Raylı sistemler için uyarlanmış telsiz iletişim protokol modeli ve başarım analizi
Madified wireless communication protocol model for railway systems and its performance analyses
Yazar: HAKAN VELİOĞLU
Danışman: PROF. DR. SELÇUK PAKER
İstanbul Teknik Üniversitesi / Bilişim Enstitüsü / İletişim Sistemleri Ana Bilim Dalı / Uydu Haberleşmesi ve Uzaktan Algılama Bilim Dalı
Derece: Yüksek Lisans, 2016
Konu: Elektrik ve Elektronik Mühendisliği = Electrical and Electronics Engineering ; İletişim Bilimleri = Communication Sciences

Özet:
Raylı sistemler, yük ve yolcu taşımacılığı anlamında günümüzde de popüler olmakla birlikte trenlerin de yük taşıma kapasiteleri ve bununla birlikte yolculuk hızları da zaman içinde ilerleme göstermektedir. Bu ilerlemeye karşın ray üstündeki taşıtların hareketlerini takip eden sistemlerse aynı ivme ile gelişim göstermediğinden günümüzde kullanılan tren takip sistemleri ray üstündeki bir taşıtın tam olarak konumunu verememekte ve bununla birlikte trenler de birbirlerinin konumunu bilememektedir.
Raylı sistem taşımacılığında taşıtlar belirli bir ray hattı üstünde hareket etseler bile farklı bir ray devresine geçecekleri zaman ray makasları öncesinde, trendeki makinistin ışıklı ya da elektro-mekanik sinyallere göre merkezden yönlendirilmesi ile trenin hareketi yönetilmektedir. Makinistin herhangi bir sebeple sinyali görememesi ya da sinyali yanlış yorumlaması durumunda çarpışma gibi istenmeyen sonuçların oluşması muhtemeldir.
Deniz taşımacılığında da taşıtların olumsuz hava koşulları ya da coğrafi şartlar neticesinde birbirlerini görememeleri durumlarına çözüm olarak kendi konum bilgilerini etraflarındaki gemilere otomatik olarak sürekli anons ettikleri bir sistem kullanılmaktadır. Bu sayede hareket halindeki bir gemi görüş alanı içerisinde olmasa bile etrafındaki diğer gemilerinin konumlarını ve hareket planlarını bilmekte ve kendi hareket planını da buna göre belirleyebilmektedir. Ayrıca belirli dinleme istasyonları ile kıyı emniyeti birimleri de gemileri takip edebilmektedir.
Tez çalışması ile raylı sistemler üstündeki taşıtların da konum ve hız gibi bilgilerini, bununla birlikte ray makası, hemzemin geçidi, ışıklı sinyal gibi sabit birimlerin de kendi konum ve durum bilgilerini; denizcilikte kullanılan model referans alınarak anons ettikleri bir model önerilmiştir.
Tez kapsamında önerilen modelin başarımını ölçmek içinde bilgisayarlı benzetim hazırlanarak gerçek yaşantıda karşılaşılması çok zor olan ortamlarda, önerilen modelin nasıl çalıştığı gözlemlenmiş ve modelin en olağan dışı ortamlarda bile verimli çalışabileceği görülmüştür.
Önerilen model içerisine, trenlerde, istasyonlarda ya da ray elemanlarında olabilecek normal dışı durumlara karşı, diğer trenlerin haberdar olabilmesi ve buna göre acil durum ekiplerinin müdahale edebilmesi amacı ile düzenlemeler de dahil edilmiştir.
Önerilen model ile belirli bir ray hattında klasik sistemler ile güvenlik sebebi ile sadece tek bir tren hareket ettirilebilirken artık trenin anlık konum bilgisine ulaşılabildiği için birden fazla trenin çalıştırılması mümkün olabilmektedir. Ayrıca önerilen model içerisinde ki acil durum mesajlarıyla yapılacak kurtarma müdahalelerinin daha verimli olması da sağlanmaktadır.

Abstract:
Railway transportations popularity has been increasing since the early days. Carrying large amounts of load on railway is still an efficient method of transportation. Moreover, railway transportation is much more plannable according to highway transportation. And it is also a safer transportation because of using a certain path of railway. For all of these reasons using railway transportation is a good choice where geographic conditions allow to lay rails.
Railway construction is not increased as railway transportation needs. As a result of this, old railway lines should be used in an efficient manner with safety in mind. Kilometers of single rail line must be used in both directions and there is no fault tolerence here. In order to solve this problem a system named interlocking, which is an arangement of signalling system that prevents conflicting movements through on a rail road or crossings. All the trains on a specific path are considered while building an interlocking plan thus, it is scheduled that which train will wait the other and where to wait and when to wait or go.
