RLC Throughput Distribution from an example RLC and LLC Throughput Stats report

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RLC Throughput and Coding Scheme report This report displays the coding scheme distribution correlated to the throughput values of the RLC/MAC layer transmitted on the radio interface. High throughput values usually correspond to a high percentage of Coding Scheme 2, while for lower levels of throughput CS1 usually prevails, as signaling traffic is low throughput and uses the power coding scheme. In the combined graph, the RLC throughput distribution and cumulative value are shown as colored areas. The trend lines represent the percentage of the coding schemes. In case there is no throughput in a certain interval, a default value of 50% is assigned to CS1 and CS2.

Example Throughput and Coding Scheme report

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RLC Throughput and Level report The aim of this report is to show the mean signal level (right vertical axis) for the different throughput values. The distribution of occurrences is also reported to weight the importance of the given results (left vertical axis).

Example RLC Throughput and Level report

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RLC Throughput and Quality report The aim of this report is to show the mean received quality (right vertical axis) for the different throughput values. The distribution of occurrences (left vertical axis) is also reported to weight the importance of the given results.

Example RLC Throughput and Quality report

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RLC and LLC Throughput per TBF report This graph shows the mean DL throughput at RLC and LLC level for any given TFI number.

Mean DL Throughput from an example RLC and LLC Throughput per TBF report

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DL statistics from an example RLC and LLC Throughput per TBF report

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BLER and Retransmissions report This report shows the effect of the radio link quality in terms of retransmissions in Uplink and Downlink (not all drive test collection tools support this) and Block Error Rate (BLER). For any interval of retransmission rate values (0 ≤ x < 1%, 1 ≤ x < 2%, etc.), the distribution (vertical bars) and the cumulative (trend line) are represented. A similar approach applies to Block Error Rate.

RLC Retransmission Rate from an example BLER and Retransmissions report

Block Error Rates from an example BLER and Retransmissions report

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3 Using GSM analyses The GSM module of SVS includes the applications Validation and Quick Analysis and DT Handover Settings and Power Control Analysis, which are available from the menu.

DT Design Validation and Quick Analysis The set of reports contained in this application gives a summary of voice traffic performance and allows a verification of the radio network design. Drive Test Call Details report This report provides a complete overview of the content of the drive test in terms of performed calls, call duration, performance indicators (call setup success rate, call completion rate, handover success rate and location update success rate) and a detailed list of the procedural messages. Various attributes of the file decoded from specific messages or sequences of messages can be counted or combined to give general indications, such as number of call setup procedures, call setup success rate and call completion success rate, or Location Update success rate in idle mode.

Example of the Call Details report www.actix.com

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Design Validation report Combining several of the elements described above into a single stream of results will give a clear and efficient view of the overall quality of the network design. It also let us efficiently identify the nature of the problems along the test route. In the report, the following information elements have been combined: RxQual: the quality is considered acceptable if RxQual is greater than or equal to three. RxLev: the level is considered sufficient if greater than or equal to –85 dBm. Server Dominance: the serving cell is considered dominant if there are less than three neighbors within 5 dB from the server. Based on these criteria the report calculates the percentage where a good design is shown. "Good design" is defined to be the condition where all three criteria are met. When at least one criterion fails, this is classified as bad design. After the display showing the percentage of good design, details on the sources of problems are shown. Combining the information, a diagnosis can be performed (for example, all points where the level is considered good and the quality is not acceptable may be classified as points of interference).

Example of the Design Validation report

Measurement Reports contain information concerning receive level, and in dedicated mode, the quality and timing advance. www.actix.com

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The statistical distribution of these elements allows engineers to assess the overall quality of the radio conditions on the test route. Peaks in the distribution of level and quality measurements may already give some indications about particular problems or specific network settings related to power control or interference. Timing advance distribution helps in identifying average serving distance (the distance to the BTS serving the test mobile) and can eventually indicate abnormally distant connections.

Neighbors within 5 dB from server report To understand further the quality of the network design, this report analyzes the relative level of reported neighbors along the drive test route.

