Alfred Project Draft

8/6/2019 Alfred Project Draft

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Research and development of a communication system for the NRZs Signaling system.

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Table of Contents

Table of Figures ................................................................................................... 3

Abstract ................................................................................................................ 4

Definition Of Terms .............................................................................................. 5

INTRODUCTION .................................................................................................. 8

1.0.1 The state of railway signaling in the world ........................................... 8

1.1 Background ............................................................................................. 9

1.2 Justification ........................................................................................... 10

1.2.1 The NRZ Signaling System Communication System ...................... 10

1.2.1.1 The siding ....................................................................................... 101.2.1.2 The Signaling System ........................................................................ 13

1.3 STATEMENT OF THE PROBLEM ........................................................... 14

2.0 OBJECTIVES ........................................................................................... 15

This project aims to achieve the following objectives:..................................... 15

3.0 LITERATURE REVIEW ............................................................................ 16

3.1 Introduction ........................................................................................... 16

3.2 Fiber optic weighed against wireless technologies ................................ 16

The OSI Reference Model .................................................................................. 19

3.2 Comparison of Wireless technologies ................................................... 22

3.3 WIMAX THE WIRELESS SOLUTION ................................................... 24

2.3.1 THE PHYSICAL LAYER ................................................................ 24

A. Channel coding .............................................................................................. 25

B. Interleaving .................................................................................................... 25

C. Modulation ..................................................................................................... 25

D. Data mapping ................................................................................................ 25

E. Space/Time Encoder (MIMO encoder) .......................................................... 27

F. Subcarrier allocation/Pilot insertion ................................................................ 27

G. IFFT and Digital-to-Analog (D/A) ................................................................... 27

Mobile WiMAX Physical layer Specification ................................................ 28


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Frequency Band and Channel Allocation ........................................................ 28

10 MHz BW / OFDMA..................................................................................... 29

TDD- Time Division Duplex ............................................................................ 29

Modulation schemes ....................................................................................... 29

3.3.2 BASIC FUNCTIONALITY OF MAC LAYER IN WIMAX .................. 30

QUALITY OF SERVICE ..................................................................................... 32

3.4 WHAT MAKES MOBILE WIMAX TICK .............................................. 35

3.5 MOBILE IP ............................................................................................ 37

Mobile WiMAX Network Architecture.................................................................. 40

4.0 DESIGN OF THE NETWORK .................................................................. 44

4.1 Abstract ................................................................................................. 44

4.2 DESIGN METHODOLOGY ................................................................... 44

4.4 THE NETWORK LAYER ....................................................................... 45

4.4.1 Design requirements list ................................................................. 46

4.4.2 Below is a layout of the proposed signaling system: ...................... 47

4.5 Simulation ............................................................................................. 48

Limitations .......................................................................................................... 49

Conclusion and Discussion ................................................................................ 50

APPENDIXA...................................................................................................... 51

Home Agent (CTC router) Configuration ........................................................ 51

References ......................................................................................................... 53


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Table of Figures

Figure 1: The devastating accident in Dete ..................................................... 9

Figure 2: DXR200 Digital Microwave backbone ............................................. 12

Figure 3: Illustration of the OSI Model ............................................................ 20

Figure 4: Graphical comparison of Wireless WAN Technologies ................ 21

Figure 5: Comparison of Wireless Technologies .......................................... 21

Figure 6: Functional Stages of the MIMO enabled mobile WiMAX physical

layer ................................................................................................................... 24

Figure 7: Channel Allocation in the Physical Layer ...................................... 29

Figure 8: Specified layer 2 and layer 1 sublayers.......................................... 31

Figure 9: Quality of Service in Mobile WiMAX ............................................... 34

Figure 10: Illustration of Mobile IP triangle routing ....................................... 39

Figure 11: Network Reference Model ............................................................. 40

Figure 12: Logical Illustration of a Nodal Subnetwork .................................. 47

Figure 13: Packet Tracer 5.3 Illustration of the entire logical network ........ 48


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Abstract

In this project, it has been undertaken, to develop a robust, reliable and real time

wireless communication system for a Communication Based Train Control

system for the National Railways of Zimbabwe.

An, all IP, architecture has been chosen and on it, Mobile WiMAX has been used

as the backhaul and last mile access technology. This is because of its

outstanding properties which have exposed in this project and found to meet the

requirements of the NRZ signaling system. It is also part of the design to make

the communication system, upgradable to IEEE 802.16m and integratable with

useful technologies like Global Positioning System (GPS).

The network has been designed to support the following services:

VOIP, real-time video streaming, file transfer and web browsing.

Security has been enabled on the network layer and every layer three device has

been locked to keep the network as free as possible from hackers. The network

has been isolated from the internet thereby reducing the likelihood of

unauthorized access.

A simulation of this design has been incorporated in the project.


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Definition Of Terms

Backhaul - comprises the intermediate links between the core of the

network and the small subnetworks at the "edge" of the entire hierarchical

network.

CBTC - Communication Based Train Control.

Internet Protocol (IP) - a protocol used for communicating data across a

packet-switched internetwork using the Internet Protocol Suite, also

referred to as TCP/IP.

last mile - the final leg of delivering connectivity from a communications

provider to a customer.

Global Positioning System (GPS) is a space-based global navigation

satellite system that provides reliable location and time information in all

weather and at all times and anywhere on or near the Earth when and

where there is an unobstructed line of sight to four or more GPS satellites.

Voice over Internet Protocol (VoIP, Voice over IP) - a family of

methodologies, communication protocols, and transmission technologies

for delivery of voice communications and multimedia sessions over

Internet Protocol (IP) networks, such as the Internet.

Siding a part of the railway where a rail station is located

Bandwidth - is a bit rate measure of available or consumed data

communication resources expressed in bits/second or multiples of it.


