Railway Signalling Based on Wireless Sensor Networks

Railway Signalling Using Wireless Sensor Networks

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From the Desk of

Sandeep Patalay


Sandeep Patalay

Senior Systems Engineer, CMC Ltd

[email protected]

Abstract Railway Signalling is safety critical domain, where still traditional technology

is in use. There are many reasons for using traditional technology; one of the main

reasons being the proven Safety performance of the older systems (Relay Based). As

the rail traffic is increasing and with higher speed of trains there is an acute need for

modernization of Railway Signalling Technology. Even with the advent of

Microprocessor based technology, the problems have not been solved. This article

proposes the use of Wireless sensor networks in Railway Signaling domain which

combines the Ground base signalling and the On–Board Signalling, which is suitable

for high Speed Railway Traffic. The article gives brief idea of the architectures of a

Sensor Node, Driver node, Gateway Node and Base Station. It discusses the network

Architectures and the Routing algorithms to be used in the sensor networks. It also

discusses the design of Control laws (Interlocking Logic) for safe movement of trains

and also the failsafe techniques to be used in the design of such Technology. The

article also describes the challenges in using the Concept of Wireless Sensor

Networks in Railway Signalling Domain.


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Contents

1. INTRODUCTION.........................................................................................................................3

1.1. ABOUT INDIAN RAILWAYS .....................................................................................................3 1.2. SIGNALLING SYSTEMS............................................................................................................3 1.3. SIGNALLING CONCEPTS..........................................................................................................4

1.3.1. Route Relay Interlocking ..................................................................................................4 1.3.2. Solid State Interlocking (SSI) or Computer Interlocking System (CIS) ............................5

2. EXISTING INTERLOCKING SYSTEMS AND THEIR LIMITATIONS.............................7

2.1. ROUTE RELAY INTERLOCKING (RRI) .....................................................................................7 2.2. COMPUTER BASED INTERLOCKING SYSTEM (SSI) ..................................................................8

2.2.1. Challenges faced by Computer based Interlocking Systems.............................................9

3. PROPOSED ARCHITECTURE OF SIGNALLING SYSTEMS IN RAILWAYS............... 11

3.1. WIRELESS SENSOR NETWORKS ............................................................................................ 11 3.1.1. Sensor Node.................................................................................................................... 11 3.1.2. Gateway Node................................................................................................................. 12 3.1.3. Base Station .................................................................................................................... 12 3.1.4. Driving Node .................................................................................................................. 14

3.2. NETWORK ARCHITECTURE................................................................................................... 14 3.2.1. Routing Algorithms......................................................................................................... 16

3.2.1.1 Flat routing algorithm ......................................................................................................... 16 3.2.1.2 TinyOS beaconing .............................................................................................................. 17 3.2.1.3 Pulse routing algorithm....................................................................................................... 17

4. FAILSAFE TECHNIQUES ....................................................................................................... 19

4.1. FAIL SAFE TECH USED IN THE DESIGN OF CONTROL LAWS (INTERLOCKING LOGIC) ............ 20 4.1.1. Geographical Method..................................................................................................... 20 4.1.2. Boolean Equation Method .............................................................................................. 20

5. CHALLENGES IN USING WIRELESS SENSOR NETWORKS IN RAILWAY

SIGNALLING ...................................................................................................................................... 21

6. FUTURE WORK AND CONCLUSIONS ................................................................................ 23

6.1. GLOSSARY OF TERMS........................................................................................................... 24

List of Figures FIGURE 1: TYPICAL RELAY CIRCUIT .........................................................................................................5 FIGURE 2: A TYPICAL SOLID STATE INTERLOCKING SYSTEM IN A STATION YARD ...................................6 FIGURE 3: TYPICAL RRI INSTALLATION....................................................................................................8 FIGURE 4: TYPICAL SSI INSTALLATION.....................................................................................................9 FIGURE 5: TYPICAL ARCHITECTURE OF A SENSOR NODE ........................................................................ 12 FIGURE 6: TYPICAL ARCHITECTURE OF A BASE STATION........................................................................ 13 FIGURE 7: TYPICAL ARCHITECTURE OF A DRIVING NODE ....................................................................... 14 FIGURE 8: FUTURISTIC MODEL USING SENSOR NETWORKS IN RAILWAY SIGNALLING ............................ 15 FIGURE 9: ROUTING TREES ..................................................................................................................... 16 FIGURE 10: FAILSAFE HARDWARE FOR SENSOR NODE............................................................................ 19


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1. Introduction

The railway signalling domain is a safety critical domain, where safety is

given utmost importance. The railway signalling domain is mostly operated using

traditional technology, which is considered safe and time proven. The New advances

in technology have not been able to solve age old problems of safety and reliability.