Example of the close neighbors report www.actix.com

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The number of neighbors that are within a 5 dB range from the serving cell receive level (including, of course, any neighbor for which the level would be above the server) are counted, giving indicator of the server dominance in each point of the test route. Typically, one or two neighbors within this range may be considered a "sane" situation. More than two from the server typically indicates that there is no clear dominant server cell in the area; therefore this is a potentially risky area, subject to interference. Two different reports have been designed to take into consideration the case of singleband networks and dual-band networks. In the latter case, the general approach is very similar to the single band but the count of neighbors within 5 dB from the server no longer makes sense, since the 1800 layer will usually be at a level lower than the 900 layer. So the analysis counts only those neighbors within 5 dB in the same band.

Handover settings and power control The set of reports contained in this application is aimed at verifying the handover and power control procedures and to help optimize the related parameters.

HO Quality report When evaluating the quality of the handover procedures in a network , the classic indicator is the handover success rate. This is based on a ratio between Handover Command and Handover Complete signaling messages.

Example of the Handover Quality report

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To gain a better view of the efficiency and appropriateness of the handover procedures, we need to compare the quality of the radio link before and after the handover. The time window is five seconds before and after the handover. A statistical comparative analysis of these average values indicates if the handovers effectively improve the quality of the radio links. In fact, under normal conditions, the quality after a handover should improve. This is visualized on a graph with: A statistical distribution of average quality before and after handover. The distribution after handover normally shows a shift of the distribution towards lower RxQual—that is, higher quality. As a reference, also the overall quality distribution in the entire trace is shown. A cumulative distribution of the previous indicators.

HO Level report An interesting indicator in analyzing handover settings along a drive test route is the level difference between the source cell and the target cell. The distribution of the level is shown in the graphic (again the time window is five seconds before and after the handover). The resulting dominant value should correspond to the handover margin defined in the handover-algorithm parameter settings.

HO Interval report Measuring the time elapsed between two consecutive handovers is a good indicator of both the quality of the handover settings and the quality of the network design. Too many handovers at very short intervals will inevitably influence the quality of the communications as perceived by users. There is always some loss of speech information during a handover.

Example of the Handover Interval report

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HO Classification report Besides the quality difference, level difference and handover intervals described above, a fast and efficient analysis process is to classify handovers based on their type. Unfortunately, the cause value for which the handover was triggered—which is the ideal indicator in this type of analysis—is only available on the A interface in the network structure. It is, however, possible to produce similar results to those obtained from A-interface HO analysis. The handover types can be computed from a combination of parameters, on which a statistical analysis is then run in a second step.

Example of the Handover Classification report

Handovers can be classified according to the average value of the following parameters or indicators, in a window corresponding to the averaging window set in the handover algorithm parameterization: Average quality before handover Average level before handover Average server and target cell level difference before handover By comparing these average values to the trigger levels defined in the handover algorithm settings, we can create a matrix of handover types. A normal handover (better cell) should occur in the following conditions: Average quality is acceptable (in the report: less than or equal to 3) Average level is sufficient to guarantee the stability and quality of the call (in the report: greater than or equal to –92 dBm) Level difference between server and target cell exceeds a reasonable handover margin (in the report: greater than 6 dB) www.actix.com

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MS Tx Power Distribution report The statistical distribution of transmit power lets you evaluate the quality of the network RF design and power control tuning along the drive test route.

Example of the Power Distribution report

In a classical Power Control setting, power will always be at a maximum at call setup or after a handover. Only after sufficient measurement reports have been received by the BTS will the power be adjusted down to the adequate level. So, a normal statistical distribution would show the highest power levels as dominant. The distribution should decrease until the lowest level is attained, as all intermediate levels are necessary steps to reach a stable state. Note that some equipment manufacturers propose specific settings or algorithms to optimize the power level during handovers. The influence of these algorithms and their efficiency can be visualized on the MS Tx Power distribution plot, since not all intermediate steps are used to reach a stable power level in this case. So, the distribution will not be regularly decreasing towards lower Tx power levels, but will present gaps. These will be more or less visible, depending on the environment—urban with low Tx levels, or rural with higher Tx levels. From the total power samples, it is possible to compute the average transmit power for the entire drive-test route. This is a good indicator of the general design and behavior of the network. It is also a perfect indicator for competitive benchmarking, as the average power level directly influences the MS battery life. In dual-band networks, transmit power values in the 900 band and in the 1800 band can easily be separated. In fact, all 900-band power values are odd values, while 1800-band power values are even values. It is therefore very easy to compare results from the tow bands, or alternatively, to analyze them separately.

