The candidate confirms that the work submitted is their own and the appropriate credit has
been given where reference has been made to the work of others.
I understand that fail to attribute material which is obtained from another source may be
considered as plagiarism
(Signature of student)_______________________
Performance Evaluation of GPRS
Ross Williamson
Computing and Management Studies (Bsc)
Session (2004/2005)
Summary
The overall objective of this project was to evaluate GPRS though simulation experiments. GPRS is
a relatively new technology that at current hasn't had extensive work done on its performance
characteristics. As GPRS is now in the main stream of users but still is yet to be fully exploited as
user take up has been slow, it is a good time to see how GPRS can perform. This can be done by first
gaining a detailed background into the the way that GSM and GPRS are implemented. Then the
correct methods of performance evaluation need to be reached so that experiments can be designed to
likely performance measure of GPRS. All this can then lead us to find out whether GPRS can
succeed in its proposed role in telecommunications evolution.
2
Acknowledgements
I would like to thank my supervisor Dr. Karim Djemame for his continuous advice and support
thought out the whole of this project and indeed as his role as my personal tutor during my time at
Leeds.
I would also like to say a big thank you to my girlfriend Susie Lacey for her support in keeping me
motivated to do this report even when things went against me and stress levels were rising.
3
Table of ContentsSummary.....................................................................................................................................................2Acknowledgements....................................................................................................................................3Table of Contents.......................................................................................................................................4Chapter 1.0 - Introduction..........................................................................................................................6
1.1 Aims.................................................................................................................................................61.2 Minimum Requirements..................................................................................................................61.3 Schedule and Milestones.................................................................................................................61.4 Deliverables.....................................................................................................................................8
Chapter 2.0 - Background Research..........................................................................................................92.1 Introduction......................................................................................................................................92.2 GSM Introduction............................................................................................................................9
2.2.1 Mobile Station (MS)..............................................................................................................102.2.2 GSM Architecture..................................................................................................................102.2.3 Radio Interface.......................................................................................................................12
2.3 GPRS..............................................................................................................................................132.3.1 GPRS architecture..................................................................................................................142.3.2 Radio Interface.......................................................................................................................152.3.3 GPRS Channel Types.............................................................................................................152.3.4 Coding Schemes.....................................................................................................................16
2.4 Performance Evaluation................................................................................................................172.4.1 Mathematical Modelling........................................................................................................172.4.2 Direct Experimentation..........................................................................................................182.4.3 Simulation..............................................................................................................................18
Chapter 3.0 - Simulation Design.............................................................................................................203.1 General Simulation Parameter Settings........................................................................................203.2 Performance Metrics.....................................................................................................................20
3.2.1 Application Response Time...................................................................................................213.2.2 IP Datagram Delay.................................................................................................................21
3.3 Performance Simulations..............................................................................................................213.3.1 Test 1 – Mean application response time..............................................................................213.3.2 Test 2 – Effect of Coding schemes........................................................................................22
Chapter 4.0 - Implementation .................................................................................................................234.1 Introduction....................................................................................................................................234.2 Sgoose Simulator...........................................................................................................................23
4.3 Simulator Findings....................................................................................................................244.3.1 Test 1 – Application response Time......................................................................................244.3.2 Test 2 – Coding schemes.......................................................................................................26
Chapter 5.0 - Evaluation..........................................................................................................................295.1 Minimum requirements.................................................................................................................295.2 Project Management......................................................................................................................30
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5.3 Project Success..............................................................................................................................305.4 Project Improvements....................................................................................................................31
Acronyms ................................................................................................................................................32Appendix A – Personal Reflection..........................................................................................................34Appendix B – Schedule............................................................................................................................35Appendix C – Additional Simulation Results.........................................................................................36Bibliography..........................................................................................................................................37
5
Chapter 1.0 - Introduction
1.1 Aims
The aim of this project is to evaluate the performance of the General Packet Radio Service (GPRS)
using a network simulator. GPRS is now become widely implemented within the consumer and
business mobile communications markets. However it is still not clear as to whether the proposed
performance achievements of GPRS are accurate in everyday activities. This project aims to
investigate whether the theoretical performance of GPRS can be implemented. This project will also
look at how a user will view GPRS and whether it is something that performs for them.
1.2 Minimum Requirements
The minimum requirements were agreed as:
� To gain an understanding of wireless networks especially GPRS
� To design simulation experiments using a network simulator
� To implement simulation experiments using a network simulator
� To find the performance metrics that could help evaluate simulation experiments
� To analyse performance results as to provide an evaluation of the perceived performance
The following was possible extensions to the above minimum requirements.
� Design simulation experiments for similar technologies such as EDGE or HSCSD
� Compare results of similar technologies with those of GPRS
� Analyse performance with respect to other technologies
1.3 Schedule and Milestones
The following Gantt chart explains the schedule of this project.
6
Preliminary Investigation
Background Reading
Network Simulator training
Design Simulation Experiments
Progress Meeting
Run simulations
Collect and sort data
Analyse data draw conclusions
Write up report
Submit final project
11/10 25/10 8/11 22/11 6/12 20/12 3/01 17/01 31/01 14/02 28/02 14/03 28/03 11/04 27/04
Figure 1.0 Gantt is taken from the mid project report.