Here we give brief of the signalling domain and signalling concepts

1.1. About Indian Railways

Railways traverse through the length and breadth of the country covering 63,140

route kms, comprising broad gauge (45,099 kms), meter gauge (14,776 kms) and

narrow gauge (3,265 kms). As the principal constituent of the nation's transport

system, Indian Railways own a fleet of 2, 16,717 wagons (units), 39,236 coaches and

7,739 number of locomotives and manage to run 14,444 trains daily, including about

8,702 passenger trains. They carry more than a million tonne of freight traffic and

about 14 million passengers covering 6,856 number of stations daily.

Harnessing the potential of these vast and widespread assets to meet the growing

traffic needs of developing economy is no easy task and makes Indian Railways a

complex cybernetic system. Over the years, Railways have built up an elaborate and

well established manual information system to help them monitoring their moving

assets. Supported by a dedicated voice communications network, it collects and

transmits information from the remotest corners of the country to control centres, at

the highest level. The size and complexity of their operations, growing traffic and

changing technologies, placed inevitably a heavy burden on this manual information

system. Need for its modernization was therefore felt for sometime.

1.2. Signalling Systems

The most important part of the railways to carry out operations like safe

movement of trains and communications between different entities is Signalling. The

Railway signalling is governed by a concept called Interlocking. A railway

interlocking system controls the traffic in a railway station, and between adjacent

stations. The control includes train routes, shunting moves and the movements of all

other railway vehicles in accordance with railway rules, regulations and technological

processes required for the operation of the railway station.

The are different types of Interlocking Systems available like cabin Interlocking

System (Mechanical Interlocking), Panel Interlocking System (PI), Route Relay

Interlocking System (RRI) and Solid Sate Interlocking System (SSI) also known as

Computer Interlocking System (CIS). The cabin Interlocking system is obsolete and

the Panel interlocking is slowing becoming obsolete. The Route Relay Interlocking

System is the widely used system. In the present age of Information technology, the

relay based technology is slowly being phased out and replaced with SSIs, but there

are operational issues with Computer based interlocking systems.


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1.3. Signalling Concepts

A station yard consists of Signals, Track Circuits and Points. These elements

are the deciding factors in the safe movement of trains. For Safe movement of trains,

some of the factors such as the track on which the train travels is unoccupied until a

safe distance, no Conflicting Movement with any other train(s) Etc. are considered..

The presence of the train on certain portion of the track is detected by a device called

Track Circuit. The object which gives the information to the train driver is the signal.

The Object used to divert the direction or set the direction of the train is a point. All

these objects such as Signals, track Circuits and points etc form input to a Centralized

system, which monitor the state of these devices and based on the Interlocking rules

and Commands given by the station master decide the safe movement of trains inside

a station yard. So all the elements in the yard are interlocked with one another, thus

the term INTERLOCKING comes in to existence. The Control laws or better known

as Interlocking rules which decide the safe movements of trains have evolved over a

period of 150 years of experience gained in operating trains. These Controls laws are

extremely complex.

1.3.1. Route Relay Interlocking

In Route Relay Interlocking or popularly known as RRI, the Control Rules are

implemented using Relays. These relay based circuits implement all types of Logic

and take inputs from Signals, Points and Track Circuits Etc. in the form of relays. The

Command to set and clear the route for the train is taken in the form of button form

the Station master’s console (Control cum Indication panel). When a command is

given the RRI checks if the command given is safe and takes necessary action, but if

the command given by the station master is invalid and unsafe it does not execute it.

The output of the Interlocking Logic is also a relay, which in turn drive the

signals and Point Machines associated with points. RRI till date is the safest system

implemented, because it implements the proven interlocking rules and also since the

Relays used in RRI are inherently failsafe, they (Contacts) drop to safe state due to

gravity even when power supply is not available or in any kind of malfunction.

The relays circuits are build using the station Control Table as the input

document and the interlocking rules as the Logic. The Control table decides the

possible movements of the train inside a station yard and its relationship with other

stations.

Example of Typical Control law or Interlocking Equation:

ASSIGN ~59EMTEZ * (L60HS * 59NWC + L60AHS * 59RWC + ~59TPS *

R62VS) TO R62VS;


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Implementation of the above equation using relays:

Figure 1: Typical Relay Circuit

1.3.2. Solid State Interlocking (SSI) or Computer Interlocking System (CIS)

An Interlocking System When built using Electronics replacing traditional

Mechanical Levers and Electro mechanical relays is called as Solid state Interlocking

System. The Same Interlocking rules or control equations used in RRI form the basis

here also. The relays used to form the logic circuits in RRI are replaced by software

variables. The field inputs are collected using digital input cards and outputs are

given using digital output cards. The processing is done by a processor where the

virtual relays (Software Variables) are evaluated using the Interlocking equations,

which are now in digitized form either as Algorithms, Boolean equations or state

charts in the processor memory. These algorithms now being executed by the

processing unit take appropriate action.

