benchmarking of gprs and gsm

BENCH MARKING OF GPRS NETWORK KEY PERFORMANCE INDICATORS IN

MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE

CHAPTER 1

INTRODUCTION

Communication is the word coined for the practical application of scientific knowledge in

the world. The advancement in communication cannot be justified unless it is used for leveraging

the user’s purpose. Communication today imbibed the accomplishment of several tasks of varied

complexity, almost in all aspects of life, the project here is meant for making the mobile

communication easy and fast.

1.1OBJECTIVE

With the increasing complexity of today’s mobile networks and rising demands of their

subscribers ,the telecommunication industry seeking for mobile communication experts able to

provide project management under tight cost constraints and high quality requirements.

The aim of the project is to benchmark the different network key performance indicators

in different areas by conducting drive testing and compare the standards of different network

performances in different areas considering the different KPI’s.

Network performance will be gauged with the help of drive test log values. RF network

KPIs are calculated and comparative analysis will be done with TRAI benchmark values.

1EIET



CHAPTER 2

FLOW CHART & DESCRIPTION

2.1 FLOW CHART

This Project involves,

• Hands-on exercise on conducting drive testing of existing mobile network.

• Obtaining test log files and exporting to excel sheets for benchmark analysis.

• It also involves formulation and calculation of network KPIs and performs benchmark

analysis.

The figure below gives the pictorial representation of the procedure followed in the project

development.

Figure 2.1 Flowchart for the project development.

2EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE2.2 FLOW CHART DESCRIPTION

2.2.1 CONFIGURATION

In the configuration process, we have to configure the tools such as mobiles, modem and

GPS to the software through laptop. All the devices are individually added and they are

configured through their properties according to the desired output. Two mobiles are used

containing same operator SIMs. On the workspace we have to configure the road maps,

highway’s, water bodies.

2.2.2 CONDUCTING DRIVE TEST AND OBTAIN LOG FILES

After configuring the tools, then we have to go for the drive test in our desired area i.e. we

have to start from one place and return to the same place. While going through the area, start the

drive test tool from the place where we are started and end at the same place. After drive test we

have to obtain the log files of the drive test of particular area.

2.2.3 EXPORTING LOG FILES TO RESPECTIVE EXCEL SHEET

We have to export the obtained log files through the software tool to our desired excel

sheet. For the different operators respective excel sheets are obtained. The excel sheets contain the

information of the samples of respective operators which are used to calculate the network key

performance indicators.

2.2.4 CALCULATION OF COLLECTED SAMPLES OF NETWORK KEY

PERFORMANCE INDICATORS

As we obtained the respective excel sheets performance indicators are calculated through

their respective formulae to the samples for their respective operators.

2.2.5 BENCHMARKING OF DIFFERENT NETWORKS PERFORMANCE ANALYSIS

BASED ON TRAI BENCHMARKS

The performance indicators of different operators are calculated through their respective

samples. The obtained performance indicators with their respective benchmarks are compared in

the excel sheet of the different operators.

3EIET



CHAPTER 3

INTRODUCTION TO MOBILE COMMUNICATIONS

3.1 REVOLUTION IN TELECOM

The telephone has long been important in modern living, but it use has been constrained

by connecting wires. The advent of mobile radio telephony and particularly the cellular radio has

removed this restriction and led to explosive growth in mobile throughout the world. The phone is

really on move now.

With the phenomenal and unprecedented growth of more than forty fold in just ten years, a

strong demand for mobile cellular services has created an industry which now accounts for more

than one third of all telephone lines. It is expected that mobile phone will soon exceed the

traditional fixed line phones. In fact the trend of fixed and mobile convergence is already being

talked about.

3.2 CONCEPT OF MOBILE COMMUNICATION

Fixed telephones, using wired access network, are meant to be used at a particular location

only. We can have telephones at our office/business and our residence. The fixed telephones are

linked to a place but the modern day life style demands that we should have telephone facility

while on move also. Mobile communication facilitates telephonic conversation in a fast moving

vehicle. This means that phones moves along with a person thereby moving telephone is linked to

a person and not to a place. In these words our reach becomes broader and world shrinks into a

Global village. Wireless communication is all around us. The day is not far off; the future

generations will wonder as to “why wires are required for a telephone to work!!!”

3.2.1 MOBILE COMMUNICATION OBJECTIVES

The important objectives of the mobile communication are

• Any time anywhere communication

• Mobility & Roaming

• High capacity & subs. density

• Efficient use of radio spectrum

• Seamless Network Architecture

4EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• Low cost

• Innovative Services

• Standard Interfaces

3.2.2 HISTORY OF MOBILE COMMUNICATION

•1946 Appeared in St .Louis USA (By AT & T) at 150 MHz band – FM – 120 KHz BW

•1960 450 MHz Band FM – 30 KHz BW

•1970 BELL LAB introduced Cellular Principle

•1979 Advanced Mobile Phone System in US

•1985 Total Access Communication System (TACs in UK)

•1986 Nordic Mobile Telephony Systems (NMT)

•1990 Digital Systems

3.3 DIFFERENT GENERATIONS – ANALOG AND DIGITAL SYSTEMS

1946-

1960s 1980s 1990s 2000s

Appearance 1G 2G 3G

Analog Digital Digital

Multi Standard Multi Standard Unified Standard

Only mobile

voice services

Mostly for voice

services & data delivery

possible

Voice and data

(breaking data barrier )-

mainly data

. Terrestrial Terrestrial Terrestrial

& Satellite

Figure 3.1Different generations of Mobile communications

5EIET



Cellular systems are described in multiple generations, with third and fourth generation (3G and

4G) systems just emerging.

• 1G systems: These are the analog systems such as AMPS that grew rapidly in the 1980s and are

still available today. Many metropolitan areas have a mix of 1G and 2G systems, as well as

emerging 3G systems. The systems use frequency division multiplexing (FDMA) to divide the

bandwidth into specific frequencies that are assigned to individual calls.

• 2G systems: These second-generation systems are digital, and use either TDMA (Time Division

Multiple Access) or CDMA (Code Division Multiple Access) access methods. The European

GSM (Global System for Mobile communications) is a 2G digital system with its own TDMA

access methods. The 2G digital services began appearing in the late 1980s, providing expanded

capacity and unique services such as caller ID, call forwarding, and short messaging. A critical

feature was seamless roaming, which lets subscribers move across provider boundaries.

• 3G systems: 3G has become an umbrella term to describe cellular data communications with a

target data rate of 2 M bits/sec. The ITU originally attempted to define 3G in its IMT-2000

(International Mobile Communications-2000) specification, which specified global wireless

frequency ranges, data rates, and availability dates. However, a global standard was difficult to

implement due to different frequency allocations around the world and conflicting input. So, three

operating modes were specified. In general, a 3G device will be a personal, mobile, multimedia

communications device that supports speech, color pictures, and video, and various kinds of

information content. There is some doubt that 3G systems will ever be able to deliver the

bandwidth to support these features because bandwidth is shared. However, 3G systems will

certainly support more phone calls per cell.

• 4G Systems: On the horizon are 4G systems that may become available even before 3G matures

(3G is a confusing mix of standards). While 3G is important in boosting the number of wireless

calls, 4G will offer true high-speed data services. 4G data rates will be in the 2-Mbit/sec to 156-

Mbit/sec range, and possibly higher. 4G will also fully support IP. High data rates are due to

advances in signal processors, new modulation techniques, and smart antennas that can focus

signals directly at users. OFDM (orthogonal frequency division multiplexing) is one scheme that

can provide very high wireless data rates. OFDM is described under its own heading.

6EIET



Figure 3.2 Services offered in different generations.

Figure 3.3 Techniques in different Cellular generations.

3.4 DEVELOPMENT AND INTRODUCTION OF THE GSM STANDARD

The chronological development of GSM standard is given below.

Year Events/Decisions/Achievements

7EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE1982 CEPT (CONFERENCE EUROPEAN POSTS AND TELEGRAPHS) Decides to

establish Grouped special mobile (the initial origin of the GSM) to develop a set of

common standards for future pan European cellular mobile network.

1984 Establishment of three working parties (WP1-3) to define and describe the services

offered in a GSM PLMN (GSM Public Land Mobile Network) the radio interface,

transmission, signaling protocols, interfaces and network architecture.

1986 A so called permanent nucleus is established to continuously coordinate the work,

which is intensely supported by industry delegates.

1987 Initial memorandum of understanding (MOU) signed by network operator

organizations (representing 12 countries) with major objectives as:

* Coordinating the introduction of the standard and time scales.

* Planning of service introduction

* Routing, billing, and tariff coordination.

1988/89 With the establishment of the European telecommunication

To Standards Institute (ETSI), the specification work was mooted to

1991/92 This international body. GSM becomes a technical committee within ETSI and splits

up to into GSM groups 1-4, later called Special Mobile Groups (SMG) 1-4, which are

technical sub Committees. GSM finally stands for Global system for Mobile

Communications

1990 The GSM specifications for 900 MHz band are also applied to a Digital cellular

system on the 1800 MHz band (DCS1800), a PCN application initiated in the United

Kingdom.

1991 The GSM Recommendations comprise more than 130 single documents.

1992 Official commercial launch of GSM service in Europe.

1993 The GSM- MOU has 62 members (signatories) in 39 countries worldwide.

1993 The end of 1993 shows one million subscribers to GSM networks, however more than

80% of them is to be found in Germany alone.

8EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE1993 First commercial services also start outside Europe: Australia, Hongkong.

The features and benefits expected in the new system were

• Superior speech quality

• Low terminal, operational, and service costs

• A high level of security (confidentiality and fraud prevention)

• International roaming

• Support of low terminal hand portable terminals

• A variety of new services and network facilities.

3.5 CONSTRAINTS IN IMPLEMENTATION

A host of services viz., tele services, supplementary services, and value added services are

being promised by GSM networks. There are certain impairments in realizing an effective mobile

communication system which has to meet the twin objectives of quality and capacity. The

following are the some of the problem areas in deploying a GSM network, which demand

extensive planning and engineering.

(a) Radio frequency Utilization

High spectrum efficiency should be achieved at reasonable cost .The bandwidth on radio

interface i.e. between the user equipment and the Radio transceiver, is to be managed effectively

to support ever increasing customer base with very limited number of radio carriers. For high BW

services e.g. MMS, as the GSM evolves towards 3G, more spectrums is demanded. Bandwidth

management is the key area, which decides the success or otherwise of a mobile operator.

(b) Multipath radio environment

The most significant problem in mobile radio systems is due to the channel itself. In

mobile radio systems, indeed, it is rare for there to exist one strong line of sight (LOS) path

between transmitter and receiver. Usually several significant signals are received by reflection and

scattering from buildings, etc...And then there are multiple paths from transmitter to receiver.

9EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEThe signals on these paths are subject to different delays, phase shifts, and Doppler shifts,

and arrives at the receiver in random phase relation to one another. The interference between

these signals gives rise to a number of deleterious effects. The most important of these are fading

and dispersion .Fading is due to the interference of multiple signals with random relative phase

that causes variations in the amplitude of the received signal. This will increase the error rate in

digital systems, since errors will occur when the signal-to-noise ratio drops below certain

Figure 3.4 Multipath Radio environments.

threshold. Dispersion is due to differences in the delay of the various paths, which disperses

transmitted pulses in time. If the variation of the delay is comparable with the symbol period,

delayed signals from an earlier symbol may interfere with the next symbol; causing Inter-symbol

interference (ISI).The counter measures for fading include diversity reception and equalization.

(c) Mobility management

The principal characteristic of mobile networks, which distinguishes them from

conventional fixed networks, is that the identity of calling and called subscribers is not associated

with a fixed geographical location. The subscribers establish a wireless connection with the

nearest base station, and can make or receive calls as they roam. Mobility management is

concerned with how the network supports this function. When a call is made to mobile customer,

the network must be able to locate the mobile customer. Network attachment process which

includes a location updating process is the answer for the mobility management. In the location

update process, the network databases are updated dynamically, so that the mobile can be reached

10EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEto offer the services. If this process is not done efficiently, it will result in poor call management

and network congestion.

(d) Services

International roaming shall be provided. Advanced PSTN services should be provided

consistent with ISDN services albeit at limited bit rates only. Encryption should be used to

improve security for both the operators and the customers.

(e) Network aspects

ITU identification and numbering plans should be used an international signaling system

should be utilized. There should be a choice of charging structure and rates. No modification shall

be required to the PSTN due to its interconnection to GSM signaling and control information

should be protected.

(f) Cost

The system parameters should be chosen to limit costs, particularly mobiles and handsets.

In a competitive environment, cost is the deciding factor for the survival of an operator.

11EIET



CHAPTER-4

BANDWIDTH MANAGEMENT

4.1 INTRODUCTION

Radios move information from one place to another over channels, and radio channel is an

extraordinarily hostile medium to establish and maintain reliable communications. The channel is

particularly messy and unruly between mobile radios. All the schemes and mechanisms we use to

make communications possible on the mobile radio channel with some measure of reliability

between a mobile and its base radio station are called physical layer, or the layer 1

procedures. The mechanisms include modulation, power control, coding, timing, and host of other

details that manage the establishment and maintenance of the channel. The radio channel has to be

fully exploited for maximum capacities and optimum quality of service.

Band width is a scarce natural resource. The bandwidth has to be managed for maximum

capacity of the system and interference free communications. The spectrum availability for an

operator is very limited. The uplink or down link spectrum is only 25 MHz, Out of this 25 MHz,

124 carriers of each 200 KHz are generated. These carriers are to be shared amongst different

operators. And as a result each operator gets only a few tens of carriers; making spectrum

management a challenging area. The following figure shows the radio connectivity between the

mobile equipment and the Radio transmitter/receiver.

Radio interface

12EIET

MOBILE SWITCHRADIO

CONTROLLER

RADIO

TRANSCEIVER



MOBILE

Figure 4.1 Radio Communication between mobile and Tx/Rx

For effective management of bandwidth, for conservation of spectrum and quality of radio link;

the following access techniques are implemented on the radio interface.

• Cellular structures and Frequency Reuse

• Multiple access Technologies

• Voice coding technologies

• Bandwidth effective Modulation scheme.

4.2 CELLULAR STRUCTURES AND FREQUENCY REUSE

Traditional mobile service was structured similar to television broadcasting: One very

powerful transmitter located at the highest spot in an area would broadcast in a radius of up to

fifty kilometers. The scenario changes as the mobile density as well as the coverage area grow.

The answer to tackle the growth is coverage extensions based on addition of new cells. The

Cellular concept structured the mobile telephone network in a different way. Instead of using one

powerful transmitter many low-powered transmitter were placed throughout a coverage area. For

example, by dividing metropolitan region into one hundred different areas (cells) with low power

transmitters using twelve conversations (channels) each, the system capacity could theoretically

be increased from twelve to thousands of conversations using one hundred low power transmitters

while reusing the frequencies.

The cellular concept employs variable low power levels, which allows cells to be sized

according to subscriber density and demand of a given area. As the populations grow, cells can be

added to accommodate that growth. Frequencies used in one cell cluster can be reused in other

cells. Conversations can be handed over from cell to cell to maintain constant phone service as the

user moves between cells.

4.2.1 CELLS

13EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEA cell is the basic geographic unit of cellular system. The term cellular comes from the

honeycomb areas into which a coverage region is divided. Cells are base stations transmitting

over small geographic areas that are represented as hexagons. Each cell size varies depending

upon landscape. Because of the constraint imposed by natural terrain and man-made structures,

the true shape of cell is not a perfect hexagon.

(a) Cellular System Characteristics

The distinguishing features of digital cellular systems compared to other mobile radio systems

are:

Small cells

A cellular system uses many base stations with relatively small coverage radii (on the

order of a 100 m to 30 km).

• Clusters and Frequency reuse

The spectrum allocated for a cellular network is limited. As a result there is a limit

to the number of channels or frequencies that can be used. A group of cells is called a cluster. All

the frequencies are used in a cluster and no frequency is reused within the cluster. And the total

set of frequencies is repeated in the adjacent cluster. Like that the total service area, i.e. may be a

country or a continent, can be served with a small group of frequencies. Frequency reuse is

possible because the signal fades over the distance and hence it can be reused .For this reason

each frequency is used simultaneously by multiple base-mobile pairs; located at geographically

distant cells. This frequency reuse allows a much higher subscriber density per MHz of spectrum

than other systems. System capacity can be further increased by reducing the cell size (the

coverage area of a single base station), down to radii as small as 200m.