The main milestones in this project are to learn the network simulator program because at this stage
all the required knowledge in terms of background research would have been done. This then takes
us onto the next phase of the project which is the information processing phase. This is where
experiments will be designed and implemented and then analysed with performance measure in
mind. The final stage of the project will then commence which is the write up. As can be seen from
the Gantt chart this is an ongoing activity but would only have been note taking up till now are this
last stage is where all the information have been sourced. These three milestones relate directly to
the requirements for this project.
However this schedule has been revised throughout the project due to unforeseen circumstances. See
Appendix B for fuller explanation of schedule changes.
7
1.4 Deliverables
The deliverables for this project have been changed from simulation scripts and this report to only
include this report. This is due to a change in the planned simulation program.
8
Chapter 2.0 - Background Research
2.1 Introduction
Before this project can begin to examine and design performance experiments a clear understanding
of the GPRS technology is needed. Also as GPRS is built upon GSM (Global System for Mobile
communication) technology an introduction to this is also needed. This chapter explains the two
main parts to this project, them being GPRS and performance evaluation.
2.2 GSM Introduction
GSM is one of many similar radio networks set-up to initially carry voice traffic of wide
geographical areas. GSM has become the most popular and is used throughout Europe and Americai.
However other systems are in place such as Personal Digital Cellular (PDC) which is used in Japan.
GSM is based on the Time Division Multiple Access (TDMA) and can now be considered to be
moving from 2G (2nd Generation) to 3G (3rd Generation). This is where GPRS comes in being
thought of at 2.5G allowing for a slower migration between the two to allow user to become aware of
the increasing possibilities.
GSM is a Circuit-switched network which is designed primarily for voice communications. The
network establishes a connection between multiple switching points and creates a circuit for the
duration of the call. Even if no conversation is taking place, the circuit is in use. This is not an
efficient use of resources available in the network, unless there is a constant flow of information
going through the circuit while connectedii. GSM creates these circuits by using TDMA and FDMA
as shown below.
TDMA takes a particular radio frequency and splits it into a set number of time sequences (eight in
GSM) this allows for several transmissions to take place on one frequency as they will each be in a
different time slot and so never be transmitting at the same timeiii. GSM takes this idea and extends it
to also use different frequencies as well. GMS is organised into eight time slots and has 200kHz
9
channels operating at 900MHz, 1800MHz and 1900Mhziv.
2.2.1 Mobile Station (MS)
A mobile station commonly associated to a mobile phone contains the hardware and software needed
to interface with a GSM radio network and also a Subscriber Identity Module (SIM)v. The SIM card
holds the personal details of the phone owner as well as the user’s access details to the radio network.
The MS is also given a Equipment Identify (EI) at manufacture.
Each MS will be assigned a number of different addresses for authentication and identification
purposes. These arevi
� International Mobile Subscriber Identity (IMSI)
� Temporary Mobile Subscriber Identity (TMSI)
� Mobile Station international ISDN number (MSISDN)
� Mobile Station Roaming Number (MSRN)
The numbering follows the rules of ISDN with regard to the MSISDN but also includes the addition
of the IMSI which is used to provide the MS with a unique identification. However the IMSI is not
revealed so that the identity of a MS can’t be revealed. This is done by a TMSI being assigned to an
MS which is only relevant in the local VLR (see below) this is also changed regularly to help
maintain confidentiality of the MS. The MSRN is used to make a link between the MS current
location and the MSC (Mobile services Switching Centre). The MSRN is made up of such things as
the country code, local area code, MSC identification and the subscriber number.
2.2.2 GSM Architecture
10
Figure 2.0 GSM architecturevii
GSM comprises of three main sections, Radio Subsystem (RSS), Network and Switching Subsystem
(NSS) and the Operation Subsystem (OSS). The RSS comprises two further sections that make up
both sides of the wireless transmission; these are the Mobile station and the Base Station Subsystem
(BSS)viii. The BSS is a network of linked radio cells (hence 'cell phone') that together cover a certain
geographical area. Each BSS has a Base Transceiver Station (BTS); these are assigned a set of
operating radio channels which vary from nearby cells to avoid interference. The BTS operates in a
hierarchical structure which next leads to the BSC (Base Station Controller) for which there will be
fewer in number. The BSC is responsible for handling the switching of MS's between cells and for
adjusting power control to allow for distance the MS is from the BTS.
The BSC then connects to the further few MSC which are located in the NSS. The MSC is used to
control the calls coming from other networks, such as from different technologies and companies.
The NSS as well as having the MSC also houses a number of databases which are used to identify
MS on and visiting the network. These databases include the Home Location Register (HLR),
Visitor Location Register (VLR)ix,
The HLR is a database that holds all the information on subscribers to that MSC. It also stores
11
information on which MSC the MS is currently located. The HLR is also connected to the
Authentication Centre (AuC) located in the OSS. The HLR uses the AuC to look up authentication
parameters and ciphering keys. The VLR is used each time an MS is turned on in a radio cell. This
database requests information from the MS and in turn request information about that MS from its
HLR. This also allows the HLR to be informed about where the MS is currently connected. The
HLR and VLR are similar in the information that they hold but differ by that the HLR holds
information on permanently subscribed users whereas the VLR is used for MS visiting that MSC.
