zte traffic statistic analysis

GBO_012_E1_0 Traffic Statistic Analysis

Course Objectives:

Understand the significance of GSM traffic statistics in

network optimization

Grasp common indices in GSM traffic statistics

Use GSM traffic statistics for problem analysis and

locating

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Contents

1 Traffic Statistics Overview......................................................................................................................... 1

1.1 Traffic Statistics Significance............................................................................................................ 1

1.2 Traffic Statistics Principles and Functions ........................................................................................ 1

1.3 OMCR Performance Management.................................................................................................... 2

1.3.1 System Functions ................................................................................................................... 2

1.3.2 Implementation Process ......................................................................................................... 4

2 Traffic Statistics Functions ........................................................................................................................ 7

2.1 Performance Analysis Report............................................................................................................ 7

2.2 Traffic Statistics Analysis Functions ................................................................................................. 7

2.3 Observation Task Management ....................................................................................................... 12

2.4 Signaling Tracing ............................................................................................................................ 12

2.5 Call Tracing..................................................................................................................................... 12

3 Traffic Statistics Indices........................................................................................................................... 15

3.1 Traffic Statistics Index Category and Content................................................................................. 15

3.1.1 Original Key Performance Indices....................................................................................... 15

3.1.2 Combined Indices................................................................................................................. 15

3.2 Counters and Signaling Points ........................................................................................................ 15

3.2.1 SDCCH Counters and Signaling Points ............................................................................... 15

3.2.2 TCH Counters and Signaling Points .................................................................................... 18

3.2.3 Handover Counters and Signaling Points............................................................................. 24

3.3 Traffic Statistics Index Definitions.................................................................................................. 27

4 Problem Locating and Analysis Through Traffic Statistics .................................................................. 31

4.1 Traffic Statistics Analysis Preparations........................................................................................... 31

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4.2 Traffic Statistics Analysis Solution..................................................................................................31

4.2.1 Overview...............................................................................................................................31

4.2.2 Common Traffic Statistics Analysis Procedures and Method...............................................32

4.2.3 Combination with Other Network Optimization Methods ...................................................32

4.3 TCH Call Drop Problems ................................................................................................................33

4.3.1 TCH Call Drop Types ...........................................................................................................33

4.3.2 Call Drop Signaling Points ...................................................................................................33

4.3.3 TCH Call Drop Solutions .....................................................................................................35

4.4 Handover Problems .........................................................................................................................39

4.4.1 Handover Problem Analysis .................................................................................................39

4.4.2 Querying Handover Indices ..................................................................................................39

4.4.3 Handover Problem Solutions ................................................................................................39

4.5 TCH Congestion Problems ..............................................................................................................41

4.5.1 TCH Congestion Problem Analysis ......................................................................................41

4.5.2 TCH Congestion Problem Solutions.....................................................................................42

4.6 SDCCH Congestion Problem ..........................................................................................................44

4.6.1 SDCCH Congestion Problem Analysis.................................................................................44

4.6.2 SDCCH Congestion Problem Solutions ...............................................................................44

4.7 TCH Allocation Problem .................................................................................................................46

4.7.1 TCH Allocation Process........................................................................................................46

4.7.2 TCH Allocation Failure Signaling Points .............................................................................47

4.7.3 TCH Assignment Failure Problem Analysis .........................................................................49

5 Traffic Statistics Cases..............................................................................................................................51

5.1 Call Drop Problem...........................................................................................................................51

5.2 Handover Problem...........................................................................................................................52

5.3 TCH Congestion Problem................................................................................................................53

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5.4 SDCCH Congestion Problem.......................................................................................................... 54

5.5 SDCCH Allocation Problem ........................................................................................................... 55

5.6 TCH Allocation Failure Problem .................................................................................................... 56

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1 Traffic Statistics Overview

1.1 Traffic Statistics Significance

Traffic statistics is very important in network optimization, which helps to understand

various network performance indices. All communication networks should be

monitored and measured, providing quantified service indices. The GSM network

performance is monitored and measured through traffic statistics by various network

elements.

Common traffic statistics indices include: call drop rate, congestion rate, handover

success rate, TCH assignment success rate, radio system connection rate, traffic, and

channel availability.

Traffic statistics is an important part of network optimization and maintenance. It

provides a platform for network problem analysis. Through the traffic statistics analysis,

users can grasp the radio network running situation, which facilitates network planning,

network optimization, and fault handling, and helps operators to gain more profit.

1.2 Traffic Statistics Principles and Functions

Traffic statistics analysis is an important function of OMCR. The following lists main

functions of OMCR:

Performing BSS configuration management

Handling various problems in BSS running

Performing performance analysis, statistics, and adjustment for BSS

Providing access interface for upper-level Network Management Center (NMC)

Figure 1.2-1 shows the position of OMCR in Public Land Mobile Network (PLMN)

system.


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Figure 1.2-1 OMCR Position in PLMN System

The OMC system is of the client/server structure, as shown in Figure 1.2-2. The

application is realized by the application server. The client can not directly establish

communication with BSS, and it only provides functions of inputting operation

instructions and outputting operation results for users,

Figure 1.2-2 Structure of OMC and BSS

1.3 OMCR Performance Management

1.3.1 System Functions

OMCR performance management involves measurement task management,

observation task management, QoS alarm monitoring, performance analysis, and report


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system.

Creating measurement task: to create a new measurement task.

Modifying measurement task: to modify a created measurement task.

Deleting measurement task: to delete a created measurement task.

Pausing measurement task: to make a created measurement task pause.

Resuming measurement task: to resume a created measurement task.

Querying counter for a measurement task: to query the current value of a counter

of a measurement object in a measurement task.

Querying historical data for a measurement task: to query the historical data of a

measurement task, the query condition might contain object, time, etc.

Creating observation task: to create an observation task.

Deleting observation task: to delete an observation task.

Pausing observation task: to make an observation task pause.

Resuming observation task: to resume an observation task.

Modifying QoS alarm threshold: to modify the alarm threshold.

Pausing QoS task: to make a started QoS task pause.

Resuming QoS task: to resume a paused QoS task.

Event observation function

Synchronization function: to perform synchronization when consistency exist in

the following aspects, which is caused by abnormalities:

♦ Inconsistency between data of measurement tasks;

♦ Inconsistency between data of observation tasks;

♦ Inconsistency between QoS management object data and OMC data.

Configuring report: to complete the CS-configured report and output it in Excel

form.

Configuring GPRS report: to complete the PS-configured report and output it in

Excel form.


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Performance report: to display the performance data collection and analysis result

in the form of report and output it in Excel form.

Outputting report in Excel form: to output report in Excel form.

Graphical analysis function of report: to analyze report data in the form of line

chart, table, and pie chart, etc.

Customizing performance indices: to customize indices (if an index that user

requires does not exist in the default report, the index can be customized).

Performance report template: to provide the report template. User can save the

customized indices or system indices in the template for making report.

Creating automation report task: to provide the function of generating report

automatically.

Modifying automation report task: to modify a created automation report task.

Deleting automation report task: to delete a created automation report task.

Automation report log management: to view the automation report generation

situation through the report log management.

Timed performance data dump: to perform timed dump for performance data to

guarantee that the data volume in the database does not increase without any

restriction.

1.3.2 Implementation Process

Performance management client: Graphical User Interface (GUI) is provided for

various performance-related operations to display the measurement task information,

and to display performance data in the form of list, Excel report, and graphics.

Performance management Local Management Function (LMF) end: Various

performance management operations from the client are received and processed. For

any operation to be sent to the foreground, such as creating a measurement task,

convert it into the corresponding operation primitive command of Common

Management Information Service (CMIS) and send it to the LAF end for processing.

For other commands, such as performance data query and performance data dump,

directly access the memory or database to complete the operation and then return the

result to the client. The performance management LMF end also receives and processes


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CMIS response message from the LAF end and the reported observation event

message.