• Small, battery-powered handsets

In addition to supporting much higher densities than previous systems, this

approach enables the use of small, battery-powered handsets with a radio frequency that is lower

than the large mobile units used in earlier systems.

14EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• Performance of handovers

In cellular systems, continuous coverage is achieved by executing a “handover”

(the seamless transfer of the call from one base station to another) as the mobile unit crosses cell

boundaries. This requires the mobile to change frequencies under control of the cellular network.

(b) Co-channel cells and interference

Radio channels can be reused provided the separation between cells containing the same

channel set is far enough apart so that co-channel interference can be kept below acceptable levels

most of the time. Cells using the same channel set are called Co-channel cells. Co-channel cells

interfere with each other and quality is affected. Within the service area (PLMN), specific channel

sets are reused at a different location (another cell). In the example, there are 7 channel sets: A

through G. Neighboring cells are not allowed to use the same frequencies. For this reason all

channel sets are used in a cluster of neighboring cells. As there are 7 channel sets, the PLMN can

be divided into clusters of 7 cells each.

• Co-channel interference

Frequencies can be reused throughout a service area because radio signals typically

attenuate with distance to the base station (or mobile station). When the distance between cells

using the same frequencies becomes too small, co-channel Interference might occur and lead to

service interruption or unacceptable quality of service.

As long as the ratio Frequency reuse distance = D

Cell radius R

is greater than some specified value, the ratio

Received radio carrier power = C

Received interferer radio carrier power I

will be greater than some given amount for small as well as large cell sizes; when all signals are

transmitted at the same power level. The average attenuation of radio signals with distance in

most cellular systems is a reduction to about 1/16 of the received power for every doubling of

distance (1/10000 per decade).

The frequency reuse distance known as separation distance is also known as the signal-to-

noise ratio. The figure on the opposite page shows the situation. At the base station, both signals

15EIET



from subscribers within the cell covered by this base station and signals from subscribers

covered by other cells are received. Interference is caused by cells using the same channel set.

The ratio D/R needs to be large enough in order for the base station to be able to cope with the

interference. A co-channel interference factor Q is defined

As Q=D/R = √ 3K

where

D is Frequency reuse distance

R is the cell radius and

K is the reuse factor or the number of cells in a cluster.

Figure 4.2 Illustration of Cellular frequency concept

Capacity / performance of trade-offs

When engineering a cellular network, the most important trade-off to make is the one

between call capacity and performance.

16EIET



Relationship between K and Performance

The performance of a cellular network can be expressed in quality of service. That is the

value of Q shall be higher to achieve an acceptable quality of service. This means a low (co-

channel) interference level in the network.

The relationship between the reuse factor K and the network performance are: if K increases,

then the co-channel interference decreases, and so the performance increases (note that there is a

fixed relationship between K and ratio D/R).

• Relationship between K and Cell Capacity

The other key relationship in cellular networks is the one between the reuse factor K and

call capacity. First of all, call capacity depends on the number of available channels. In GSM, a

limited number of frequencies is available (for GSM: 124 frequencies, and for GSM-1800: 374

frequencies). The frequencies are grouped into frequency sets. If K increases, the number of

frequencies per set (and so per cell) decreases, and so the call capacity per cell.

The value of K in GSM cellular networks varies between 4 and 21. Note that in real networks, K

is not a constant within the whole PLMN area, but varies depending on the traffic capacity

needed in certain regions. Typically, K is high in urban regions and low in rural regions.

If K increases, then performance increases

If K increases, then call capacity decreases per cell

The number of sites to cover a given area with a given high traffic density, and hence

the cost of the infrastructure, is determined directly by the reuse factor and the number of traffic

channels that can be extracted from the available spectrum. These two factors are compounded

in what is called spectral efficiency of the system. Not all systems allow the same performance

in this domain: they depend in particular on the robustness of the radio transmission scheme

against interference, but also on the use of a number of technical tricks, such as reducing

transmission during the silences of a speech communication. The spectral efficiency, together

with the constraints on the cell size, determines also the possible compromises between the

capacity and the cost of the infrastructure. All this explains the importance given to spectral

efficiency.

17EIET



4.3 MULTIPLE ACCESS TECHNOLOGIES

Cellular systems divide a geographic region into cells where a mobile unit in each cell

communicates with a base station. The goal in the design of cellular systems is to be able to

handle as many calls as possible (this is called capacity in cellular terminology) in a given

bandwidth with some reliability.

In any cellular system or cellular technology, it is necessary to have a scheme that enables

several multiple users to gain access to it and use it simultaneously. As cellular technology has

progressed different multiple access schemes have been used. They form the very core of the

way in which the radio technology of the cellular system works. A mix of Frequency Division

Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with

frequency hopping, has been adopted as the multiple access schemes for GSM. GSM chose a

combination of TDMA/FDMA as its method.

The FDMA part involves the division by frequency of the total 25 MHz bandwidth into

124 carrier frequencies of 200 kHz bandwidth. One or more carrier frequencies are then assigned

to each BS. Each of these carrier frequencies is then divided in time, using a TDMA scheme,

into eight time slots. One time slot is used for transmission by the mobile and one for reception.

They are separated in time so that the mobile unit does not receive and transmit at the same time.

Figure 4.3.1 FDMA/TDMA based radio channel concept.

18EIET



Figure 4.3.2 FDMA Technique in GSM.

Figure 4.3.3 TDMA Technique in GSM.

4.4 DIGITAL MODULATION OF GSM RADIO : GMSK

19EIET



The radio connectivity between the mobile station and the Radio transceiver is made by

transmitting carrier .The digital information generated by the system or the network is to be

imparted to the radio carrier by suitable digital modulation technique.

If the amplitude of a carrier is shifted with binary information, it is said ASK is employed,

wherein the amplitude of the carrier is switched between their full-on and full-off conditions. If

the carrier frequency is shifted with the binary information, this is equivalent to shifting between

two or more carriers of different frequencies. This is FSK and is widely used in analog cellular

systems for signaling functions. There is no limit to the number of carrier frequencies that can be

shifted, but the use of two frequencies, quite close together, is the universal implementation of

FSK. As with FSK ,the shift between various carriers differing from each other only in their

relative phase(PSK).There are many varieties of PSK ,and each is broadly distinguished from the

others by the number of allowed phases .

4.4.1 GAUSSIAN MINIMUM SHIFT KEYING (GMSK)

The modulation specified for GSM is GMSK with BT=0.3 and rate 270 5/6 k bauds.

GMSK is a type of constant envelope FSK, where the frequency modulation is a result of a

carefully contrived phase modulation .The most important feature of GMSK is that it is a

constant –envelope variety of modulation. This means there is a distinct lack of AM in the

carrier with a constant limiting of the occupied bandwidth.

The constant amplitude of the GMSK signal makes it suitable for use with high efficiency

amplifiers. An easy way to understand the GMSK signal is to first investigate its precursor,

Minimum–Shift Keying (MSK).The following figure indicates the steps in generating an MSK

signal.

How the data is treated in GMSK is explained below:

The waveforms are all aligned together in phase. Little scales are placed are placed in the

figure to help make the phase relationships between the waveforms clearer.

• 10 bits of the data stream {1101011000} is considered for analysis.

• The data stream is divided into odd and even bit streams:(“odd bits” and “even

bits”).In creating odd bits and even bits ,each alternate odd and even bit in data is

hold for two bit times. Staggering odd bits and even bits already helps to create a

waveform with minimal AM. For convenience odd bits and even bits are made to 20

EIET



take the values 1or1. In GSM case, if the data rate (in waveform “data”) is 270.833

kbps, then the staggered odd bits and even bits will have half the rate135.4 kbps.

• The fourth and fifth wave forms in the following figure are the high freq and the

low freq versions, respectively, of the carrier. Since MSK is a form of FSK, finally

modulated carrier needs two diff. Frequency components (low and high).the MSK

signal is created by shifting between these two frequencies.

• The MSK signal is created starting with bit number 2, with the help of the truth

table given below along with the waveforms. At any instant the odd and even bit

values are taken from the table and follow the rules as given in the truth table to

obtain the MSK waveform at that instant.

• Either the high or the low frequency versions of the carrier is picked corresponding

to the instant under consideration and also according to the sense instructions(+or-)

the wave form is to be turned up or down.

21EIET



Figure 4.4 GMSK wave forms

Smooth phase transitions can be noticed, as the MSK waveform changes its frequency one

from the other. These high and low frequencies shall be as close together as possible in the

frequency domain.

To make a GMSK signal from an MSK signal ,the stretched data waveforms (each135.4

kbps) have to be filtered with a Gaussian filter of an appropriate bandwidth defined by the BT

product(Bandwidth*Time).In GSM case ,BT is 0.3,which makes B=81.3 kHz when T is 3.7

micro sec (T=1/270.833).

4.5 SPEECH CODING IN GSM

Due to the restricted transmission capacity on the radio channel, it is desirable to

minimize the number of bits we need to transmit. The information is transmitted within pulses,

so that the content, the representation of the originally continuous audio signal, is compressed in

the time domain when it is transmitted over the radio path. Inside the receiver, the information is

decompressed, or expanded, in order to regenerate the continuous audio signal. The device that

transforms the human voice into a digital stream of data suitable for transmission over the radio

interface and regenerates an audible analog representation of the received data (voice) is called a

speech codec.

4.5.1 HOW THE SPEECH CODING WORKS IN GSM

Sound (human voice) is converted to an electrical signal by the microphone. To digitize

this analog signal, it is sampled at 8 KHz rate. The signal is sampled after filtering. Every 125

micro seconds, a value is sampled from the analog signal and quantized by a 13 bit word. The

125 micro sec sampling intervals are derived from a sampling frequency of 8 KHz, which are

8000 samples per second. A sampling rate of 8000 samples per second means that the output of

Analog to Digital converter delivers a data rate of 8000x 13bps=104 Kbps.104 Kbps data is far

22EIET



too high to be economically transmitted over the radio interface; considering the Bandwidth

scarcity. Band width has to be shared by number of users for costing advantages. The speech

coder will have to do something to significantly reduce this rate by extracting irrelevant

components in the data stream. The speech coder has to search for excess baggage we can safely

remove from the bit stream scheduled for transport over the radio path. GSM uses to processes

to strip redundant fat from the data representing voice traffic. The compression algorithm used in

GSM is a procedure called RPE-LTP.

4.5.2 REGULAR PULSE EXCITATION AND LONG TERM PREDICTION (RPELTP)

Every 20ms, 160 sampled values from the ADC are taken and stored in an intermediate

memory. An analysis of a set of data samples produces eight filter coefficients and an excitation

signal for a time-invariant digital filter. This filter can be regarded as a digital imitation of the

human vocal tract, where the finer coefficients represent vocal modifiers(e.g., teeth, tongue,

pharynx)and the excitation signal represents the sound(e.g., pitch , loudness) or the absence of

sound that we pass through the vocal tract(filter). A correct setting of filter coefficients and an

appropriate excitation signal yields a sound typical of the human voice.

The procedure, so far, has not performed any data reductions. The reductions come in

further steps, which take advantage of certain attributes of the human ear and vocal tract .The

160 samples, transformed into filter coefficients, are divided into four blocks of 40 samples

each. Each block represents a 5-ms period of voice. These blocks are sorted into four sequences.

Where each sequence contains very forth sample from the original 160 samples. Sequence

number 1 contains samples 1, 5, 9, 13…., 37, sequence number 2 contains samples 2, 6, 10,

14, .38, Sequence number 3 contains samples3, 7, 11, 15,39, and Sequence number 4 contains

samples 4, 8, 12, 16…40. The first reduction in data comes when the speech encoder selects the

sequence with the most energy.

This linear predictive coding (LPC) and regular pulse excitation (RPE) analysis has a

very short memory of approximately 1ms. A more long-term consideration of neighboring (or

adjacent) blocks in time is not performed here, There are numerous correlations in the human

voice, especially in long vowels such as the in car, where the same sound recurs in succeeding 5-

23EIET



ms samples. Taking the similarity of sounds between adjacent samples (Adjacent 5-ms blocks)

into account can significantly reduce the amount of data required to describe the human voice.

This second reduction task is performed by a LTP Function.

4.5.3 LONG-TERM PREDICTION ANALYSIS (LTP)

The LTP function accepts a sequence selected by the LPC/RPE analysis. Upon accepting

sequence, it then looks among all the previous sequences passed to it (which will reside in

another intermediate memory for 15ms) for the earlier sequence that has the highest correlation

to ( bears the greatest resemblance to ) the current sequence. It can be said that the LTP function

looks for the one sequence from among those already received that is most similar to the

sequence just received from the LPC/RPE. Now it is only necessary to transmit a value

representing the difference between the two sequences, along with a pointer to tell the receiver

on the other end of the radio channel, which sequence it should select among its recently

received sequences for comparison. The receiver knows which differential values it has to apply

to which sequences. The transmission of the whole sequence is not necessary, only the

difference between sequences, This further reduces the data on the channel.

The speech coder issues a block of 260bits (a speech frame) once every 20ms. This

corresponds to net data rate of 13kbps, a data reduction of a factor of eight. Speech transcoding

is a task that requires a large number of calculations at high speeds. It is, therefore, an ideal

application for digital signal processing (DSP) techniques.

24EIET



CHAPTER-5

GSM NETWORK ARCHITECTURE

5.1 INTRODUCTION

A GSM system is basically designed as a combination of three major subsystems:

• The Base Station Subsystem(BSS)

• The Network Subsystem (NSS)

• The Operation Support Subsystem (OSS)

25EIET



Figure 5.1 Architecture of GSM Network.

In order to ensure that network operators will have several sources of cellular

infrastructure equipment, GSM decided to specify not only the air interface, but also the main

interfaces that identify different parts. There are three dominant interfaces, namely, an interface

between MSC and the base Transceiver Station (BTS), and an Um interface between the BTS

and MS.

5.2 GSM NETWORK STRUCTURE

Every telephone network needs a well-designed structure in order to route incoming

called to the correct exchange and finally to the called subscriber. In a mobile network, this

structure is of great importance because of the mobility of all its subscribers. In the GSM system,

the network is divided into the following partitioned areas.

• GSM service area

• PLMN service area

• MSC service area

26EIET



• Location area

• Cell site

The GSM service is the total area served by the combination of all member countries

where a mobile can be serviced. The next level is the PLMN service area. There can be several

within a country, based on its size. The links between a GSM/PLMN network and other PSTN,

ISDN, or PLMN network will be on the level of international or national transit exchange. All

incoming calls for a GSM/PLMN network will be routed to a gateway MSC. A gateway MSC

works as an incoming transit exchange for the GSM/PLMN. In a GSM/PLMN network, all

mobile-terminated calls will be routed to a gateway MSC. Call connections between PLMNs, or

to fixed networks, must be routed through certain designated MSCs called a gateway MSC. The

gateway MSC contains the interworking functions to make these connections. They also route

incoming calls to the proper MSC within the network. The next level of division is the

MSC/VLR service area. In one PLMN there can be several MSC/VLR service areas. MSC/VLR

is a role controller of calls within its jurisdiction. In order to route a call to a mobile subscriber,

the path through links to the MSC in the MSC area where the subscriber is currently located. The

mobile location can be uniquely identified since the MS is registered in a VLR, which is

generally associated with an MSC.

The next division level is that of the LA’s within a MSC/VLR combination. There are

several LA’s within one MSC/VLR combination. A LA is a part of the MSC/VLR service area

in which a MS may move freely without updating location information to the MSC/VLR

exchange that control the LA. Within a LA a paging message is broadcast in order to find the

called mobile subscriber. The LA can be identified by the system using the Location Area

Identity (LAI). The LA is used by the GSM system to search for a subscriber in an active state.

Lastly, a LA is divided into many cells. A cell is an identity served by one BTS. The MS

distinguishes between cells using the Base Station Identification code (BSIC) that the cell site

broadcast over the air.

5.3 MOBILE STATION (MS)

The MS may be a stand-alone piece of equipment for certain services or support the

27EIET



connection of external terminals, such as the interface for a personnel computer or fax. The MS

includes mobile equipment and a subscriber identity module (SIM).MS does not need to be

personally assigned to one subscriber. The SIM is a subscribe module which stores all the

subscriber-related information. When a subscriber’s SIM is inserted into the ME of an MS that

MS belongs to the subscriber, and the call is delivered to that Ms. The ME is not associated with

a called number-it is linked to the SIM. In this case, any ME can be used by a subscriber when

the SIM is inserted in the MS.