The final part is the OMC (Operating and Maintenance Centre), this is connected to everything in the
NSS and the BSC. This is used as an overall control that is responsible for monitoring the whole
network to maintain best possible quality of service. The OMC is out of the scope for this project so
doesn't need to be discussed further.
2.2.3 Radio Interface
The radio interface connects the MS with the BTS and due to the reliability and speed of the modern
wired network directly affects the majority of the performance within the network. This is why it has
to be the most optimized for the greatest efficiency. As mentioned above GSM is based on TDMA
however it is extended further by also incorporating Frequency Division Multiple Access (FDMA).
FDMA takes a range of frequencies and divides them up into equal slots so that different users can
transmit at the same time because they will be sending on different frequencies. It is also helpful to
reduce interference between radios cells as the sets of frequencies used for nearby cells will not be
the same. In GSM 900, which as mentioned earlier operates just above and below the 900MHz band,
the bandwidth is divided into an equal uplink and downlinks each receiving 25MHz with 20 MHz
separating them to cut interference. These 25MHz channels are then divided into 200 kHz slots
allowing for 124 different carriers see figure 2.1. This example is also true of GSM 1800 expect that
GSM 1800 has a 75MHz assigned for uplink and downlink which in turn allows for 374 carrier
frequencies.
12
Figure 2.1 FDMA and TDMAx
On each of the carrier frequencies described above exists TDMA as well, meaning that the 124
carrier frequencies in GSM 900 are each spilt into 8 time slots which forms one TDMA frame. The
TDMA frame is 4.614ms long which therefore means that one time slot is 0.577ms longxi. This
means that each transmission channels has to be described as a frequency and also a time slot
number.
2.3 GPRS
GPRS is a technology that runs at 2.5G with regard to GSM evolution. GPRS is about using the
current GSM radio resources more efficiently so that GSM can support the increasing need for
packet switched data. By introducing packet switching users are only using the radio interface when
they are sending or receiving data unlike before where more of a circuit switched approach was used.
Addressing is done by placing the information in the header of each information packet. The main
designs of GPRS have made it so that it will lie over the top of the conventional GSM system but
provide faster data rates of up to 117kbit/s compared to the GSM 9.6kbit/sxii. GPRS is designed to
carry data which by nature usually means short bursts of data (which makes it able to cope with IP
(internet protocol) easily) rather then GSM which are designed to carry voice which need a low but
continuous data stream.
13
GPRS is the link between 2G and 3G UTMS (Universal Mobile Telecommunication System). 3G is
going to be totally packet switched including voice calls to make maximum use of bandwidth and to
provide greater user speeds for new application demands such as video calls. GPRS supports the
ability to have IP end to end in the GSM network which allows for new applications to be realised
before the jump to complete packet switching.
2.3.1 GPRS architecture
Figure 2.3 GPRS architecturexiii
GPRS doesn't change the GSM architecture at all but add to more nodes that look after the control of
the packet switching. Both of these nodes are situated between then MSC and its interfaces to other
networks within the NSS. These two new nodes are Gateway GPRS Support Node (GGSN) and the
Serving GPRS Support Node (SGSN)xiv.
The GGSN goes directly between the SGSN and external network connections. This allows for
routing to be provided to the SGSN with the correct addresses identified from the different incoming
protocols from the different network types. The GGSN then passes data to the SGSN which lays
between the MSC and the GGSN. The SGSN is the main part of GPRS as the MSC is to GSM. This
is where packet addresses are read and there destination decided by mapping the address onto the
14
correct ISMI which is found from the databases connected to the MSC. The SGSN also looks after
the security of GPRS subscribers which encryption and authentication.
2.3.2 Radio Interface
As GPRS is an overlay in GSM the same FDMA and TDMA allocations are used however the way
that these channels gets assigned changes to incorporate GPRS. This is done by allocating a certain
number of channels for GPRS use. These channels are then called the Packet Data Channels
(PDCHs). PDCHs can be allocated in a fixed or dynamic way depending on operator preferencexv.
This means that is at certain times of the day packet data increases and voice data drops the number
of available GPRS channels could be increased instantly to allow for the extra number of users.
GPRS uses at least one PDCH as a Master Packet Data Channel (MPDCH) which is always assigned
and is used to provide the control for the slave channels which will usually change in number with
load demanded.
2.3.3 GPRS Channel Types
Once a PDCH has been assigned is may be used for several different operations. These operations
are divided into different channels types which are as follows.
Packet Common Control Channel (PCCCH), this is used to define the common control operations
needed by GPRS. It is further divided into 4 channels, Packet Random Access Channel (PRACH),
Packet Paging Channel (PPCH), Packet Access Grant Channel (PAGCH) and the Packet Notification
Channel (PNCH).