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Layer 3 Switches - are basically routers that switch based on Layer 3

information, the basic difference being processing speed and/or the way

they do the switching; Level 3 switches use ASICs/hardware instead of the

CPU/software that a router would.

Router - is an electronic device that interconnects two or more computer

networks, and selectively interchanges packets of data between them.

Each data packet contains address information that a router can use to

determine if the source and destination are on the same network, or if the

data packet must be transferred from one network to another.

WiMAX (Worldwide Interoperability for Microwave Access) - is a

telecommunications protocol that provides fixed and fully mobile internet

access.

MIMO - is the use of multiple antennas at both the transmitter and receiver

to improve communication performance. It is one of several forms of smart

antenna technology.

WAN - wide area network is a data network that covers a broad area

(i.e., any network whose communications links cross metropolitan,

regional, or national boundaries)

QoS Quality of Service refers to resource reservation control

mechanisms rather than the achieved service quality. Quality of service is

the ability to provide different priority to different applications, users, or

data flows, or to guarantee a certain level of performance to a data flow.


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For example, a required bit rate, delay, jitter, packet dropping probability

and/or bit error rate may be guaranteed.

OSI Model - The Open Systems Interconnection model is a product of

the Open Systems Interconnection effort at the International Organization

for Standardization. It is a way of sub-dividing a communications system

into smaller parts called layers. A layer is a collection of conceptually

similar functions that provide services to the layer above it and receives

services from the layer below it.

COA - Care of Address is a temporary IP address for a mobile device.

This allows a home agent to forward messages to the mobile device. As

separate address is required because the IP address of the device that is

used as host identification is topologically incorrect - it does not match the

network of attachment. The care-of address splits the dual nature of an IP

address, that is, its use is to identify the host and the location within the

global IP network.

Handover orHandoffrefers to the process of transferring an ongoing call

or data session from one channel connected to the core network to

another.


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INTRODUCTION

1.0.1 The state of railway signaling in the world

Communication-Based Train Control (CBTC) is the safe control of urban rail

vehicles using data communication as the means of tracking train locations and

sending speed and stopping information to trains. First generation CBTC

systems used low frequency, low data rate transmission to ensure reliable

information exchange. Two trends are driving the next generation of CBTC

technology. First is the desire to move away from communication equipment

installed between the rails to the use of radios. Second is the desire for

interoperability to enable the industry to buy train and trackside equipment from

different suppliers and operate that equipment seamlessly. Interoperability and

high performance moving blocks are key requirements for the Asia Pacific

market, particularly China. The Alcatel official website remarks in one of its white

papers.

The remark is a direction to show where the tide of railway technology is going. It

is only wise to ride on this tide and to design systems that can be in step with the

latest technological advancements.

It is the scope of this project to do just that in the case our local railway.

The National Railways of Zimbabwe (NRZ), the sole railway operator in the

nation, has had a lot of money lost and a lot of lives lost on its railway. One


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incident that stands out is the Dete train accident in February of 2003, where two

trains collided head on. This has been due to a number of causes. The main

causes however, can be prevented. It is only noble that the causes that are

preventable be nipped in the bud.

Figure 1: The devastating accident in Dete

1.1 Background

The Signaling and control system of the NRZ has over the years become almost

defunct because of thefts, lack of maintenance and aging systems. The system

has unfortunately failed when it is needed the most. This has resulted in loss of

lives, goods and potential revenue due to accidents and declining confidence of

the business sector in this rather wide logistics network.

In light of the unfortunate turn of events, it has become necessary and important

that a solution be drawn out to provide a robust, efficient, real time

communication system for the signaling system.


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1.2 Justification

In this project, it is proposed that the existing communication system for the

signalling system be replaced with a robust, efficient, real time communication

system which is secure from vandals.

1.2.1 The NRZ Signaling System Communication System

The NRZ makes use of a DXR200 backbone microwave communication system

to convey its diverse kinds of signals throughout its network. Below is an

illustration of how the backbone microwave bandwidth is apportioned.

The railway network makes use of points known as sidings to do a varying

number of essential operations. It is important that the general relevance of the

siding be explained in order to understand the structure of the NRZ

communication system.

1.2.1.1 The siding

The following are the operations of the siding:

1. Trains stop in order to give way to the particular train which is

accorded the right of way

2. Collection and delivery of wagons is done


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3. Repeaters of the backbone microwave system are located

4. Passengers board or disembark the train

5. The NRZ staff is based

6. Any maintenance work is coordinated from here

It is therefore clear that siding play an important role in the communication

system of NRZ. Below is a basic layout of the signaling module found on a typical

siding.


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Figure 2: DXR200 Digital Microwave backbone

Relays CTC

Control

cubicle

PLC Hub RouterDATA

CARD

Axle counter

UHF RADIO (for

voice

Four

Wire

Card

Local dial exchange

(LDE) the PABXSubscription

Card

Subscription

CardTrack Routing


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1.2.1.2 The Signaling System

A

s trains traverse a railway route they are regulated by control signals which are

relayed from a central location known as the Central Train Control (CTC). In

Zimbabwe, there are two main routes namely the Harare-Mutare route and the

Dabuka-Harare route. The CTC for the Harare-Mutare route is located in Harare.

It is also the route on which the DXR200 digital Microwave backbone is installed.

The CTC for the latter is at Dabuka and it is currently using analogue backbone

to convey signaling instructions.

At present, the train control signals are relayed by the Backbone microwave to

the different sidings from where they are then relayed over fiber optics cable to

the traffic lights which are located along the railway. When the train driver sees

the signal they know how to proceed along the route.

The control signals can tell the driver to slow down, to proceed or to stop

depending on the color of the traffic light signal.

This system has its disadvantages, which are the following:

It is one way communication and the driver cannot acknowledge receipt of

the signal.