Performance management Local Access Function (LAF) end: The CMIS operation

request from the LMF end, such as creating the measurement task, is received. After

necessary validity check, it is forwarded to the foreground for processing. The response

from foreground is received and returned to the LMF end for processing. The

observation event reported from foreground is received and forwarded to the LMF end

for processing. The performance data reported from foreground is received and

processed, and such data are stored in the database through calling the database

interface.

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2 Traffic Statistics Functions

2.1 Performance Analysis Report

ZTE OMCR has powerful network performance index statistics functions, which

includes:

Creating performance report

Defining report template

Automatically generating report

Customizing user index formula

Setting timed automatic dump for performance data

It also provides statistics functions for traffic of different types, different levels, and

different ranges.

2.2 Traffic Statistics Analysis Functions

ZTE OMCR provides many types of measurement task management functions.

Basically, ZTE traffic statistics falls into the following six types:

Circuit Switching (CS) service basic measurement

BTS measurement

Radio measurement

A-interface measurement

Resource occupation measurement

Packet Switching (PS) service measurement

1. CS basic measurement

CS service basic measurement (commonly used)

The basic measurement task is the measurement used to generate the basic

performance report, which includes information such as the resource situation,

service situation, and channel quality. It gives an overall description of the


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entire network situation.

Q3 module measurement

In Q3 module measurement, some service data statistics are performed

according to modules. Such statistics are mostly related to assignment,

handover, and CPU load, and are mainly performed for RMM (for SMM, only

the CPU load statistics is performed).

Q3 cell BTS measurement counters

The BTS power, signal level, signal quality, and call distance are measured

according to cells. The measurement values of all carriers in the cell are

accumulated in the carrier power control counter and then reported.

2. BTS measurement

Power control measurement

According to the following two conditions, BTS decides whether the power

control should be performed for MS or BTS:

♦ The receiving level and receiving quality in MS measurement report

♦ The receiving level and receiving quality measured by BTS

BTS then performs statistics according to different power control reasons.

Carrier service measurement (commonly used)

It takes the carrier as the measurement unit and measures information related

to channel activation, assignment, handover success or handover failure,

carrier interference band, and maximum and minimum uplink/downlink signal

level. It helps to understand the carrier situation, facilitating adjustment and

maintenance.

Paging measurement

It performs statistics for discarded pagings and queue length, with the carrier

as the measurement unit.

Carrier-level basic measurement

It measures the carrier occupation situation, including:

♦ The number of times of TCH being occupied and the relevant occupation


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duration

♦ The number of times of SDCCH being occupied and the relevant

occupation duration

3. Radio measurement

Cell radio measurement (commonly used)

It measures the interference on each channel (in idle state and in the state of

being occupied) in the cell and the service quality.

Radio access measurement

It measures the MS’s radio random access process, including the number of

access attempts due to different reasons, the number of processing times, and

the number of successes.

SDCCH measurement

It measures the SDCCH allocation, occupation, assignment, and usage,

reflecting the SDCCH service situation.

TCH/F measurement

It measures TCH/F-related resource allocation, occupation, assignment, and

usage. It is used in channel configuration and relevant parameter adjustment.

SAPI3 measurement

It measures the number of point-to-point short message link establishments

and the number of received messages and sent messages.

RMM assignment measurement

It describes the RMM assignment situation, including assignment attempts

due to various reasons, assignment executions, and queuing.

RMM call drop measurement

It measures the number of RMM call drops on various channels in the

signaling flow, RMM call drop causes. It helps to understand the network

running situation, facilitating network performance evaluation and network

parameter adjustment.

Handover cause measurement


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It measures the number of incoming handover (outgoing handover) attempts

due to various causes.

General handover measurement

It performs statistics for handover due to various causes, including the number

of handover attempts, executions, successes, and failures. It reflects the

handover success rate.

Handover synchronization measurement

It measures the number of handovers of various synchronization modes.

Adjacent cell handover measurement (commonly used)

It measures the handovers between some cells and their adjacent cells.

Paging measurement

Abis interface signaling statistics

It measures the number of signalings transmitted at Abis interface.

Radio resource availability measurement

It measures the radio channel resource usage of each cell.

HR statistics measurement

It is a basic measurement added according to users’ requirement. It mainly

describes the TCH/H resource usage and service situation.

4. A-interface measurement

A-interface signaling statistics measurement

It measures the number of signalings generated by BSC, forwarded by BSC,

and received by BSC. The measurement is performed according to signaling

name and signaling type.

A-interface assignment, call drop, and handover statistics measurement

It measures the number of assignments, call drops, and handovers at

A-interface, and performs statistics for failures according to corresponding

types. It facilitates problem locating and handling.

SCCP connection measurement and terrestrial circuit resource availability


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measurement

It measures the SCCP link establishment situation and the availability of

terrestrial circuit resource.

5. Resource occupation measurement

TRX LAPD link measurement

It measures the signaling interaction on the LAPD signaling link of each TRX.

The measurement task is performed when the LAPD board transmits or

receives message, and the measurement unit is TRX LAPD link.

O&M LAPD link measurement

It measures the signaling interaction on each O&M LAPD signaling link. The

measurement task is performed when the LAPD board transmits or receives

message, and the measurement unit is O&M LAPD link.

SCCP link measurement

It measures the signaling interaction on each SCCP signaling link, and the

measurement unit is SCCP link.

Processor load measurement

It measures the CPU load, occupied memory, service load, and file system

load of each SMM and each RMM. The measurement unit is module. The

measurement task is performed periodically (usually 5 minutes).

6. PS service measurement

PS basic measurement

This measurement task generates the PS basic performance report, which

contains information of the resource usage, service situation, and channel

quality, etc. comprehensively describing the PS network situation.

NS measurement

It takes a single Network Service Virtual Connection (NSVC) as the

measurement entity and measures data transmission of NSVC link, signaling

interaction, and abnormality if there is any.

BSSGP measurement


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It measures the message transceiving situation on BSSGP layer at Gb interface

in GPRS service.

NSE measurement

It takes a single NSE as the measurement entity and measures the number of

paging, the number of state indication messages, and the number of signaling

BVC resetting.

PS traffic statistics measurement

It performs statistics for the cell services, including the radio block usage and

changes in coding schemes.

Resource management measurement

It performs statistics related to BSC system resource usage, including the

channel resource usage measurement and measurement of request, access, and

assignment.

2.3 Observation Task Management

The observation task is started according to certain task scheduling rules. Data

collection for the observation object is triggered by the observation event, in other

words, once an observation event is triggered, the observation report is generated and

reported immediately, and real-time analysis for the observation object can be

performed.

2.4 Signaling Tracing

The signaling tracing is performed during the commissioning and debugging process

(with low traffic), to realize BSC tracing from one terminal and save the tracing file at

the same time for future analysis. It facilitates checking the signaling flow to locate the

signaling fault.

2.5 Call Tracing

To perform the call tracing, MSC must initiate the tracing of specific resource first,

MSC then sends message to activate BSC to perform call tracing. After that, press the

Start button at the call tracing client interface to receive the call tracing message sent


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from the foreground.

Note:

This document mainly introduces the traffic statistics analysis. For OMCR operation

details, refer to relevant materials.

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3 Traffic Statistics Indices

3.1 Traffic Statistics Index Category and Content

3.1.1 Original Key Performance Indices

SDCCH congestion rate

SDCCH assignment success rate

TCH congestion rate

TCH assignment success rate

TCH call drop rate

Handover success rate

Voice channel availability

3.1.2 Combined Indices

Traffic call drop ratio

Radio system connection rate

Worst cell ratio

3.2 Counters and Signaling Points

3.2.1 SDCCH Counters and Signaling Points

1. C11603 Number of SDCCH call attempts

Meaning:

This counter counts the number of SDCCH call attempts in the cell. The call

attempt includes the following cases that require SDCCH allocation: normal

originated call, location update, call reestablishment, IMSI request, IMSI

detachment, and short message request.