The PRACH is used by the MS to initiate a packet transfer and upload the data. The PPCH is used to
by the BSS to page a MS therefore notifying it that a data transmission is about to take place. The
PAGCH is used to tell the MS what resources are to be assigned to it in the establishment phase of a
15
data transfer. The PNCH is used for the multicast service of GPRS where a message is broadcast.
This sends a message to all the subscribed MS to tell them that a transmission is going to start. It
also tells other MS that may be interested in the broadcast when it and on what resources it is
startingxvi.
The second group of channel type is the Packet Broadcast Control Channel (PBCCH) this simply
transmits system information to GPRS terminals in a radio cell.
The final group is the Packet Traffic Channel (PTCH), this is where the user’s data is carried and is
divided into two further sub channels. The Packet Data Traffic Control (PDTCH) is used for data
transfer. It is assigned temporarily to one MS or group of MS to allowed data to be sent and
received. To control the data that is sent on the PDTCH a Packet Associated Control Channel is
used. This provides feedback to the MS such as acknowledgement messages and resource
assignment so the data transfer is run on the correct frequency/time slots at the maximum speed
available.
2.3.4 Coding Schemes
GPRS has four different coding schemes that it can use at the radio interface. These different
schemes offer different benefits but also draw backs to their use. Each coding scheme had a different
amount of error protection which when removed speeds up the throughput but can lead to a larger
number of errors in the wireless transmissions. The coding schemes are chosen dynamically to give
the user the best possible data rate depending on the radio conditions. The further the MS is from the
BTS the further the signal has to travel through the air. This means that there is an increased chance
in another MS transmitting at the same time as it may not have realised that there is a transmission in
progress. By increasing the error correction the MS and the BSC have a chance to save the corrupted
message and so mean that it doesn’t have to be resent. This helps save on radio resources for other
people but slows down the throughput of the user because they are now attaching more errors bit to
their transmission and so less data bits.
16
This means that the CS-1 coding scheme is used when the MS is furthest away from the BTS or when
the greatest amount of error correction is needed. CS-1 has the lowest throughput with only
9.05kbit/s achievable per time slot. This works up to CS-4 which has no error correction but is likely
only to be useful in a small area because of this. The most common coding scheme is CS-2 as this
has a good mix of throughput and error correctionxvii.
GPRS Coding Schemes
Timeslots 1 2 3 4 5 6 7 8
CS1 9.20 18.40 27.60 36.80 46.00 55.20 64.40 73.60
CS2 13.55 27.10 40.65 54.20 67.75 81.30 94.85 108.40
CS3 15.75 31.50 47.25 63.00 78.75 94.50 110.25 126.00
CS4 21.55 43.10 64.65 86.2 107.75 129.30 150.85 172.40
Speeds are given in kbps
Figure 2.4 Coding Schemes
2.4 Performance Evaluation
Performance evaluation can be spilt into three main areas, these are mathematical modelling, direct
experimentation and simulation. In order to find the most appropriate technique to use for this
project the advantages and disadvantages of are outline below.
2.4.1 Mathematical Modelling
Mathematical Modelling equations build what is being modelled before real case parameters are put
in to determine the results. There are different types of models that can be used example of such are
probabilistic models, queuing models, Markov models and Petri net modelsxviii. The probabilistic
model is often used for network modelling communication networks because it has the following
benefitsxix
� task arrival times to a computer are random
� user inputs are random
17
� packets arriving to a network are random
� errors on communication links are random
One advantage of mathematical modelling is that it is relatively quick to set up and evaluate a
system. However the disadvantages are that the user has to have a solid background in mathematics
theory and that many systems often have to be simplified in their mathematical interpretation of
them.
2.4.2 Direct Experimentation
The proposed system is built and then released to either an enclosed area or everywhere. Results can
then be collected on its performance. This method while the best for evaluating real world systems is
very time consuming as the systems have to be built and distributed before data can be collected.
This method does have the most flexibility in testing as inputs can be changed according to real
world use and not only according to what the user thinks are real world uses. The disadvantages are
that the required resources needed are substantial and that the time taken to gather results can be long
running into many months or years.
2.4.3 Simulation
“A simulation is the imitation of the operation of a real-world process or system over time. Whether
done by hand or on a computer, simulation involves the generation of an artificial history of a system
[...] to draw inferences concerning the operation characteristics of the real system." xx
Simulation is done by writing a computer program that can turn a set of given inputs into the likely
real world outputs. Simulators are easier to use than doing mathematical modelling because the user
tells the program what the system is and its constraints and inputs. The program then works out the
maths of the described system. Simulators can often provide great detail on modelling the system as
the detail is only constrained by the effort of the user. Some advantages to using simulation are:
18
� Specialist previous knowledge is usually limited to learning a computer programming language
which is not too difficult
� Can be accurate and enable a wide range of different implementations as well as implementations
to be changed quickly
� Tests can be re-run easily to build up results
� Random input data is easily generated
However simulation does have its disadvantages in that it can take time to actually run the tests but
this is not as much as direct experimentation and also simulations are just that simulating what you
think the conditions will be like in the real world. It is there for hard to program in events for which
the user hasn’t considered or more random events.