The CTC terminal cannot monitor the behaviour of the train driver, that is,

whether they are following instructions or not.


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The system cannot offer real time updates to CTC terminal of position,

speed, direction and condition of a train on the track. Lack of such

information increases the risk of accidents of the rail.

The traffic lights are on a small section of he rail and hence if the driver

lapse concentration, they cannot revisit the signal again as they would

have passed it, there by making traversing the railway network more

risky.

1.3 STATEMENT OF THE PROBLEM

The NRZs signaling communication system is not real time in nature and

is being vandalized and hence signals are no longer being relayed to the

respective traffic lights along the railway effectively.


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2.0 OBJECTIVES

This project aims to achieve the following objectives:

1. To develop a communication system which offer two way

communication between the central train control (CTC) terminal

and any train on the network

2. To enable real time communication between the CTC terminal and

any particular train.

3. To make the communication system modules securable by

making them less bulky.

4. To offer bandwidth that does not bottle neck the throughput of the

communication system

5. To make modify the existing backbone to offer more bandwidth

hence more communication services.

6. To provide a reasonably cost effective network

7. To provide for future expansion and integration with other

futuristic technology

8. To make the network easy to troubleshoot and maintain


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3.0 LITERATURE REVIEW

3.1 Introduction

In this chapter, fiber optic is analyzed and weighed with wireless technologies.

The wireless technology of choice, Mobile WiMAX, is then carefully singled out

on the basis of relevancy to the problem statement. The technology of choice is

then analyzed to show how it is structured and its key features are explained.

3.2 Fiber optic weighed against wireless technologies

It will be unjust to simply discredit the application of optical fiber without making a

full analysis of its merits and demerits.

The following is a look into the importance of Optical fiber:

An optical fiber has very clear cut advantages over wire and formerly, radio, and

this is why in the telecommunications industries copper has significantly been

replaced by fiber optic systems.

The following are the main advantages of optical fibers

1. Attenuation in fiber is markedly lower than that of coaxial cable of

twisted pair and is constant over a very wide range so transmission

within wide range of distance is possible without repeaters.


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2. Smaller size and lighter weight. Optical fibers are considerably

thinner than coaxial cable or bundled twisted pair cable. So they

occupy much less space.

3. Electromagnetic isolation. Electromagnetic waves generated from

electrical disturbances or electrical noises do not interfere with light

signals as a result, the system is not vulnerable to interference,

impulse noise or crosstalk.

4. No physical electrical connection is required between the sender

and the receiver.

5. The fiber is much more reliable, because it can better with stand

environmental conditions such as pollution radiation and salt which

produce corrosion. Moreover it is nominally affected by nuclear

radiation. Its life is longer than that of copper wire.

6. Almost there is no crosstalk in optical fibers and have transmission

is more secure and private as it is very difficult to tap into a fiber.

7. Greater bandwidth. Bandwidth of the optical fiber is higher than that

of an equivalent wire transmission line.

8. Fibers are very good dielectrics, hence isolation coating is not

required.

9. Data rate is much high in a fiber and hence much more information

can be carried by each fiber than by equivalent copper cables.

10. Cost per channel is lower than that of an equivalent wire cable

system.


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11. Due to non inductive and non conductive nature of a fiber, there is

no radiation and interference on other circuits and systems.

12. Greater Repeater spacing. Fiber optic systems can achieve a data

rate of 5Gbps over a distance of 111km without repeaters.

13. The raw material is available in plenty.

The disadvantages of fiber optics are as follows:

1. Installation technicians must protect their eyes. The densities of optical

energy emitted by the light sources and by the extremity of the fibre

are sufficient to damage the retina permanently before the victim

notices.

2. Damage at any point along the laid fiber optic impairs further down

stream transmission.

3. Installing and repairing a fiber optic cable is labour intensive and in

most cases includes excavation or working close to high voltage

cables.

4. Since considerable amounts of vandalisms occur in the dark, fiber

optic cables can be mistaken for, the much sort after, copper cables

and a subsequently liable to vandalism.

5. Fiber optic does not support last mile mobile communication hence can

only be used for fixed point to point communication.

6. Expensive initial installation cost.


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These few disadvantages listed above make fiber optics a lesser option as

compared to the latest wireless LAN technologies, when considering the problem

at hand.

The wireless LAN technologies have the following advantages

1. No cable laying cost

2. No significant cable requirements

3. Ease to install , maintain and repair

4. Provide real time access and communication.

5. The installable units are not bulky and are not recyclable hence do not

attract much attention from vandals.

6. A wireless system can be troubleshot from a remote location hence

whenever a problem occurs at any unit it is quickly identified and

corrected quickly

Wireless LAN/WAN technologies have Internet Protocol (IP) based architecture.

This architecture is described in a hierarchical layered network architecture which

has set protocols for every layer, Open Systems Interconnections (ISO)

Reference Model.

The OSI Reference Model

The OSI reference model is a set of guidelines that application developers can

use to create and implement applications that run on a network. It also creates a


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framework for creating and implementing networking standards, devices, and

internetworking schemes. It has 7 different layers, divided into two groups.

1. The top 3 layers define how the application within the end stations will

communicate with each other and with users.

2. The bottom 4 layers define how data is transmitted end to end.

The diagram below shows the layout of the Model.

Wireless LAN/WAN technologies differ in the way they specify the bottom 3

layers, giving them properties which vary as we move from one technology to the

other.

Figure 3: Illustration of the OSI Model


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In this project, the communication subnet is the area of interest as the upper 4

layers are the same for all IP based devices and are relevant to the end devices

only.