Calculation formula:


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C11603 (number of SDCCH call attempts) = C10101 (number of SDCCH

occupation attempt (for assignment)) + C10104 (number of SDCCH

occupation attempt (for handover))

2. C11604 Number of SDCCH overflows

Meaning:

This counter counts the number of SDCCH call attempts that fail to occupy

the SDCCH channel. If SDCCH channel is requested successfully but the

actual assignment fails, this counter does not count.


C11604 (number of SDCCH overflows) = C10103 (number of SDCCH

occupation failures (for assignment)) + C10106 (number of SDCCH

occupation failures (for handover)

3. C11644 Number of SDCCH assignment successes

Meaning:

This counter counts the number of MS successfully accessing SDCCH after

BSC sending the immediate assignment message IMM_ASS.

After BSC responds to the channel request message and successfully activates

SDCCH, BSC sends the immediate assignment message IMM_ASS to MS to

notify MS to use this channel. After MS receives the message, MS sends the

SABM frame to BTS on SDCCH, and BTS sends the ES_IND message to

BSC.

If BSC receives the correct EST_IND message within specified time, it

indicates that the SDCCH assignment succeeds, and the counter accumulates.

Measurement point:

The counter counts when BSC receives the correct EST_IND message or the

assignment completion message.

4. C11645 Number of SDCCH assignment failures

Meaning:

This counter counts the number of MS failing to access SDCCH after BSC

sending the immediate assignment message IMM_ASS.


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After BSC responds to the channel request message and successfully activates

SDCCH, BSC sends the immediate assignment message IMM_ASS to MS to

notify MS to use this channel. After MS receives the message, MS sends the

SABM frame to BTS on SDCCH, and BTS sends the ES_IND message to

BSC.

If BSC receives the incorrect EST_IND message or T3101 is timeout, then the

SDCCH assignment fails, and the counter and C10115 accumulates

simultaneously.

Measurement point:

The counter counts when BSC receives the incorrect EST_IND message or

when T3101 is timeout.

5. C11605 Number of SDCCH call drops

Meaning:

This counter counts the number of call drops during the call process when

SDCCH is assigned but TCH is not occupied.


C11605 (number of SDCCH call drops) = C10643 (number of SDCCH call

drops)

6. C11606 SDCCH traffic during busy hour

Meaning:

This counter counts the total traffic on SDCCH during busy hour.


C11606 (total SDCCH busy time) = C11504 (total SDCCH busy time)

Total traffic = Total SDCCH busy time / statistics period


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MS BTS BSC

CHL_REQ

CHL_RQD

A1

CHL_ACT

CHL_ACT_ACK

A2

IMM_ASS_CMD

IMM_ASS

SABM

EST_IND

A3

SDCCH congestion

SDCCH assignment success

Figure 3.2-1 SDCCH Performance Measurement Signaling Measurement Point

3.2.2 TCH Counters and Signaling Points

1. C11607 Number of available voice channels

Meaning:

This counter counts the number of TCHs that can be assigned normally,

including the number of available TCH/Fs and the number of available

TCH/Hs.


C11607 (number of available voice channels) = C11507 (average number of

available TCH/Hs) + C11513 (average number of available TCH/Fs)

2. C11608 Number of unavailable voice channels

Meaning:

This counter counts the number of TCHs that can not be assigned normally,

including the number of unavailable TCH/Fs and the number of unavailable

TCH/Hs.



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C11608 (number of unavailable voice channels) = C11508 (average number of

unavailable TCH/Hs) + C11514 (average number of unavailable TCH/Fs)

3. C11609 Number of call attempts on voice channel (excluding handover)

Meaning:

This counter counts the number of call attempts on TCH (occupation attempt)

after all SDCCHs are occupied in the cell. The call attempt includes cases that

the calling/called party attempts to establish a call, including TCH being

assigned as SDCCH during the Very Early Allocation (VEA) and excluding

various handover situations. The TCH channel includes the TCH/F channel

and the TCH/H channel.


C11609 (number of call attempts on voice channel (excluding handover)) =

C10301 (number of TCH/F occupation attempts (signaling) (for assignment))

+ C10320 (number of TCH/F occupation attempts (voice) (for assignment)) +

C10351 (number of TCH/F occupation attempts (data) (for assignment)) +

C10401 (number of TCH/H occupation attempts (signaling) (for assignment))

+ C10420 (number of TCH/H occupation attempts (voice) (for assignment)) +

C10451 (number of TCH/H occupation attempts (data) (for assignment))

4. C11610 Number of voice channel overflows (excluding handover)

Meaning:

This counter counts the number of overflows of calls for TCH after all

SDCCHs are occupied in the cell. The overflow includes cases that the

calling/called party fails to establish a call on TCH after occupying SDCCH,

including TCH being assigned as SDCCH during the Very Early Allocation

(VEA) and excluding various handover situations. The TCH channel includes

the TCH/F channel and the TCH/H channel.


C11610 (number of voice channel overflows (excluding handover)) =

C10303 (number of TCH/F occupation failures (signaling) (for assignment)) +

C10322 (number of TCH/F occupation failures (voice) (for assignment)) +

C10353 (number of TCH/F occupation failures (data) (for assignment)) +


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C10403 (number of TCH/H occupation failures (signaling) (for assignment))

+ C10422 (number of TCH/H occupation failures (voice) (for assignment)) +

C10453 (number of TCH/H occupation failures (data) (for assignment))

5. C11611 Number of call attempts on voice channel (including handover)

Meaning:

This counter counts the number of call attempts on TCH (occupation attempt)

after all SDCCHs are occupied in the cell. The call attempt includes cases that

the calling/called party attempts to establish a call, including TCH being

assigned as SDCCH during the Very Early Allocation (VEA) and including

various handover and direct retry situations. The TCH channel includes the

TCH/F channel and the TCH/H channel.


C11611 (number of call attempts on voice channel (including handover)) =

C11609 (number of call attempts on voice channel (excluding handover)) +

C10304 (number of TCH/F occupation attempts (signaling) (for handover)) +

C10323 (number of TCH/F occupation attempts (voice) (for handover)) +

C10354 (number of TCH/F occupation attempts (data) (for handover)) +

C10404 (number of TCH/H occupation attempts (signaling) (for handover)) +

C10423 (number of TCH/H occupation attempts (voice) (for handover)) +

C10454 (number of TCH/H occupation attempts (data) (for handover))

6. C11612 Number of voice channel overflows (including handover)

Meaning:

This counter counts the number of overflows of calls for TCH after all

SDCCHs are occupied in the cell. The overflow includes cases that the

calling/called party fails to establish a call on TCH after occupying SDCCH,

including TCH being assigned as SDCCH during the Very Early Allocation

(VEA) and including various handover and direct retry situations. The TCH

channel includes the TCH/F channel and the TCH/H channel.


C11612 (number of voice channel overflows (including handover)) =

C11610 (number of voice channel overflows (excluding handover)) + C10306


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(number of TCH/F occupation failures (signaling) (for handover)) + C10325

(number of TCH/F occupation failures (voice) (for handover)) + C10356

(number of TCH/F occupation failures (data) (for handover)) + C10406

(number of TCH/H occupation failures (signaling) (for handover)) + C10425

(number of TCH/H occupation failures (voice) (for handover)) + C10456

(number of TCH/H occupation failures (data) (for handover))

7. C11613 Number of voice channel being occupied (excluding handover)

Meaning:

This counter counts the number of TCH being occupied successfully

(excluding handover), including voice occupation and data occupation. The

TCH channel includes the TCH/F channel and the TCH/H channel.


