Abstract:
Aircrafts throughout their journeys need to be in contact with air traffic controllers. Long haul ocean crossing flights are increasing steadily every year due to growing demands. The increasing air traffic over the ocean is a serious concern for the controllers where the aircrafts are beyond the eye of the radar and ground stations that track their motion in air. The present communication between the cockpit and the controller is a high frequency radio communication where the position of the aircraft is sent to the air traffic controller through a third party radio operator who acts as an intermediary between them. Relatively the half duplex nature of the system and the use of a single frequency channel add complexity to the problem.
Though the modern communications is going through an era where graphical interaction systems are being developed, the communication in an oceanic haul is still voice communication.
This project focuses to improve and analyze the performance of data communications in exchanging information between the pilot and the controller in oceanic haul long routes. The controller pilot data link communications is taken for analysis in its performance in oceanic routes.
Introduction:
Aviation has seen rapid developments. Modern airliners are now heavily computerized monsters. They now fly higher and faster. The role of an air traffic control is massive in the field of aviation. Since they provide information about other planes approaching nearer to them and maintain spacing between planes. Pilots usually communicate with the controllers using radio aids that allow them to use normal voice communications, which is the very existing problem in the industry of aviation.

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Demand for air traffic has been increasing steadily in recent times. Increasing flights to the United States of America have led to the increase in air traffic over the oceans which have also intensified the job of the air controllers. With Nearly hundreds of carriers crossing the Atlantic every day, the responsibility of the air traffic control are huge, because they are the only point of reference to a pilot for navigating and knowing information about the traffic prevailing around him. The Traditional HF voice communication system has drawbacks which limits the points of communication between a pilot and an air traffic controller. Call sign confusion, interference and many more factors are hurdles to a good ground to air communications.
Effective communication management is primary concern in developing co-operative multi user interaction systems. Each individual agent must know what, when and how to communicate. Effective Utilization of shared communication resources also should also be taken in to account, when there is limited resource available. This needs more attention when the demand exceeds the capacity of the system, and a priority is required
This project focuses on how to improve communication between them, taking in to account various parameters that affect the transmission in a high frequency radio communication. Apart from, the attempt to replace traditional voice communication, to a text based communication using CPDLC (Controller Pilot Data Link Communication) in an oceanic haul is also developed,that would be a base to the future aviation researches .
Background:
The main idea is to achieve data link communication in a oceanic haul where there are no radar coverages and grond station contacts.This research was carried out in past ,over few years back.The CPDLC is a continuously on-going research project.Some of the old papers are still being researches to achieve text based information exchanges through datalink commnications.However due to the advent of growth in technology each and every day,advances in research also take a new shape than and there. This has motivated this project to evaluate the performance issues of applications in the aeronautical telecommunication networks ,which is the backbone network in aviation .
Aims and Objectives :
This thesis aims to :
To develop a robust and reliable communication model using adhoc systems
Research the various performance issues in using the ATN application CPDLC in long haul routes.
The main objectives of this project is to
Reduce the work load of the air traffic controllers by developing text data communications using adhoc networks
Reduce workload for pilots
Prevent congestion of air traffic
Preventing air accidents due to misinterpretation of communications from the controller
Research Methodology:
The background for this thesis includes a lot of introduction and information about how ground to air communications take place.In order to review the present standards of communication,to analyze their nature and also to understand its properties ,a very intense search has been made and during this phase ,references to various technical publications ,articles expert opinions were read and references from various international organizations like ICAO and Eurocontrol were taken up to write this thesis.After careful analysis, performance analyziz was made and implementing text based communication was studied.The next part of this project will discuss all necessary things needed to understand this thesis.
Literature survey :
The History of communications in aviation is vast .We would be discussing only the current developments in modern technology and reviewing some of the applications developed in relations to this thesis.
The standards in Aviation are monitored by certain governing bodies that are responsible for the safety and security of the air transport.The International Civil Aviation Organisation (ICAO) is the major governing body and specialized agengy of the United Nations for standard and recommended procedures in civil aviation.
Aeronautical telecommunication Network:
ICAO having endorsed a proposal for the future communications in aviation that uses digital datalinks to supplement voice communication to provide a good and reliable communication,has led to the development of Aeronautical telecommunication network.It is an international infrastructure that manages digital data transfer between the aircraft and the air traffic control.In fact it is a dedicated network for aviation to provide seamless ground to air as well as ground to ground communication for the aircrafts.It is designed to support the increasing air traffic due to increasing demands.
Development Of ATN:
Air traffic management needed enhancements due to its rapid increase in traffic .as a result a single controller cannot handle such a large amount of traffic .Hence the concept of distributed traffic handling were followed by the controllers to handle the rising amount of traffic.Efforts were also taken to automate traffic handling.When critical analysis were made regarding the automation of traffic,it became apparent that air traffic automation requires more exchange of computer to computer data interchange that included transfers between aircraft and ground computers.Apart from that automation also required infrastructures that need to be integrated with aircraft and ground stations.The real automation of this air traffic could be possible only data computers rather than being independent processors were also a networking peers with their respective ground stations.These operational objectives when combined together led to a dedicated forming of a complete network exclusively for the field of aviation,that came to be known as the Aeronautical telecommunications network.
Data communications have been deployed both(air to ground and ground to ground ) for many years not only for air traffic management but also for administrative and ground operations.Besides the aeronautical telecommunications support a wide environment where tcket sales,crew duty rotas,weather forecasts are all done.Certain standards and protocols are used. The international air transport associationhave developed a set of OSI profiles called the aeronautical OSI profile,that can be used over the ATN.
Benefits of ATN over the traditional networks:
The ATN has scored over the traditional voice communications with a number of advantages that would make air travel more safe and secure.the following are some of the advantages over which
Are described below:

ATN offers more clarity in communications.As a result retransmission rate is reduced much and interpretation errors are minimized in a long distance communication
The Communication channel is made use more effieciently to transmit data between the computers rather than usage of channel in voice communication.
It gives increased possibility to connect two end users without knowing their locations in a global data communication network environment.
It minimizes work load for the pilots and controllers ,since a number of preformatted and short text messages are available .These standard message are very well known to the pilots.These standards are globally fixed and they mean the same everywhere in world.Hence communication becomes easy ,eliminating misinterpretation errors completey.
The multitude for accommodation of various ground systems like ATSC,AOC,AAC,APC are easily done using ATN.
ATN provides a unique solution to satisfy a wide range of data communications therby providing much ease and effiecient useage of the available resources.
It provides a robust an reliable service to achieve higher availability and integrity that should be need for an improved operational capability.
It provides for timely exchange of ATS information between ground facility improving security and productivity of the entire system

The above reasons justify that the ATN has unique features and its applications are widely important in both ground to air as well us ground to ground communications
The ATN environment:
Operating in a global environment is always a challenge for a network .The ATN is designed to operate in extreme environment considering rough weather and other circumstances.Apart from lot of constraints,the ATN has been designed to operate under four major situations .Each one is termed an element.The first element is to capability of the network to transfer data to an aircraft without the sender knowing where actually the aircraft is, in other words hidden identity.This network mobility is essentially the powerful feature of the Aeronautical telecommncation network.The next element is to use the available multiple links installed in aircraft simultaneously.In this case the specification of the cost,link and speed preferences are advised b the applications that ATN uses when forwarding the data to an air borne system.It is these preferences that determine which link to be chosen to send and receive data over the other available links.The third element is to accommodate all available low bandwidth links both grond and airlinks in this case.These links are normally used to compress the data to be sent.The last element is the standardization of the applications and all services used by the ATN so that they remain the same throughout the world and do not cause further confusion in supporting applications.
The ATN Mobility:
An important feature in network is that all the other end users must know the availability of the datalinks so that congestion can be reduced.Hence it is necessary in a network to let to know the end users about the links and the multiple paths that reach all the aircraft airborne.Broadcasts in a network can lead to a serious problem of flooding. Hence inorder to satisfy network mobility the ATN designers have stipulated a concept called the backbone routing technology.This effiecient technology restricts the broadcasting of information to avoid flooding but at the same time allows the ground system to send information about the availability of the links and other information to the nearest aircraft that are with ints reach. This simple figure would easily demonstrate the backbone routing implementation.
The router r0 is the aircraft router that is airborne.Now when it is necessary to communicate with the router R7 .the ground router has to update all the backbone routers that the aircraft is aavailable for contact.It is also noted that only the routers in home domain are updated and all the others remain unnoticed.Hence considering this situation R1 R2 R 3 are updated and the others are left unnoticed .. Hence routers not in home domain do not know what is happening. This helps to avoid flooding .
The squares in the figure represent applications used by the ATN. The CPDLC application of the ATN is discussed in the later chapters as how messages are transferred.Here application H1 when needs to communicate with aircraft airborne,has the router R8 in the home.R 8 does not know any information about the aircraft,so it forwards to the nearsest backbone router R2.R2 knows the information about the aircraft hence it sends the data to R7 through R1. Thus data traverses.In similar case, application H2 needing to communicate sends the data to R16,From there the backbone router R5 takes over.But R5 does not have any information about the aircraft hence instead of forwarding the packet to R5 it sends to R4 and then the same process takes place. Both the aircraft and the ground routers at the ground station communicate using a protocol called inter domain routing protocol which forms the basis of this inter domain routing.This is the protocol that redistributes data over the entire network without causing flooding and thereby enabling all the communication .