Train is guided by light signals or elector-mechanical signals by the central office according to interlocking table. Signalling equipments determine when the train engineer will stop or move the train. However, train engineer moves the train by assuming the rail scissors are positioned correct by the central office and does not even know whether the rail scissors are at the correct position or not. On the central office side they don't know the trains exact position. A trains position only be known while it is passing on axle counters or track circuits.
The logic behind train circuits is calculating the voltage difference between two rail line. Voltage is appliad to one rail line and when the train moves on that line it shortcircuits the  other line. Receving the voltage on the other line means that there is a train on that prat of the track. Another method of track circuit is receiving the audio frequency signal given from one end of the rail which is based on the frequency shift of the signal at the other end. Ray circuits, said at least one train on the track but can not give an exact knowledge of the location information on the track.
The axle counters are special electronic circuits which are counting train axles passing over the switch circuits. In case of entering train axles and exiting train axles are equal, the central office assumes that train had left the field and the track is available for new trains. The important part of axle counters are how many bits are used for counters. For example if a train which has 256 axles passes over an 8 bit coded axle counter means no train passed over it and the circuit is empty which is an dangerous case. On the other hand axle counters doesn't give the speed information of trains.
While operating high-speed train traditional axle counters and ray circuits and visual signalling systems are useless. For this reason a wireless communication technology which is based on GSM and named GSM-R has developed. GSM-R technology is used for voice transmission and also data transmission between central office and the train engineer but it has a severe capacity problem. When the number of trains increases the transmission could be delayed or blocked and this can be result dangerous situations becasue of high speed. Moreover, there is no protocol definition for transmitting position data of train. For this reason central office still does not know the exact location of train.
Despite the nowadays improved technology central offices still doesn't know the exact position or speed of railway vehicles or state of ray circuits and signalling lights. Moreover train engineers has no knowledge of the railway state or other trains location information which is not in engineers field of view. If a train has an emergency condition on railroad, central offices has no information of this situation and also other trains has no knowledge of the emergencey case. For this reason we couldn't send more than one train to a signle railway because of the security considerations. On the other hand, a train engineer obeys only the light signals and there is no other support systems. If the signal or circuit has broken the engineer has no chance to realize this. Moreover if some emergency situation happened to a train there is no fast and reliable way to know the exact speed or position of it and this increases the intervention time to train.
Watercrafts may not see each other because of adverse weather conditions or geographical conditions and this can also result unwanted accidents same as railway transportation. For this reason, all vessels are equipped with a device that broadcasts the navigational data continuously, so a ship could know the vessels around itself without the need of its crews visual view. Moreover, centers offices can also know positions of vessels by receiving stations which are positioned at coastline. This system is called AIS (Automatic Identification System) whose technical specification is developped by ITU (Internation Telecommunication Union) and protocol messages are managed by IMO (Internation Maritime Organization).
In the AIS system all vessels send its navigation information and GNSS position and receiving vessels determines its navigation with received information. All the vessels are free with their movement and AIS system has no direct access to ships navigation system.
In order to improve railway tracking and getting exact location or speed information of a train, getting position and state information of a rail switch, grade crossing or light signal, a new model whose design is referenced from AIS system is proposed in this thesis. Signal specifications are kept out of scope and assumed same as in the AIS model. So signal coverage calculations are same with AIS. Protocol definitions, message specifications and broadcast access algorithms are defined in the thesis. While TDMA (Time Division Multiple Access) channel access method is used, time synchronisation is also an important issue so it is also defined in the thesis.
After the protocol definitions explained, some key perfomance metrics such as message collisions are studied. Moreover, a computer simulation over GNU Octave was written to measure the performance metrics and it is found that the proposed model works well in extreme conditions like heavy loaded railway traffics or with high speed trains. This happens becasue of self correction methods and special roles for the ground units whihch are defined in the thesis.
The proposed system needs cheap hardware and could be used with existing solutions as a support system. While only one train can move a railway in the traditional system, multiple trains can be operated over a railway with the proposed system. The center office can get the realtime position of a vehicle and ground stations. Moreover, rescue operations can be made much more quickly and efficiently in the proposed system, because there is a special definition for emergency messages so head center can know where the emergent case occurs and also what the case is.
The proposed system has soma parts to improve like security. Neither AIS nor the proposed system has an security definition and all signal sources are assumed legitimate. However an unwanted train signal could be broadcast as a malicious activity. This may have no harmfull effect in open sea transportation but this may result much more dangerous situations in railway transportation.