Figure 4: Graphical comparison of Wireless WAN Technologies

Burst Speed Average User

Throughput

Capacity Other Features

GPRS 53 Kbps 30-40 Kbps

EDGE 200 Kbps 100 - 130 Kbps Double that of GPRS Backward compatible

with GPRS

UMTS 384 Kbps 220 - 320 Kbps Increased over EDGE

for high bandwidth

Simultaneous voice and

data operation enhanced

security, QoS,

multimedia support, and

reduced latency

UMTS - HSDPA 2 to 3 Mbps 550 - 1100Kbps 2.5 to 3.5 times over

WCDMA

Backward compatible

with WCDMA

CDMA2000

1XRTT

144 Kbps 50 - 70Kbps

CDMA2000

1XEV-DO

800 Kbps 300 - 500 Kbps Optimized for data,

VoIP in

developmentMobile WiMAX

IEEE 802.16e

8 Mbps IN 5 MHz 1 Mbps+ in 5

MHz channel20% higher than

HSDPA or EV-DO

VOIP, Video

conferencing

Figure 5: Comparison of Wireless Technologies


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3.2 Comparison of Wireless technologies

It is imperative to highlight the expectation of the wireless technology of choice

before picking one suitable for the situation at hand. The following are the

expectation of the wireless technology:

y It should be able to offer non line of sight coverage of its cell.

y It should be able to offer bandwidth higher than 2Mbps in order to avoid

bottlenecking of the communication system.

y It should offer the required bandwidth with mobility at distances of at least

a kilometer from the base station.

y It should support an IP based mesh network.

y the technology should be compatible with the handset devises on the

market.

y Relatively cheap to deploy

y Futuristic that is, can remain compatible with future technologies for the

longest period.

With this criterion in mind, the most probable technology is WiMAX and more

specifically mobile WiMAX IEEE 802.16e.

Mobile WiMAX, built from the onset to fulfill requirements for mobile broadband

applications, presents the following advantages over all other mobile and

broadband technologies:

Mobile IP (MIP) algorithms at its core include elements such as home agents that

allow seamless handover of services when a subscriber moves from one


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coverage area to another. With a complete set of IP functions and interfaces as

part of the standard, Mobile WiMAX enables the delivery of IP based services,

while maintaining end-to-end quality of service (QoS). Core networks based on

IP routers and switches are lower cost and easier to install and operate than

other alternatives. As todays multimedia services are IP based, all IP networks

can easily support the provisioning and QoS for the different services.

Scalable Transmission Coding by offering several options for each device,

mobile WiMAX maximizes the performance, service availability and quality. Each

device can communicate with the closest base station using one of various

transmission coding schemes depending on signal quality, interference, its

internal processing capabilities, and many other parameters. The coding also

adapts periodically to match the current status of the device.

Spectral Efficiency combining the latest transmission coding schemes with

several channel size options (up to and including 20 MHz) and the ability to

group sub-carriers allows operators to use their available spectrum in the most

efficient manner.

Advanced over-the-air QoS offering multimedia services, which combine voice,

data, and video in a single air link to numerous users means that QoS is critical

for the proper operation of the network. As WiMAX is all IP, QoS correlation

between the IP network and broadband services, most of which are IP based, is

straightforward. QoS over-the-air is part of the mobile WiMAX standard in which

a design transmission scheduler is used to ensure proper QoS for each and

every service.


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Non line-of-sight (NLOS) and Smart Antennas enabling communication through

walls and other physical obstacles in both urban and rural environments, mobile

WiMAX is a true NLOS technology. Mobile WiMAX maximizes the number of

services delivered and their quality regardless of operating environment by

employing smart antenna technology including beam forming capabilities, power

control and other standard-defined parameters.

3.3 WIMAX THE WIRELESS SOLUTION

Due to its long-range and high-bandwidth transmission, IEEE 802.16 has also

been considered in areas where it can serve as the backbone network with long

separation among the infrastructure nodes.

2.3.1 THE PHYSICAL LAYER

Figure 6: Functional Stages of the MIMO enabled mobile WiMAX physical layer


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A. Channel coding

The channel coding stage includes randomization, coding and puncturing. Initially

the input data is randomized in order to avoid long runs of ones and zeros. The

output of the data randomizer is encoded with a convolutional encoder whose

constraint length is 7, and the native code rate is 1/2. The puncturing block

punctures the output of the convolutional encoder to produce higher code rates.

B. Interleaving

The interleaving stage uses a block interleaver to interleave the encoded bits.

This maps adjacent encoded bits onto separated subcarriers, thus minimizing the

impact of burst errors caused by spectral nulls (interestingly, such interleaving is

not present in the 802.11a/g standard).

C. Modulation

The modulation block converts a sequence of interleaved bits into a sequence of

complex symbols depending on the chosen modulation scheme (QPSK, 16QAM,

and 64QAM).

D. Data mapping


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In order to understand the operation of the data mapping block, it is necessary to

explain OFDMA system and a number of its specific terms.

Slot: This is the minimum possible data allocation unit in the OFDMA PHY. For

DL PUSC, one slot represents one subchannel over two OFDMA symbols. For

UL PUSC, one slot represents one subchannel over three OFDMA symbols

Data region (or data burst): a data region of a user is a two dimensional

allocation of a group of contiguous logical subchannels (which will later be

physically distributed when the distributed permutation is chosen), in a group of

contiguous slots. The size of the data region will depend on the number of

subchannels allocated to each user and the user packet size. Values of 4 (UL)

and 5 (DL) are used for the allocated subchannels, and a user packet size of 120

bytes is assumed. The first step in the data mapping process is to segment the

sequence of modulation symbols into a sequence of slots. Each slot contains a

number of modulation symbols. For example, in DL PUSC each slot contains 48

symbols. The second step is to map the slots into a data region, so that the

lowest numbered slot occupies the lowest numbered subchannel among the

allocated subchannels. The mapping of slots continues vertically to the edge of

the data region, and then moves to the next available OFDMA slot.