SIM is needed in order to access the services provided by the GSM PLMN. MS can be

installed in Vehicles or can be portable or handheld stations. The MS may include provisions for

data communication as well as voice. A mobile transmits and receives message to and from the

GSM system over the air interface to establish and continue connections through the system.

Different type of MS’s can provide different type of data interfaces. To provide a

common model for describing these different MS configuration, ”reference configuration” for

MS, similar to those defined for ISDN land stations, has been defined. Each MS is identified by

an IMEI that is permanently stored in the mobile unit. Upon request, the MS sends this number

over the signaling channel to the MSC. The IMEI can be used to identify mobile units that are

reported stolen or operating incorrectly.

Just as the IMEI identities the mobile equipment, other numbers are used to identify the

mobile subscriber. Different subscriber identities are used in different phases of call setup. The

Mobile Subscriber ISDN Number (MSISDN) is the number that the calling party dials in order

to reach the subscriber. It is used by the land network to route calls toward an appropriate MSC.

The international mobile subscribe identity (IMSI) is the primary function of the subscriber

within the mobile network and is permanently assigned to him. The GSM system can also assign

a Temporary Mobile Subscriber Identity (TMSI) to identity a mobile. This number can be

periodically changed by the system and protect the subscriber from being identified by those

attempting to monitor the radio channel.

5.3.1 FUNCTIONS OF MS

The primary functions of MS are to transmit and receive voice and data over the air

interface of the GSM system. MS performs the signal processing function of digitizing,

encoding, error protecting, encrypting, and modulating the transmitted signals. It also performs

28EIET



the inverse functions on the received signals from the BS.

In order to transmit voice and data signals, the mobile must be in synchronization with the

system so that the messages are the transmitted and received by the mobile at the correct instant.

To achieve this, the MS automatically tunes and synchronizes to the frequency and TDMA

timeslot specified by the BSC. This message is received over a dedicated timeslot several times

within a multiform period of 51 frames. We shall discuss the details of this in the next chapter.

The exact synchronization will also include adjusting the timing advance to compensate for

varying distance of the mobile from the BTS.

The MS monitors the power level and signal quality, determined by the BER for known

receiver bit sequences, from both its current BTS and up to six surrounding BTS’s. This data is

received on the downlink broadcast control channel. The MS determines and send to the current

BTS a list of the six best-received BTS signals. The measurement results from MS on downlink

quality and surrounding BTS signal levels are sent to BSC and processed within the BSC. The

system then uses this list for best cell handover decisions.

MS keeps the GSM network informed of its location during both national and international

roaming, even when it is inactive. This enables the system to page in its present LA. The MS

includes an equalizer that compensates for multi-path distortion on the received signal. This

reduces inter-symbol interference that would otherwise degrade the BER.

Finally, the MS can store and display short received alphanumeric messages on the liquid

crystal display (LCD) that is used to show call dialing and status information. These messages

are limited to 160 characters in length.

• Power Levels

These are five different categories of mobile telephone units specified by the European

GSM system: 20W, 8W, 5W, 2W, and 0.8W. These correspond to 43-dBm, 39-dBm, and 37-

dBm, 33-dBm, and 29-dBm power levels. The 20-W and 8-W units (peak power) are either for

vehicle-mounted or portable station use.

The MS power is adjustable in 2-dB steps from its nominal value down to 20mW (13

dBm). This is done automatically under remote control from the BTS, which monitors the

29EIET



received power and adjusts the MS transmitter to the minimum power setting necessary for

reliable transmission.

5.3.2 SIM CARD

As described in the first chapter, GSM subscribers are provided with a SIM card with its

unique identification at the very beginning of the service. By divorcing the subscriber ID from

the equipment ID, the subscriber may never own the GSM mobile equipment set. The subscriber

is identified in the system when he inserts the SIM card in the mobile equipment. This provides

an enormous amount of flexibility to the subscribers since they can now use any GSM-specified

mobile equipment. Thus with a SIM card the idea of “Personalize” the equipment currently in

use and the respective information used by the network (location information) needs to be

updated. The smart card SIM is portable between Mobile Equipment (ME) units. The user only

needs to take his smart card on a trip. He can then rent a ME unit at the destination, even in

another country, and insert his own SIM. Any calls he makes will be charged to his home GSM

account. Also, the GSM system will be able to reach him at the ME unit he is currently using.

The SIM is a removable SC, the size of a credit card, and contains an integrated circuit chip

with a microprocessor, random access memory (RAM), and read only memory (ROM). It is

inserted in the MS unit by the subscriber when he or she wants to use the MS to make or receive

a call. As stated, a SIM also comes in a modular from that can be mounted in the subscriber’s

equipment.

When a mobile subscriber wants to use the system, he or she mounts their SIM card and

provide their Personal Identification Number (PIN), which is compared with a PIN stored within

the SIM. If the user enters three incorrect PIN codes, the SIM is disabled. The PIN can also be

permanently bypassed by the service provider if requested by the subscriber. Disabling the PIN

code simplifies the call setup but reduces the protection of the user’s account in the event of a

stolen SIM.

5.4 IDENTIFICATION NUMBERS

5.4.1 INTERNATIONAL MOBILE SUBSCRIBER IDENTITY(IMSI)

An IMSI is assigned to each authorized GSM user. It consists of a mobile country code

(MSC), mobile network code (MNC), and a PLMN unique mobile subscriber identification

number (MSIN). The IMSI is not hardware-specific. Instead, it is maintained on a SC by an 30

EIET



authorized subscriber and is the only absolute identity that a subscriber has within the GSM

system. The IMSI consists of the MCC followed by the NMSI and shall not exceed 15 digits.

5.4.2 TEMPORARY MOBILE SUBSCRIBER IDENTITY (TMSI)

A TMSI is a MSC-VLR specific alias that is designed to maintain user confidentiality. It is

assigned only after successful subscriber authentication. The correlation of a TMSI to an IMSI

only occurs during a mobile subscriber’s initial transaction with an MSC (for example, location

updating). Under certain condition (such as traffic system disruption and malfunctioning of the

system), the MSC can direct individual TMSIs to provide the MSC with their IMSI.

5.4.3 MOBILE STATION ISDN NUMBER (MSISDN)

The MS international number must be dialed after the international prefix in order to

obtain a mobile subscriber in another country. The MSISDN numbers is composed of the

country code (CC) followed by the National Significant Number (NSN), which shall not exceed

15 digits.

5.4.4 MOBILE STATION ROAMING NUMBER (MSRN)

The MSRN is allocated on temporary basis when the MS roams into another numbering

area. The MSRN number is used by the HLR for rerouting calls to the MS. It is assigned upon

demand by the HLR on a per-call basis. The MSRN for PSTN/ISDN routing shall have the same

structure as international ISDN numbers in the area in which the MSRN is allocated. The HLR

knows in what MSC/VLR service area the subscriber is located. At the reception of the MSRN,

HLR sends it to the GMSC, which can now route the call to the MSC/VLR exchange where the

called subscriber is currently registered.

5.4.5 INTERNATIONAL MOBILE EQUIPMENT IDENTITY (IMEI)

The IMEI is the unique identity of the equipment used by a subscriber by each PLMN and

is used to determine authorized (white), unauthorized (black), and malfunctioning (gray) GSM

hardware. In conjunction with the IMSI, it is used to ensure that only authorized users are

granted access to the system. An IMEI is never sent in cipher mode by MS.

5.5 BASE STATION SYSTEM (BSS)

31EIET



The BSS is a set of BS equipment consisting of a Radio transmitter/receiver called BTS

(Base Transceiver Station)and a controller called BSC (Base Station Controller).The BSS is

viewed by the MSC through a single A interface as being the entity responsible for

communicating with MSs in a certain area. The radio equipment of a BSS may be composed of

one or more cells. A BSS may consist of one or more BTS. The interface between BSC and BTS

is designed as an A-bis interface. The BSS includes two types of machines: the BTS in contact

with the MSs through the radio interface and the BSC, the latter being in contact with the MSC.

The function split is basically between transmission equipment, the BTS, and managing

equipment at the BSC. A BTS compares radio transmission and reception devices, up to and

including the antennas, and also all the signal processing specific to the radio interface. A single

transceiver within BTS supports eight basic radio channels of the same TDM frame. A BSC is a

network component in the PLMN that function for control of one or more BTS. It is a functional

entity that handles common control functions within a BTS.

A BTS is a network component that serves one cell and is controlled by a BSC. BTS is

typically able to handle three to five radio carries, carrying between 24 and 40 simultaneous

communication. Reducing the BTS volume is important to keeping down the cost of the cell

sites. An important component of the BSS that is considered in the GSM architecture as a part of

the BTS is the Transcoder/Rate Adapter Unit (TRAU). The TRAU is the equipment in which

coding and decoding is carried out as well as rate adoption in case of data. Although the

specifications consider the TRAU as a subpart of the BTS, it can be sited away from the BTS (at

MSC), and even between the BSC and the MSC.

The interface between the MSC and the BSS is a standardized SS7 interface (A-

interface) that, as stated before, is fully defined in the GSM recommendations. This allows the

system operator to purchase switching equipment from one supplier and radio equipment and the

controller from another. The interface between the BSC and a remote BTS likewise is a standard

the A-bis. In splitting the BSS functions between BTS and BSC, the main principle was that only

such functions that had to reside close to the radio transmitters/receivers should be placed in

BTS. This will also help reduce the complexity of the BTS.

5.5.1 FUNCTIONS OF BTS

32EIET



Figure 5.2 Base Transceiver Station Function.

As stated, the primary responsibility of the BTS is to transmit and receive radio signals

from a mobile unit over an air interface. To perform this function completely, the signals are

encoded, encrypted, multiplexed, modulated, and then fed to the antenna system at the cell site.

Trans-coding to bring 13-kbps speech to a standard data rate of 16 kbps and then combining four

of these signals to 64 kbps is essentially a part of BTS, though it can be done at BSC or at MSC.

The voice communication can be either at a full or half rate over logical speech channel. In order

to keep the mobile synchronized, BTS transmits frequency and time synchronization signals

over frequency correction channel (FCCH and BCCH logical channels. The received signal from

the mobile is decoded, decrypted, and equalized for channel impairments.

Random access detection is made by BTS, which then sends the message to BSC. The

channel subsequent assignment is made by BSC. Timing advance is determined by BTS. BTS

signals the mobile for proper timing adjustment. Uplink radio channel measurement

corresponding to the downlink measurements made by MS has to be made by BTS.

5.5.2 BTS-BSC CONFIGURATIONS

There are several BTS-BSC configurations: single site; single cell, single site, multi cell and

multisite, multi cell. These configurations are chosen based on the rural or urban application.

These configurations make the GSM system economical since the operation has options to adapt

the best layout based on the traffic requirement. Thus, in some sense, system optimization is

possible by the proper choice of the configuration. These include Omni directional rural

33EIET



configuration where the BSC and BTS are on the same site; chain and multi drop loop

configuration in which several BTSs are controlled by a single remote BSC with a chain or ring

connection topology; rural star configuration in which several BTSs are connected by individual

lines to the same BSC and sectored urban configuration in which three BTSs share the same site

and are controlled by either a collocated or remote BSC. In rural areas, most BSC’s are installed

to provide maximum coverage rather than maximum capacity.

5.6 TRANSCODER (TXCDR)

Depending on the relative costs of a transmission plant for a particular cellular operator,

there may be some benefit, for larger cells and certain network topologies, in having the

transcoder either at the BTS, BSC or MSC location. If the transcoder is located at MSC, they are

still considered functionally a part of the BSS. This approach allows for the maximum of

flexibility and innovation in optimizing the transmission between MSC and BTS.

The transcoder is the device that takes 13-Kbps speech or 3.6/6/12-Kbps data multiplexes

and four of them to convert into standard 64-Kbps data. First, the 13 Kbps or the data at 3.6/6/12

Kbps are brought up to the level of 16 Kbps by inserting additional synchronizing data to make

up the difference between a 13-Kbps speech or lower rate data, and then four of them are

combined in the transcoder to provide 64 Kbps channel within the BSS. Four traffic channels

can then be multiplexed on one 64-Kpbs circuit. Thus, the TRAU output data rate is 64 Kbps.

Then, up to 30 such 64-Kpbs channels are multiplexed onto a 2.048 Mbps if a CEPT1 channel is

provided on the A-bis interface. This channel can carry up to 120-(16x 120) traffic and control

signals. Since the data rate to the PSTN is normally at 2 Mbps, which is the result of combining

30-Kbps by 64-Kbph channels, or 120- Kbps by 16-Kpbs channels.

5.6.1 BASE STATION CONTROLLER (BSC)

The BSC is connected to the MSC on one side and to the BTS on the other. The BSC

performs the Radio Resource (RR) management for the cells under its control. It assigns and

release frequencies and timeslots for all MSs in its own area. The BSC performs the inter cell

handover for MSs moving between BTS in its control. It also reallocates frequencies to the BTSs

in its area to meet locally heavy demands during peak hours or on special events. The BSC 34

EIET



controls the power transmission of both BSSs and MSs in its area. The minimum power level for

a mobile unit is broadcast over the BCCH. The BSC provides the time and frequency

synchronization reference signals broadcast by its BTS’s. The BSC also measures the time delay

of received MS signals relative to the BTS clock. If the received MS signal is not centered in its

assigned timeslot at the BTS, The BSC can direct the BTS to notify the MS to advance the

timing such that proper synchronization takes place. The functions of BSC are as follows.

The BSC may also perform traffic concentration to reduce the number of

transmission lines from the BSC to its BTSs, as discussed in the last section.

5.7 NETWORK AND SWITCHING SUBSYSTEMS(NSS)

5.7.1 MOBILE SWITCHING CENTER (MSC) AND GATEWAY MOBILE SWITCHING

CENTER (GMSC)

The network and the switching subsystem together include the main switching functions

of GSM as well as the databases needed for subscriber data and mobility management (VLR).

The main role of the MSC is to manage the communications between the GSM users and other

telecommunication network users. The basic switching functions of performed by the MSC,

whose main function is to coordinate setting up calls to and from GSM users. The MSC has

interface with the BSS on one side (through which MSC VLR is in contact with GSM users) and

the external networks on the other (ISDN/PSTN/PSPDN). The main difference between a MSC

and an exchange in a fixed network is that the MSC has to take into account the impact of the

allocation of RRs and the mobile nature of the subscribers and has to perform, in addition, at

least, activities required for the location registration and handover.

The MSC is a telephony switch that performs all the switching functions for MSs located

in a geographical area as the MSC area. The MSC must also handle different types of numbers

and identities related to the same MS and contained in different registers: IMSI, TMSI, ISDN

number, and MSRN. In general identities are used in the interface between the MSC and the MS,

while numbers are used in the fixed part of the network, such as, for routing.

5.7.2 FUNCTIONS OF MSC

As stated, the main function of the MSC is to coordinate the setup of calls between GSM

mobile and PSTN users. Specifically, it performs functions such as paging, resource allocation,

35EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECElocation registration, and encryption.

Specifically, the call-handling function of paging is controlled by MSC. MSC

coordinates the setup of call to and from all GSM subscribers operating in its areas.

The dynamics allocation of access resources is done in coordination with the BSS. More

specifically, the MSC decides when and which types of channels should be assigned to which

MS. The channel identity and related radio parameters are the responsibility of the BSS; The

MSC provides the control of interworking with different networks. It is transparent for the

subscriber authentication procedure.

The MSC supervises the connection transfer between different BSSs for MSs, with an

active call, moving from one call to another. This is ensured if the two BSSs are connected to the

same MSC but also when they are not. In this latter case the procedure is more complex, since

more than one MSC in involved. The MSC performs billing on calls for all subscribers based in

its areas. When the subscriber is roaming elsewhere, the MSC obtains data for the call billing

from the visited MSC. Encryption parameters transfers from VLR to BSS to facilitate ciphering

on the radio interface are done by MSC. The exchange of signaling information on the various

interface toward the other network elements and the management of the interface themselves are

all controlled by the MSC. Finally, the MSC serves as a SMS gateway to forward SMS messages

from Short Message Service Centers (SMSC) to the subscribers and from the subscribers to the

SMSCs. It thus acts as a message mailbox and delivery system.