SSIs are required to replace the existing RRI and PI Systems Since the

traditional systems are very expensive and difficult to maintain because of the huge

number of relays and mechanical levers used. SSIs are a better solution to the older

systems since they are costing only ¼ the cost of RRI or PI and the maintenance cost

is negligible and are easy to maintain.


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Figure 2: A Typical Solid State Interlocking System in a Station Yard


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2. Existing Interlocking systems and their Limitations

Here we discuss the existing systems used for railway traffic control and their

system architectures. We also list out the limitations these systems have in the current

scenario

2.1. Route Relay Interlocking (RRI)

In traditional RRI (Route Relay Interlocking) systems the interlocking logic is

implemented through electromechanical relays. In a typical four road station the

number of relays used to implement this type of logic would in the order of 1000

relays and wiring is so complex that the time taken to install and commission a RRI is

very long. The testing of the system requires the total station to be setup and testing

done during normal train operation. The maintenance of RRI systems is costly and

complex. So the need for a better system which would reduce the number of relays

and maintenance was needed.

A brief list of issues that explain why RRIs are not suitable in the present age of

Information technology

1. The Relays used to build the logic circuits are bulky and take a lot of space

2. The relay wiring is very huge and it may take years to complete an installation

3. The wiring from the field object such as Signals, Points and tracks to the

Relay Room and entire relay wiring is done using copper cables, which is

expensive and it amounts to 50% of the RRI installation cost

4. The testing of RRI is still an informal process that take months to verify and

validate the installation

5. Any change to the station yard such as adding an additional line requires most

or entire RRI wiring to be changed or replaced, which take years to complete

6. Maintenance of the system is very difficult

Due to the above listed reasons, we conclude that RRI is not acceptable to present day

scenarios where traffic needs are growing continuously and the demand for speed of

trains in continuously going up


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Figure 3: Typical RRI Installation

2.2. Computer based Interlocking System (SSI) In SSI system the relays used to implement the interlocking logic in RRI

would be simulated by software variables and only the final Output driving relays are

needed, so the number of relays is reduced to ¼ of the total RRI relays. The

Installation time is also greatly reduced to 1/5 of the RRI installation time and the

testing can be simulated and be done even at the factory. Thus the need for a SSI

System aroused. The Control Laws or the Interlocking equations are modified as

software algorithms and are stored in the embedded system memory. The control

table of the station yard which gives the possible movements of the trains in the yard

is stored as look up tables in software.

Advantages of SSI over RRI:

1. The space taken by SSI system is minimal when compared to RRI

2. Entire logic circuits are simulated in software, no need of Bulky relays

3. Relay wiring cost is saved

4. Installation time comes down drastically to months

5. Verification and Validation of Software is a formal documented process

6. Any change to the Station yard can be quickly addressed by changing the look

up tables and can virtually be done in a matter of hours

7. Maintenance of the system is very easy


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Figure 4: Typical SSI Installation

2.2.1. Challenges faced by Computer based Interlocking Systems

1. The wiring from the field object such as Signals, Points and tracks to the SSI

Rack is still done using Copper cables which amounts to huge costs

2. The hardware reliability and availability factor is low compared to the system

availability given by RRI

3. The fail safe mechanisms employed in processor based equipment is not

standard and often get untested during V&V activities

4. Lack of formal methods in developing the control algorithms (Interlocking

Logic)

5. Lack of domain Knowledge in Signalling and Traditional Route Relay

Interlocking Systems, This creates a technological gap between the software

programmers and the Domain consultants. This leads to Errors in software,

which might lead to unsafe failures of the system

6. Extending the working scope of the Interlocking systems for monitoring and

other non-Interlocking functions, which leads to degraded performance of the

system

7. Employing Non-Formal Interlocking principles instead of traditional RRI

Principles leads to software complexity. For Ex: The Geographical method


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needs every system that is installed for new Yard needs validation, which is

not practicable.

8. Since the software and hardware is so complex, complete test of the system is

not possible and most of the faults are revealed at the field Installation stage or

during normal working of the system in field.

9. The software is to be changed for every yard, the software structure should be

in a generic form, but we seldom see a generic form and at this stage errors

creep in.

10. The lack of standardization in the railway working principles and the core

Interlocking principles, the software developers are forced to do changes in the

software for every yard in Different railway zones.

11. Increase in the complexity of the software leads to difficulty in testing, since

most of the Interlocking systems are sequential machines they are error prone

and are very difficult to test.