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E. Space/Time Encoder (MIMO encoder)

The Space/Time Encoder stage converts one single input data stream into

multiple output data streams. How the output streams are formatted depends on

the type of MIMO method employed.

F. Subcarrier allocation/Pilot insertion

At this stage all data symbols are mapped to a data region and assigned to their

corresponding logical subcarriers. The next step is to allocate the logical

subcarriers to physical subcarriers using a specific subcarrier permutation; pilots

are also inserted at this point.

G. IFFT and Digital-to-Analog (D/A)

The final stage is to convert the data into analogue form (in the time-domain) for

use in the radio front end. A guard interval is also inserted at this stage. A link-

speed is defined as a combination of a modulation scheme and a coding rate.

The peak data rate D is calculated as below:

D=NDNbRFECRSTC/Ts

where ND, Nb, RFEC, RSTC,, and Ts denote the number of assigned data

subcarriers to each user, the bits per sub-carrier, the FEC coding rate, the space-

time coding rate, and the OFDMA symbol duration respectively. On the UL, more

subchannels are used for control purposes, and more pilots are assigned to a

subchannel.


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Mobile WiMAX Physical layer Specification

Frequency Band : 2.3GHz

Channel Bandwidth: 9MHz

Duplex: TDD / 5msec frame

Multiple Access: 10 MHz BW / OFDMA

Modulation: QPSK, 16QAM, 64QAM

Channel Coding : CTC (Convolutional Turbo Codes)

Cell Coverage : ~1km

Maximum Data Rate

Sector throughput : DL : 18 Mbps, UL : 6 Mbps

User throughput : DL : 3 Mbps, UL :1 Mbps

MIMO adaptive beam foaming antenna

Frequency Band and Channel Allocation

Frequency bands for the Mobile WiMAX network: 2.3GHz ~ 2.4GHz

Divided into three bands for service provision (SP)

A channel bandwidth is 9MHz and guard band between service

provision is 4.5MHz

Channel allocation in the Mobile WiMAX network


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Figure 7: Channel Allocation in the Physical Layer

10 MHz BW / OFDMA

It Minimizes multi-path interference to increase spectral efficiency

It is Robust against multipath delay spread

It has No intra-cell interference (orthogonal multiple access)

It offers High degree of freedom in resource allocation

TDD- Time Division Duplex

It is a better choice than FDD in that:

It is not sensitive to the Doppler Effect

It minimizes guard band to increase spectral efficiency

There is no need for pair bands

It has flexible Downlink and Uplink resource allocation (time zone) according to

traffic request

This (TDD/OFDMA) provides high spectral efficiency support

Modulation schemes

Hierarchical modulation is implemented to make the signal more rugged to fading

QPSK (Quadrature Phase-Shift Keying)


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16QAM (Quadrature Amplitude Modulation)

64QAM

CTC (Convolutional Turbo Code)

This maximizes data rate

3.3.2 BASIC FUNCTIONALITY OF MAC LAYER IN WIMAX

The MAC layer consists of three sublayers:

1) the service-specific convergence sublayer(CS)

2) MAC common part sublayer(MAC CPS)

3) Security sublayer.

1) The main functionality of the CS is to transform or map external data from

the upper layers into appropriate MAC service data units (SDUs) for the

MAC CPS. This includes classification of external data with the proper

MAC service flow identifier (SFID) and connection identifier (CID). An SDU

is the basic data unit exchanged between two adjacent protocol layers.

2) The MAC CPS provides the core functionality for system access,

allocation of bandwidth, and connection establishment and maintenance.

This sublayer also handles the QoS aspect of data transmission.

3) The security sublayer provides functionalities such as authentication,

secure key exchange, and encryption.


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For the PHY layer, the standard supports multiple PHY specifications, each

handling a particular frequency range.

Figure 8: Specified layer 2 and layer 1 sublayers


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QUALITY OF SERVICE

QoS provisioning is one of the essential features in IEEE 802.16. However, there

are differences in the standard specifications, specifically, in IEEE 802.16-2004

and IEEE 802.16e.

A service flow is defined as a one-way flow of MAC SDUs on a connection

associated with specific QoS parameters such as latency, jitter, and throughput.

These QoS parameters are used for transmission and scheduling. Service flows

are typically identified by SSs and BSs based on their SFID. There are three

basic types of service flows: provisioned service flows, admitted service flows,

and active service flows.

Aprovisionedservice flow is defined in the system with an SFID, but it

might not have any traffic presence. It may be waiting to be activated for

usage.

An admitted service flow undergoes the process of activation. In

response to an external request for a specific service flow, the BS/SS will

check for available resources based on the QoS parameters to see if it

can support the request. If there are sufficient resources, the service flow

will be deemed admitted. The resources assigned to this service flow may

still be used by other services.


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A service flow will be active when all checks are completed and the

resources are allocated. Packets will flow through the connection

allocated to the service flow.

The use of service flows is the main mechanism used in QoS provisioning.

Packets traversing the MAC sublayer are associated with service flows as

identified by the CID when QoS is required.

