**3.1.2 Physical level**

472 Wireless Communications and Networks – Recent Advances

management of a wireless communications channel at the application layer. The architecture proposed is currently being deployed in some railway companies from Spain (Euskotren and ETS from Basque Country, and Renfe from Spain), will be presented (Gutiérrez et al., 2010). It is an innovative general purpose wireless communication channel which allows the train to communicate with the railway control centres in such a way that the applications or services are unaware of communication issues such as: establishment and closure of the communication, management of the state of connectivity,

This new wireless communication architecture has to respond to the demand for communication and transmission of information from any application, so it will have to take into account the nature of the information to be sent. The information exchanged between two applications (one on earth and the other on a train) may have different urgency degrees depending on their purpose or treatment with respect to the exchanged information. In fact, there is information that needs to be transmitted at the time that it is generated, for example in case of positioning information or alarms in some critical train operation elements. On the other hand, there may be less urgent information whose transmission can be postponed, such as train CCTV images, or audio files used by the background music. In addition, the

urgent or priority information is usually smaller than the non-priority information.

illustrates the technologies used to implement the architecture in two real scenarios.

In this section the core components of the mentioned wireless communications architecture are described. This architecture allows a full-duplex transmission of information between applications and devices deployed in the trains, and applications that are in the railway control centres. The description of the architecture will be made at two levels: conceptual and physical level. The first level defines the basic concepts of the architecture, and the second one

From a conceptual point of view, two issues are especially important: the elements that manage architecture's behaviour and the ways in which the different applications

Our architecture hosts both terrestrial and train-side applications, so in order to manage its behaviour two main entities are defined: Terrestrial Communication Manager (TCM) and On-board Communication Manager (OCM). The former manages terrestrial aspects of the architecture and the latter train-side issues. Although the managers have a different physical

Due to the information transmission needs and for a correct and optimized used of the communication architecture, two types of communications are distinguishes: "slight" and

**3.1 Towards a train-to-earth wireless communications architecture** 

prioritization of information and so on.

**3.1.1 Conceptual level** 

(terrestrial and on-board) transmit the information.

Delivery and reception of the information,

 Dynamic train addressing, Medium access control, Security and Encryption, and Communication error management.

location, both of them have nearly the same responsibilities:

In this section the technological aspects of the wireless architecture are described. They refer to the protocols and the communication technologies used for the development of the trainto-earth architecture (showed in Fig. 1).

Fig. 1. Train-to-earth wireless communications architecture.

It is important to point out that the protocols and technologies for the development of the new architecture have been selected with regard to: standardization, robustness, security, scalability and compatibility with existing and potential applications and systems. The major aim has been the ease of integration of any application or system into the new communication architecture. Concerning with this objective, Web Services constitute the transport technology for the communication between final applications and the "local" Communication Managers. All the information is interchanged in XML format, in order to allow future extensions.

On the other hand, the communication between the Terrestrial (TCM) and the On-board Communication Managers (OCM) is based on REST (Representational State Transfer) technology. This communication technology uses the HTTP (HyperText Transfer Protocol) protocol and XML formatted messages. This solution is similar to traditional XML Web Services but with the benefit of a low overload and computational resources consumption.

Although the information interchanged between the TCM and the OCM is not encrypted, using the HTTP protocol allows the easy migration to HTTPS (HyperText Transfer Protocol Secure) that offers encryption and secure identification. It can be seen that every communication has to go through two core elements that can result in the loose of channel availability in case of failure. This problem is tackled by means of the use of web services because this solution deploys support web services in a way similar to traditional web architectures. It can be said that the selected technologies and architectures are well known and broadly used in different application areas or contexts, but they are novel in the railway 'train-to-earth' communication field.

In order to establish a wireless communications channel between the trains and the railway control centres, mobile and radio technologies have been selected (Yaipairoj et al., 2005). In this case, slight and heavy communications use different technologies due to different transmission characteristics.

Due to de necessity of delivery of information in real-time, mobile technologies such as GPRS/UMTS/HSPA (Gatti, 2002) are used for the *slight communications*. These technologies do not offer a great bandwidth nor a 100% coverage and they have a cost associated to the information transmission. Despite this, these technologies are a good choice for the delivery of high-priority and small sized information. The selection of the specific technology (GPRS/UMTS/HSPA) depends on whether the service is provided or not, (by a telecommunications service provider), and the coverage in a specific area. ). To increase coverage availability, the hardware installed in each train has two phone cards belonging to different telephone providers. This allows switching from one to the other depending on coverage availability. Therefore, the idea is to have a predetermined operator, and only switch to the second when the former is unable to send.