12. With Increasing speed of trains, there needs to be a direct communication with

the on board computer of the train (Engine), so that there is less human

involvement and thus less human errors. But Interlocking systems are mostly

not capable of sending commands to the on board computer of the train

(Engine)


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3. Proposed Architecture of Signalling Systems in Railways

As Discussed in the above chapter, the existing systems used for signalling in

railways have limitations in terms of Operations and Technology. These systems have

not used the latest advances in the field of Information Technology. There is need to

upgrade the existing Railway Signalling Infrastructure and addition of new

technologies like fail safe wireless communications which shall combine both the

ground based signalling (Interlocking Systems) and the Locomotives (On Board

Computers of the train), so that the operation speed of the trains can be increased and

thus leading to safe systems with very low accident probability, better utilization of

the track and increased profits to railways. In this chapter we shall propose the

futuristic model of signalling in railways using the most recent advance in the

Wireless Sensor Networks (WSN). We shall also propose a formal approach to be

taken in making Control Algorithms for safe movement of trains

3.1. Wireless Sensor Networks

A wireless sensor network (WSN) is a wireless network consisting of

spatially distributed autonomous devices using sensors to cooperatively monitor

physical or environmental conditions, such as temperature, sound, vibration, pressure,

motion or pollutants, at different locations. The development of wireless sensor

networks was originally motivated by military applications such as battlefield

surveillance. However, wireless sensor networks are now used in many civilian

application areas, including environment and habitat monitoring, healthcare

applications, home automation, and traffic control.

3.1.1. Sensor Node

Each node in a sensor network is typically equipped with a radio transceiver or

other wireless communications device, a small microcontroller, and an energy source,

usually a battery. The envisaged size of a single sensor node can vary from shoebox-

sized nodes down to devices the size of grain of dust, although functioning 'motes' of

genuine microscopic dimensions have yet to be created. The cost of sensor nodes is

similarly variable, ranging from hundreds of dollars to a few cents, depending on the

size of the sensor network and the complexity required of individual sensor nodes.

Size and cost constraints on sensor nodes result in corresponding constraints on

resources such as energy, memory, computational speed and bandwidth.

As per the above definition of a sensor node, sensor node can be used in

railway signalling scenario to detect the presence of train, serving the purpose of track

circuits, to detect the aspect of the signal and its health and detect the position of

points and alsodetect the presence of vehicles at level crossing gates etc. When the

sensors detect the event being monitored (Presence of train, Change of aspect in a

signal, Movement in a point, Movement near a Level Crossing gate etc), the event

needs to be reported to one of the base stations, which can take appropriate action.

Depending on the exact application, different objective functions will require different

data-propagation strategies, depending on things such as need for real-time response,

redundancy of the data (which can be tackled via data aggregation techniques), need

for security, etc.


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Apart form the above discussed points, a sensor node can also be installed in

on board systems like train engine to monitor the different aspects like speed, brake

pressure etc. so that these can be used by the ground based equipment (Interlocking

Systems) to process the data and take appropriate action in case of abnormal

conditions.

Sensor nodes communicate with each other by wireless means, using the IEEE

802.11b wireless technology. The deployed network has typical inter-nodal distances

of 400m, and the furthest reliable communications range we have achieved in the field

is about 600m, therefore nodes at the edges of the network must use nodes between

themselves and the gateway as data relays when communicating with the gateway. To

fulfill this requirement, the nodes form an ad hoc network, where each node aims to

form as many communications links to other nodes in the network, where possible.

Figure 5: Typical Architecture of a Sensor Node

3.1.2. Gateway Node

Gateway nodes are important elements in a sensor network since they provide

the ability to establish long range reach-back communication in order to retrieve

critical data to remote locations. Gateways connect sensor clusters to wired networks.

All communication with the user within the sensor network goes through the gateway

node. The data is then transmitted to the base station where all the processing of the

data takes place

3.1.3. Base Station

The Base station receives the data from all the sensor nodes through the

gateway and processes them. The base station in a railway signalling scenario consists

of wired network connection to the gateway(s). It consists of a processing unit which

is responsible for executing the Control algorithms (Interlocking Rules) and the

station specific data to allow safe movement of trains. As opposed to the traditional

interlocking systems, the base station here can be used to communicate to the onboard

computer of the train and facilitate safe speeds and proper braking distance without

manual intervention. This base station can also be use to know the geographical

location of trains and send to it to the central office where all traffic can be monitored

efficiently. The base station logs all the events in its memory eliminating the need for


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an external data logger as in the case of traditional interlocking systems. Since the

Base station is connected to Internet, any authorized person can monitor the station

yard sitting in a remote location.

In case of Remote train control or driverless trains, the Information from

sensor nodes must participate in algorithms which result in commands to traction

motors, brakes, and doors Etc. One of the safety-critical applications for sensors in

railroading is separation management. Current train control systems -- freight or

passenger, manual or automatic -- rely on 'block signalling,' by which separation is

assured under the online influence of a centralized authority arranged to allow only

one train at a time to occupy a given block (a segment of track of fixed length).

Information about the location of trains is crudely quantized based on train length and

the size of each block.

All of the requisite sensors, signalling, and processing devices are derived

from proven failsafe technologies, as are the accompanying software methodologies.

Sensor nodes, some with built-in intelligence, play an important role in the

architecture of autonomic train separation. Furthermore, there are daunting safety

requirements that characterize railroading applications. Railway vehicles with their

traditional wiring harnesses suffer limitations in supporting vital control functions.