C11613 (number of voice channel being occupied (excluding handover)) =

C10302 (number of TCH/F being occupied successfully (signaling) (for

assignment)) + C10321 (number of TCH/F being occupied successfully (voice)

(for assignment)) + C10352 (number of TCH/F being occupied successfully

(data) (for assignment)) + C10402 (number of TCH/H being occupied

successfully (signaling) (for assignment)) + C10421 (number of TCH/H being

occupied successfully (voice) (for assignment)) + C10452 (number of TCH/H

being occupied successfully (data) (for assignment))

8. C11614 Number of voice channel being occupied (including handover)

Meaning:

This counter counts the number of TCH being occupied successfully

(including handover), including voice occupation and data occupation. The

TCH channel includes the TCH/F channel and the TCH/H channel.


C11614 (number of voice channel being occupied (including handover)) =

C11613 (number of voice channel being occupied (excluding handover)) +

C10305 (number of TCH/F being occupied successfully (signaling) (for

handover)) + C10324 (number of TCH/F being occupied successfully (voice)

(for handover)) + C10355 (number of TCH/F being occupied successfully


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(data) (for handover)) + C10405 (number of TCH/H being occupied

successfully (signaling) (for handover)) + C10424 (number of TCH/H being

occupied successfully (voice) (for handover)) + C10455 (number of TCH/H

being occupied successfully (data) (for handover))

9. C11657 Number of TCH assignment successes

Meaning:

This counter counts the number of the Establish Indication message or the

Assignment Complete message being received after TCH assignment

succeeds. TCH assignment includes assignment of signaling, voice, and data.


C11657 = C10314 + C10345 + C10364 + C10414 + C10445 + C10464

10. C11658 Number of TCH assignment failures

Meaning:

This counter counts the number of the Establish Indication message or the

Assignment Complete message being not received after TCH assignment

succeeds. TCH assignment includes assignment of signaling, voice, and data.


C11658 = C10315 + C10346 + C10365 + C10415 + C10446 + C10465

11. C11615 Number of call drops on voice channel

Meaning:

This counter counts the number of call drops due to radio reasons after the

TCH channel is assigned successfully, including call drops during handover

and call drops after handover. The TCH channel includes the TCH/F channel

and the TCH/H channel.


C11615 (number of call drops on voice channel) = C10644 (number of call

drops on TCH/F) + C10645 (number of call drops on TCH/H)

12. C11616 Total traffic on voice channel

Meaning:


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This counter counts the total traffic of all TCH channels. The TCH channel

includes the TCH/F channel and the TCH/H channel.


C11616 (total traffic channel busy time) = C11511 (total TCH/H busy time) +

C11517 (total TCH/F busy time)

Total traffic on voice channel = C11616 (total traffic channel busy time) / statistics

period

TCH performance measurement signaling statistics point:

M S BTS BSC

CHL_REQ

CHL_RQD

A1

CHL_ACT

CHL_ACT_ACK

A2

IM M _ASS_CM D

IM M _ASS

SABM

EST_IND

A3

Figure 3.2-2 Immediate Assignment Flow

MS BTS BSC

B1CHL_ACT

ASS_CMD

B2ASS_CMD

ASS_CMD

SABM

EST_IND

B3

MSC

CHL_ACT_ACK

UA

ASS_COMASS_COM

ASS_COMB4

Figure 3.2-3 Common Assignment Flow


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3.2.3 Handover Counters and Signaling Points

1. C11617 Number of handover requests

Meaning:

This counter counts the total number of requests for intra-BSC

inter-cell/intra-cell handover and inter-BSC inter-cell handover. This counter

counts when the request is for incoming handover.


C11617 (number of handover requests) = C10912 (number of BSC-controlled

inter-cell incoming handover executions) + C10915 (number of

MSC-controlled incoming handover executions (common)) + C10917

(number of MSC-controlled incoming handover executions (forced release)) +

C10919 (number of MSC-controlled incoming handover executions (queuing))

+ C10982 (number of intra-cell handover successes)

2. C11618 Number of handover successes

Meaning:

This counter counts the total number of successes of intra-BSC

inter-cell/intra-cell handover and inter-BSC inter-cell handover. This counter

counts when the incoming handover succeeds.


C11618 (number of handover successes) = C10913 (number of

BSC-controlled inter-cell incoming handover successes) + C10920 (number of

MSC-controlled incoming handover successes) + C10982 (number of

intra-cell handover successes)

3. C11619 Number of dual-band handover call attempts

Meaning:

This counter counts the number of handover attempts between GSM 900

system and DCS 1800 system, including handovers in the same layer and

handovers between different layers.


C11619 (number of dual-band handover call attempts) = C10934 (number of


25

incoming handover executions from undefined layer (900 → 1800)) + C10939

(number of incoming handover executions from undefined layer (1800 →

900)) + C10952 (number of hetero-frequency incoming handover executions

from the same layer (900 → 1800)) + C10957 (number of hetero-frequency

incoming handover executions from upper layer (900 → 1800)) + C10962

(number of hetero-frequency incoming handover executions from lower layer

(900 → 1800)) +C10967 (number of hetero-frequency incoming handover

executions from the same layer (1800 → 900)) + C15972 (number of

hetero-frequency incoming handover executions from upper layer (1800 →

900)) + C15977 (number of hetero-frequency incoming handover executions

from lower layer (1800 → 900))

4. C11620 Number of dual-band handover successes during busy hour

Meaning:

This counter counts the number of handover successes between GSM 900

system and DCS 1800 system, including handovers in the same layer and

handovers between different layers.


C11620 (number of dual-band handover successes) = C10935 (number of

incoming handover successes from undefined layer (900 → 1800)) + C10940

(number of incoming handover successes from undefined layer (1800 → 900))

+ C10953 (number of hetero-frequency incoming handover successes from

the same layer (900 → 1800)) + C10958 (number of hetero-frequency

incoming handover successes from upper layer (900 → 1800)) + C10963

(number of hetero-frequency incoming handover successes from lower layer

(900 → 1800)) + C10968 (number of hetero-frequency incoming handover

successes from the same layer (1800 → 900)) + C15973 (number of

hetero-frequency incoming handover successes from upper layer (1800 →

900)) + C15978 (number of hetero-frequency incoming handover successes

from lower layer (1800 → 900))

Handover performance measurement signaling statistics point:


26

MS BTS BSC

C1CHL_ACT

C2ASS_CMD

ASS_CMD

SABM

EST_IND

C3

MSC

CHL_ACT_ACK

UA

ASS_COMASS_COM

HO_PREFORM

MEAS_REP

MEAS_RES

Figure 3.2-4 Internal Handover Flow (Intra-Cell Handover)

Figure 3.2-5 BSC-Controlled Inter-Cell Handover Flow


27

MS BTS BSC

D1CHL_ACT

HO_REQ

D2

SABM

HO_DETECT

D3

MSC

CHL_ACT_ACK

UA

HO_COMHO_COM

HO_COM

HO_REQ_ACK

HO_DETECT

HO_ACCESS

PHY_INFO

Figure 3.2-6 External Handover Flow

3.3 Traffic Statistics Index Definitions

1. SDCCH congestion rate

SDCCH congestion rate (%) = (number of SDCCH overflows / number of SDCCH

call attempts) × 100% = C11604 / C11603 × 100%

2. SDCCH call drop rate

SDCCH call drop rate (%) = (number of call drops on SDCCH / number of

SDCCH being occupied successfully) × 100% = C11644 / (C11644 + C11645) ×

100%

3. SDCCH assignment success rate

SDCCH assignment success rate (%) = number of SDCCH assignment successes /

number of SDCCH assignment attempts × 100%

4. TCH congestion rate

TCH congestion rate (excluding handover) (%) = number of TCH overflows

(excluding handover) / number of call attempts on TCH (excluding handover)

× 100% = C11610 / C11609 × 100%

TCH congestion rate (including handover) (%) = number of TCH overflows

(including handover) / number of call attempts on TCH (including handover)


28

× 100% = C11612 / C11611 × 100%

5. TCH assignment success rate

TCH assignment success rate (%) = voice channel allocation success rate

(excluding handover) = number of service channel being occupied (excluding

handover) / number of call attempts on voice channel during busy hour (excluding

handover) × 100%

6. TCH call drop rate

TCH call drop rate (excluding handover) (%) = number of call drops on voice

channel / number of voice channel being occupied (excluding handover) ×

100% = C11615 / C11613 × 100%

TCH call drop rate (including handover) (%) = number of call drops on voice

channel / number of voice channel being occupied (including handover) ×

100% = C11615 / C11614 × 100%

Trigger point: after the ASSIGMENT COMPLETE message.