The simulator that this was going to be using is ns2 xxi however (see appendix B for explanations) the
simulator that will now be used is SGOOSExxii. This comes from a company called AIX COM which
is a German company that has grown out of a research project with a German university. They have
a GRPS simulator which is currently free and is being updated regularly. The advantages of this
simulator is that experiments are very easy to set up because there is less coding needed while small
detailed changes can still be made. The disadvantages are that it is not as flexible as ns2 in designing
experiments giving limited number of experiments that can be done. Also the output of this program
is text only which means that analysis will have to be done manually to provide the output graphs.
See implementation chapter for greater analysis of the design of this network simulator.
19
Chapter 3.0 - Simulation Design
3.1 General Simulation Parameter Settings
It has been assumed that there will be no delay in the core and external network, e.g. public Internet.
This is because the main delay in GPRS networks is at the air interface whereas in well structured IP
networks the delay is minimal in comparison.
Also due to the limitations of the GPRS simulator only 10 GSM MS are permitted for the
experiments. The number of radio cells is limited to one cell only. This means performance that is
affected by hand off procedures when MS's move between cells can’t not be modelled. However as
seen in REF simulations that are done over singles cells also hold for multiple cells.
The number of parameters to be changed for each experiment will be kept to a minimum so that
comparison between results and with other findings can be more efficiently done. Also this helps in
the complexity of the analysis once the tests have been run.
These performance simulations will use a variety of different predicted application types which may
well be used on a MS. These include WWW requests, E-Mail, and WAP sessions. The details of the
default parameters will now be explained. These are as seen in the config.ini file of the simulator.
The simulation time will be 20000.0 GSM/GPRS seconds. Also it the chances of an error occurring
have been set at 10% which is what network operators currently allow for. The .... fill in rest!
3.2 Performance Metrics
With each test that is performed a number of performance metrics will be calculated. These will
enable analysis of the results to be comparable across each of the experiments. The definitions for
these are.
20
3.2.1 Application Response Time
This is the length of time in seconds that it takes for the user initiating a request for the particular
application to completely received that request to the MS.
3.2.2 IP Datagram Delay
This is the time distance between the time of the IP datagram being offered to the GPRS bearer and
the time it is forwarded to the peer IP layer. The time taken between transmission and reception of
each IP datagram is calculated.
3.3 Performance Simulations
3.3.1 Test 1 – Mean application response time
Objectives:
To find out the length of time taken to fulfil common application requests. The applications that will
be used are WWW, E-Mail and WAP. Once the mean time has been found this will be cross
referenced with the maximum time that users are willing to wait for these applications to respond.
This will allow a user orientated look at whether the performance of GPRS is good enough for its
most likely uses.
Method:
To start with just one MS will be used and the mean response time will be taken for each application.
There will then be an increase in MS's until the maximum supported of 10 is reached. The tests will
not assume that a the simulation will only be dealing with one application. There will be a traffic
mix of 60% of traffic assigned to WAP, 30% to e-mail and 10% to WWW. This will mean that when
the tests are run each application will be sharing the simulation with the other applications. This has
been done to better reflect a real life situation with the traffic distributed as expected outside the
simulations.
21
3.3.2 Test 2 – Effect of Coding schemes
Objectives:
To see how the change in coding schemes would effect the performance of GPRS for the user.
Method:
The down link IP delay will be used to see how the change in coding schemes from CS – 1 to CS – 4
changes the performance of the download. The number of users will be increase from 1 to 10 as
before to see how each coding scheme can handle different numbers of users also. This is because of
the higher number of users the more errors that are likely to occur and so the greater the amount of
error correction is likely to be needed.
22
Chapter 4.0 - Implementation
4.1 Introduction
This chapter looks at how the previously designed experiments have become real results. The
network simulation tool that was used to run the simulations will be introduced and explained how it
is set up and configured so that consistent results are found.
4.2 Sgoose Simulator
Sgoose works from one executable and two configuration files. The executable program is what
produces the outputs and data. This program takes the details from the two text input files to feed
them into its GPRS architecture. The first configuration file holds all the information that is required
to set-up the simulation. This is the file that has all the main changes in. The second configuration
file holds details of all the simulations that can be run and the output formatting details of them. This
file is not edited as often with the only real change that can be made is to turn simulations on or off.
One of the drawbacks with Sgoose is that it is only a demo version because the full version requires
payment. This means that there are fewer features, the cut down features include a graphical user
interface which would be used to set-up and run the experiment quickly without the need for directly
manipulating the configuration files. Also this GUI provides output without having to manually form
the results. This drawbacks are only superficial in that they don't stop the simulations being done but
make the usability harder. Features that do effect the simulations are the fact that only a maximum
of 10 MS can be modelled at once. This however is still enough for an insight into the trends of the
experiments. I have already explained that single cell simulations tend to hold for multicellular
similar simulations so this drawback that the demo version has is not so important.