Figure 6: Typical Architecture of a Base Station

Central

Processing Unit

Communication

Unit for Gateway

Nodes

Data Logging Unit

Communication

Unit for Internet

Operator’s

Console unit

Gateway

Node

Internet


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3.1.4. Driving Node

A driving node is similar to a Sensor node, the only difference is that it drives

the objects in the station yard and the on-board systems, based on the commands

received from the base station through gateway node. The driving node consists of a

radio transceiver or other wireless communications device, a small microcontroller,

and an energy source, usually a battery and driving channels that generate signals to

drive the field objects. The driving node is used to drive the DC motors of point,

Glow a Signal Aspect and also operate Level Crossing gates.

Figure 7: Typical Architecture of a Driving Node

3.2. Network Architecture

Availability of data is the most critical part of the Railway Signalling system.

Therefore a reliable architecture of Sensor Network shall be used. Mesh networking

is a way to route data between nodes. It allows for continuous connections and

reconfiguration around broken or blocked paths by “hopping” from node to node until

the destination is reached. A mesh network whose nodes are all connected to each

other is a fully connected network. Mesh networks differ from other networks in that

the component parts can all connect to each other via multiple hops, and they

generally are not mobile. Mesh networks can be seen as one type of ad hoc network.

Mesh networks are self-healing: the network can still operate even when a node

breaks down or a connection goes bad. As a result, a very reliable network is formed.

This concept is applicable to wireless networks, wired networks, and software

interaction.

Wireless mesh networks is the most topical application of mesh architectures.

Wireless mesh was originally developed for military applications, but have undergone

significant evolution in the past decade. Wireless mesh networking has seen three

distinct radio configurations of mesh technology, each incorporating iterative

improvements allowing for greater reliability and versatility. As the cost of radios

plummeted, single radio products evolved to support more radios per mesh node with

Microcontroller

Po

wer so

urce

Transceiver

External

Memory

Ou

tpu

t Ch

an

nels (O

utp

ut

Ca

rd)


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the additional radios providing specific functions- such as client access, backhaul

service or scanning radios for high speed handover in mobility applications. The mesh

node design also became more modular - one box could support multiple radio cards -

each operating at a different frequency. As a result, a whole new set of applications

are being enabled by third generation mesh networking technology. These include real

time video surveillance, border security or voice communication inside underground

mines.

Figure 8: Futuristic Model using Sensor Networks in Railway Signalling

In Figure 8, it can be seen that how wireless sensor networks are used to

perform railways signalling. The Sensor nodes are used to detect the presence of train,

aspect of the signals, Position of points, speed of the train etc. The sensor network

normally constitutes a Wireless ad-hoc network, meaning that it each sensor supports

Sensor Node Sensor Node Sensor Node

Sensor Node Sensor Node

Gateway

Node

Base

Station

Output Channels

Driving Node

Train Engine

Sensor Node

1T 2T

S1 S2

S3

S4

3T

P1

Driving Node


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a multi-hop routing algorithm (several nodes may forward data packets to the base

station).

3.2.1. Routing Algorithms

A number of routing protocols for sensor networks have been proposed in the

literature over the last few years. Many of the protocols draw inspiration from similar

protocols for wireless ad-hoc networks. Since the challenges for sensor networks are

different from those of ad-hoc networks, several interesting variations are introduced.

In addition, many novel routing mechanisms have been proposed specially for sensor

networks. The following lists some of the sensor network routing algorithms suitable

for Railway Signalling Scenario.

3.2.1.1 Flat routing algorithm

Flat routing protocols are similar to the conventional multihop ad-hoc routing

protocols. Each sensor node determines its parent node(s) to forward data packets.

The nodes are not organized into hierarchical clusters as is done in the hierarchical

protocols. The advantage of this approach is that all the nodes can reach the base

station irrespective of their position.

The most common way of routing in a sensor networks is routing trees (multi

hop routing). A routing tree is a collection of sensor nodes with the base station as the

root of the tree. Sensor ‘A’ is the parent for sensors ‘B’ and ‘C’. Sensor nodes

transmit all there results to there parent nodes only. It is the responsibility of the

parent node for forwarding them to the base station. A child can keep track of several

parent nodes, and depending on the power levels or the quality of the communication

links a child node can change its parent node.

Figure 9: Routing Trees


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Routing structures such as routing trees is well suited when there are only a

few number of nodes in the network. A data gathering schedule is a way the data

packets are collected from all the sensors and routed to the base station with

maximum lifetime. The main assumption of this algorithm is that the location of the

sensors, base station and energy values of the sensor nodes are known priori. In this

model the lifetime of the system is intrinsically connected to the data gathering

schedule. During each round a sensor will collect its own, neighbor’s data and

possibly aggregate it and send it to the base station.