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MS Tx Power Step distribution To investigate further the behavior of the Power Control algorithm—and to verify that it is working according to the scenario imagined by the optimization engineers when defining its parameterization—we analyze the various power steps. A statistical distribution of these steps will then reveal the behavior of the Power Control mechanism. A typical setting is to decrease the power by steps of 2 dB until a suitable level is reached, and to increase the level by steps of 4 to 6 dB if the quality or the level of the radio link becomes insufficient. This will appear clearly on a statistical distribution, where these specific values will be used most. Other step values may show on the plot, and will give an indication of the potential improvement of power usage by power optimization algorithms during handover, for example. In fact, during a handover, the typical approach is to return to maximum transmit power on the new cell. This will induce positive power steps that can be quite important, especially in an urban environment where cells are generally very small, and consequently very low power levels may be used.

MS Tx Power Step Interval Distribution report Much in the same way that in the previous distribution we looked at the changes in transmit power in terms of power steps, we can gain a better understanding or appreciation of the Power Control mechanism by looking at its behavior in time. Usually, the Power Control algorithms include parameters that control the interval between two consecutive power changes. This is to allow the Mobile Station to effectively change its transmit power, and for the Base Station to make enough control measurements that the power has effectively been reduced or increased.

Example of the Power Step Interval report

The statistical distribution of these intervals should clearly show a peak corresponding to the interval value defined in the parameter settings. Smaller values are usually related to handovers, while bigger values indicate that the power has been stabilized. The latter is a good indication of the proper tuning of the Power Control algorithm, but also gives a view on the dynamics of the radio link. A drive test at low speed in an area of good coverage will show the occurrence of longer intervals, while for a high-speed drive test along a highway, the power will almost never stabilize. www.actix.com

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4 Combined analysis User applications, IP protocols and radio link A complete set of indicators is available for the analysis of user applications (http, WAP, email, FTP, etc.) and the protocols (TCP, UDP, ICMP, etc. on top of IP). These let you investigate the user perception of the quality of the offered services. You can build specific queries and reports to combine the performances with the radio link status. The different equipment vendors use similar methodologies for their drive test data testing products. However, they do use different terminology. Analyzer uses a common terminology, independent from any individual vendor terminology. From the drive test files you can obtain the following attributes (the actual number of available attributes will depend on the collection device):

Measurements

Attributes

Connection Measurements

Connection Setup Time Connection Duration Connection Failures Successful Connections Dropped Connections Connection IP Address

Application Measurements

Application Throughput UL and DL (Average, Peak, Minimum, Instantaneous) Application Received Bytes Application Transmitted Bytes Application Elapsed Time Application Delay UL and DL (Average, Peak, Minimum, Instantaneous) Application Round Trip Time (RTT) Application Round Trip Time Max

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Measurements

Attributes

Protocol Measurements

IP Throughput

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ICMP Throughput PPP Throughput TCP Throughput Protocol Received Bytes Protocol Transmitted Bytes Protocol Elapsed Time

…and others specific to single applications (Wen, Ping, FTP, WAP).

Using an IP sniffer allows an independent collection of many of the previous attributes, complementing them with other parameters such as: For IP

IP Address (source and destination) Header and Payload Length Indicators of QoS class (precedence, delay, reliability)

For TCP

TCP port number (source and destination: it allows the identification of the application) TCP connection establishment TCP connection closure TCP connection establishment abort TCP retransmissions TCP transmissions timeout Session Payload Length Acknowledges Time with no acknowledge TCP Session duration …and others.

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Example of reports combining application and radio analysis

Radio network and IP/application layers Analyzer provides the following features for advanced evaluation of radio link and subscriber-perceived performance: Detailed attributes for every network layer up to the application layer. User-definable queries allow custom metric calculation based on finite-state event detection and time-series attributes. User-definable reports to build ad-hoc statistical summaries of subscriberperceived performance matrices. While investigating the data, users can also benefit from the complete set of internal tools and features provided with Analyzer, such as: Mapping, charting and reporting modules Message and protocol stack browsers Time-series and multi-dimensional statistical query module Network element database Filtering and binning module Finite-state event detection engine

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Data merging and synchronization / correlation module Open data import and export module These features are described in the online help. Some examples of troubleshooting using ad hoc analysis are provided below. You can use these as the basis for creating other types of analysis, depending on the specific type of investigations required.