Bandwidth grant services define bandwidth allocation based on the QoS

parameters associated with a connection. In downlink transmissions a BS has

sufficient information to perform scheduling, but in uplink transmissions a BS

performs the scheduling of various service transmissions based on information

gathered from SSs. In such cases an SS will request uplink bandwidth from the

BS, and the BS will allocate bandwidth on an as needed basis. For proper

allocation of bandwidth, five services are defined to support different types of

data flows:


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QoS Category Applications QoS specificationsUGS

Unsolicited Grant Service

VOIP y Maximum Sustained

rate

y Maximum Latency

Tolerance

y Jitter TolerancertPS

Real-Time Polling Service

Streaming audio or Video y Minimum Reserved

Rate

y Maximum Sustained

Rate

y Maximum Latency

Tolerance

y Traffic Priority

ErtPS

Extended real time Polling

service

Voice with Activity Detection

(VOIP)

y Minimum Reserved

Rate

y Maximum SustainedRate

y Maximum Latency

Tolerance

y Jitter Tolerance

y Traffic Priority

NrtPS

Non real time Polling Service

File Transfer Protocol (FTP) y Minimum Reserved

Rate

y Maximum Sustained

Rate

y Traffic Priority

BE

Best Effort

Data Transfer, Web Browsing,

etc

y Maximum Sustained

Rate

y Traffic Priority

Figure 9: Quality of Service in Mobile WiMAX

Unsolicited grant service (UGS) is designed to support real-time constant bit

rate (CBR) traffic such as VoIP; this provides fixed size transmission

opportunities at regular time interval without the need for requests or polls.


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Real-time polling service (rtPS) is designed to support variable bit rate (VBR)

traffic such as MPEG video. In this service the BS offers the SS periodic request

opportunities to indicate the required bandwidth.

Non-real-time polling service (nrtPS) is for delay-tolerant data service with a

minimum data rate, such as FTP. This service allows an SS to use contention

request and unicast request opportunities for bandwidth request. Unicast request

opportunities are offered regularly in order to ensure that the SS has a chance to

request bandwidth even in a congested network environment.

Best effort (BE) service does not specify any service related requirements.

Similar to nrtPS, it provides contention request and unicast request opportunities,

but it does not provide bandwidth reservation or regular unicast polls.

3.4 WHAT MAKES MOBILE WIMAX TICK

Mobile WiMAX standard offers scalability in both radio access technology and

network architecture; thus, it provides flexibility in network deployment and

service offerings. There are several key features supported by mobile WiMAX:

QoS in that it defines service flow to enable end-to-end IP-based service

mapping and also provides mechanisms for optimal scheduling on a frame-by-

frame basis


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Flexible spectrum allocation in that it is scaled to work in different

channelization from 1.25 to 20 MHz complying with diverse requirement in

different countries

Enhanced security in that new authentication was added

High data rate with the MIMO antenna techniques and flexible sub

channelization schemes, which can support peak rates of 63 Mb/s in downlink

and 28 Mb/s uplink per sector Power consumption and handoff are two critical

issues for mobile applications. Mobile WiMAX

provides two modes for power

efficient operation, sleep mode and idle mode. Sleep mode aims to minimize a

mobile users power consumption and also provide flexibility that allows a mobile

user to scan BSs to collect handoff related information. In idle mode a mobile

user can traverse multiple BSs and periodically capture downlink broadcast

messages without registration to any specific BS. This eliminates the need for an

inactive mobile user to hand off. Mobile WiMAX provides three handoff

mechanisms: hard handoff (HHO), fast base station switching (FBSS), and

macro-diversity handover (MDHO). HHO is mandatory, while FBSS and MDHO

are optional. In both FBSS and MDHO, a mobile user and BS maintain a so

called active set, a list of BSs involved with the mobile users handoff. An anchor

BS is defined from the active set. In FBSS a mobile user only communicates with

the anchor BS, and the handoff involves the transition to a new anchor BS. In

MDHO a mobile user communicates with all BSs in the active set, and different


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operations are defined for uplink and downlink transmission during handoff.

Multicast and broadcast services (MBS) are supported in Mobile WiMAX.

3.5 MOB

ILE IP

IP (i.e., Mobile IPv4) was designed to provide a way to support host mobility. A

standard Mobile IP proposed by the Internet Engineering Task Force (IETF) over

IP version 4 (IPv4) consists of the following functional entities:

Mobile node (MN): A host or router that can travel around the Internet while

maintaining any ongoing communication session.

Home agent (HA): A router that maintains a list of registered MNs. It is used to

forward MN-addressed packets to the appropriate visiting network when MNs are

away from home.

Foreign agent (FA): A router with an interface in an MNs visiting network, which

assists the MN in informing its HA of its current care-of address.

Care-of address (CoA): A local IP address that identifies the MNs current

location.

Collocated CoA: An externally obtained local IP address temporarily assigned

to the MN.

Correspondent node (CN): A peer host with which an MN communicates.

Home address: A permanent IP address that is assigned to an MN.

Tunnel: The path taken by an encapsulated data packet. It leads packets from

the HA to the FA.


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Mobile IP uses two IP addresses: the fixed (permanent) home address and the

CoA for the mobility of an MN. The operation of Mobile IP is based on the

cooperation of the three major processes: agent discovery, registration, and

tunneling.

1. Agent discovery: A process by which an MN determines its new

attachment point or IP address as it moves within the wireless/IP network. When

an MN is connected to its home network, it works exactly as a traditional node in

a fixed place. When an MN detects its movement to a foreign network, it obtains

a CoA

by directly reading it from an agent advertisement from its associated FA

or a collocated CoA by contacting Dynamic Host Configuration Protocol (DHCP)

on the local network.

2. Registration: A process performed as an MN enters and remains in a

foreign network. This process involves requesting services for the MN from the

associated FA and informing the associated HA of its new CoA. The MN informs

the HA directly if it obtains a new collocated CoA. Registration consists of an

exchange of two messages, a registration request and a registration reply,

between the MN and its HA. This process enables the HA to associate each new

CoA to the MNs home address. This process is also called binding update.

3. Tunneling: A process by which Mobile IP tunnels data packets, whether it

is away from its home network or not. In the tunneling process, the HA

encapsulates the data packets by using an IP-within-IP approach. In the IP-

within-IP approach, the HA inserts a new IP header, the MNs CoA, in front of the

IP header of a data packet addressed to the MNs home address. When using an


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FA CoA, when an FA receives the encapsulated data packet, it merely has to

eliminate the tunnel header and deliver the rest to the MN. If a collocated CoA is

used, the HA sends the encapsulated data packet to the MN directly, and the MN

does the decapsulation itself.