5.7.3 VISITOR LOCATION REGISTER (VLR)

The VLR is collocated with an MSC. A MS roaming in an MSC area is controlled by the

VLR responsible for that area. When a MS appears in a LA, it starts a registration procedure.

The MSC for that area notices this registration and transfers to the VLR the identity of the LA

where the MS is situated. A VLR may be in charge of one or several MSC LA’s. The VLR

constitutes the databases that support the MSC in the storage and retrieval of the data of

subscribers present in its area. When an MS enters the MSC area borders, it signals its arrival to

the MSC that stores its identify in the VLR. The information necessary to manage the MS is

contained in the HLR and is transferred to the VLR so that they can be easily retrieved if so

required.

• Data Stored in VLR

The data contained in the VLR and in the HLR are more or less the same. Nevertheless

36EIET



the data are present in the VLR only as long as the MS is registered in the area related to that

VLR. Data associated with the movement of mobile are IMSI, MSISDN, MSRN, and TMSI.

The terms permanent and temporary, in this case, are meaningful only during that time interval.

Some data are mandatory, others are optional.

5.7.4 HOME LOCATION REGISTER (HLR)

The HLR is a database that permanently stores data related to a given set of subscribers.

The HLR is the reference database for subscriber parameters. Various identification numbers

and addresses as well as authentication parameters, services subscribed, and special routing

information are stored. Current subscriber status including a subscriber’s temporary roaming

number and associated VLR if the mobile is roaming, are maintained.

The HLR provides data needed to route calls to all MS-SIMs homes based in its MSC

area, even when they are roaming out of area or in other GSM networks. The HLR provides the

current location data needed to support searching for and paging the MS-SIM for incoming calls,

wherever the MS-SIM may be. The HLR is responsible for storage and provision of SIM

authentication and encryption parameters needed by the MSC where the MS-SIM is operating. It

obtains these parameters from the AUC.

The HLR maintains record of which supplementary service each user has subscribed to

and provides permission control in granting services. The HLR stores the identification of SMS

gateways that have messages for the subscriber under the SMS until they can be transmitted to

the subscriber and receipt is knowledge.

Some data are mandatory, other data are optional. Both the HLR and the VLR can be

implemented in the same equipment in an MSC (collocated). A PLMN may contain one or

several HLRs.

5.7.5 AUTHENTICATION CENTER (AUC)

The AUC stores information that is necessary to protect communication through the air

interface against intrusions, to which the mobile is vulnerable. The legitimacy of the subscriber

is established through authentication and ciphering, which protects the user information against

unwanted disclosure. Authentication information and ciphering keys are stored in a database

within the AUC, which protects the user information against unwanted disclosure and access.

In the authentication procedure, the key Ki is never transmitted to the mobile over the air 37

EIET



path, only a random number is sent. In order to gain access to the system, the mobile must

provide the correct Signed Response (SRES) in answer to a random number (RAND) generated

by AUC.

Also, Ki and the cipher key Kc are never transmitted across the air interface between the

BTS and the MS. Only the random challenge and the calculated response are transmitted. Thus,

the value of Ki and Kc are kept secure. The cipher key, on the other hand, is transmitted on the

SS7 link between the home HLR/AUC and the visited MSC, which is a point of potential

vulnerability. On the other hand, the random number and cipher key is supposed to change with

each phone call, so finding them on one call will not benefit using them on the next call.

The HLR is also responsible for the “authentication” of the subscriber each time he

makes or receives a call. The AUC, which actually performs this function, is a separate GSM

entity that will often be physically included with the HLR. Being separate, it will use separate

processing equipment for the AUC database functions.

5.7.6 EQUIPMENT IDENTIFY REGISTER (EIR)

EIR is a database that stores the IMEI numbers for all registered ME units. The IMEI

uniquely identifies all registered ME. There is generally one EIR per PLMN. It interfaces to the

various HLR in the PLMN. The EIR keeps track of all ME units in the PLMN. It maintains

various lists of message. The database stores the ME identification and has nothing do with

subscriber who is receiving or originating call. There are three classes of ME that are stored in

the database, and each group has different characteristics.

• White List: -contains those IMEIs that are known to have been assigned to valid

MS’s. This is the category of genuine equipment.

• Black List: - contains IMEIs of mobiles that have been reported stolen.

• Gray List: - contains IMEIs of mobiles that have problems (for example, faulty

software, and wrong make of the equipment). This list contains all MEs with faults

not important enough for barring.

Interworking Function

GSM provided a wide range of data services to its subscribers. The GSM system interface

with the various forms of public and private data networks currently available. It is the job of the

38EIET



IWF to provide this interfacing capability.

The IWF, which in essence is a part of MSC, provides the subscriber with access to data rate and

protocol conversion facilities so that data can be transmitted between GSM Data Terminal

Equipment (DTE) and a land-line DTE.

• Echo Canceller (EC)

EC is used on the PSTN side of the MSC for all voice circuits. The EC is required at the

MSC PSTN interface to reduce the effect of GSM delay when the mobile is connected to the PSTN

circuit. The total round-trip delay introduced by the GSM system, which is the result of speech

encoding, decoding and signal processing, is of the order of 180 ms. Normally this delay would not

be an annoying factor to the mobile, except when communicating to PSTN as it requires a two-wire

to four-wire hybrid transformer in the circuit. This hybrid is required at the local switching office

because the standard local loop is a two-wire circuit. Due to the presence of this hybrid, some of the

energy at its four-wire receive side from the mobile is coupled to the four-wire transmit side and

thus retransmitted to the mobile. This causes the echo, which does not affect the land subscriber but

is an annoying factor to the mobile. The standard EC cancels about 70ms of delay.

During a normal PSTN (land-to-land call), no echo is apparent because the delay is too

short and the land user is unable to distinguish between the echo and the normal telephone “side

tones” However, with the GSM round-trip delay added and without the EC, the effect would be

irritating to the MS subscriber.

5.8 OPERATION SUBSYSTEM (OSS)

The OSS provides alarm-handling functions to report and log alarms generated by the

other network entities. The maintenance personnel at the OSS can define that criticality of the

alarm. Maintenance covers both technical and administrative actions to maintain and correct the

system operation, or to restore normal operations after a breakdown, in the shortest possible time.

39EIET



Figure 5.3 Operation and Maintenance Centre.

The fault management functions of the OSS allow network devices to be manually or

automatically removed from or restored to service. The status of network devices can be checked,

and tests and diagnostics on various devices can be invoked. For example, diagnostics may be

initiated remotely by the OSS. A mobile call trace facility can also be invoked. The performance

management functions included collecting traffic statistics from the GSM network entities and

archiving them in disk files or displaying them for analysis. Because a potential to collect large

amounts of data exists, maintenance personal can select which of the detailed statistics to be

collected based on personal interests and past experience. As a result of performance analysis, if

necessary, an alarm can be set remotely.

The OSS provides system change control for the software revisions and configuration data

bases in the network entities or uploaded to the OSS. The OSS also keeps track of the different

software versions running on different subsystem of the GSM.

CHAPTER 6

CALL AND MOBILITY MANAGEMENT

6.1 INTRODUCTION

40EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEMobility management entails the GSM system keeping track of the mobile while on the move.

The mobility management is implemented through mobility management (MM) sub-layer which

is present in layer 3 of the protocol stack at MS and MSC. The functions performed by the

mobility management are:

Subscribe Data management at MSC/VLR-

Subscriber data from HLR are retrieved by MM at the time of first location updating of the

subscriber. Dynamic data change for a subscribe is also managed by the MM.

• Services provided to upper layers-

MM provides basic means of transportation of upper CM sub-layer messages between MS

and the network Handover procedures ensure smooth transition from one radio network to

another.

• Subscriber Authentication and confidentiality Management –

MM procedures ensure data confidentiality of a subscriber MM procedures provide a

means for to ensure data confidentiality at radio interface.

Mobility management is implemented through MM procedures, which are broadly classified in to

two groups –

I) MM Common Procedure

II) MM Specific Procedure

A MM specific procedure can only be started if no other MM specific procedure is running

or no MM connection exists between the network and the mobile station. The end of the running

MM connection has to be awaited before a new MM specific procedure can be started.

During the lifetime of a MM specific procedure, if a MM connection establishment is

requested by a CM entity this request will either be rejected or be delayed until the running MM

specific procedure is terminated (this depends upon implementation). Any MM common

procedure (except IMSI detach) may be started during MM specific procedure.

6.2 MM COMMON PROCEDURES

1.TMSI REALLOCATION PROCEDURE

41EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEThe purpose of the TMSI reallocation procedures is to provide identify confidentiality i.e. to

protect a user against being identified and located by an intruder. TMSI is used for identification

within the radio interface signaling procedures instead of IMSI. Usually the TMSI reallocation is

performed at least at each change of a location area. The reallocation of TMSI can be performed

explicitly after predetermined no. of accesses by MS to the network or implicitly by a location

updating procedure. TMSI reallocation can be initiated by the network at any time whilst RR

connection exists between the network and the mobile station.

In case of TMSI reallocation procedure initiated by the network, network sends TMSI

reallocation command message to the MS containing new TMSI/LAI MS on receiving the

message stores new TMSI and LAI in SIM and deletes the older entries and sends TMSI

reallocation complete message to the network.

2. AUTHENTICATION PROCEDURE

Authentication Triplets: At network side, authentication procedure requires authentication triplets.

Authentication triplet consists of:

• Random number RAND (128 bits)

• Signed response SRES (32 bits)

• Ciphering key (64 bits)

While initiating authentication procedure, if network has no authentication triplet or all

triplets have been used, it requests AUC for the same. The index of currently used triplet is known

as CKSN. (Ciphering key sequence number).

3. IDENTIFICATION PROCEDURE

The identification procedure is used by the network to request a MS to provide specific

identification parameters to the network e.g. IMSI, IMEI.

In case MS update location in the system using TMSI, but due to data base failure, TMSI

at network end is no more available; network initiates identification procedure and asks for IMSI.

If network is unable to receive identity response, it clears all the ongoing MM

connections and releases radio resources.

42EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE4. IMSI DETACH PROCEDURE

The purpose of this procedure is to indicate the network that MS has switched off. This

enables the network not to page for the subscriber and invokes other applicable supplementary

service (e.g. call forwarding etc.)

5. CIPHERING PROCEDURE

Ciphering procedure is needed to encrypt data transmission over radio interface. When

MSC needs to indicate ciphering on radio interface, it sends Cipher Mode Command message to

BSS. This message contains Kc and a list of permitted algorithms. BSS stores Kc for this session

and sends Cipher mode message to the MS. BSS enable encryption and uses Kc and permitted

algorithm to encrypt /decrypt data.

In case ciphering is completed successfully the network receives a Cipher Mode Complete

command from the MS. This message contains the algorithm used for ciphering. MSC stores this

information and proceeds for further activities like call setup etc.

In case MS is not able to support the ciphering it sends Cipher Mode Reject command to the

network. At MSC the encryption control is operator controlled. If ciphering is mandatory and

network receives a Cipher Mode Reject command from the MS, MSC clears all the ongoing MM

connection and then releases radio resources.

6. ABORT PROCEDURE

The procedure is used to abort any ongoing or established MM connection.

6.3 MM SPECIFIC PROCEDURES

The MM specific procedures are

• Normal location updating – MS moves from one LA to other LA.

• Periodic location updating -- It is used to periodically notify the availability of the MS to

the network.

• IMSI attach procedure – It is used to indicate the IMSI as active in the network.

43EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE MS initiates location-updating procedure by sending a Location Updating Request message

to the network. MS starts a guard timer and enters state location updating initiated. After this, the

network initiates authentication and ciphering procedure. After successful authentication and

ciphering location updating procedure proceeds further.

To limit the number of unsuccessful location updating attempts, an Attempt Counter is

maintained at MS. Attempt counter is incremented when a location updating procedure fails.

Attempt counter is reset when MS is powered on /a SIM is inserted / location update is

successfully completed.

If the Location updating is successfully accepted by the network a Location Updating

Accept message is transferred to the MS. Implicit TMSI reallocation procedure is also invoked.

MS on receiving the Location Updating Accept stores the received LAI, stops the guard

timer, reset the attempt counter and sets the update status of SIM to update. MS at all times

maintains a list of forbidden LAI’s and PLMN’s in SIM. If the LAI or PLMN identity contained

in the Location Updating Accept message is a member of any of the “forbidden lists” then any

such entries are deleted in MS.

After location-updating procedure is over, the RR connection is released. The network

initiates the release. MS waits for the release and if within time out the connection is not released,

MS aborts it.

MM procedures make use of certain MAP (Mobile Application Part). For example

location updating procedure, which is an MM specific procedure make use of MAP procedures

like: Down-loading of subscriber related data from HLR to VLR through MAP procedure on C

interface. Thus LU procedure of MM makes use of a MAP procedure also. Similarly

authentication procedure, which is a MM common procedure, makes use of MAP procedure to

retrieve authentication triplets from AuC.

6.4 CONNECTION MANAGEMENT

The mobility management (MM) sub-layer provides services to different entities of upper

connection management (CM) sub-layer. The different entities of CM sub-layer requesting for

44EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEservice to MM sub-layer could be-call control (CC), short message service (SMS) or

supplementary service (SS). An MM connection is established and released on request from CM

entities. Different CM entity communicates with their pier entities using different MM

connection.

An MM connection requires an RR connection. Several MM connections may be active at the

same time and all simultaneous MM connection for a given MS use the same RR connection.

Different MM connections are identified by different protocol discriminator (PD) and transaction

identifier (TI) value. MM connection establishment may be initiated by the MS or the network.

For an MS to initiate MM connection its updating status should be updated and MM sub-

layer should be either in IDLE or ACTIVE state. If any MM specific procedure is running then a

new MM connection establishment will either be rejected or delayed (depending upon

implementation). If no RR connection exists between MS and network, RR connection is

established by sending CM service request message to the network and MM sub- layer enters wait

for o/g MM Connection State. If RR connection already exists and an MM connection is active,

CM service Request message is sent and MM sub-layer enters wait for additional o/g MM

connection state. The CM service Request message contains the type of CM service requested

(o/g call, Emergency call, SMS, SS). If the network can accept the CM service request, a CM

Service Accept message is sent to MS and on recovering this message MM connection become

active and CC message can be transferred by CM entity. If network cannot accept CM service

request from MS, a CM service Reject message is sent to MS.

In case of MT (mobile terminated) call (incoming voice call or short message to the MS) CM

sub–layer entity in the network request MM sub layer to establish a MM connection.

The MM sub-layer is informed after completion of paging procedure. Now these can be two

scenarios depending upon whether RR connection to the desired MS already exists or not.

(a) RR connection to the desired MS already exists

This could be the case when MS is in conversation mode and there is a short message for the

MS. When RR connection between the network and desired MS exists and also no MM specific 45

EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEprocedure is running (is no location updating etc. is in progress) the network establishes new MM

connection over same RR connection with new PD/TI combination. Before establishment of a

new MM connection the network may initiate any of MM common procedure like authentication,

ciphering etc. and wait for their successful completion.

(b) RR connection to the desired MS does not exist

MM sub layer first requests to establish an RR connection and on connection establishment

MM sub-layer may initiate any of MM common procedure (authentication, ciphering etc.) Upon

successful completion of any such procedure MM sub layer informs the requesting CM sub-layer

entity.

If RR connection establishment is unsuccessful or any of the MM common procedure fails,

this is indicated to CM sub-layer with appropriate error cause.

A CM sub-layer entity, after having been advised that a MM connection has been

established, requests the transfer of CM massages. The CM messages passed to MM sub-layer are

sent to these other side of the interface with PD & TI set according to source entity. Upon

receiving CM message, the MM sub-layer on the side of the interface distributes it to the relevant

CM entity according to the PD & TI value.

After the information transfer between CM entities is over, an established MM connection

can be released by local CM entity. The release of CM connection is then done locally in the MM

sub-layer. After the release of last MM connection by its user, the MM sub-layer decides to

release RR connection requesting RR sub-layer.

6.5 CALL PROCESSING

In this we discuss the call processing aspect and look into specifics case of a mobile

originated (MO) call and a mobile terminated (MT) call. We also look into short message (SMS)

and voice mail service (VMS) as implemented IMPCS pilot project.

6.5.1 RF CHANNEL OVERVIEW

RF channel play important role in call processing case. These are basically three types of

RF control channel.

46EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• Broadcast control channel

The broadcast channels are points to multi-point channel, which are defined only for

down-link direction (BTS to mobile station). They are divided into:

• BCCH (Broad cast control channel): BCCH acts as a beacon. It informs the mobile

about system configuration parameters . Using this information MS choose the best cell to

attach to. BCCH is always transmitted on full power and it is never frequency hopped.

• FCCHC (Frequency correction channel): MS must tune to FCCH to listen to BCCH.

FCCH transmits a constant frequency shift of the radio carrier that is used by the MS for

frequency correction.

• SCH (synchronization channel): SCH is used to synchronize the MS in time .SCH

carries TDMA frame number and BSIC (Base Station Identity Code)

• Common control channels

Common control channels are specified as point to multi-point, which operate only in one

direction either in up-link or down-link direction.

• PCH (Paging Channel): PCH is used in down-link direction for sending paging message

to MS whenever there is incoming call.

• RACH (Random Access Channel): RACH is used by the MS to request allocation of a

specific dedicated control channel (SDCCH) either in response to a paging message or for

call origination /registration from the MS. This is an up-link channel and operates in point

to point mode.

• AGCH (Access Grant Channel): AGCH is a logical control channel which is used to

allocate a specific dedicated control channel (SDCCH) to MS when MS request for a

channel over RACH. AGCH is used in downlink direction.

• Dedicated Control Channel

Dedicated control channel are full duplex, point to point channel. They are used for

signaling between the BTS and certain MS. They are divided into

47EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• SACCH (Slow Associated Control Channel): the SACCH is a duplex channel, which is

always allocated to TCH or SDCCH. The SACCH is used for

• Radio link supervision measurements.

• Power control.

• Timing advance information.

In 26 frame traffic multi-frame 13th frame (frame no .12) is used for SACCH.SACCH is used

only for non-urgent procedures.

• FACCH (Fast Associated Control Channel): FACCH is requested in case the

requirement of signaling is urgent and signaling requirement cannot be met by SACCH.

This is the case when hand-over is required during conversation phase. During the call

FACCH data is transmitted over allocated TCH instead of traffic data. This is marked by a

flag known as stealing flag.

• SDCCH (Stand Alone Dedicated Control Channel): The SDCCH is a duplex, point to

point channel which is used for signaling in higher layer. It carries all the signaling

between BTS & MS when no TCH is allocated to MS. The SDCCH is used for service

request, location updates, subscriber authentication, ciphering. Equipment validation and

assignment of a TCH.

6.5.2 MOBILE ORIGINATED (MO) CALL

There are four distinct phase of a mobile originated call-

• Setup phase.

• Ringing phase.

• Conversation phase.

• Release phase.

Out of these phases the setup phase is the most important phase and includes authentication of

the subscriber, Ciphering of data over radio interface, validation of mobile equipment, validation

of subscriber data at VLR for requests service and assignment of a voice channel on A-interface

by MSC. Whenever MS wants to initiate on outgoing call or want to send an SMS it requested for

48EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEa channel to BSS over RACH. On receiving request from MS, BSS assigns a stand-alone

dedicated control channel (SDCCH) to MS over access grant channel (AGCH). Once a SDCCH

has been allocated to MS all the call set up information flow takes place over SDCCH.

A connection management (CM) entity initiates a CM Service Request message to the

network. Network tries to establish MM connections between the MS and the network and upon

successful establishment of MM connection a CM Service Accept message is received by MS

from the network. MS now sends a Call Set up Request to the network which contains the dialed

digits (DD) of the called party. As the call setup message is received at the MSC/VLR certain

check are performed at MSC/VLR like- whether the requested service is provisioned for the

subscriber or not, whether the dialed digits are sufficient or not, any operator determined barring

(ODB) does not allow call to proceed further etc. As these checks are performed at MSC/VLR a

Call Proceeding Message is sent from the network towards the MS. After all the checks are

successfully passed MSC sends Assignment command to the BSS which contains a free voice

channel on A-interface On getting this message BSS allocates a free TCH to the MS and informs

the MS to attach to it. MS on attaching to this TCH informs the BSS about it. On receiving a

response from the BSS, MSC switches the speech path toward the calling MS. Thus at the end of

Assignment the speech path is through from MS to MSC. It is important to note that at this stage

mobile has not connected user connection as yet. MS at this stage does not listen anything.

After assignment MSC sends a network set-up message to the PSTN requesting that a call

be set up. Included in the message are the MS dialed digits (DD) and details specifying which

trunk should be used for the call. The PSTN may involve several switching exchanges before

finally reaching the final local exchange responsible for applying the ringing tone to the

destination phone. The local exchange will generate the ringing tone over the trunk, or series of

trunk (if several intermediate switching exchange are involved), to the MSC. At this point in time

MS will hear ringing tone. The PSTN notifies the MSC with a network-alerting message when

this event occurs. MSC informs the MS that the destination number is being alerted. It is

important to note that this is primarily a status message to the MS. The MS hears the ringing tone

from the destination local exchange through the established voice path.

49EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE When the destination party goes off hook, PSTN informs the MSC of this event. At this

point, MS is connected to the destination party and billing is started. MSC informs the MS that

connection has been established and MS acknowledges the receipts of the connect message.

Under normal condition, the termination of a call is MS initiated or network initiated. In this

scenario, we have assumed that MS initiates the release of the call by pressing “end” button and

MS send a disconnect message to the MSC. The PSTN party is notified of the termination of the

call by a release message from the MSC. The end- to- end connection is terminated. When MSC

is left with no side task (e.g. charging indication etc.) to complete a release message is sent to the

MS. MS acknowledges with a release complete message. All the resources between MSC and the

MS are released completely.

6.5.3 MOBILE TERMINATED (MT) CALL

The different phases of a mobile terminated call are

• Routing analysis.

• Paging.

• Call setup.

• Call release.

The phases of mobile terminated (MT) call are similar to a mobile originated (MO) call

except routing analysis and paging phase. Call to a mobile subscriber in a PLMN first comes to

gateway MSC (GMSC). GMSC is the MSC, which is the capable of querying HLR for subscriber

routing information. GMSC need not to be part of home PLMN, though it is normal practice to

have GMSC as part of PLMN in commercially deployed networks.

GMSC opens a MAP (Mobile Application Part) dialogue towards HLR and Send / Routing

/ Info-Request (SRI request) specific service message is sent to HLR. SRI request contains

MSISDN of the subscriber. HLR based on location information of this subscriber in its database,

opens a MAP dialogue towards VLR and sends Provide / Roaming / Number-request (PRN

request)to the VLR. VLR responds to PRN request with PRN response message, which carries an

MSRN(mobile subscriber roaming number), which can be used for routing toward visiting MSC

in the network. HLR returns MSRN to GMSC (MSC that queried HLR) in SRI response message.

50EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEOn getting MSRN the GMSC routes the call towards VMSC the purpose of this entire exercise is

to locate where the terminating mobile subscriber is.

The MSRN received at GMSC is in international format (Country Code + Area Code +

subscriber number). Normally, based on the routing info at GMSC, the call may be routed out of

the GMSC towards VMSC of the terminating subscriber, in which case appropriate signalling

protocol (MF or ISUP) depending on the nature of connecting of GMSC with subsequent

exchange along the route will apply. If at VMSC the terminating mobile subscriber is found to be

free (idle), paging is initiated for terminating mobile subscriber. MSC uses the LAI provided by

the VLR to determine which BSS’s should page the MS. MSC transmit a message to each of these

BSS requesting that a page be performed. Included in the message is the TMSI of the MS. Each of

the BSS’s broadcasts the TMSI of the mobile in a page message on paging channel (PCH).

When MS detects its TMSI broadcast on the paging channel, it responds with a channel

request message over Random Access Channel (RACH). Once BSS receives a channel request

message, it allocates a stand –alone Dedicated Control Channel (SDCCH) and forwards this

channel assignment information to the MS over Access Grant Channel (AGCH). It is over this

SDCCH that the MS communicates with the BSS and MSC until a traffic channel assigned to the

MS. MS transmits paging response message to the BSS over the SDCCH. Included in this

message is MS TMSI and LAI. BSS forwards this paging response message to the MSC. Now

Authentication and Ciphering phases are performed to check the authenticity of MS and encrypt

data over radio interface.

On the network side after paging is initiated, while waiting for paging response, a defensive

timer called, ”Early ACM” timer is run at MSC to avoid network timeouts. On successfully

getting paging response, a setup message is constructed to be sent towards terminating MS. In

case paging fails due to authentication failure or when the subscriber is out of radio-coverage, the

call is cleared.

In case CLIP is not subscribed by the terminating mobile subscriber, calling number is not

included in set-up message. In case CLIP is subscribed and PI value in calling number parameter

indicates “presentation allowed” the number is included in the set-up message. In case CLIP is

51EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEsubscribed but PI received in calling number parameter indicates “presentation restricted” then

number is included only if CLIRO is also subscribed to.

MS on receiving the set-up message performs compatibility checking before responding

to the set-up message – it is possible that MS might be incompatible for certain types of call set-

ups. Assuming that MS passes compatibility checking, it acknowledges the call setup with set-up

confirm message. After getting set-up confirm message from the MS, MSC performs assignment

phase (similar to one discussed in MO call) and a voice path is established from MSC to the MS.

MS begins altering the user after it receives the traffic channel assignment. MS send alerting

message to the MSC .MSC upon receiving the alerting indication from the MS begins generating

an audible ringing tone to the calling party and sends a network alerting via GMSC to the PSTN.

Prior to this the calling party heard silence.

At this point in the call, MS is alerting the called party by generating on audible tone. One

of the three events can occur-calling party hangs-up, mobile subscriber answers the phone, or the

MSC times out waiting for the mobile subscriber to the answer the call. Since radio traffic channel

is a valuable resource, GSM does not allow a MS to ring forever.

In the present scenario we have assumed that the mobile subscriber answers the phone.

The MS in response to this action stops alerting and sends a connect message to the MSC. MSC

removes the audible tone to the PSTN and connects the PSTN trunk to BSS trunk (terrestrial

channel) and sends a connect message via GMSC to the PSTN. The caller and the called party

now have a complete talk path. This event typically marks the beginning of the call for billing

purposes. MSC sends connect acknowledge message to the MS.

The release triggered by the land user is done in similar way as the release triggered by

mobile user. MSC receives a release message from the network to terminate end-to-end

connection. PSTN stops billing the calling landline subscriber. MSC sends a disconnect message

towards the MS and MS responds by a Release message. MSC release the connection to the PSTN

and acknowledges by sending a Release Complete message to PSTN. Now the voice trunk

between MSC and BSS is cleared, traffic channel (TCH) is released and the resources are

completely released.The mobile-to-mobile call scenario is a combination of phases encountered in

mobile originated (MO) and mobile terminated (MT) call.

52EIET



CHAPTER 7

GPRS

7.1 INTRODUCTION TO GPRS

In response to customer demand for wireless Internet access – and as a stepping-stone to

3G networks – many GSM operators are rolling out general packet radio service (GPRS). This

technology increases the data rates of existing GSM networks, allowing transport of packet based

data. New GPRS handsets will be able to transfer data at rates much higher than the 9.6 or 14.4

kbps currently available to mobile-phone users. Under ideal circumstances, GPRS could support

rates to 171.2 kbps, surpassing ISDN access rates. However, a more realistic data rate for early

network deployments is probably around 40 kbps using one uplink and three downlink timeslots.

Unlike circuit-switched 2G technology, GPRS is an “always-on” service. It will allow

GSM operators to provide high speed Internet access at a reasonable cost by billing mobile-phone

users for the amount of data they transfer rather than for the length of time they are connected to the

network. This application note looks in detail at the challenges of measuring and optimizing GPRS

networks. It builds directly on the first paper in this series, 'Understanding General Packet Radio

Service (GPRS),’ AN-1377, which explains the new protocols, procedures, and other technology

changes that GPRS introduces to GSM networks.

7.2 GPRS MEASUREMENT MODEL

This discusses the measurements used to evaluate the performance of GPRS networks

using drive test tools. GPRS measurements, which map into the model are divided into three

categories: data performance, signal quality, and RF performance.

Data Performance

This category emphasizes data-transfer-quality measurements (as perceived by customers)

and GPRS layer-specific measurements. Data performance measurements are used to establish

quality benchmarks and to detail the performance of individual layers. We further divide the data

performance into two sub-categories:

53EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• IP/Application layer measurements are made by simulating user application models and

measuring the parameters directly perceived by the user (such as throughput and delay).

• GPRS layer measurements are made at the layers below the application layer (for example, at

the RLC/MAC/LLC layers) and are hidden to the user. These measurements offer insight about

events on the GPRS layers that can impact the application layer performance.

In both cases, the measurements are made using a test mobile connected to a laptop PC with

special data measurement software. We also need a server for end-to-end data measurements.

Signal Quality

This category consists of physical layer measurements and a subset of RLC and MAC

layer measurements. The measurements are made using a test mobile phone.

RF Performance

This category consists of network-independent measurements such as interference,

scanning, and spectrum analysis. The measurements require sophisticated RF test tools such as

DSP-based RF measuring receivers.

Measurement model

Data performance Signal quality RF performance

Application layer GPRS layers

Phone + Server based measurements Phone-based Receiver-based

Figure 7.1. GPRS drive test measurement model

The relationship between the GPRS measurement model for data performance and the

various protocol stacks is shown in Figure 7.2. Performance is measured at three layers: end-to-

54EIET



end data performance at the application layer, GPRS layer data performance at the GPRS layers,

and RF quality performance at the air interface.

Client (GPRS mobile phone) Server

Figure 7.2. GPRS measurement model on the protocol stacks

End-to-end data performance

Data performance at the application layer is measured end-to-end; that is, we simulate a

“real world data pattern and send it to the other “end” – a test server, which performs the

measurements and stores or sends the results back to the client, a mobile phone (MS). These

measurements are made to quantify the user perception of data performance, and they are

analogous to the voice quality measurements in GSM networks. We can also get information on

the IP layer, depending on the type of server used and where it is placed in the network.

GPRS layer data performance

The data from the application layers is first processed by the GPRS layers and headers are

added before it is sent onto the air interface. To a certain extent (depending on quality of service

levels), the GPRS layers are capable of providing data performance information (such as LLC and

RLC layer performance).

55EIET



Signal quality and RF performance

These measurements are primarily physical layer measurements that provide signal level

and quality information. The category may include optimization measurements such as

interference monitoring and scanning.

Using drive test tools, we can make these measurements simultaneously. Consequently,

when low application throughput is measured, the GPRS layers, signal quality, and RF

performance measurements help to determine the cause of the problem.

7.3 DATA PERFORMANCE MEASUREMENTS

Classes of service

Customers have different data usage requirements. Some need only low data rates for

specific Web-based transactions; others need moderate data rates for applications such as e-

mail; and still others need high data rates for tasks such as transferring or downloading large,

Web-based files. Often customers have different requirements at different times.

GPRS has the flexibility to support dynamic management of network resources and

therefore different service levels. Customers will have the option of deciding what quality of

service they need and will know (at least in theory) what they are paying for. Service level

choices will likely be divided into a hierarchy of quality bands or classes such as platinum, gold,

silver, and bronze.

On the system side, ETSI has defined a set of quality of service (QoS) classes to support

the implementation of tiered service levels:

• Precedence class

• Reliability class

• delay class

• Peak throughput class

• mean throughput class

56EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEThese QoS parameters will play a key role in ensuring that customers get the data

performance they expect.

Measurement objectives

Data performance is measured at the application and GPRS layers. Each set of measurements has

its own objectives.

At the application layer

A goal of performance measurements at this layer is to simulate the user model, first by

simulating the data applications (such as Web browsing, e-mail, and file transfers), and then by

modeling the data load. We need to simulate the asymmetrical data transfer pattern (more data in

the downlink, less in the uplink) of real world applications.

A second objective is to provide real time stamped measurements. This requires getting

performance information in real time. In order to quantify and benchmark these measurements,

we need to refer to standards such as ETSI's QoS parameters. At present these parameters are

focused primarily on the application layer, and so benchmarking GPRS network performance

against these standards will provide a good way to quantify user perception of data services.

At the GPRS layers

Performance measurements at the GPRS layers are more complex, because many protocol

layers are working simultaneously (for example, the RLC, MAC, and LLC). To identify a

protocol layer of concern, we need to simulate the different QoS levels and carry out

measurements at the different layers. Because data packets travel through different nodes in the

GPRS network (BSS, SGSN, GGSN, PDN), we use GPRS layer measurements to identify the

problem nodes. One additional objective of GPRS layer measurements is to provide control over

the protocol layers.