7. Handover success rate

Handover success rate (%) = (number of handover successes / number of

handover requests) × 100% = C11617 / C11616 × 100%

Dual-band handover success rate (%) = (number of dual-band handover

successes / number of dual-band handover requests) × 100% = C11619 /

C11618 × 100%

8. Voice channel availability

Voice channel availability (%) = number of available service channels / number of

configured service channels × 100% = C11607 / (C11607 + C11608) × 100%

9. Traffic call drop rate

Traffic call drop rate = voice channel traffic during busy hour × 60 / number of call

drops on voice channel during busy hour

Service channel traffic: total traffic of voice channels in all cells

Unit: minute, indicating the average interval between two call drops.

10. Radio system connection rate


29

Radio system connection rate = (1 - TCH congestion rate) × (1 – SDCCH

congestion rate)

It reflects the network resource usage.

11. Worst cell ratio

Worst cell ratio = number of worst cells / number of cells × 100%

The worst cell satisfies the following conditions:

During busy hour, traffic per TCH channel > 0.1 Erl, call drop rate on voice

channel > 3% or congestion rate on voice channel > 5%.

31

4 Problem Locating and Analysis Through Traffic Statistics

4.1 Traffic Statistics Analysis Preparations

The GSM system provides comprehensive traffic statistics functions, facilitating

maintenance personnel to analyze and control the network performance. It should be

noticed that the network performance analysis is not based on the traffic statistic data

of a certain day or a certain time, instead, the analysis requires traffic statistic data of a

period of time. Usually, the average traffic statistic data during busy hours in one week

is used to evaluate the network performance. Traffic statistics indices are correlated, for

example, in a cell with high congestion rate, the incoming handover success rate is low.

Any abnormal traffic statistics index might indicate the existence of network problem,

which makes the network maintenance to be performed in time.

Before the traffic statistics is performed, at least one week’s traffic statistic data should

be obtained, including data related to BSC performance measurement, cell

performance measurement (TCH measurement and SDCCH measurement), and

inter-cell handover performance measurement.

Also, information of the network structure, site distribution, and carrier configuration

and capacity should be grasped before the traffic statistics is performed. It is advised to

use ASSET and MAPINFO to print the site distribution map, in which the site name,

cell direction, and BCCH frequency point are marked (information such as BSIC, cell

traffic, antenna height, and transmission power can also be marked in the map),

facilitating comparison in the traffic statistics.

4.2 Traffic Statistics Analysis Solution

4.2.1 Overview

According to the characteristics of traffic statistic data, the traffic statistics analysis is

implemented from the entire network performance measurement to the cell

performance measurement, from major index analysis to minor index analysis. In other

words, during the traffic statistics analysis process, the entire network performance is


32

analyzed first, after deciding which performance indices are poor, perform analysis for

cells that might cause the problems. Network optimization operations can be used in

the traffic statistics analysis for problem locating and analysis.

4.2.2 Common Traffic Statistics Analysis Procedures and Method

General principle: From global range to local range, from network indices of one day

to network indices of one week, from a single index to correlated indices, from major

cells to minor cells.

TOP10 Analysis Method

Application principle

All statistic data are mutually correlated. The standard of evaluating whether an

index is good is not fixed, thus it is difficult to define an exact value as the

threshold distinguishing a good index and a bad index for different systems.

Method description

Use Excel or other software tools to implement data sorting and filtering functions,

to sort traffic statistic data from bad to good. In this way, it is easy to associate

various data and find the internal law, and to locate the problem.

The network quality can be evaluated by examining and comparing the BSC-level

traffic statistics report.

♦ If it is found in the BSC-level traffic statistics report that an important index

(such as the call drop rate or handover success rate) is abnormal, check the

cell-level traffic statistics report for further analysis.

♦ Check cells where only a single index exceeds the standard range or the

absolute number of faults (call drop, congestion, and handover failure) is

large, to decide whether further handling should be performed.

4.2.3 Combination with Other Network Optimization Methods

Drive Test (DT): Simulate the mobile call process to analyze coverage, quality,

handover, and signaling.

Call Quality Test (CQT): Dial a large number of calls at different places.

Signaling tracing: Collect signaling through the signaling testing instrument or the


33

signaling tracing function in OMCR.

4.3 TCH Call Drop Problems

4.3.1 TCH Call Drop Types

Usually, TCH call drop problems falls into the following three categories:

Call drop due to radio link failure

Call drop due to LAPD link failure

Call drop due to handover failure

4.3.2 Call Drop Signaling Points

1. Call drop due to radio link failure

Figure 4.3-1 shows the signaling point of call drop due to radio link failure.

MS

BTS Pn P0 MSC

DT1/BSSM CLR_CMD[reason

=normal.]

I/RLM/DATA REQ

CHAN REL[reason]CHAN REL

[reason]

FACCH/I/RR

DISC

FACCH/[LAPDm]

FACCH/[LAPDm]

UAREL IND[-/-]

I/RLM DT1/BSSM

CLR_CMP[-/-]

I/DCM

DEACT SACCH[-/-]I/DCM

RF CHAN REL[-/-]

RF CHAN REL ACK[-/-]

I/DCM

Meas Report Meas Result

Meas Result

厖 .

CON Fail Ind App/Release CMDDT1/BSSM

CLR_REQ[-/-]

?

C10602 number of radio link failures (on SDCCH)C10608 number of radio link failures (on TCH/F signaling)C10614 number of radio link failures (on TCH/F voice)C10620 number of radio link failures (on TCH/F data)C10626 number of radio link failures (on TCH/H signaling)C10632 number of radio link failures (on TCH/H voice)C10638 number of radio link failures (on TCH/H data)

?

Figure 4.3-1 Call Drop due to Radio Link Failure

2. Call drop due to handover failure

Figure 4.3-2, Figure 4.3-3, and Figure 4.3-4 show signaling points of call drops

due to handover failure.


34

A1

BSCBTS:TRXMS

ASSIGNMENT COMMAND

CHANNEL ACTIVATE

A2

CHANNEL ACTIVATE ACK

SET T3107

T3107 Timeout

Figure 4.3-2 Call Drop due to Intra-Cell Handover Failure

A1

BSCOld BTS:MS

HANDOVER COMMAND

CHANNEL ACT

A2

CHANNEL ACT ACK

New BTS

HANDOVER COMMAND SET T3103

T3103 Timeout

Figure 4.3-3 Call Drop due to Intra-BSC Inter-Cell Handover Failure

Figure 4.3-4 Call Drop due to Inter-BSC Handover Failure


35

4.3.3 TCH Call Drop Solutions

The TCH call drop is usually due to the following causes:

Equipment hardware fault

Poor radio signals, causing poor coverage in some area

Interference (intra-network interference or external interference)

Inappropriate radio parameter settings

Unbalance between uplink and downlink

Unstable transmission or problems in repeater and tower amplifier

This section introduces solutions for call drop problems due to the above causes

respectively.

1. Call drop due to hardware fault

Traffic statistics analysis

♦ Cell performance report: SDCCH availability and TCH availability are

abnormal, which are lower than 100%; TCH allocation failure rate is high,

which is higher than 10%.

♦ BTS measurement: locate call drops and assignment failure at carrier level.