Now there is yet another to note in this backbone routing. Preference based routing is necessary to allow multiple paths in a network to be available simultaneously.Some applications may require quality of service for cost and safety for performance isues.In such cases it is required to route over the other available routes.here in this considered case,aircraft router has both VHF and satellite conection .In case if the VHF is not available then a satellite connection is used to transfer data . accordingly the aircraft router R9 will take its turn to send information through the router R4 which is in ground .all entries are stored in packets by the routers they pass through.By this the packets can be examined by the other router for the shortest path to be taken to the destined aircraft it should be sent.Thus the mobility of the ATN works,adding more reliability to the communication system .
ACCOMODATION OF LOW BANDWIDTH :
As specifeied earlier the design of ATN is designated to accommodate the low bandwidth froundlinks to the network.Accordingly,the throughput for the groundlnks to operate in ATN is expected to operate in a low bandwidth than the available system today.Hence a compression methodology has to be used for thi purpose.The present VDL2 comunication used in aeronautical communication a channel that can transfer 32 k bits / second is shared by multiple aircraft using the carrier sense multiple access method.It is a notable feature that ATN has let also multiple compressions to occur in its routers .The rate of compression is not a fixed one.It is determined when an aircraft enters the region of an ATN router .An open system interconnection network connectionless protocol is used for this kind of purposes.It reduces hundred bytes of header information to six btes.Another technique used to compress the header is the deflate technique.A deflate algorithm uses the similar kind of mechanisms used by the ZIP used in compressing text files.It compresses both the header and the data units available to the aircraft .this way by compressing the data units,low bandwidth channels are made use by the ATN effieciently.
APPLICATIONS OF THE ATN:
The standardization of ATN by the international civl aviation organisatio has led to the development of four applications.These four applications are of more significance in the aviation industry since they help in navigation of aircrafts when air borne .These applications are

Context Management
Automatic dependent surveillance system
Controler pilot data link communications
Digital-Inflight information system

The digital inflight information system is used to get updates from the ground sation required for the flight such as weather udates and other traffic conditions in terminal area ehich is essential during landing
The controller pilot data communications is a essential development and this thesis would deal mainly with this application that exactly replaces the voice communications with the data messages to the extent it could do .
The automatic dependent surveillance system is highky essential for the transmission of digital information regarding the position of the aircraft to the radar at the ground station so that automated networks and systems can maintain adequate separation of aircrafts in air.The separation of aircrafts depending up on their size and type is an important consequence in the field of Aviation where it constitutes to the safety of the planes going at higher altitudes .
Context management is another directional service,where specific features could be provided to the aircraft on logging in to that particular air traffic system,associated with the network address of the server at the ground station.Thus aircraft being guided till the terminal control are where the approach controller would take care after with the secondary radars being used for assistance .
These applications would surely contribute to the effective communication between the controllers and the pilots who are air borne.
The functionality of The Aeronautical Telecommunication Network :
The functionality of an aeronautical network seems to be little when compared to that of voice transmission networks,but it offers more robust connectivity and more integrity in communication between two end systems that are either fixed or mobile or even adhoc,taking in to consideration for transition paths and end to end delays.
ATN Components:
Notably ATN supports many Full dupliex communication system. These are some of the supportable forms of the communications

Airline systems and ATS systems
Airline and aircraft systems
ATS and aircraft systems
ATS systems interconnected
Airline syatems inter connected together

Airline systems refer to the maintainence syatems that the airlines deploy such as dispatch deliveries and others whereas aircraft systems refer to the systems actually in aircraft that help to navigate the syatems
The above figure shows the typical environment of an ATN
Subnetworks,Routers and the Endsyatems are the major components that constitute to the building of an ATN.
Subnetworks:
Subnetworks which form a part of the comuication network. A subnetwok forms the major transferring medium between the sytems in the ATN.They are infact a major component of ATN .Usually packet switching netwrks are the subnetworks used in ATN. A lot of grond as well as air borne systems are equipped with capability of supporting this features in ATN.
Subnetworks can be of two types.