Application layer measurements

We need to understand the data performance model at the application layer of testing

before we delve into the bits and pieces of the performance parameters.

57EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEIf we take a broad view of the GPRS data communication model, we see the application

layer sitting on top. An application at one “end” (or node) of the network – the GGSN, for

example – communicates with a similar application at the other end, perhaps using TCP or UDP

protocols for acknowledged or unacknowledged modes of transfer, respectively.

In this model, the application layer at the mobile station (MS) communicates with the IP layer of

the MS and passes the application-layer datagram to the IP layer. This IP layer (which is standard

TCP/IP) forwards the information to the GGSN by way of the different GPRS nodes and protocol

layers. At the GGSN, the datagram information received from the MS is returned to the IP

datagram level. Then the IP layer at the GGSN communicates with the IP layer at the other end of

the call (at the PC) through the Public Data Network (PDN) IP interfaces.

When we measure data performance at the application layer, we want to send data from one end

of our network and measure its performance at the other end, with the goal of understanding the

end-to-end performance as experienced by the user.

Figure7.3. Data transfer at the Application and IP layers.

Measurement configuration at the application layer

58EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEThere are two ways to carry out data performance measurements at the application layer:

the loopback method and the end-to-end measurement method.

Loopback measurement

This approach requires a fixed unit at the network node, which collects the data coming

in from the mobile and then sends the data back to the mobile (creating a loopback). The mobile

compares the data sent with the data received. The process also can be reversed by simulating

data traffic and sending it from the fixed unit at the network to the mobile and then back to the

network.

This approach raises several issues. For example, in drive testing the network, the

objective is to make real time measurements, and we are interested in position-specific

performance data. The loopback approach does not precisely support real world application

models such as simultaneous, asymmetrical data transfer. Rather than measure uplink and

downlink separately, it measures uplink and downlink as a total loop. Consequently, we do not

know whether a problem exists on the uplink or the downlink.

End-to-end measurements

At the application layer, end-to-end measurements can be described as follows: One node

transmits data and another node receives the data and measures its performance. For our test

purposes, one end node is the mobile and the other is a measurement server. This server can be a

located at the GGSN or somewhere in the PDN (Internet world).

Since the measurements are made end-to-end, in the uplink the server measures the data

received from the mobile and sends back the results. In the downlink the measurements are made

by the same software that generated the uplink data.

Quality of service (QoS) parameters

Whatever parameters we measure, the ultimate objective at the application layer is to get

the user perspective. Thus it is essential to benchmark performance broadly against certain

standards.

59EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEETSI GPRS recommendations define quality of service for users according to specific parameters,

including reliability, throughput, and delay. Our data performance measurements therefore focus

on these parameters.

Reliability measurements

Reliability has been defined as the “probability of service data units (SDUs) getting

corrupted, lost, duplicated, and received out-of-sequence.”

Service data units are IP datagrams at the application layer. ETSI has defined five classes

of reliability at different interfaces. Since our application-layer measurements are made from end-

to-end, these reliability classes are significant.

If we know the negotiated reliability class of the phone, for example, we can make error

performance measurements, correlate them with the assigned reliability class, and benchmark

performance against the standard figures. Similarly, by performing absolute data-performance

measurements, we can determine the highest achievable class of reliability in the network and

based on the measurements assign reliability classes to customers.

Because SDU reliability is affected not just by errors but also by missequence and loss of

SDUs, reliability measurements also help us optimize routing paths, packet control parameters

such as segmentation and compression, and system timers for controlling buffer overflow.

The table in Figure 7.4 shows the level of reliability desired for the five reliability classes

defined by ETSI. Each class defines the interfaces on which protection and acknowledgments are

mandatory.

60EIET



Figure 7.4. ETSI reliability classes

For non-real time traffic, the highest reliability is required because the application

layer cannot handle data corruption, loss, and other such problems. If traffic is real time, the

application layer can manage retransmissions and thus the lower layers require less reliability.

Typically reliability classes 2 – 4 are preferred, with class 2 the ideal. The reason for this

preference is that we can manage physical link reliability (quality) on the Gn interface and so

ensure reliability on the GTP interface, which is generally in UDP mode. This helps ensure that

retransmissions will not occur and thus provides a better throughput.

The LLC layer data transfer goes through the air interface as does the RLC. The

probability of data corruption is low on the LLC layer if the RLC is reliable, but the probability of

missequencing is high as a high degree of segmentation takes place at the LLC and RLC layers.

With different levels of reliability possible, the application layer must be able to detect

errors when it operates at a higher reliability class. If errors are not corrected by the GPRS layers,

then they should be by the application layer. For test purposes, we cannot provide protection at

the application layer, so we cannot make the corresponding error performance measurements at

this layer. Rather, errors must be measured using UDP on the IP layer at the data reception and

generation ends.

Quantifying reliability

61EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEETSI has attempted to quantify reliability based on the probability of SDUs being lost,

duplicated, out of sequence, or corrupted. The SDUs that get corrupted by errors on the interfaces

can be delivered to the end application and thus to users in that same corrupted form. Due to

problems on the interfaces and in the retransmission process, SDUs also may be duplicated or

arrive at the end node out of sequence.

GPRS is not a store-and-forward service. Customers are real time data users, so GPRS

provides buffers to store information along the route to compensate for resource shortages. Delays

in getting resources or transporting the information further depend on the protocols and equipment

being used. The SDUs stored in the buffer may be discarded by a GPRS node if the holding timer

expires; if this happens these SDUs will be lost. Thus, to meet a specified level of reliability, we

must overcome a number of measurement challenges.

Reliability measurement challenge:

Synchronization

One of the biggest challenges in measuring reliability is time synchronization. With an

end-to-end measurement approach, we need to synchronize the time that the data is sent with the

time it is received. In the downlink, synchronizing measurements is generally not a problem. But

when the datagrams are sent in the uplink, they reach the measurement server only after some

period of time has elapsed. The server then must send the results back to the MS, which adds

more delay. As a result, the time at which the data was sent and the time at which the results are

received can differ significantly. The position of the mobile may also have changed at this point

and thus the serving cell.

We can resolve this problem in a couple of ways:

• We can synchronize the absolute timing at the MS and the server (for example, using the

Global Positioning System). The MS can then timestamp every datagram. The server then returns

the datagram with the MS timestamp and adds its own. The timestamps can be correlated to the

right position.

62EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• The MS also can stamp every datagram sent in the uplink with the latitude and longitude.

The same latitude and longitude is returned by the serve in the measurement results, providing the

correct positions.

If there is no synchronization, we can reduce the SDU size, which speeds the transmission

of measurement results from the server back to the MS. We can also reduce the LLC block size.

Both of these actions reduce the time it takes to transmit results back to the MS allowing results

to be much faster.

Figure 7.5 Differences in the time data is sent and measurement results are received create a need

for synchronization. Since GGSNs are located indoors, it is important to locate the GPS antenna

outdoors in view of GPS satellites.

Reliability measurement challenge: SDU size

When we make data performance measurements, another challenge is to decide the SDU

size. In practice, SDU size depends on the application in use. If we look at different layers in the

protocol stack, we see the packet sizes that exist in GPRS. The size of an IP datagram can range

from 1 octet to 65,535 octets. When a datagram arrives at the SNDCP/LLC layer at the SGSN, it

is segmented into 1520 octets. There are other possibilities for controlling packet size in a general

way. ETSI defines two SDU sizes for reliability measurements: 128 octets and 1024 octets.

Whether we use the smaller or larger size will affect our measurements. If SDUs are small in size,

we have to send a larger number of them. Even if a few of them get corrupted, we will still

achieve good reliability percentages. On the other hand, if we use the larger size for our SDUs, we

will send fewer of them. In this case, even if only a small number of SDUs get corrupted, the

reliability percentage for our data transmission will drop. Further, retransmission of large SDUs

adds delay and thus reduces throughput. On the other hand, the larger SDUs require less overhead

and so help increase application layer throughput.

An appropriate SDU reliability test involves two steps:

63EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• Simulate SDU performance – We can simulate IP datagrams based on different possible

applications, measure their reliability, and correlate the results with our throughput measurements.

• Measure raw bit errors – Measuring the raw bit errors gives us an indication of link (MS to

GGSN) reliability. Then, correlating the raw bit error measurements with simulated SDUs of

different size provides a good estimate of the reliability of the link for different applications.

Agilent uses this two-step process to measure IP layer BER performance.

Throughput factors

One major performance parameter that is evident to customers is throughput. Throughput

is the rate at which data is expected or received over a period of time. Throughput is measured in

bits/second. Simply stated, it is the data rate achieved against expectations. A combination of

several factors determines throughput link reliability, compression, retransmission mechanisms,

and delay.

Link reliability

Errors in data transmission at the link can trigger retransmissions – that is, the same data

being transmitted again. The result is a reduction in throughput. Throughput can be defined as

either raw or effective.

• Raw throughput indicates the rate at which data is received. The data may contain errors and

therefore would not be usable. The IP layer throughput is generally considered the raw

throughput.

• Effective throughput is the rate of correctly received data. This is generally associated with the

application layer.

Compression

This technique is deployed at the SNDCP layer at the SGSN to reduce the size of the data.

Compression increases throughput on the air interface layer, which cannot offer high throughput

due to physical layer limitations. Compression affects our measurements at the application layer.

Although it increases throughput on the air interface, it may not do the same at the application

64EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECElayer, because the SNDCP has to decompress the data before passing it to the application layer,

where limitations on the mobile phone's buffering Retransmission mechanisms

These mechanisms are used to compensate for data impairments. But retransmissions are

optional and depend on the negotiated QoS class. If retransmissions are on, then SDU size plays

an important role in determining the throughput. If retransmissions are not on, then GPRS layer

throughput will be high, but if errors are present, the application layer throughput will suffer. In

this case, since the lower layers are not compensating for errors, the application layer must.

Another important consideration is the air interface cell reselection done by the MS when it is in

the packet transfer mode. Cell reselection results in a complete transfer of TBF (temporary block

flow) rather than segmented LLC blocks. With many reselections there will be more TBF level

retransmissions, which again impact our throughput capabilities may exist.

Delay

There are several causes of delay in packet transmission and reception. Delays can be

caused by buffering. Because the interface types are different and each interface has a

different maximum transmission unit (MTU) capacity, the data must be buffered. If buffering is

excessive, however, buffers may overflow and SDUs will be lost. This triggers retransmission of

the lost packets, which affects the throughput. Delay can also occur if acknowledgements are not

received within a specified period (established by timers), again triggering retransmission.

Peak throughput

Defined by ETSI, peak throughput is measured at the Gi and R reference points in units of

octets per second. It specifies the maximum rate at which we expect data to be transferred across

the network for an individual PDP context. There is no guarantee that this peak rate can be

achieved or sustained for any time period; sustaining peak throughput depends on the capability

of the MS and the availability of radio resources. The network may limit customers to a

negotiated peak data rate, even if additional transmission capacity is available. The peak

throughput is independent of the delay class, which determines the per-packet GPRS network

transit delay. The peak throughput classes are defined in Figure 7.6

65EIET



Figure7.6. ETSI peak throughput classes

Measuring throughput

Throughput is measured at the application layer and at the GPRS layers. Throughput is

affected by factors such as compression and reliability classes, which can create significant

differences at the application layer and the GPRS layers.

Assigning a high reliability class to data will ensure that throughput at the GPRS layers is

the same as throughput at the application layer, as error detection and retransmission will be

enabled at all layers. Lower reliability classes often result in higher throughout at the GPRS

layers and lower throughput at the application layers, as errored SDUs can be passed to the

application layer, which then must retransmit the data. Additionally, if the application layer is

working in acknowledged mode, the estimated throughput rate will be accurate, but the actual

throughput will be lower. This mode requires using TCP at the application layer. With TCP, the

probability of detecting errors at the application layer goes down because only error-free data gets

through. So, although we get a correct estimate of throughput, we compromise our reliability

measurements. A more suitable approach to achieving throughput is to keep the application layer

in unacknowledged mode (thus using UDP). This scenario gives us high data rate and the ability

to measure errors. By taking into account the number of errors versus the number of data packets

received, we can calculate the application layer throughput. This level of measurement flexibility

is possible only by making end-to-end measurements.

End-to-end throughput measurements

End-to-end throughput measurements are made at the IP layer and the application layers.66

EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECE• IP Layer Throughput

This measurement requires the use of UDP. Since UDP does not manage reliability, all the

data coming in at the IP layer is transferred to the application layer. Recall that TCP and UDP are

in the layer between the IP and application layers. The application layer is a test client, so we can

measure the data coming in from the IP layer that is, the IP throughput. At this stage we don't

know how reliable the data coming in is, so the IP throughput measurement can include bad

packets.

• Application layer throughput

This measurement can be done in both TCP and UDP modes. When TCP is used, reliability is

included in the throughput measurement at the application layer. If UDP is used (which allows us

to measure the IP layer performance, as discussed in the previous section), then the application

layer throughput is calculated using the IP layer throughput and the bad or errored data at the

application layer (that is, the measured IP BER).

In UDP mode, we therefore get both IP layer and application layer throughput information, as

illustrated in Figure 7.7. In TCP mode, we measure only the application layer throughput.

Figure7.7. End-to-end throughput measurement at the IP and application layers. IP BER, lost

packets, and out-of-order packets are also shown.

Delay measurements

67EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEGPRS is not a store and forward service. It buffers data only temporarily during

transmission resource assignment and handling of impairments. Data buffering can result in delay.

Delay can vary with time and load. Delay performance is based on the mean delay/second for

data transfer through the GPRS network. High rates of delay can occur during momentary

problems, such as a moment of peak traffic. To account for this, ETSI defines a 95-percentile

delay, which is the maximum allowable delay for 95 percent of the SDUs that are delivered over

the GPRS network. The delay parameter defines the end-to-end transfer delay incurred in the

transmission of SDUs through the GPRS network. It is measured from mobile data terminal

interface (“R”) to the Gi interface at the SGSN, as shown in Figure 7.8

Figure7.8. Measurement of one-way delay

To make absolute delay measurements (particularly one- way delay), we need to

synchronize the absolute timing at the transmit and receive ends. This can be achieved by using

GPS receivers at both ends or by using some other proprietary time synchronization technique.

Once the clocks synchronize, the transmit end ill timestamp the SDU and send it to the receive

end. The receive end captures the SDU, adds its timestamp, reads the attached transmit end

timestamp, and measures the delay (which is the time that has elapsed between the transmit and

the receive timestamps).

ETSI defines delay measurements from R at the mobile to the Gi interface. If we want to

benchmark against ETSI standards, the measurement server needs to be at the GGSN, within the

firewall of the GPRS network (Figure 7. 9). We can also locate the server in the PDN to identify

PDN nodes that may be causing excessive delay.

68EIET



Figure7.9. Absolute delay measurement

Round trip time (RTT)

Since end-to-end delay requires time synchronization, which becomes difficult in certain

implementations, RTT is another method of getting some information about the delay. The RTT is

the amount of time measured from the point in time at which the last bit of the data packet

leaves the application to the point in time at which the last bit of the acknowledgement is

received. Thus RTT includes the time in both directions. Measurements of RTT are useful in

understanding the impact on data performance of packet fragmentation and buffer size.

A combined analysis on both RTT and one way delay allows us to troubleshoot problems

of latency in the network.

QoS simulation

Simulation capability is another critical element of data performance measurements.

Customers are assigned QoS levels for different classes of service when they subscribe to a GPRS

service. These parameters are also negotiated during PDP context activation. Data performance

among QoS levels will vary significantly. If the GPRS network can achieve high QoS levels,

those levels can be assigned to customers for a higher price. To verify the data performance, we

69EIET


MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST ECEneed the ability to simulate multiple data transactions at the different QoS levels. Because QoS

levels are associated with measurement parameters at the GPRS layers, we cannot simulate data

transfer patterns at the application layer. Therefore, we require multiple handsets (or different

SIM cards) to make data calls at the different QoS levels simultaneously. Our multi-handset test

model for measuring QoS levels should include a data performance test suite that can control the

multiple phones and generate similar data-transfer patterns on each phone. With each phone

assigned a different QoS level, the performance can be verified at each level and appropriate steps

taken to correct any problems.