Figure 10: Illustration of Mobile IP triangle routing

Mobile IP uses triangle routing (i.e., tunnel) as shown in the diagram above. In

triangle routing, data packets sent from the CN (a fixed terminal) to the MN is

sent to the MNs HA first using standard IP routing. The HA encapsulates the

data packets and tunnels the data packets to the MNs CoA. At the associated

FA, the data packets are detunneled and sent to the MN. Although triangle

routing is simple and easy to use, it is inefficient since it takes a route from a CN

to an HA and then to an MN. The overhead of the HA could be a system

performance bottleneck due to large data traffic passing through.

Optimized routing is proposed to solve the performance problem with triangle

routing in Mobile IP. In optimized routing, the MN informs the CN of its CoA that


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has been tunneled to the MN directly without any assistance from the HA. Mobile

IP with optimized routing allows every CN to cache and use binding copies

between the associated HA and the MN.

Mobile WiMAX Network Architecture

Figure 11: Network Reference Model

The network reference model (NRM), identifies key functional entities and

reference points over which the network interoperability specifications are

defined. The WiMAX NRM differentiates between network access providers

(NAPs) and network service providers (NSPs). The NAP is an entity that provides


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WiMAX radio access infrastructure, while the NSP is the entity that provides IP

connectivity and WiMAX services to WiMAX subscribers according to some

negotiated service level agreements (SLAs) with one or more NAPs. The network

architecture allows one NSP to have a relationship with multiple NAPs in one or

different geographical locations. It also enables NAP sharing by multiple NSPs.

The WiMAX NRM, as illustrated above, consists of several logical network

entities: MSs, an access service network (ASN), and a connectivity service

network (CSN), and their interactions through reference points R1

R8. Each MS,

ASN, and CSN represents a logical grouping of functions as described in the

following:

Mobile station (MS): generalized user equipment set providing wireless

connectivity between a single or multiple hosts and the WiMAX network. In this

context the term MS is used more generically to refer to both mobile and fixed

device terminals.

Access service network (ASN): represents a complete set of network

functions required to provide radio access to the MS. These functions include

layer 2 connectivity with the MS according to IEEE 802.16 standards and WiMAX

system profile, transfer of authentication, authorization, and accounting (AAA)

messages to the home NSP (HNSP), preferred NSP discovery and selection,

relay functionality for establishing layer 3 (L3) connectivity with MS (i.e., IP


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address allocation), as well as radio resource management. To enable mobility,

the ASN may also support ASN and CSN anchored mobility, paging and location

management, and ASN-CSN tunnelling.

Connectivity service network (CSN): a set of network functions that provide

IP connectivity services to WiMAX subscriber(s). The CSN may further

comprises network elements such as routers, AAA proxy/ servers, home agent,

and user databases as well as interworking gateways or enhanced broadcast

services and location-based services.

A CSN may be deployed as part of a green field WiMAX NSP or part of an

incumbent WiMAX NSP. The following are some of the key functions of the

CSN:IP address managementAAA proxy or serverQoS policy and admission

control based on user subscription profilesASN-CSN tunnelling support

Subscriber billing and interoperator settlementInter-CSN tunnelling for roaming

CSN-anchored inter-ASN mobilityConnectivity to Internet and managed WiMAX

services such as IP multimedia services (IMS), location-based services, peer-to-

peer services, and broadcast and multicast services Over-the-air activation and

provisioning of WiMAX devices

Base station (BS): a logical network entity that primarily consists of the radio

related functions of an ASN interfacing with an MS over-the-air link according to

MAC and PHY specifications in IEEE 802.16 specifications subject to applicable


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interpretations and parameters defined in the WiMAX Forum system profile. In

this definition each BS is associated with one sector with one frequency

assignment but may incorporate additional implementation-specific functions

such as a DL and UL scheduler.

ASN gateway (ASN-GW): a logical entity that represents an aggregation of

centralized functions related to QoS, security, and mobility management for all

the data connections served by its association with BSs through R6t. The ASN-

GW also hosts functions related to IP layer interactions with the CSN through R3

as well as interactions with otherASNs through R4 in support of mobility.

Typically multiple BSs may be logically associated with an ASN. Also, a BS may

be logically connected to more than one ASN-GW to allow load balancing and

redundancy options. The WiMAX network specification defines a single

decomposed ASN profile (ASN C) with an open R6 interface as well as an

alternative ASN profile B that may be implemented as an integrated or a

decomposed ASN in which R6 is proprietary or not exposed.


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4.0 DESIGN OF THE NETWORK

4.1 Abstract

In this section the knowledge that has been explained in the previous

chapter is applied to a theoretical network of 5 sidings and one Central

Train Control Station to meet the objectives of the project.

4.2 DESIGN METHODOLOGY

This project aims at solving the problem by replacing the traffic light system, with

a robust and efficient Communication based train control (CBTC) system which is

secure from vandalism and can convey the signal in real time.

Wireless technology has been preferred because of the ease associated in

setting it up as well as that it is less prone to vandalism and it is not bulky. It is

also a state of the art wireless technology 4G.

It is in the designers best interest to make the modification to the existing NRZ

communication system as little as possible at the same time maintaining a high

level of relevancy to the problem at hand.

It is therefore sensible to modify the signaling module at the siding by replacing

the hub, CTC cubicle and the relay with a switch, DHCP server and a WiMAX

Base Station. The Backhaul of the network has also had to be replaced with a

Mobile WiMAX one. This has not in any way put a requirement on the


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construction of new infrastructure to house the new technology but the

technology can be comfortably installed on the existing infrastructure

In this modification much effort has been put in making the IP mobility possible,

making the network secure from unauthorized access, Introducing a full coverage

of radius 3 kilometers and enabling downlink and uplink of signals by a train while

in motion.