23
4.3 Simulator Findings
The simulations that were run will now be explained before they are evaluated along side other
research results.
4.3.1 Test 1 – Application response Time
To recap this test was looking at the response times of three predicted uses of GPRS on a MS. The
applications that are in question are World Wide Web requests, E-mail requests and WAP sessions.
The simulation was configuration for 1 to 10 MS with application response time measured in
seconds.
As is shown below the application that performs the best is WAP with the lowest waiting time just
under a steady 2 seconds. E-mail comes next with a slow increase in response time as the number of
MS increases but still doesn't move far from the 4 seconds mark. However the slowest application
for responding to user requests is WWW with an increasing response from about 5 seconds for every
request.
The reason for WWW being the slowest and in fact WAP being the fastest comes do to the type of
service that they are. WWW would be retrieving data that has been optimized for PC's connecting
over a minimum of a reliable 56Kb/s modem. This contrasts WAP which is designed to be used on a
mobile phone and often is not even in colour. This would mean that WAP would be a better
application service for operators to focus on. However the problem with this is that WWW is by far
the more popular and more versatile in what can be done. Also most phones these days have colour
screens which would help enhance a WWW page. WAP does also have the advantage that it is
cheaper because of the smaller download size because of the less information being sent. So if only
simply information is needed that can be displayed in a textural way WAP is probably the answer.
24
Figure 4.1 Test 1 – Application response time
As we can see from Appendix C This graph is similar. However their simulation used a larger
number of MS for which i was limited. It is interesting to note the change in direction on 10 MS
onwards. This effects WWW and e-mail but not really WAP. This shows the benefit of having
applications that are tailor to more mobile needs as WAP whereas WWW and e-mail are the same as
would be found on the wired Internet. This produces an adverse effect on the response times as users
increase. The differences in our results are that I have found that WWW requests take slightly longer
then report in Appendix C. This could be to do with the difference in simulators used or slightly
different configuration settings for example they have used slightly different traffic mix
considerations allowing for slightly less WWW traffic.
This simulation needs to be looked at not only with the raw results in mind but also more importantly
what the user expects and will wait for. People have different patience times, for example people
are more likely to wait longer for an e-mail to send because they are usually just leaving it to send
because they are not getting any information from it. Also having put the effort into writing the e-
mail they are willing to wait to make sure it gets sent. This is different to WWW where people will
only wait a certain amount of time for a web page to download before trying a different one. As
people are looking for information when using WWW they are more impatient. Due to the fact that
25
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5
6
7
8
Application Response Time
WWW
WAP
Number of Mobile Stations
App
licat
ion
Res
pons
e T
ime
people are aware the the content of WAP is below that of WWW they are prepared to wait even less
time for a response. The user excepted response times for these three applications are around 10 - 15
seconds for WWW so that user trail of thought is not broken. As already outlined users are prepared
to wait longer for e-mail data, the average time that a user will wait is about 120 seconds. WAP
traffic is similar to WWW in that users want it quickly so a time of about 10 seconds would be what
the user expects to be achieved.
As we can see from the results all these applications fall within the required levels for the time that a
user will wait for the task to be completed. This shows the worth that GPRS can bring to GSM
added these functions that work within user acceptance guild lines filling an important gap in
usability until full acceptance of 3G has happened.
4.3.2 Test 2 – Coding schemes
This test was interested in looking at the difference in performance for users depending on the
amount of error correction that is used. This is influenced by the distance that the MS is situated
from the BTS.
As shown below there is a large difference in the IP delay (measured in seconds) between the
different coding schemes. The difference between CS-1 and CS-2 is bigger then the difference
between CS-2 and CS-3 which would suggest this is why CS-2 is the most common coding scheme
used, due to the balance in performance and error correction.
This has implications for users because if they happen to live further from a BTS then there
performance is likely to be permanently effected. This slows uptake to only the people that can get
the benefits of a CS-4 coding scheme. Also with the exponential growth that is indicated on the
graph by CS-1 if the cell area happens to by very large or have busy sections in it then the numbers
of users will really harper use of GPRS.
26
Figure 4.2 Test 2 – Coding Schemes
As shown in Appendix C after 10 MS the changes in delay tend to level off not increasing as much as
is shown in the above graph. However what is clearly shown is that Stuckmann has come to the
same results as I have in seeing that predicted change in IP delay through the different coding
schemes. The performance of GPRS starts off well being lower than a second which would be an
acceptable delay, however the delay does increase substantially which each new user that is added.
This means that in a real life example where the number of users could be changing very quickly the
perceived delay in IP would just around. This would mean the user getting frustrated because some
times they would have very little delay but then this would change to a large delay. The problem
with this is that the user will not be able to work out an acceptable time for the delay. This peoples
perceptions of how long things take arrived at though experience. With the jumping in delay from
less then a second to more then 3.5, as shown for CS-1, that is a 300% difference.