70EIET

BENCH MARKING OF GPRS NETWORK KEY PERFORMANCE INDICATORS IN ECE MOBILE SERVICE URBAN AREA BY CONDUCTING DRIVE TEST

CHAPTER 8

KEY PERFORMANCE INDICATORS

8.1 INTRODUCTION

For radio network optimization it is necessary to have key performance indicators. These

KPIs are parameters that are to be observed closely when the network monitoring process is

going on. Mainly, the term KPI is used for parameters related to voice and data channels, but

network performance can be broadly characterized into coverage, capacity and quality criteria

also that cover the speech and data aspects.

The performance of the radio network is measured in terms of KPIs related to voice

quality, based on statistics generated from the radio network. Drive tests and network

management systems are the best methods for generating these performance statistics.

The most important of these from the operator's perspective are the BER (bit error rate),

the FER (frame error rate) and the DCR (dropped call rate).

The BER is based on measurement of the received signal bits before decoding takes

place, while the FER is an indicator after the incoming signal has been decoded. Correlation

between BER and FER is dependent on various factors such as the channel coding schemes or

the frequency hopping techniques used. As speech quality variation with the FER is quite

uniform, FER is generally used as the quality performance indicator for speech. The FER can be

measured by using statistics obtained by performing a drive test. Drive testing can generate both

the uplink and the downlink FER.

The dropped call rate, as the name suggests, is a measure of the calls dropped in the

network. A dropped call can be defined as one that gets terminated on its own after being

established. As the DCR gives a quick overview of network quality and revenues lost, this easily

makes it one of the most important parameters in network optimization. Both the drive test

results and the NMS statistics are used to evaluate this parameter. At the frame level, the DCR is

measured against the SACCH frame. If the SACCH frame is not received, then it is considered to

be dropped call. There is some relation between the number of dropped calls and voice quality. If

the voice quality were not a limiting factor, perhaps the dropped call rate would be very low in 71

EIET


the network. Calls can drop in the network due to quality degradation, which may be due to

many factors such as capacity limitations, interference, unfavorable propagation conditions,

blocking, etc. The DCR is related to the call success rate (CSR) and the handover success rate.

The CSR indicates the proportion of calls that were completed after being generated, while the

handover rate indicates the quality of the mobility management/RRM in the radio network.

KPIs can be subdivided according to the areas of functioning, such as area level, cell level

(including the adjacent level), and TRX level. Area-level KPIs can include SDCCH requests, the

dropped SDCCH total, dropped SDCCH A-bis failures, outgoing MSC control handover (HO)

attempts, outgoing BSC control HO attempts, intra-cell HO attempts, etc. Cell-level KPIs may

include SDCCH traffic BH (av.), SDCCH blocking BH (av.), dropped SDCCH total and

distribution per cause, UL quality level distribution, DL quality/level distribution etc. The TRX

level includes the likes of UL and DL quality distribution.

8.2 NETWORK PERFORMANCE AND MONITORING

The whole process of network performance monitoring consists of two steps:

• Monitoring the performance of the key parameters,

• Assessment of the performance of these parameters with respect to capacity and

coverage.

First the radio planners assimilate the information/parameters that they need to monitor. The

KPIs are collected along with field measurements such as drive tests. For the field

measurements, the tools used are ones that can analyze the traffic, capacity, and quality of the

calls, and the network as a whole. For drive testing, a test mobile is used. This test mobile keeps

on making calls in a moving vehicle that goes around in the various parts of the network. Based

on the DCR, CSR, HO, etc., parameters, the quality of the network can then be analyzed. Apart

from drive testing, the measurements can also be generated by the network management system.

And finally, when 'faulty' parameters have been identified and correct values are determined, the

radio planner puts them in his network planning tool to analyze the change before these

parameters are actually changed or implemented in the field.

72EIET


8.3 NETWORK PERFORMANCE ASSESSMENT

The performance indicators are listed below:

• Amount of traffic and blocking

• Resource availability and access

• Handovers (same cell/adjacent cell, success and failure)

• Receiver level and quality.

• Power control.

8.3.1 COVERAGE

Drive test results will give the penetration level of signals in different regions of the

network. These results can then be compared with the plans made before the network launch. In

urban areas, coverage is generally found to be less at the farthest parts of the network, in the

areas behind high buildings and inside buildings. These issues become serious when important

areas and buildings are not having the desired level of signal even when care has been taken

during the network-planning phase. This leads to an immediate scrutiny of the antenna locations,

heights and tilt. The problems are usually sorted out by moving the antenna locations and

altering the tilting of the antennas. If optimization is being done after a long time, new sites can

also be added.

Coverage also becomes critical in rural areas, where the capacity of the cell sites is

already low. Populated areas and highways usually constitute the regions that should have the

desired level of coverage. A factor that may lower the signal level could be propagation

conditions, so study of link budget calculations along with the terrain profile becomes a critical

part of the rural optimization. For highway coverage, additions of new sites may be one of the

solutions.

8.3.2 CAPACITY

Data collected from the network management system is usually used to assess the

capacity of the network. As coverage and capacity are interrelated, data collected from drive tests

is also used for capacity assessment. The two aspects of this assessment are dropped calls and

73EIET


congestion. Generally, capacity-related problems arise when the network optimization is taking

place after a long period of time. Radio network optimization also includes providing new

capacity to new hot-spots, or enhancing indoor coverage. Once the regional/area coverage is

planned and executed in the normal planning phase, optimization should take into consideration

the provision of as much coverage as possible to the places that would expect high traffic, such

as inside office buildings, inside shopping malls, tunnels, etc.

8.3.3 QUALITY

The quality of the radio network is dependent on its coverage, capacity and frequency

allocation. Most of the severe problems in a radio network can be attributed to signal

interference. For uplink quality, BER statistics are used, and for downlink FER statistics are

used. When interference exists in the network; the source has to be found out. The entire

frequency plan is checked again to determine whether the source is internal or external. The

problems may be caused by flaws in the frequency plan, in the configuration plans (e.g. antenna

tilts), inaccurate correction factors used in propagation models, etc.

8.4 PARAMETERS CONSIDERED FOR THE DRIVE TEST

The following parameters are primarily considered while doing the drive test:

• Call success ratio (CSR): CSR is the number of successful attempts to make a call.

Ideally, a network should be capable of accepting all the calls attempted to be made. The

ideal value of CSR is 1 i.e. the network should be capable of accepting 100 % of the calls

made. CSR is found out through a long call.

CSR = succeeded attempts/ total number of attempts

• Rx level: It is the received signal strength i.e. it is the strength of the signal received by

the receiver cell phone. It can be found out through a long call as well as a short call. The

acceptable value of Rx level is at least -95 dbm.

• Rx quality: Rx quality is also known as speech quality. It is the quality of the speech

received by the receiver cell phone. It is indicated by the Bit Error Rate or the BER. For a

network to have good performance, the Rx quality should lie between 0-5. If the Rx

74EIET


quality exceeds 6, then the network’s performance is not acceptable. It can be found out

through a long call as well as a short call.

• C/I ratio: The C/I ratio or the Carrier to Interference ratio is an important parameter in

analyzing the quality of a network. The C/I ratio gives a relationship between the carrier

of the network and the interference it is facing. Theoretically the C/I ratio should be at

least 18 db but in practical cases 12 db is also acceptable.

• Handover success rate (HO Rate): HO rate is the number of successful handovers made

by a cell phone. It is the ratio of the number of successful handovers made to the total

number of attempts to make a handover. To find out the HO rate, the mobile should be in

dedicated mode i.e. the mobile should be on call.

HO rate = number of successful handover attempts/ total number of handover attempts.

Through put : the speed of the data that is received and transmitted by the network at

that particular location. It is calculated by dividing the total average speed to the no.of

throughputs.

Apart from these parameters the following parameters are also considered:

• Frame Error Rate;

• Cell Site Database- Site Configuration, Latitude & Longitude of the site location,

• BSIC,

• LAC,

• Hopping Frequencies,

• Non-Hopping Frequencies;

• MAIO,

• Antenna Parameters like Tilt, Pattern, Gain, and Azimuth/Orientation.

75EIET


8.5 PARAMETER TUNING

The ending of the assessment process sees the beginning of the complex process of fine-

tuning of parameters. The main parameters that are fine-tuned are signaling parameters, radio

resource parameters, handover parameters and power control parameters.

Network planning will have used standard propagation models and correction factors

based on some trial and error methods that may be valid for some parts of the network and

invalid for other parts. Then, during network deployment, some more measurements are made

and the parameters are fine-tuned again. Once the network goes 'live', the drive test and NMS

statistics help in further fine-tuning of the parameters, and it is at this point that a set of default

parameters is created for the whole network. However, as the network is inhomogeneous, these

default parameters may not be sufficiently accurate in all regions, thereby bringing down the

overall network quality and leading to a reduction in revenue for the network operator.

Radio network optimization must be a continuous process that begins during the pre-

launch phase and continues throughout the existence of the network.

76EIET


CHAPTER 9

DRIVE TESTING

9.1 INTRODUCTION

The Indian telecommunication industry, with about 650 million mobile phone

connections as of May 2010, is the second largest telecommunication network in the world. The

Indian telecom industry is the fastest growing one in the world and it is projected that India will

have a 'billion plus' mobile users by Jan 2012. The Indian telephone lines have increased from a

meager 40 million (approx.) in the year 2000 to an astounding figure now. The main drivers for

this extraordinary growth are because of Government’s Telecom reforms and the stupendous

success of GSM standard, which is the most popular standard for mobile telephony systems in

the world.

GSM differs from its predecessor technologies in that both signaling and speech channels

are digital, and thus GSM is considered a second generation (2G) mobile phone system. RF

Network Planning & Optimization is an ongoing activity for all wireless networks because of its

highly growing market demand. By gathering, analyzing network data and revising network

parameters using proper RF Planning and Optimization, efficient and effective cellular

communication is achieved.

RF performance parameters such as the received signal strength, receive voice quality,

carrier to interference ratio, etc., are defined for the efficient and effective functioning of the RF

network. The Drive Testing (DT) is performed in GSM network to ensure the availability,

integrity, & reliability of the network. How to optimize the BTS coverage area successfully is the

real challenge. As we move further ahead, the need for better technologies and reliability of

services, integration and cost effective solutions have become a necessity for service providers.

If the optimization is successfully performed, then the QOS, reliability and availability of RF

Coverage area will be highly improved resulting in more customers and more profits to the

mobile telecom service providers.

77EIET


Figure 9.1 Integrated drive-test solutions consisting of a digital receiver and phone. A GPS

receiver provides location information.

9.2 WHAT IS DRIVE TEST?

Drive testing is the most common and maybe the best way to analyze Network

performance by means of coverage evaluation, system availability, network capacity, network

retainibility and call quality. Although it gives idea only on downlink side of the process, it

provides huge perspective to the service provider about what’s happening with a subscriber point

of view.

The drive testing is basically collecting measurement data with a phone, but the main

concern is the analysis and evaluation part that is done after completion of the test. Remember

that you are always asked to perform a drive test for not only showing the problems, but also

explaining them and providing useful recommendations to correct them.

Drive Test, as already mentioned, is the procedure to perform a test while driving. The vehicle

does not really matter; you can do a drive test using a motorcycle or bicycle. What matters is the

hardware and software used in the test.

• A notebook - or other similar device (1)

• With collecting Software installed (2),

78EIET


• A Security Key - Dongle - common to these types of software (3),

• At least one Mobile Phone (4),

• One GPS (5),

• A Scanner – optional (6).

Also is common the use of adapters and / or hubs that allow the correct interconnection of all

equipment.

The following is a schematic of the standard connections.

Figure 9.2 Schematic diagram of drive test.

The main goal is to collect test data, but they can be viewed / analyzed in real time (Live) during

the test, allowing a view of network performance on the field. Data from all units are grouped by

collection software and stored in one or more output files (1).

Figure 9.3 Drive test output from various sources.

79EIET


GPS: collecting the data of latitude and longitude of each point / measurement data, time,

speed, etc. It is also useful as a guide for following the correct routes.

• MS: mobile data collection, such as signal strength, best server, etc ...

• SCANNER: collecting data throughout the network, since the mobile radio is a

limited and does not handle all the necessary data for a more complete analysis.

The minimum required to conduct a drive test, simplifying, is a mobile device with software to

collect data and a GPS. Currently, there are already cell phones that do everything. They have a

GPS, as well as a collection of specific software. They are very practical, but are still quite

expensive.

9.3 DRIVE TEST ROUTES

Drive Test routes are the first step to be set, and indicate where testing will occur. This

area is defined based on several factors, mainly related to the purpose of the test. The routes are

predefined in the office.

A program of a lot of help in this area is Google Earth. A good practice is to trace the

route on the same using the easy paths or polygons. The final image can then be brought to the

driver.

Figure 9.4 Drive test route map

Some software allows the image to be loaded as the software background (geo-

referenced). This makes it much easier to direct routes to be followed.

80EIET


It is advisable to check traffic conditions by tracing out the exact pathways through which

the driver must pass. It is clear that the movement of vehicles is always subject to unforeseen

events, such as congestion, interdicted roads, etc. Therefore, one should always have on hand -

know - alternate routes to be taken on these occasions.

Avoid running the same roads multiple times during a Drive Test (use the Pause if

needed). A route with several passages in the same way is more difficult to interpret.

9.4 DRIVE TEST SCHEDULE

Again depending on the purpose, the test can be performed at different times - day or

night. A Drive Test during the day shows the actual condition of the network - especially in

relation to loading aspect of it. Moreover, a drive test conducted at night allows you to make, for

example, tests on transmitters without affecting most users.

Typically takes place nightly Drive Test in activities such System Design, for example

with the integration of new sites. And Daytime Drive Test applies to Performance Analysis and

also Maintenance.

Important: regardless of the time, always check with the responsible area which sites are

with alarms or even out of service. Otherwise, your job may be in vain.

9.5 TYPES OF CALLS

The Drive Test is performed according to the need, and the types of test calls are the

same that the network supports - calls can be voice, data, video, etc.. Everything depends on the

technology (GSM, CDMA, UMTS, etc. ...), and the purpose of the test, as always.

A typical Drive Test uses two phones. A mobile performing call (CALL) for a specific

number from time to time, configured in the Collecting Software. And the other, in free or IDLE

mode, i.e. connected, but not on call. With this, we collect specific data in IDLE and CALL

modes for the network.

The calls test (CALL) can be of two types: long or short duration.

• Short calls should last the average of a user call - a good reference value is 180 seconds.

Serve to check whether the calls are being established and successfully completed (being

a good way to also check the network setup time).

81EIET


• Long calls serve to verify if the handovers (continuity between the cells) of the network

are working, i.e. calls must not drop.

9.6 TYPES OF DRIVE TEST

The main types of Drive Test are:

• Performance Analysis

• Integration of New Sites and change parameters of Existing Sites

• Marketing

• Benchmarking

Tests for Analysis Performance is the most common, and usually made into clusters

(grouping of cells), i.e., an area with some sites of interest. They can also be performed in

specific situations, as to answer a customer complaint.

In integration testing of new sites, it is recommended to perform two tests: one with the

site without handover permission - not being able to handover to another site thus obtaining a

total visualization of the coverage area. The other, later, with normal handover, which is the final

state of the site.

Depending on the type of alteration of the site (if any change in EIRP) both tests are also

recommended. Otherwise, just perform the normal test. Marketing tests are usually requested by

the marketing area of the company, for example showing the coverage along a highway, or at a

specific region/location.

Benchmarking tests aims to compare the competing networks. If the result is better, can

be used as an argument for new sales. If worse, it shows the points where the network should be

improved.

82EIET


CHAPTER 10

DRIVE TEST TOOL: JDSU E6474A v15.2

Agilent technologies have introduced the industry’s first integrated test solution that in a

single protocol analysis tool, seamlessly combines mobile device data captured from a RF

interface and from a mobile terrestrial network. Troubleshooting and optimizing today’s

networks requires a broad understanding of the network performance over multiple interfaces.

Rapid growth in the number of subscribers and in-data network usage has challenged the

radio access network in both RF capacity and data throughput performance measuring across the

last hop from the base station to the mobile device is essential for troubleshooting and

optimization and without visibility to the air interface, network operators must manually

correlate data from independent drive test and protocol analysis tools.