On the other hand, for the *heavy communications*, WiFi radio technology has been chosen. This technology allows the transmission of large volumes of information, does not have any costs associate to the transmission and its deployment cost is not very expensive. In this case, a private net of access points is needed. This net does not need to cover the complete train route because the heavy communications are thought for the transmission of big amounts of information at the end of train service (for example the video recorded by the security cameras).

Although each separate technology can't achieve 100% coverage of the train route, the combination of both comes very close to complete coverage (Pinto et al., 2004). As the application layer protocols are standard, other radio technologies such as TETRA or WiMAX (Aguado et al, 2008) can easily substitute the ones selected now. These technologies can achieve a 100% coverage and neither one has a transmission cost. However, there are certain limitations such as the cost of deploying a private TETRA network, and the cost and the stage of maturity of the WiMAX technology

#### **3.2 How to manage wifi based broadband communications**

As it was explained previously, there are some railway applications that need a high bandwidth to interchange large amount of information without time restrictions (real time communication is not needed). This kind of communications will enable 'train-to-earth' information exchange for train side systems update/maintenance and multimedia information download/upload such as videos or pictures.

Although the information interchanged between the TCM and the OCM is not encrypted, using the HTTP protocol allows the easy migration to HTTPS (HyperText Transfer Protocol Secure) that offers encryption and secure identification. It can be seen that every communication has to go through two core elements that can result in the loose of channel availability in case of failure. This problem is tackled by means of the use of web services because this solution deploys support web services in a way similar to traditional web architectures. It can be said that the selected technologies and architectures are well known and broadly used in different application areas or contexts, but they are novel in the railway

In order to establish a wireless communications channel between the trains and the railway control centres, mobile and radio technologies have been selected (Yaipairoj et al., 2005). In this case, slight and heavy communications use different technologies due to different

Due to de necessity of delivery of information in real-time, mobile technologies such as GPRS/UMTS/HSPA (Gatti, 2002) are used for the *slight communications*. These technologies do not offer a great bandwidth nor a 100% coverage and they have a cost associated to the information transmission. Despite this, these technologies are a good choice for the delivery of high-priority and small sized information. The selection of the specific technology (GPRS/UMTS/HSPA) depends on whether the service is provided or not, (by a telecommunications service provider), and the coverage in a specific area. ). To increase coverage availability, the hardware installed in each train has two phone cards belonging to different telephone providers. This allows switching from one to the other depending on coverage availability. Therefore, the idea is to have a predetermined operator, and only

On the other hand, for the *heavy communications*, WiFi radio technology has been chosen. This technology allows the transmission of large volumes of information, does not have any costs associate to the transmission and its deployment cost is not very expensive. In this case, a private net of access points is needed. This net does not need to cover the complete train route because the heavy communications are thought for the transmission of big amounts of information at the end of train service (for example the video recorded by the

Although each separate technology can't achieve 100% coverage of the train route, the combination of both comes very close to complete coverage (Pinto et al., 2004). As the application layer protocols are standard, other radio technologies such as TETRA or WiMAX (Aguado et al, 2008) can easily substitute the ones selected now. These technologies can achieve a 100% coverage and neither one has a transmission cost. However, there are certain limitations such as the cost of deploying a private TETRA network, and the cost and

As it was explained previously, there are some railway applications that need a high bandwidth to interchange large amount of information without time restrictions (real time communication is not needed). This kind of communications will enable 'train-to-earth' information exchange for train side systems update/maintenance and multimedia

'train-to-earth' communication field.

switch to the second when the former is unable to send.

the stage of maturity of the WiMAX technology

**3.2 How to manage wifi based broadband communications** 

information download/upload such as videos or pictures.

transmission characteristics.

security cameras).

With the purpose of providing an innovative broadband communications architecture suitable for the railway, a number of WiFi networks have to be settled in places where the trains are stopped long enough to ensure the discharge of a certain amount of information. This is: stations in the header that starts or ends a tour and garages. In this way, WiFi coverage is not complete, but broadband communications are designed to update large amount of information, which, usually do not need to take place in real time.

Therefore, at this point it has to be taken into account aspects such as bandwidth, coverage or communications priorities. The existing broadband management systems, which are used in other (non mobile) environments, do not satisfy all the needs of the railway applications (California Software Labs, 2008; Marrero et al., 2008). Furthermore, some additional problems have to be solved on this environment. In one hand, it is necessary to find a mechanism to locate the trains because they don't have a known IP address all the time. A dynamic IP assignment is used for every WiFi network so a train obtains a different IP address every time it is connected to a network, and a certain IP address could be assigned to different trains in different moments. On the other hand, there are several applications that want to transmit information to/from the trains at the same time. This implies the existence of a bandwidth monopolization problem.

To tackle these challenges, it is necessary a smart intermediate element which manages when the applications (both terrestrial and train-side) can communicate with each other. That is to say: a Broadband 'train-to-earth' Communications Manager, whose design and functional architecture is described below.