It is however imperative to note that the mobile WiMAX Standard IEEE802.16m,

which is in its Final stages of refining, will increase radius of coverage to 50Km

thereby decreasing the number of base stations while increasing coverage. In

integrating this technology it is the base station Modules only which will need to

be upgraded keeping the rest of the design unchanged

4.4 THE NETWORK LAYER

The objective is to come up with a network which dynamically assigns IP

addresses to trains and communicates with them in real time.

As stated earlier, there are three important components of the railway

communication system. These are:

1. The locomotive

2. The central train control

3. The siding


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4.4.1 Design requirements list

1. A locomotive must communicate directly with the Central train control at all

times in real time without loss of signal

2. A locomotive must communicate directly with the sidings bound the route

it is travelling on in real time

3. A locomotive must transmit data on the its condition and of the train in real

time to Central train control without fail.

4. The Central Train Control should be able to send in real time broadcasts

and multicasts when necessary.

5. In the event of simultaneous communication to one terminal, both

sessions should be upheld.

6. The communication with Central Train Control should have first preference

in the event of congestion

7. The siding must communicate with CTC in real time all the time.

8. Have a Backhaul which supports the Bandwidth requirements of the

services offered

After coming up with the network requirements, the next stage is the design of

the network in accordance with the network requirements.


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4.4.2 Below is a layout of the proposed signaling system:

Below is the layout of a nodal subnetwork

Signal from microwave backbone

WiMAX base station

Locomotive on rail

Figure 12: Logical Illustration of a Nodal Subnetwork

A nodal *subnet should be allowed 10 host addresses and these are allocated as

follows:

y 6 hosts for rail vehicles i.e. delivery trains and in transit trains.

y 1 host for the *subnet DHCP Server

y 1 host for the sidings

y 2 hosts for maintenance personnel on the rail

The routers in the illustration below represent a backhaul network which has a

Fixed WiMAX (IEEE802.16d) Point to point link. The link in question supports

63Mb/s downlink and 28Mb/s for uplink.

Base Station

Switch

Siding

Loco

1

in

Loco

2

out


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4.5 Simulation

Below is an illustration of the logical network

Figure 13: Packet Tracer 5.3 Illustration of the entire logical networkPlease see the configuration file for the above, in the Appendices


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Conclusion and Discussion

The use of Communication Based Train Control with the Fourth Generation

Radio Access is a very useful advancement. This is so because the high data

rates achievable with this technology enable expansion in the services that can

be taken advantage of by the NRZ to making their rail both safer and more

profitable.

A service such as CCTV can be comfortably incorporated on the network without

requiring further upgrades in the communication system. Some communication

based control systems can also be added on to the network as well thereby

becoming a source of revenue since these other control systems can belong to

other companies.

With an efficient Communication System, More trains can traverse the railway

per given time thereby increase the capacity of the parastatal to generate

revenue.

At the evaluation of this system, it is true to say, it meets the demands of the

problem statement. It provides a solution which is robust, efficient, real time and

safe from vandalism.


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APPENDIX A

Home Agent (CTC router) Configuration

en

config t

hostname Home_Agent

Banner motd # You have reached A restricted resource. NO unauthorized

access!!!#

enable secret nrzrouter1

line con 0

password nrz1

login

exec-timeout 0

exit

int f0/0

desc gateway of Foreign_Network_1

ip address 192.168.0.14 255.255.255.0

no shut

router mobile

exit

ip mobile home agent

router eigrp 1986

network 192.168.0.0 255.255.255.0


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network 10.0.0.0 255.255.255.0

redistribute mobile

ip dhcp excluded-address 192.168.0.17 192.168.0.31

ip dhcp pool FN_POOL_3

default-router 192.168.0.31

exit

exit

copy run start


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References

1. The NRZ Eastern region, signalling department

2. Cisco Broadband Wireless Gateway Release 2.0 for Cisco IOS Release 12.4(24)YG

3. Cisco IOS IP Configuration Guide

4. Broadband For All by WiMAX Technology: Amit Kumar,M.Phil.*;Jyotsna Sengupta,

Ph.D.; and Yunfei Liu, Ph.D.

5. Enhanced Mobility Support for Roaming Users: Extending the IEEE 802.21 Information

Service; Karl Andersson1, Andrea G. Forte2, and Henning Schulzrinne2

6. Air Interface for Fixed Broadband Wireless Access Systems, IEEE STD 802.16 2004,

October, 2004.

7. Air Interface for Fixed and Mobile Broadband Wireless Access Systems, IEEE

P802.16e/D12,

8. February, 2005.9. Hassan Yagoobi, Scalable OFDMA Physical Layer in IEEE 802.16 WirelessMAN, Intel

Technology Journal, Vol 08, August 2004.

10.WiMAX End-to-End Network Systems Architecture - Stage 2: Architecture Tenets,

Reference Model and Reference Points, WiMAX Forum, December, 2005.

11.Mobile WiMAX Part II: A Comparative Analysis, WiMAX Forum, 2006; L.J.Cimini,

Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency

Division Multiplexing, IEEE Trans.Comm., vol.COM-33, no. 7, pp 665-675, June 1985.

12.7 Richard Van Nee and Ramjee Prasad, OFDM for Wireless Multimedia

Communications, Artech House,2000.J. Andrews, A. Ghosh, R.Muhamed,

Fundamentals of WiMAX. Prentice Hall, 1st Edition, 2007.13.802.16e Mobile WiMAX MAC IP CORE www.seasolve.com

Documents

Alfred Project Draft