Agilent’s E6474A drive test tool has revolutionized and simplified end to end

troubleshooting. The software allows users to correlate signaling procedures from the air

interface and radio access network interfaces in a single view to detect and troubleshoot

problems from the mobile phone to the network.

The benefits of using this drive test tool are:

• Automatic correlation of data collected from both the radio and network interfaces to find

end-to-end performance issues more easily.

• Mobile device and network combined protocol decoding as well as call trace groupings to

enable a complete understanding of mobile access network behaviors.

• Detection of lost and delayed messages from the air interface.

• Isolation of base station with RF performance, capacity and interference problems to

perform root cause analysis.

• Evaluation of overall RF performance.

10.1 DRIVE TEST PRE REQUIREMENTS

83EIET


Before starting the drive test, the following data is to be collected from the BTS:

• Height of Antenna

• Antenna Azimuth – Orientation

• Antenna tilt

• Checking of RF Sectorization

• Verification of serving area by existing Antenna orientations

• VSWR & TX Power of DRX

Also, the following data from the OMC-R is to be collected:

• BCCH frequency

• Hopping Frequency

• MAIO & HSN

• Neighbor List

10.2 DRIVE TEST PROCEDURE

After collecting the required information from the BTS and the OMC-R, the drive test is

started. The equipment is set up in a vehicle and long calls as well as short calls are generated.

A long call is a call which is generated as well as terminated by the user himself. A short call

is a preprogrammed call generated by the system for a very small duration, say 10 seconds or

more.

A long call is used to measure the handover success rate as well as the Rx quality, while CSR

and Rx level are measured on a short call.

The drive test is done over a distance of 3 km or more from the starting point. Various

parameters are observed and recorded during the drive test.

The drive test procedure is as follows:

84EIET


• Tool may be setup for two mobiles – One for Long call and another for short

calls (2 minutes).

• In the route map following are to be enabled for Analysis.

• Rx Level

• RX Quality

• Survey Markers (like H/O, DCR & H/O symbols)

• Cell site Database.

• Call statistics for the Calls in the Point -1 to be enabled.

• Conduct the Drive Test – covering all sectors by observing the following

Parameters:

• Rx Level

• Rx Quality

• Interference on BCCH & Hopping Frequencies.

• Handovers & Drop Calls

• Observe whether the nearest sector is serving or not.

The data, as per the requirements are observed and recorded. The data is analyzed for

performance.

10.3 CONFIGURING THE DRIVE TEST TOOL

10.3.1 HARDWARE CONFIGURATION

The Hardware window shows the hardware devices which are to be added to the drive test tool.

• One mobile for short call configuration

• One mobile for Long call configuration

• GPS

File ProjectManager NewProject Name

85EIET


ViewSystem panelsHard ware window(right click)Add DevicePhone(short call)

ViewSystem panelsHard ware window(right click)Add DevicePhone(Long call)

ViewSystem panelsHard ware window(right click)Add DeviceGPS

Figure 10.1 Configuration of Hard ware Devices.

10.3.2 CONFIGURING THE CALLS

In Sequencer window we specify the type of test to be done by the each device i.e. mobiles

For Short Call:

ViewSystem panelSequencer (right click)Service model (right click)Parallel sequenceShort call

For Long Call:

ViewSystem panelSequencer (right click)Service model (right click)Parallel sequenceLong call

86EIET


Figure 10.2 Configurations of Calls

10.3.3 CONFIGURING SHORT CALL PROPERTIES

Short CallCALL_CONTROL_TESTViewProperties

• Number of times to run: Infinite

-The number of times for a call to run throughout the Drive Test if after a disruption.

• Inter Call Idle time: 5 sec

-Time duration between the calls

• Auto Dial: Yes

-Makes the call automatically after 5 sec (Inter Call Idle time)

• Call Statistics: Yes

-Display of No. of Dropped calls, Good calls etc., (Call Analysis)

• Immediate Dial: Yes

-To dial immediately after disconnection.

87EIET


• Continuous Call: No

-Since the Short Call would be terminated and re-initiated throughout the Drive Test, it is

configured as No

• Call Duration: 40 sec

-Duration of the Short Call should be minimum to make a trail for every sector or cell

• Call Setup: 20 sec

-Time given to setup or answer a call, if it exceeds call will be terminated.

• Call number: Any number

-Destination or called party number

• Auto Answer: No

-If it is Yes, then the mobile would be only in incoming mode (doesn’t suit for Drive

Test)

• COM Port: COM 58

-Number of port that to be connected to PC

• Voice MOS Test: No

-It is the Voice Mean Opinion score Test, not required for the Drive Test because person

doing the Drive Test don’t speak throughout the Test.

88EIET


Figure 10.3 Configurations of Short Call Properties

10.3.4 CONFIGURING LONG CALLS PROPERTIES

Long CallCALL_CONTROL_TESTViewProperties

• Number of times to run: Infinite

-The number of times for a call to run throughout the Drive Test if after a disruption.

• Inter Call Idle time: 5 sec

-Time duration between the calls

• Auto Dial: Yes

-Makes the call automatically after 5 sec (Inter Call Idle time)

• Call Statistics: Yes

-Display of No. of Dropped calls, Good calls etc.,(Call Analysis)

• Immediate Dial: Yes

-To dial immediately after disconnection.

• Continuous Call: Yes

-Since the Long Call would be operated throughout the Drive Test, it is configured as Yes

• Call Duration: NILL

89EIET


-As the Long Call is operated throughout the Drive Test the Call duration need not be

specified.

• Call Setup: 20 sec

-Time given to setup or answer a call, if it exceeds call will be terminated.

• Call number: Any number

-Destination or called party number

• Auto Answer: No

-If it is Yes, then the mobile would be only in incoming mode (doesn’t suit for Drive

Test)

• COM Port: COM 57

-Number of port that to be connected to PC

• Voice MOS Test: No

-It is the Voice Mean Opinion score Test, not required for the Drive Test because person

doing the Drive Test don’t speak throughout the Test.

90EIET


Figure 10.4 Configurations of Long Call Properties

10.3.5 CONFIGURING OF MAP AND CELL SITE DATA

MAP:

ViewCommon ViewsMapOpen map file (Load from Destination address in PC)

Cell site Data:

ToolsOptionsCell siteOpen cell site data file (Load from Destination address in PC)

Hyderabad map like streets, state highways, water bodies, national highways etc with cell

sites given below

• Cell sites near Gachibowli are shown below

Figure 10.5 Cell Sites of Gachibowli.

10.3.6 CONFIGURING THE DATA ITEMS

Configuring the Data Items Selection of parameters like Rx level, Rx Quality, C\I ratio of both

Short call and Long call which we want to display in the map through different colors and

different ranges which are available in Data items window.91

EIET


For Short Call:

ViewSystem PanelData itemsShort CallTraceGSM signal -Rx Level

-Rx Quality

For Long Call:

ViewSystem PanelData itemsLong CallTraceGSM signal -Rx Level

-Rx Quality

-C\I Ratio

Figure 10.6 Configurations of Data Items

10.3.7 MAP LEGEND

Map Legend shows the display of Configured Data Items Selected like Rx level, Rx Quality, C\

I ratio of both Short call and Long call in the map through different colors and different ranges

as shown in the Legend Window in the below figure.

92EIET


Figure 10.7 Map Legends in the Drive Test.

CHAPTER 11

DATA COLLECTION IN DRIVE TEST

11.1 OBSERVATIONS AND RECORDINGS

Drive testing is the most common and maybe the best way to analyze Network performance

by means of coverage evaluation, system availability, network capacity, network retainibility and

call quality. Although it gives idea only on downlink side of the process, it provides huge

perspective to the service provider about what’s happening with a subscriber point of view. The

data, as per the requirements are observed and recorded. The data is analyzed for performance.

The following shots have been taken while conducting the drive test.

• Drive test is nothing but collection of samples.

93EIET


Figure 11.1 Collection of samples of the Drive Test.

• GPS location or Vehicle position on the map is indicated with red pointer as shown

below. Five parameters like Rx quality, Rx level of short and long call and C/I ration of

long call shown below on the map.

94EIET


Figure 11.2 Measure of 5 parameters in the Drive Test.

• BSNL drive test from RTTC Gachibowli to Nanakramguda , Khajaguda via Outer ring

road and back to RTTC via Cyberabad Commissioner office. In the map red color

indicates the bad quality. A map legend (Indications of 5 parameters and their ranges) is

shown on right side of screen.

Figure 11.3 Drive Test From RTTC Gachibowli To Nanakramguda

• BSNL user events like H.O success, good call etc can be seen below.

95EIET


Figure 11.4(a) BSNL Hand Over’s in Drive Test.

Figure 11.4(b) BSNL Drive Test signal strength.

96EIET


• BSNL short call and Long call views is given below.

Figure 11.5 (a) BSNL Short Call View in Drive Test.

Figure 11.5(b) BSNL Long Call View in Drive Test.

97EIET


11.2 DRIVE TEST ANALYSIS

11.2.1 BENCH MARKS OF TRAI

Every leading network service provider in the market should follow the Benchmarks by

the “TELECOM REGULATORY AUTHORITY OF INDIA”. A network is said to be good

if it satisfies the benchmarks of TRAI.

Downlink Parameters:

• Rx Level > 95 %

• RX Quality > 95 %

• C/I Ratio > 98%

• Handover success rate > 98%

• Call setup success rate > 98%

• Call Completion success rate > 98%

• Drop call rate < 3%

11.2.2 FORMULATION & CALCULATION

(1) Rx Level:

Rx Level = Total samples (> -95 dbm) *100 %

Total no. of samples collected

(a) BSNL:

Total no. of samples collected= 5225

Total no. of samples collected (> -95 dbm) =5101

Rx Level = 5101

--------- *100 % = 97.62%

5225

(2) Rx Quality:

98EIET


Rx Quality= Total samples (0-5) * 100 %


(a) BSNL:


Total no. of samples collected (=0) =264






Total no. of samples collected (0-5) =264+84+199+362+2008+1553

=4470

Rx Quality= 4470

--------- *100 % = 85.5%

5225

(3) Carrier to Interference Ratio (C\I):

C\I = Total samples (> 9 db) *100 %


• BSNL:


Total no. of samples collected (> 9 db) =3863

C\I = 3863

99EIET


-------- *100 % = 96.8%

3990

(4) Hand Over Success Rate:

HSR = Total no. of successful H.O’s *100 %

Total no. of H.O commands

• BSNL:

Total no. of Hand Over Commands = 52

Total no. of Successful Hand Over’s=50

HSR = 50

---- *100 % = 96.1%

52

(5) Call Analysis:

(i) Call Setup Success Rate:

Rate of calls which are successfully established.

CSSR= No. of calls successfully setup * 100

Total no. of calls attempted

(ii) Call Completion Success Rate:

Rate of calls which are successfully established and disconnected by the user.

CCSR= No. of calls setup and disconnected * 100

Total no. of calls Attempted

100EIET


(iii) Drop Call Rate:

DCR= No. of Dropped calls * 100

Total no. of calls Established

S.no Parameter BSNL

1 No of Call attempts 21

2 Successfully Established 20

3 No. of Blocked calls 1

4 No. of Dropped Calls 0

(a) BSNL:

(i) CSSR = 20 * 100 = 95.2%

21

(ii) CCSR = 20 * 100 = 95.2%

21

(iii) DCR = 0 * 100 = 0%

20

(6) THROUGHPUT : max bit rate of data transmitted

BSNL = 70 KBPS

(7) SERVICE DELAY :

BSNL = 7.04MS

(8) FER (FRAME ERROR RATE )

BSNL = 5150 * 100 = 98.6%

5220

101EIET


CHAPTER 12

ADVANTAGES & DISADVANTAGES

12.1 ADVANTAGES

Operators would able to solve the problems of network.

We can identify and compare the performance of different operators.

Operators can provide better service to the customers.

12.2 DISADVANTAGES

Expensive.

Time taking process.

Operator has to engage engineers to collect the data (Bench marking).

102EIET


CHAPTER 13

APPLICATIONS

For every operator, customers are very important.

Before customer complaints (like call drops ,bad speech quality, no signal etc.),the

operator should able to identify the problem and rectify the problem i.e. preventive

maintenance .

corrective maintenance i.e. After receiving complaint from the customer, operator should

solve the problem.

103EIET


CHAPTER 14

FUTURE SCOPE

At present Drive Testing in GSM RF Optimization is being performing manually for the improvement of performance of the network. Instead of doing drive testing manually, there may be a scope ANMS (Automatic Network Management System) process. In this system, Drive Testing equipment can be attached to moving vehicle to serve in operator test area and it can be monitored by the server. By using the internet, all the drive data can be simultaneously collected up to date to the server.

CHAPTER 15

104EIET


RESULT

COMPARISION OF BENCH MARKS

Figure 15.5 comparision of benchmarks

CHAPTER 16

105EIET


CONCLUSION

The overall objectives of any RF design depend on a number of factors that are determined by the needs and expectations of the customer and the resources made available to the customer.

Due to the mobility of subscribers and complexity of the radio wave propagation ,most of the network problems are caused by increasing subscribers and the changing environment. Radio Network Optimization is a continuous process that is required as the network evolves. Radio Network optimization is carried out in order to improve the network performance with the existing resources. The main purpose is to increase the utilization of the network resources, solve the existing and potential problems on the network and identify the probable solutions for future network planning.

Through Radio Network Optimization, the service quality and resources usage of the network are greatly improved and the balance among coverage, capacity and quality is achieved.

106EIET


CHAPTER 17

BIBILOGRAPHY

1. Wireless communication by S.Rappaport

2. Mobile cellular telecommunication - William C.Y.Lee

3. Cellular technology for rural areas – W.C.Y.Lee

4. Umts performance Measurement by Ralf Kreher

5. GSM, GPRS and EDGE performance by Timo Halonen,Javier Romero,Julan Melero

6. GSM/EDGE: Evolution and Performance by Mikko Saily,Guillaume Sebire, Eddie Riddington

107EIET


CHAPTER 18

REFERENCES

1. http://en.wikipedia.org/wiki/GSM

2. www.jdsu.com

3. http://www.telecombuzz.org/2009/08/gsm-drive-test.html

4. www.fcc.gov (documentation from Motorola)

5. www.ofcom.org.uk

6. www.etsi.org (definitions of GPRS)

108EIET


ABBREVIATIONS

2G Second Generation AB:-Access Burst

ANSI American National Standards Institute

Auk Authentication Centre

BSC Base Station Controller

BSS Base Station System

BTS Base Transceiver Station

CC Call Control

CDMA Code Division Multiple Access

CM Connection Management

DCS 1800 Digital Cellular System 1800 (today: GSM1800)

DECT Digital Enhanced Telecommunications System

DL Down-Link

DRX Discontinuous reception

DTX Discontinuous Transmission

EDGE Enhanced Data rate for GSM Evolution

EGPRS Enhanced General Packet Radio Service

EIR Equipment Identity Register

ETSI European Telecommunications Standards Institute

FDMA Frequency Division Multiple Access

GMSC Gateway MSC

GMSK Gaussian Minimum Shift Keying

GPRS General Packet Radio Service

GSM Global System for Mobile Communications

HLR Home Location Register

HSCSD High Speed Circuit Switched Data

IMEI International Mobile Equipment Identity109

EIET


IMSI International Mobile Subscriber Identity

IP Internet Protocol

ISDN Integrated Services Digital Network

Kbits/s/slot Kilo Bits per second per slot

Kb Kilo bits

KB Kilo Bytes

ME Mobile Equipment

MM Mobility Management

MS Mobile Station

MSC Mobile Switching Centre

MSRN Mobile Station Roaming Number

OMC Operation and Maintenance Centre

PDP Packet Data Protocol

PLMN Public Land Mobile Network

PSK Phase Shift Keying

QoS Quality of Service

SCH Synchronization Channel

SIM Subscriber Identity Module

SMS Short Message Service

SMSS Short Message Service Support

SS Supplementary Service Support

SS7 Signalling System Number 7

TD/CDMA Time Division Code Division Multiple Access

TDMA Time Division Multiple Access

TMSI Temporary Mobile Subscriber Identity

TRAU Transcoding and Rate Adaptation Unit

TRX Transceiver

110EIET


UE – UT User Equipment – User Terminal

UL Up-Link

UMTS Universal Mobile Telecommunications System

USB Universal Serial

VLR Visitor Location Register

111EIET

benchmarking of gprs and gsm

Documents

Transcript of benchmarking of gprs and gsm