Ground to Ground (Fixed)
Air to Ground (Mobile)

Ground to Ground subnetworks are usually local are networks connecte with in subsytems .It can be the local airport network connected with the ATC to share weather and other advisory informations where there is no controllers employed for certain routes.
Local area networks employed with token ring ,Ethernet FDDIare nowadays used as ground subnetworks
Air to Ground networks are usually wide are networks .They are used when the air craft is air borne and hence communicates with the ATC controllers.Based up on the type of Network the ATN routers adapts itself to transfer the data packets between the adjacent subnetworks.
The various air to ground subnetworks include

Very High Frequency Subnetwork
Satellite communication
Mode S subnetwork
HF subnetworks

The HF and MODE S subnetworks are mostly used in short range communications.Mostly short haul flights that travel in the European continents are equipped with instruments that make use of the Mode S Subnetworks.Both the communications and the automatic dependent surveillance for spacing can be achieved with Mode S which is relatively cheap when compared others.
HF Networks are much more similar networks but are used in relatively average haul flights which are in continuous contact with grund stations throughout their journeys with out any oceanic travel.
Satellite communication and VHF networks are the physical transfer mediums used in Long haul oceanic flights .RElativekly costlier for communication but still used in modern days.
Routers:
The ATN routers play a major role in routing the packets to different subnetworks thus maintaining the integrity of the network.Since the air crafts change their course during their course of flight,the path taken in a network to reach that aircraft must be determined accordingly.It is the routers that decide the traversing of packets at any stage in network.The routing is performed on the basis of class of service the packets request and the availability of the links in the network.
Dynamic routing is also supported by ATN ,allowing each router to update ,due to the changing courses taken overby flight and other failures occurring unexpectedly where an alternate path is chosen.Routers operate at the third layer of the OSI layer according to a set of predefined protocols.The necessary information to route the packet are present in the header of the encapsulated packet that a router that deals with.
Routers in ATN can be broadly classified in to two types .

Intra domain routers (static or dynamic)
Inter domain routers

The intra domain routers are local routers that are used in local domains and are not to global international standards.But the inter domain routers are all set to global international standards and can handle more traffic than inter domain routers .
The ATN routers differ from the Normal routers by certai features that distinguish them.those are listed.

In a ATN router there is a possibility of applying specific policies that enable the supporting of mobile communication providing effiecient air to ground routing
The enhanced security system that ATN adheres to protect the tampering of data.
Use of compression methods to accommodate low bandwidths in air to fround links and other data networks.
Termination procedures and initialization of a new route when an aircraft enters the system or leaves is a major advantage in an ATN

End syatem:
These system integrate the host system with the network.all the application level services are coordinated to the host by these end systems to establish communications with the peer to peer system in network.
They have the capacity to communicate with another subnetworks end systes in order to provide end to end communications to the ATN applictons that handle operations. It is for this purpose they have a seven ;ayered protocol stack that hosts appropriate communication services in support to the applications of the ATN.
The above figure relates the constituent elements of the aeronautical telecommunication networks with the OSI layer protocol stack structures ,that helps in establishing peer to peer commuications with end systems.
ATN domains :
Unlike other networks ATN has also domains that are essential for routing purposes.Each domain may have inter domain router and end systems. To make the routing process simplified the administrations of the adjacent domains may combine together to form a single domain sharing a policy and hence making it single.
Administrative domains are part of ATN where they are managed by a single authority.This administrative domains are either the Civil Aviation Authority ,an Air traffic controller or an international aeronautical communication service provider (IACSP)
A routing domain may have the following characteristics.

All informations regarding the connectivity and quality of the service related with the internal systems are are exchanged without any restrictions.
With in a domain ,selected routes(the common routes to the other systems lying outside the domains ) may be advertised by a BIS router.
These routes advertised by the BIS routers to the other routers that lie outside the domains are controlled by a policy that enforced by the advertising BIS router .

The figure gives an example of an ATN domain
Thus the very basic essentials of a data network that is used to exchange informations was discussed. In next chapters VHF which is used as a physical transfer medium is discussed .