♦ Alarm statistics and dynamic channel observation: the carrier occupation is

abnormal.

♦ NMS measurement: TCH occupation duration is short.

Solution

Locate the hardware fault range according to the above statistics analysis. Check

whether there is any fault in the following parts:

♦ TRX

♦ CMM

♦ TIC

♦ Combiner

♦ Divider

♦ Tower amplifier


36

♦ Repeater

♦ The power amplifier’s output power is too low

2. Call drop due to interference (intra-network interference or external interference)


♦ Analyze the interference band level and occurrence law with the change of

time and traffic.

♦ Block carriers one by one to observe the change of interference band.

♦ Analyze handover causes, and calculate the proportion of handovers due to

poor quality in the total handovers.

♦ Through cell radio measurement, analyze distributions of the signal level and

the quality in the cell.

Solution

(1) Perform the drive test to check interfered routes and signal quality

distribution.

(2) Adjust the site transmission power and antenna downtilt or adjust the

frequency plan for relevant cells to avoid interference.

(3) Use the spectrum analyzer to analyze.

(4) Enable Frequency Hopping (FH), Discontinuous Transmission (DTX) and

power control to reduce interference.

(5) Remove equipment problems, such as TRX’s self-oscillation and antenna’s

intermodulation.

3. Call drop due to weak radio signals and poor coverage


♦ Power control measurement: the average uplink/downlink signal strength is

too low.

♦ Cell radio measurement: the proportion of receiving level being low is too

large.

♦ Handover cause measurement: the signal level is too low when handover is

initiated, the average receiving level is too low.


37

Solution

(1) Perform the drive test in area with poor coverage.

(2) Adjust the following network parameters according to the drive test result.

Site transmission power;

Antenna downtilt and height;

Minimum MS access level;

Adjacent cell relationship;

Threshold of minimum access level for handover candidate cell.

(3) Add sites

4. Call drop due to inappropriate handover parameter settings


♦ Handover parameter settings: check whether there is any inappropriate

parameter setting.

♦ General handover measurement: check the outgoing handover failure.

♦ Handover cause measurement: check handover causes and proportions of

handovers due to various causes.

♦ Adjacent cell handover measurement: find out cells from which the outgoing

handover success rate is low and to which adjacent cell the outgoing

handover success rate is low, to decide the fault cause.

♦ CS basic service measurement: the number of handovers and the number of

TCH occupation successes are out of proportion (>3).

Solution

(1) Add appropriate number of adjacent cells.

(2) Adjust handover parameters.

5. Call drop due to unbalance between uplink and downlink (tower amplifier, power

amplifier, and antenna direction)


CS basic service measurement and power control measurement: analyze the


38

average uplink/downlink receiving level.

Signaling tracing: analyze the uplink/downlink receiving level.

Solution

(1) Check the tower amplifier, CDU, RDU, and BTS boards to ensure they are

normal, and check RF connections to ensure they are normal.

(2) Remove the antenna feeder problem if there is any:

Check whether the antenna azimuth and pitch angle satisfy the design

specification.

Check feeders and jumpers to ensure they are connected correctly.

Check antenna feeder connectors to ensure they are in good contact.

Check feeder cables to ensure they are not damaged.

Check SWR to ensure it is normal.

6. Call drop due to inappropriate radio parameter settings


♦ Relevant parameter settings: Radio Link Timeout (RLT) and the minimum

access level are not set appropriately.

Solution

Modify inappropriate radio parameters.

7. Call drop due to inappropriate SCCP timer settings


During the call process, the call drop problem occurs after a fixed interval (4

minutes for example).

Solution

(1) Check the SCCP timer.

(2) TMIAS: inactivity sending timer (100 ms) = 900 × 100 ms = 90 seconds

(3) TMIAR: inactivity receiving timer = 2400 × 100 ms = 240 s = 4 minutes

(4) The call drop duration is related to the inactivity receiving timer, in other


39

words, the call is released when the timer is timeout. Modify the inactivity

receiving timer’s value to be 10 minutes. After doing that, the fault is

removed.

4.4 Handover Problems

4.4.1 Handover Problem Analysis

Cells with handover problems: all cells, a few cells

Cell handover directions: outgoing handover failures and incoming handover

failures

Handover failure objects: handover failures between service cell and multiple cells,

handover failures between service cell and a few cells.

4.4.2 Querying Handover Indices

1. Find out cells with low handover success rate.

2. Find out cells of which the number of handover failures is large.

3. Find out the number of outgoing handover failures and the number of incoming

handover failures. Decide which handover type is the primary problem.

4. Record the outgoing handover performance and incoming handover performance

of cells.

5. Observe handover failures and analyze their occurrence laws:

For the following cases, decide which one has a lower handover success rate.

♦ Handovers to all adjacent cells

♦ Handovers to a few adjacent cells

♦ Handovers triggered by any condition

♦ Handovers triggered by a few conditions

4.4.3 Handover Problem Solutions

The handover success rate might be influenced by the following aspects:

Inappropriate handover parameters


40

Equipment problems (a few carrier boards are damaged)

Interference

Coverage

Unbalance between uplink and downlink

Clock problems (the site adopts internal clock, upper-level clock is unstable or the

clock skew is large)

This section introduces solutions for handover problems due to the above causes

respectively.

1. Handover problem due to inappropriate parameter settings (parameters related to

adjacent cell planning and handover)

Solution

(1) Check PBGT threshold and handover threshold to ensure they are appropriate,

check handover function options to ensure they are appropriate.

(2) If it is found that the number of handovers and the number of TCH

occupations are out of proportion, check handover parameter settings and

make adjustment if necessary (adjusting the minimum inter-cell handover

interval, PBGT threshold, etc).

2. Handover problem due to equipment problems

Analysis objects

♦ Cells of which the incoming handover success rate is low

♦ Adjacent cells of which the outgoing handover success rate is low

Problem locating

(1) The destination cell receives the channel activation message CH ACT but

responds with the CH ACT NACK message or does not respond (TIMEOUT).

(2) The TCH availability is abnormal.

(3) The number of call drops due to terrestrial link break is large.

(4) If a cell always has high call drop rate and high congestion rate, it might

indicate that some equipment in the cell is faulty.


41

(5) Observe transmission and board alarms to check whether there is any clock

alarm.

(6) If the site’s handover access is restricted by the access level and quality, check

relevant parameter settings.

3. Handover problem due to other causes

(1) After problems related to parameter configuration, congestion, and equipment

faults are removed, perform TCH call drop rate analysis.

(2) Perform analysis for adjacent cells in parameter configuration, interference,

coverage, and uplink/downlink balance.

4.5 TCH Congestion Problems

4.5.1 TCH Congestion Problem Analysis

Usually, two indices are used for congestion problem analysis:

TCH congestion rate

SDCCH congestion rate

Congestion in a cell falls into the following three cases:

Congestion occurs on both SDCCH and TCH.

Congestion occurs on TCH, not on SDCCH.

The congestion rate on SDCCH is high, while the congestion rate on TCH is low

or there is no congestion on TCH.

After the congestion problem occurs in a cell, check whether carriers in the cell and

adjacent cells are faulty first, then perform other analyses.

The following describes congestion problem analysis in the three cases mentioned

above.

Congestion occurs on both SDCCH and TCH.

If congestion occurs on both SDCCH and TCH in the adjacent cell, then add

carriers or sites to reduce the congestion rate. If there is no congestion in the

adjacent cell, then equalize the traffic in the following sequence: adjust antenna,

modify the handover threshold, and adjust cell parameters. If there is no


42

congestion in other cells of this site, reconfigure the cell.

Congestion occurs on TCH, not on SDCCH.

In this case, adjust the handover threshold, adjust antenna, adjust cell parameters,

or reconfigure the cell.

The congestion rate on SDCCH is high, while the congestion rate on TCH is low

or there is no congestion on TCH.