3.2.1.2 TinyOS beaconing

The TinyOS embedded sensor network platform employs a very simple ad-hoc

routing protocol. The base station periodically broadcasts a route update beacon

message to the network. The beacon message is received by a few nodes that are in

the vicinity of the base station. These nodes mark the base station as their parent and

rebroadcast the beacon to their neighbours. The algorithm proceeds recursively with

nodes progressively propagating the beacon to their neighbours; each node marks the

first node that it hears from as its parent. The beacon is thus flooded throughout the

network, setting up a breadth-first spanning tree rooted at the base station. This

process is repeated at periodic intervals known as epochs.

Each network node periodically reads its sensor data and transmits the data

packet to its parent in the spanning tree. The parent node in turn forwards the packet

to its parent and soon. This process is repeated until the data finally reaches the base

station. The attractive feature of TinyOS beaconing is its simplicity–nodes do not

have to maintain large routing tables or other complicated data structures. Each node

needs to remember only its parent node in the path to the base station. By combining

the beaconing with a MAC layer scheduling scheme such as TDMA, the nodes can

conserve power by keeping their radio off most of the time. In spite of its attractive

features, the beaconing protocol suffers from one main disadvantage: it is not resilient

to node failures. If a parent node fails, then its entire subtree is cut off from the base

station during the current epoch. Moreover, the protocol results in uneven power

consumption across network nodes. The nodes nearer to the base station consume a

lot of power in forwarding packets from all the nodes in their subtree, whereas the leaf

nodes in the spanning tree do not have to perform any forwarding at all and consume

the least power.

3.2.1.3 Pulse routing algorithm

The Pulse protocol addresses the three topics of routing, energy consumption

and time synchronization in sensor networks. It uses a periodic pulse signal generated

and flooded by a pulse source to provide routing paths and synchronization to the

network. As the pulse propagates through the network nodes, a spanning tree rooted at

the pulse source is constructed. Node traffic follows the paths along this spanning

tree. A node that wants to communicate packets sends a reservation packet to the

pulse source. The reservation packet contains the address of the node sending the

packet and is used to set up reverse routes for data packets. Thus, active nodes need to

keep sending reservation packets in response to the periodic pulse signals to keep the

routes fresh. Idle nodes that do not have data to communicate and that are not needed

for forwarding packets can switch off their radios till the next pulse signal arrives and

thereby save energy. To further reduce energy consumption, the Pulse protocol is


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modified to incorporate intermediate wake-up periods. The motivation behind this

modification is that the routes in the network are established by the flooding of the

pulse signal, which is an expensive process. Instead, nodes are permitted to send

reservation packets during intermediate wake-up periods which can occur several

times between two pulse floods. This enables faster path activations with lesser

energy expenditure.

The Pulse protocol is similar to the beaconing protocol if the pulse source is

considered to be the base station. Thus it has similar merits and demerits as the

beaconing protocol. One area of improvement in the Pulse protocol is to provide a

path deactivation feature. This feature would allow nodes to deactivate paths and

conserve energy even if the intervals between wake-up periods are arbitrarily long.

This would of course trade off the fast path activation for power efficiency.


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4. Failsafe Techniques

Railway Signalling is a safety critical domain and all the equipment used here

shall be Fail Safe. The Existing failsafe techniques used in the design of hardware is

listed in Error! Reference source not found.. The table gives the advantages and

disadvantages of each type of technique. In this section we propose a new safety

technique to be used in the design of hardware applicable for Sensor node, Driver

Node and Base station.

Figure 10: Failsafe Hardware for Sensor Node

In the above figure (Figure 10), two processors are used to process the inputs

from the sensors, the two processors are of the same configuration, but have different

softwares implemented on each one. The two softwares are written in such a manner

that both use different algorithms to process the data. The Software Voter takes the

output generated by two softwares and compares it. The final level of checking is

Sensor

Inputs

Software 1

Software 2

Processor 1

Software 1

Software 2

Processor 2

V

O

T

E

R

So

ftware

Vo

ter

So

ftware

Vo

ter

System

Outputs Identical Hardware

Identical Hardware

Supervisor and

diagnostic Module

Sensor

Inputs


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done by Hardware Comparator called Voter to compare the results given by two

processing units. If the results are matching the data is transmitted to the base station

other wise appropriate action is taken to make the system Fail Safe. Above all this the

Supervisor and the diagnostic module monitors the performance of the two processing

units, such as Voltage, Error checking Etc. The same architecture can be used in the

design of a Base Station. The above architecture is unique since there are two voters

ensuring safety i.e. First level checking is done by a Software Voter and the final level

checking is done by a Hardware Voter. By adopting the above described architecture,

both failsafe operation and high reliability is ensured.

4.1. Fail Safe Tech used in the design of Control Laws (Interlocking Logic)

As Discussed in earlier sections the Railway Signalling Control Laws or

Interlocking Rules form the basis for Safe movement of trains. There are different

methods by which these Interlocking rules can be implemented in software. Here we

discuss the existing methods for design of Interlocking Rules in Software.