What Stuckmann also simulated is this same simulation but using the different modulation schemes
27
1 2 3 4 5 6 7 8 9 100
0.5
1
1.5
2
2.5
3
3.5
4
Downlink IP Delay - coding schemes
CS-1
CS-2
CS-3
CS-4
Number of Mobile Stations
Dow
nlin
k IP
del
ay (
s)
of EGPRS. EGPRS (Enhanced General Packet Radio Service) is also known as EDGE (Enhanced
Data rates for Global Evolution). EGPRS works by using a different modulation and coding schemes
to increase data rates and robustness of transmission. EGPRS can work on any GPRS network as
they are effectively the same technology but EGPRS employs 8 different modulation and coding
schemes (MCS). These help to give EGPRS 3 times the data rates of GPRS.
The results for this test with EGPRS is that the different MCS lower the overall delay through all
levels with the graphs following similar patterns. This difference comes that as the number of users
increase the IP delay doesn't increase in EGPRS as quickly as it does in GPRS and so by the time 10
users are present the difference between CS-1 and MCS-1 is about half a second. Because of the
number of MCS there is less of a jump in the performance between each one meaning that the user
would notice less change in performance with a greater diversity of cell specification be able to be
had.
28
Chapter 5.0 - Evaluation
5.1 Minimum requirements
The first minimum requirement was understand wireless networks which has been shown in my
background research of chapter two. It was important to explain the way that GSM is implemented
as GPRS is bolt on technology to this using the same ideas but extending them into packet switched
data. Following the GSM introduction in chapter two is a look at how GPRS is laid over GSM and
what it changes. The first half of this chapter shows my knowledge in wireless networks.
The second minimum requirement was to design some experiments to test the performance of GPRS
using a network simulator. These designs as have been shown in chapter three. The designs were
made with three things in mind. Firstly the way that they were going to be analysed. I designed the
experiments so that I could produce graphs that could be used to directly compare to other obtained
results from other authors and also so that comparisons can be made with other technologies. This
helps cover the aim of this project to extend the knowledge of perceived performance results in
GRPS.
The third minimum requirement was to implement these designs in a network simulator. This has
been covered in chapter four. I have used the selected simulator to run a number of tests which were
produced the data to form the objectives for my designs. I have used the performance metrics
discussed in chapter four to get out of the simulations specific data that meets the objectives of that
experiment.
The fourth requirement was to find performance metrics that could would be used to explain the
generated data. These have been defined in chapter three. There are many different performance
metrics that could have been included here but only the ones that are relevant to the experiments
themselves have been included.
29
The last requirement was to analyse the results in terms of performance. This is dealt with in chapter
four where i have shown the results of the simulations and then analysed them to explain what they
mean for the user. What I have also done here is to use other simulations to explain to see if my
results are accurate. I have also used other technologies to shown what GPRS can do in terms of
GSM evolution.
I have identified possible extensions to these minimum requirements which were the analyses into
the comparison with another technology which have been done in chapter four also to use published
findings to compare to my own which has been covered though the analyse in chapter four.
5.2 Project Management
The strategy for research on this project was to use the knowledge that I have already obtained on
wireless networking from previous years of study to expand it into the specialised field of GPRS. I
have previously study the wireless networking protocol of 801.11 which introduced me to TDMA
and FDMA which is the bases of GSM. The management of this research was fairly easy to do as
there is plenty of literature which is easy to obtain. The only small problem was the shift in work
load towards the end of the allotted time for research. I underestimated the amount that was need to
be read. This meant that I had to increase the number of hours that was done per week to reach the
required level by the schedule I have set myself.
The main project management issue came due to the simulator problems as discussed in Appendix A
and B this effect the schedule to a large amount which I have been struggling to over come since.
This has lead to things like their only being two simulations rather then the planned three. Also I
have not been able to simulate other technologies myself.
5.3 Project Success
30
This project has been a success in some departments having met all the minimum requirements and
covering the aims of the project as set out in chapter 1. The project has also been very useful in
providing me with a new life skill in performance evaluation. This is useful skill to have in the
computer field. I have also been successful with this project in terms of my increased understanding
of two everyday technologies being GSM and GPRS.
5.4 Project Improvements
This project could be improved in the following ways. The first thing that I would improve is to do
at least one more simulation. I would choose something that could be compared across other
technologies easily. This would most likely take the form of a data throughput simulation to look at
the the speed that GPRS can send data under certain conditions. Then I could use a wide range of
technologies from 2G to 3G to show how they compare. I would look for a different simulator so
that I could run simulations for more then 10 users which would provide better depth to my analysis.