Example 1 Operators are focused on verifying the service as perceived by the subscribers. To do that, it is necessary to identify the services used and evaluate the user perceived performance indicators (typically throughput and delay). It is possible to use predefined queries (provided during the training courses) that give the overall view of the single tasks (FTP sessions in this case):

List of application tasks displayed in the Statistical Explorer

The proposed drive test shows a connection (result of the PdP Context Activation), and then a ping is performed. The ping—although not a user application—is often used to provide an indication of the minimum delay that the network can support. The actual service used is an FTP download and upload of 30K and 15K. The throughput results are generally good: around 30 kbps using 3 timeslots (see below).

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Timeslot allocation statistics report

Only one task (row 6, highlighted in the previous screenshot) is not showing a performance in line with the others, and should be investigated in more detail. The following chart enables us to visualize the content of the complete drive test, and shows the user-perceived metrics (application throughput and delay), combined with the corresponding network parameters (LLC and RLC throughput):

Delay and throughput for the complete drive test

Again, task number 6 shows a throughput discontinuity, responsible for the overall low throughput.

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We can now investigate to see if radio events like cell reselection are responsible for the throughput degradation. In the screenshot below, the DL TBF number (TFI) is displayed and shows a regular pattern.

Cell reselection impact

The cell reselection has an impact on the next task but not on number 6. We can focus on the task filtering it by selecting the task in the Statistics Explorer and clicking the Filter button:

Filtering a task

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Looking at the DT GPRS Radio link performance analysis module (see below) it is clear that one of the two cells driven during that task (automatically everything has been filtered in accordance to it) has a quality problem (mean Rx Quality = 3 with mean C-value of –61 dBm):

DT GPRS Radio link performance analysis

To make this more explicit, the report on the level and quality can be run on that cell. The interference analysis graph shows what is clearly an interference problem:

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DT GPRS Radio link performance analysis

The result of the analysis is, therefore, that the application is showing a good performance, but a specific cell is showing interference. This can be eliminated, for instance by revising the frequency plan.

Example 2 This example focuses on studying the throughput on the different layers (application, TCP, IP and RLC), using the information from the drive test combined with the IP sniffer data. The first step is to display a summary with a query in the Statistics Explorer:

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Downlink throughput study for the single tasks

The task type (i.e. application in use) is obtained using the TCP source port number (that indicates the type of application that is generating the downlink traffic) and ICMP type (some pings are occurring between the FTP downloads). The focus is on the first FTP session, filtering it and using the reports of the radio link module. The radio performance is good: level and quality, RLC and LLC throughput, timeslot allocation is 3 TS all the time, the CS used is CS2 92% of the time. However, the throughput is not maintained at the maximum all the time—this is unexpected since FTP is used and 3 timeslots are constantly allocated.

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Cell Explorer filtered on the first task: level and quality are very good

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Throughput report for the first task

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Throughput and coding scheme per timeslot

If there is no radio problem, let us raise the analysis to the higher layers, displaying the attribute TCP_Data_Pending_AckDL (indicates the total bytes with an acknowledgement pending indownlink), and TCP_Network_Bytes_Acknowledged (indicates the total bytes acknowledged from every acknowledgement message):

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TCP investigation on a chart

The red ellipse above corresponds to the red square shown below. They show that the packets in downlink are no longer acknowledged, and the pending bytes accumulate until they reach the size of the TCP receiving window (equal to 16072 bytes). At that point, the receiving buffer is full and the packets would be discarded, so the transmission is stopped. In fact, the throughput goes to zero. The figure below shows that the packets are received on the PC COM port (the sequence number continues to be incremented) but the corresponding acknowledgements do not (the uplink packets maintain the same acknowledgement number): the FTP application on the PC is probably causing this. When it is able to send the updated acknowledge, the transmission restarts.

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TCP investigation on sequence numbers, acknowledges and received bytes

Also in the rest of the session there are other events like this, but they have a smaller effect on the throughput because the receiving window does not saturate again (the acknowledge restart before the pending bytes reach the window size). So, in this example, an application problem was found in that the FTP client on the PC was not able to process all the received data. In case we want to analyze other tasks, we would need to go back to the old query and disable the filter on task number 1, select another task and repeat the analysis.

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