4.1.1. Geographical Method

In the Geographical method the input to the Interlocking systems is given as

the position of the signals, points, tracks Circuits and Slots. The Interlocking is

implemented based on the generic rules such as no part of the track are shared by the

two routes at a time, Conflicting routes should not be set at a time etc. This type of

implementation requires a great knowledge of the Yard Elements and the

interconnection between them. In this method the software does not have one to one

relation ship to the relay circuits used for RRI and is very difficult validate, so this

method has failed to create the necessary confidence in the railway operators

4.1.2. Boolean Equation Method

The Boolean equation method is the implementation of the traditional relay

interlocking principles. In this method the relay circuits are implemented as Boolean

equations, so there is one to one relation ship between the relay circuits and the

software variables. Since there is a one to one relation ship between the software and

the RRI Relay circuits, Railway operator can easily validate the software entrees

made and this method gives him sufficient confidence. This method theoretically has

very high safety performance, since the Control laws once written remains constant

and only the station data is changed for every yard, but the actual implementation of

this method has limitations like the control laws are not fully tested and they are not

generic, Boolean equations are written for every yard Etc. Typically these boolean

equation are in huge numbers and very difficult to verify these equations.

As Discussed above both the methods have limitations in practical

applications, so a new method in which the control laws are generic and applicable for

all the yards is yet to be designed. In Railway Signalling using sensor networks

scenario these Control laws need modification and with merging of Ground based

signalling and ON-Board Signalling (On Train), a new set of Control laws have to be

proposed which ensure the safe movement of trains.


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5. Challenges in using Wireless Sensor Networks in Railway

Signalling

The use of Wireless Sensor Networks in a safety critical Domain like Railways

signalling poses challenges in implementation and Operation. Some of the issues and

challenges are discussed in this chapter.

1. Sensor network communications must prevent disclosure and undetected

modification of exchanged messages. Due to the fact that individual sensor

nodes are anonymous and that communication among sensors is via wireless

links, sensor networks are highly vulnerable to security attacks.

2. The gateway nodes are prone to failures just like any sensor node, and they

consume significantly more energy since they transmit over longer distances

compared with sensor-to-sensor links. Failure of a Gateway node results to

catastrophic results because, there not information regarding the yard status to

the base station

3. Sensor nodes have limited computing power and memory sizes. This restricts

the amount of intermediate result a node can hold, also the type of data

processing algorithm on a Sensor node.

4. Signals detected at physical sensors might have errors. Malfunction sensors

might repeatedly generate false signals, also there could be bias caused by the

placement of the sensor.

5. Sensor Nodes, Driver Node and Gate Way node have to work in High EMI

Environment. Since sensor networks can be deployed in different situations,

wireless medium can be greatly affected by noisy environments, and thus the

signal attenuates in regard to the noise. Note that an adversary can

intentionally interfere and cause enough noise to affect the communication. It

is vital to ensure that communication is on time to respond to emergencies.

6. Wireless sensor networks at times may add delay in sending data to the base

station due to the routing algorithms, etc, but Railway Signalling is very time

critical job, any delay in receiving the data leads to Catastrophic results.

7. If a sensor node fails due to a technical problem or consumption of its battery,

the rest of the network must continue its operation without a problem.

Researchers must design adaptable protocols so that new links are established

in case of node failure or link congestion. Furthermore, appropriate

mechanisms should be designed to update topology information immediately

after the environment changes so as to minimize unnecessary power

consumption.

8. The network should be scalable and flexible to the enlargement of the

network’s size. The communication protocols must be designed in such a way

that deploying more nodes in the network does not affect routing and

clustering. Rather, the protocols must be adapted to the new topology and


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behave as expected. In other words, the network must preserve its stability.

Furthermore, introducing more nodes into the network means that additional

communication messages will be exchanged, so that these nodes are integrated

into the existing network. This must be done in a way that a minimum number

of messages need to be exchanged among the sensor nodes, and thus battery is

not wasted unreasonably.

9. As in Wireless Sensor Networks Both Ground based signalling (Way Side

Signalling) and On-Board Signalling (Cab Signalling) get merged, so there is

the complexity of linking the ground based control laws to the inputs received

from the On-Board Sensors in the train

10. Design and development of failsafe, fault tolerant and energy saving network

routing algorithms is a complex design


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6. Future Work and Conclusions

The use of Wireless Sensor Networks in railway signalling domain was

proposed in the earlier chapters. The work ahead is of proving the concept and

making in it to reliable technology that can be implemented. The proof of concept will

require design and development of Fail Safe and Fault tolerant Sensor Node, Driver

Node, Gateway Node and the Base station and the design of network architecture that

combines both ground based and On-Board Signalling. As far as the Software in

considered, design of network routing algorithm which is Fail Safe and Fault Tolerant

and at the same time energy efficient and the design of fail safe Control laws for safe

movement of trains.