31
Acronyms AuC Authentication Centre
BSC Base Station Controller
BSS Base Station Subsystem
BTS Base Transceiver Station
EDGE Enhanced Data rates for Global Evolution
EGPRS Enhanced General Packet Radio Service
EI Equipment Identify
FDMA Frequency Division Multiple Access
GGSN Gateway GPRS Support Node
HLR Home Location Register
IMSI International Mobile Subscriber Identity
MPDCH Master Packet Data Channel
MCS modulation and coding schemes
MSC Mobile services Switching Centre
MS Mobile Station
MSISDN Mobile Station international ISDN number
MSRN Mobile Station Roaming Number
NSS Network and Switching Subsystem
OMC Operating and Maintenance Centre
OSS Operation Subsystem
PAGCH Packet Access Grant Channel
PBCCH Packet Broadcast Control Channel
PCCCH Packet Common Control Channel
PDCHs Packet Data Channels
PDC Personal Digital Cellular
PNCH Packet Notification Channel
PPCH Packet Paging Channel
32
PRACH Packet Random Access Channel
PTCH Packet Traffic Channel
RSS Radio Subsystem
SIM Subscriber Identity Module
SGSN Serving GPRS Support Node
TMSI Temporary Mobile Subscriber Identity
VLR Visitor Location Register
33
Appendix A – Personal ReflectionMy personnel reflection on this project would have to centre on software simulator issues. As
described in Appendix B problems with NS-2 lead to a large delay in the project and hence the other
parts having to be rushed. However to really reflect on this project experience I must go back before
the problems with simulator software started. Something that I have found with this project is that
more research was required at an earlier stage. However I am not talking about the background
research as included earlier in the report. I think my greatest lesson and hence biggest piece of
advice for someone thinking about entering a networking project would be do lots of research on
how much detail you will have to know about the network protocol to be studied. Had I taken the
time early on I might have realised the the type of knowledge that was expected of me to learn was in
fact the type that I am not suited to. This extra bit of research at the start might have lead me to
finding a project that didn't just capture my motivation from the start but held it all the way through.
This would have in turn lead to more work being done earlier which would mean that more time
would have been spare to incorporate software problems such that I had with NS-2. However there
is still the lesson of not taking for granted that the recommended piece of software will indeed work
and do what you need it to.
Even with the problems this project has been an eye opening one for me that I can take forward so as
not to repeat my mistakes again.
34
Appendix B – ScheduleThe schedule for this project has had to undergo some changes to allow for unforeseen events. The
schedules was significantly altered by the fact that the chosen network simulator of NS-2 is not able
to be installed. The way that NS-2 works is by having a core program which itself is made from
separate programs that integrate together. Extra parts to NS-2 are provided in modular form with the
core program having to be patched before it is installed to allow for the module updates to be
included. However the problem with this is that different people build modules with different
versions of the core program. As NS-2 is being continually updated this means that finding the
working combination can prove tricky. This project went though several problems with NS-2. The
first of these was that NS-2 doesn't work with fedora 3. The next problem was that an old version is
installed on the school of computing machines which is too old to support the module for GPRS
which allows NS-2 its compatibility with GPRS. The final problem occurred trying to get the GPRS
patch to work on a third workstation configuration. These problems shifted the schedule completely.
Not only that but progress had been made in the training of NS-2 time that was also to become
wasted. With NS-2 not working a new simulator was found in the form of Sgoose as detailed earlier
in the report.
35
Appendix C – Additional Simulation ResultsThis section includes results that I have gathered from other sources that I have used in chapter 4.0
to help verify my results.
Application response times
xxiii
Coding schemes
36
i Emmanuel Seurre, Patrick Savelli and Pierre-Jean Pietri, GPRS for Mobile Internet, London: Artech
House (2003), page 2.
ii Ricky Ng and Ljiljana Trajkovichttp://www.ensc.sfu.ca/people/faculty/ljilja/cnl/presentations/ljilja/opnetwork02/talk_358/sld005.htm10/04/05
iii Peter Stuckmann, the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003) , page 16.
iv Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 1.
v Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 11.
vi Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 12.
vii Christian Bettstetter, Hans-Jorg Vogel, and Jorg Eberspacherhttp://zzz.com.ru/zzz_original_site/art61.html 27/04/05.
viii Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 9.
ix Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 110.
x http://www.volny.cz/pavel.pilat/gsm/gsm_right.htm 22/04/05.
xiPeter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 16 figure 2.8.
xii Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 21.
xiii Christian Bettstetter, Hans-Jorg Vogel, and Jorg Eberspacherhttp://zzz.com.ru/zzz_original_site/art61.html 28/04/05.
xiv Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 23.
xv Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 26.
xvi Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 27 - 29.
xvii Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 43- 45.
xviii Prof. Lizy Kurian John http://www.ece.utexas.edu/projects/ece/lca/courses/382m/documents/perf-eval-encyclopedia.pdf (12/03/05) - Performance Evaluation: Techniques, Tools and BenchmarksxixDr. Andreas Willig http://www-ks.hpi.uni-potsdam.de/docs/engl/teaching/pet/ss2003/hpi_big_picture.pdf (12/03/05)Dr.-Ing. Andreas Willig Performance Evaluation Techniques Fundamentals and Big Picture
xx Jerry Banks, John S. Carson, Barry L. Nelson and David M. Nicol Discrete- Event System Simulation.
Prentice-Hall, Upper Saddle River, NJ, third edition, (2000).
xxi http://www.isi.edu/nsnam/ns/ 27/04/03xxii http://www.aixcom.com/Home_e.php 27/04/03
xxiii Peter Stuckmann ,the GSM Evolution – Mobile Packet Data Services ,Chichester: John Wiley & Sons
(2003), page 206.