Observe interference on SDCCH and RF loss situation. If there is serious RF

interference on SDCCH, it might cause increase in the number of invalid call

attempts and increase in the number of SDCCH RF losses. Moreover, MS

frequently occupies SDCCH or the SDCCH occupation duration increases, causing

congestion on SDCCH. In such cases, modify the frequency planning or perform

SDCCH carrier changeover.

Check whether the number of location updates (OK_ACC_PROC[LOCIATIAON_

UPDIATE]) is too large. If the location registration area’s boundary is on the two

sides of major roads in cities or area with dense population, it might cause frequent

MS location registration in the area, which increases the load of SDCCH and

causes congestion. In such cases, optimize the location registration area’s

boundary with the following methods: adjusting the cell coverage, increasing the

number of SDCCHs, modifying cell parameters, and repartitioning location

registration areas.

To solve congestion problems on SDCCH due to other reasons, adjust the cell

coverage, increase the number of SDCCHs, or modify cell parameters.

4.5.2 TCH Congestion Problem Solutions

The TCH congestion problem might be due to the following causes:

Insufficient system capacity

Large interference

Coverage

Antenna feeder problems

Inappropriate parameter settings (system messages)

This section introduces solutions for TCH congestion problems due to the above causes


43

respectively.

1. Congestion problem due to insufficient system capacity or uneven traffic

Judgment

♦ The traffic per channel is too heavy (traffic per channel > 0.6)

♦ The number of overflows is too large, and the phenomenon of long-time

full-busy state exists.

♦ The traffic is not equalized (traffic in the three cells of a site are not equalized,

or traffic at several sites are not equalized).

♦ The congestion problem is serious.

Solution

(1) Expand system capacity, or adjust carrier configurations for busy cells and

idle cells.

(2) Adjust the cell coverage (adjust the site transmission power, and adjust

antenna azimuth, downtilt, and height).

(3) Adjust cell parameters (Cell Reselection Offset (CRO), minimum MS access

level, enabling load handover).

(4) Adjust cell priorities and cell handover parameters.

2. Congestion problem due to interference (intra-network interference, external

interference)

When the interference reaches a certain level that the Carrier-to-Interference ratio

requirement can not be satisfied, the Bit Error Rate (BER) increases rapidly. It

causes interference on the assignment command and the assignment process,

which results in channel occupation failure.

In idle state, the downlink interference forces the DSC counter of MS to decrease

to 0, MS then reselects a cell with lower level. This case might also cause the

channel occupation failure.

Judgment and solution

For details, refer to the part related to interference in TCH Call Drop Solutions.

3. Congestion problem due to antenna feeder problem


44

Judgment

♦ Cell frequency sweeping: measurement result of the same frequency point by

the main receiving antenna and the diversity receiving antenna.

♦ Cell radio measurement: signal level and quality distributions.

♦ Signaling tracing: measurement report analysis.

Solution

Check the antenna azimuth and downtilt, and antenna feeder connections.

4. Congestion problem due to inappropriate parameter settings

Judgment

Check data configuration such as the minimum MS access level and parameters

related to cell reselection.

Solution

Adjust inappropriate parameter settings.

5. Congestion problem due to poor coverage

Judgment and solution

For details, refer to the part related to coverage in TCH Call Drop Solutions.

4.6 SDCCH Congestion Problem

4.6.1 SDCCH Congestion Problem Analysis

For details, refer to TCH Congestion Problem Analysis.

4.6.2 SDCCH Congestion Problem Solutions

The SDCCH congestion problem might be due to the following causes:

Inappropriate parameter settings (system messages)

Insufficient system capacity

Inappropriate location area partitions

Interference


45

This section introduces solutions for SDCCH congestion problems due to the above

causes respectively.

1. Congestion problem due to inappropriate parameter settings

Judgment

Radio access measurement:

♦ The number of immediate assignment successes / the number of immediate

assignments > 85%.

It is the ratio of the number of est_ind messages reported by MS to the issued

immediate assignment commands, which should be larger than 85%. If this

value is abnormal, it indicates that relevant parameter settings in the system

message data table are inappropriate.

♦ Radio access reason types, the number of calling times, the number of times

of being called, the number of location updates, the number of short

messages, etc.

Solution

(1) Adjust parameters, such as the retransmission times and the number of

expanded transmission timeslots.

(2) Check parameter settings related to location update (dual-band network 1800

MHz parameter settings, CRO, cell reselection hysteresis, and periodical

location update time).

(3) In the dual-band network, too many inter-office handovers might cause

increase in the number of location updates. In such cases, adjust 1800 MHz

handover parameter settings for the dual-band network, and adjust other

parameters such as CRO.

2. Congestion problem due to insufficient system capacity

To handle the capacity problem in some special circumstances, such as the location

updates at boundaries of location areas or near the railway, increase the number of

configured SDCCHs or TRX.

3. Congestion problem due to inappropriate location area partition

For this case, adjust the location area partition.


46

Note:

Inappropriate location area boundary might cause frequent location updates. For

example, if a street is taken as the boundary of a location area, pedestrians on the street

and multi-path propagation will cause frequent location updates.

4. Congestion problem due to interference

The RACH threshold is set too low, if interference exists, the system might

misjudge that there are a large amount of SDCCH occupation requests, which

causes the SDCCH congestion problem.

4.7 TCH Allocation Problem

4.7.1 TCH Allocation Process

Difference among TCH occupation, TCH allocation, and TCH assignment

TCH occupation

It refers to the channel resource usage in database in the central controller unit MP.

After the CHANNEL REQUIRE message is received, the system queries the

channel resources in MP’s database. If there is available channel resource, the

channel occupation succeeds (database is running normally); if there is no

available channel resource, the channel occupation fails. This process is actually

the process of querying and allocating channel resource according to the radio

resource data table in MP’s database. For channel occupation failures mentioned

above, only those due to no available radio resources are recorded as channel

occupation failure.

TCH allocation

After the channel is requested successfully from the database, BSC sends the

ChannelActivation FOR TCH message to BTS, that is, TCH allocation attempt.

After BSC receives the ChannelActivationAck message from BTS, the TCH

allocation succeeds. If BSC receives the ChannelActivationNack message or does

not receive the ChannelActivationAck due to timeout, it indicates that the TCH

allocation fails.

TCH assignment


47

After BSC receives the ChannelActivationAck message during the channel

allocation process, it sends the IMMEDIATE ASSIGN COMMAND message on

downlink SDCCH, and it is recorded as a TCH assignment attempt. The TCH

assignment is mainly performed between BSC and MS, and BTS only

transparently transfers relevant commands. After BSC receives the

EstablishIndication message from BTS, the TCH assignment succeeds; otherwise,

the TCH assignment fails. This process mainly involves the radio interface and is

interfered by indefinite factors, thus the assignment failure rate is high.

Note:

A successful channel allocation does not mean a successful channel assignment.

Usually, the number of TCH (or SDCCH) allocations is much larger than the number of

TCH (or SDCCH) assignments.

4.7.2 TCH Allocation Failure Signaling Points

Figure 4.7-1 TCH Allocation Failure

As shown in Figure 4.7-1, an Assignment Failure corresponds to an Assignment


48

Request, reflecting a TCH assignment failure. For Assignment Failure (1), the failure is

mainly due to no available channel; for Assignment Failure (2), the failure is due to site

fault; for Assignment Failure (3), the failure involves channel assignment failure at air

interface, which is due to coverage problem or interference.

The TCH assignment failure process is mainly controlled by T3107.

Figure 4.7-2 Assignment Success

Figure 4.7-3 Assignment Failure

Figure 4.7-4 T3107 Timeout


49

4.7.3 TCH Assignment Failure Problem Analysis

To handle TCH allocation failure due to TCH congestion, refer to TCH Congestion

Problems. This section mainly analyzes the TCH assignment failure problem.