In this article I covered the major advantages of using Wireless Sensor

Networks in Railway Signalling domain. The hostile and remote environment at

which sensor nodes are often deployed and the limited computational and energy

power along the limited storage are the factors that drive the adoption of security

solutions. Since the limited resources affect the types of security algorithms and

protocols that can be implemented in a WSN.I also discussed some challenging

directions that need special attention. Focus should be placed on designing protocols

that are scalable, flexible, fault tolerant and adaptable to dynamic changes. However,

the main challenge for researchers is to balance the trade off between resources spent

for security and the protection offered. The target is to have a spherical security

strategy with solutions that compensate each others vulnerabilities, and provide an

enhanced protection to railway signalling Network.

References

1. “An Adaptive and Fault-Tolerant Gateway Assignment in Sensor Networks”, William W. Su,

Boeing Integrated Defense Systems, Anaheim, CA 92806

2. “Cooperation and Routing in Multi-Hop Networks”, Elzbieta Beres and Raviraj Adve, Dept.

of Elec. and Comp. Eng. University of Toronto10 King’s College Road, Toronto

3. “Wireless Sensor Networks and Applications”, Dagstuhl Seminar, Alois Ferscha1, Stephan

Olariu and Tom Pfeifer, Univ. Linz, AU

4. M. Verma and V. Chandra/The Design and Development of a Fail-Safe Interlocking System

Using Microprocessors for Indian Railways," Proc. Region Ten IEEE Conf., IEEE Press, New

York, 1989, pp.511-514.

5. “The Current Status of Signal Control Systems, and Research and Development”, Yoshinori

Kon , Advanced Railway System Development Center, Research and Development Center of

JR East Group

6. “Microprocessor-Based Railway Interlocking Control with Low Accident Probability”, V.

PURNACHANDRA RAO AND P. A. VENKATACHALAM, IEEE TRANSACTIONS OS

VEHICULAR TECHNOLOGY. VOL. VT-35. NO. 3. AUGUST 1987

7. http://www.irfca.org/faq/faq-signal4.html#general


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6.1. Glossary of Terms

WSN: Wireless Sensor Network, or WSN, is a network of RF transceivers, sensors,

machine controllers, microcontrollers, and user interface devices with at least two

nodes communicating by means of wireless transmissions.

Hard Real Time System or Mission critical System: A real-time computer system

must react to inputs from controlled object and from the operator. The instant at

which a result must be produced is called a deadline. If by missing a firm deadline a

catastrophe could happen, then the deadline is called hard. A real-time computer

system that must meet at least one hard deadline is called a hard real-time computer

system or a safety-critical real-time computer system.

Railway Interlocking System: A railway interlocking system controls the traffic in a

railway station, and between adjacent stations. The control includes train routes,

shunting moves and the movements of all other railway vehicles in accordance with

railway rules, regulations and technological processes required for the operation of the

railway station.

Interlocking Logic: A term used for the logical relationships between physical

entities in the railway yard such as points, signals, track circuits, and so on. In SSI,

this is programmed in the Software; in relay-based interlocking this is hardwired into

the relay circuitry, and in ground-frame interlocking it is manifest in the mechanical

linkages between physical components.

Mechanical interlocking System: An Interlocking System When built using

mechanical linkages between Levers (Physical Entities) is called Ground-frame

interlocking System.

Panel Interlocking System: A system similar to RRI, but multiple commands are

needed to set and Lock a route for safe movement of trains

Route Relay Interlocking System (RRI): An Interlocking System When built

completely using Electro mechanical relays is called as Route Relay Interlocking

System.

Solid State Interlocking System (SSI): An Interlocking System When built using

Electronics replacing traditional Mechanical Levers and Electro mechanical relays is

called as Solid State Interlocking System.

Reliability: The reliability can be defined as the ability of an item to perform a

required function under stated conditions for a stated period of time.

Redundancy: The existence of more than one means of accomplishing a given

function. Each means of accomplishing the function need not be necessarily identical.


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Hardware (Software Diversity): Two or more different Versions of Hardware

(Software) working in a system to achieve a same result.

Failure: The termination of the ability of an item to perform a required function.

Maintainability: The ability of an item, under stated conditions of use, to be retained

in, or restore to, a state in which it can perform its required function, when

maintenance is performed under stated conditions and using prescribed procedure and

resources.

Availability: The ability of an item (Under combined aspects of its reliability,

maintainability, and maintenance support) to perform its required function over a

stated period of time.

Wayside Signalling: Ground Based Interlocking Systems used to drive the objects

located in the station yard

TinyOS: TinyOS is an open-source operating system designed for wireless embedded

sensor networks. It features a component-based architecture which enables rapid

innovation and implementation while minimizing code size as required by the severe

memory constraints inherent in sensor networks.

Railway Signalling Based on Wireless Sensor Networks

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