1. TCH assignment failure causes

Hardware fault: TRX or CDU is faulty, connections on panels are loosened,

transmission quality is poor at A-interface or Abis interface.

Interference: intra-network co-frequency/adjacent-frequency interference, causing

high BER and MS unable to establish link with the network.

Antenna feeder problems: antenna feeder is damaged, the single-polarized

antenna’s azimuth and downtilt differ from those of the antenna type, SWR is high,

or antenna feeder connection is incorrect.

Inappropriate parameter settings: Hopping Sequence Number (HSN) and Mobile

Allocation Index Offset (MAIO) are set inappropriately, T3107 is set too small,

and the configuration data at background is inconsistent with the planning data.

RxLevAccessMin is set too small, MsTxPwrMaxCch is set inappropriately.

Coverage problem: weak signal or unbalance between uplink and downlink.

Repeater problem: uplink/downlink interference or unbalance between uplink and

downlink.

Transmission problem: high transmission BER or unstable transmission.

Other causes: inconsistency between software versions or between hardware

versions, signaling cooperation problem between BSC and MSC, etc.

2. Solutions for TCH assignment failure problems

Check whether cell radio parameters are set appropriately, such as FH parameters

and frequency data. Adjust inappropriate parameters.

Check indices such as BER and idle interference band level, and adjust these

parameters to reduce radio interference.

Check the cell’s hardware, such as transceiver, combiner, divider, and RF

connections among boards. Replace the faulty hardware if there is any.

Perform the drive test and check on the site to see whether the following problems

exist: interference, incorrect antenna feeder connection, and incorrect antenna


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azimuth or downtilt. Solve the problem if there is any.

Perform comprehensive analysis for the performance report, such as the congestion

rate, handover success rate, call drop rate, the proportion of handover due to

various reasons, to locate the fault.

Check BSC version and site versions to avoid TCH assignment failure due to

version inconsistency.

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5 Traffic Statistics Cases

5.1 Call Drop Problem

[Problem Description]

On March 16th, it is found in the OMCR background statistics that the call drop rate at

site 3 in PAKTEL network increases greatly: the call drop rate (including handover) is

larger than 7%, and the number of call drops increases greatly.

[Problem Analysis]

1. Obtain the performance data and check the call drop type. It is found that there are

many call drops due to radio link failure.

2. Perform statistics for the cell’s interference bands. It is found that the proportion of

interference bands over level-3 is very high.

3. It is doubted that interference exists at the site. Check the frequency plan, it is

found that there is no co-frequency/adjacent-frequency interference in the cell and

adjacent cells. It is found through BTS measurement that the second carrier and the

third carrier have high assignment failure rate.

4. The alarm statistics indicates that there is no hardware alarm at the site, no missed

adjacent cell, and handover parameter settings are appropriate. However, it is

found through checking the site’s FH parameters that the second carrier and the

third carrier have the same MAIO.

[Solution]

Adjust the third carrier’s MAIO from 8 to 10. After doing that, the interference

disappears, the number of call drops decreases greatly, and the call drop rate becomes

normal. Figure 5.1-1 shows these indices before and after the adjustment.


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Figure 5.1-1 Indices before and after Adjustment

5.2 Handover Problem


The background performance data indicates that the handover success rate at cellA is

low.

[Problem Analysis]

Check radio parameters, it is found that the parameter settings are appropriate. It is

found through Dynamic Data Management at the background OMCR that one carrier

in cellA can not be occupied. It is diagnosed that the problem is due to hardware fault.

[Solution]

Replace the faulty board. After doing that, the handover success rate increases greatly.

Figure 5.2-1 shows relevant indices before and after the adjustment. The problem is

resolved.



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5.3 TCH Congestion Problem

Case 1


On December 21st, through the background OMCR statistics, it is found that the TCH

traffic in third sector of site106 under BSC8 becomes congested suddenly, and the TCH

congestion rate (excluding handover) is over 30%.

[Problem Analysis]

It is found through investigation that there has no congestion in the third sector before,

and the TCH congestion problem occurs suddenly. Perform statistics for surrounding

cells’ performance and alarms. It is found that site127 which is relevant to the sector is

disconnected, causing the third sector of site106 to absorb more traffic, which result in

the TCH congestion problem.

[Solution]

Make site127 to be commissioned immediately. After doing that, it is found that the

TCH traffic congestion problem in the third sector of site106 is removed. Figure 5.3-1

shows relevant indices before and after the adjustment.


Case 2


After configuring the dynamic GPRS channel at BSC1, it is found on the next day that

the congestion rate increases greatly.


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[Problem Analysis]

1. Adjust the number of dynamic GPRS channels before busy hour, and enable only

one dynamic GPRS channel for all cells.

2. Observe on the next day, it is found that there are many TCH allocation failures

(excluding handover) in cells under BSC1, causing the service channel allocation

rate to decrease (from 98% to 93%). Through signaling tracing, it is found that, for

timeslots which are originally configured as dynamic GPRS channels and later

changed to be TCH channels, channel activations all fail, as shown in Figure 5.3-2.

Figure 5.3-2 Channel Activation Failure

[Solution]

Change the dynamic GPRS channel to be static GPRS channel, and then change it to be

TCH channel. After doing these, it is found that the number of TCH allocation failures

(excluding handover) decreases and the network performance becomes normal.

Note: The dynamic GPRS channel can not be directly changed to be TCH channel, because it might

cause the changed channel unable to be occupied, which results in decrease in the TCH channel

allocation success rate. Instead, the dynamic GPRS channel should be changed to be static GPRS

channel first, and then changed to be TCH channel.

5.4 SDCCH Congestion Problem


After a site is commissioned, it is found in cell3 that SDCCH congestion occurs

suddenly, with a congestion rate higher than 35%.

[Problem Analysis]


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Through the basic measurement statistics, it is found that the SDCCH congestion

problem in cell3 is due to a large amount of location updates. Check the planning data

and LAC partition, but all are normal. Thus it is diagnosed that the problem exists in

the site’s commissioning data.

[Solution]

It is found through checking that cell3’s LAC is 8198, but LACs of the other two cells

of the site are 8199, which are incorrect and causes a large amount of location updates.

After adjusting the two cells’ LAC to be 8198, the SDCCH congestion rate becomes

normal, and the problem is resolved.

5.5 SDCCH Allocation Problem


During the process of commissioning a satellite transmission site, it is found that some

handsets can not access the network and can not call either.

[Problem Analysis]

After performing the signaling analysis, it is found that after BSC issues the

IMMEDIATE CMD message, the waiting for EST IND message from MS is always

timeout, which causes BSC to release the new channel at the local end.

The satellite transmission has a shortcoming that the delay is large. According to the

signaling analysis, the unidirectional delay from the ground station to the satellite or

from the satellite to the ground station is 150 ms. It causes some handsets unable to

receive the immediate assignment command after sending the channel request.

Therefore, the handset can not access the network or can not call.

[Solution]

Implement the immediate assignment optimization at BSC to make the channel

activation message and the immediate assignment message to be issued simultaneously.

In this way, the signaling transmission delay is reduced. After doing this, the handset

can access the network and call normally. The problem is resolved.


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5.6 TCH Allocation Failure Problem


It is found in cell3 of site1 that the call drop rate reaches 6.9%, the incoming handover

success rate is only 68.38%, and the outgoing handover success rate is 91.31%.

[Problem Analysis]

It is found through checking that there is no interference, thus it is diagnosed to be

hardware problems. After performing BTS measurement statistics for cell3, it is found

that the second carrier in cell3 has a very high TCH assignment failure rate, which is

over 30%. After performing on-site test, it is found that after the carrier is occupied, the

signal level is 20 dB lower than the BCCH carrier level.

[Solution]

Replace the carrier. After doing this, all indices become normal. Figure 5.6-1 shows

relevant indices before and after the adjustment.


zte traffic statistic analysis

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