# Towards the Internet of Smart Trains:
## 2 Communication Systems in Railway Scenarios
### Intra-Car Communication Networks
Main Technologies for Intra-Car Communication Networks  
Several technologies are embedded in the Train Access Terminal (TAT) to provide a continuous connection. Furthermore, they are able to link the train to the Internet backbone and to provide Internet on-board. Apart from the bandwidth requirement, among the criteria to select a specific technology are generally the connection quality (i.e., the signal strength), delay, throughput, security, and cost. Two major families of technologies may be considered [19; 20]:  
* Satellite solutions. Distinct types are available (i.e., Geostationary Orbit (GEO), Medium Earth Orbit (MEO), Low Earth Orbit (LEO)) with different frequency bands and that may provide unidirectional or bidirectional communications. Satellites are used for both locating trains (aided by Global Navigation Satellite Systems (GNSS) systems [21], like GPS, the European GALILEO, the Russian GLONASS or the Chinese BEIDOU) and communicating with the wayside equipment. * Terrestrial solutions. They can be grouped into two main categories: (a) technologies that rely on existing networks (i.e., public cellular networks), and (b) technologies that require ground infrastructure to be deployed: leaky coaxial cable, Wi-Fi, WiMAX, radio-over-Fiber, and optical solutions. Apart from legacy systems (usually analog), the trend of applying wireless systems in railways is still in its first decade of life. There are three types of systems. First, those based on open standards: TETRA, General Packet Radio Service (GPRS), and IEEE 802.11 family of standards; second, open standards with slight modifications on some layers (e.g., GSM-R); finally, proprietary wireless communication solutions have also its niche in the market. Traincom by Telefunken [22] or FLASH-OFDM [23] are good examples with a great acceptance in the railway sector. Nowadays, GSM-R is the most widely used communications system between trains and the different elements involved in operation and control within the railway infrastructure. It operates in 38 countries across the world, including all member states of the European Union (EU) and countries in Asia, America, and northern Africa [24]. Two frequency bands were reserved by the European Telecommunications Standards Institute (ETSI) for railway communications in Europe in 1995, which are 921-925 MHz for Downlink (DL) and 876-880 MHz for Uplink (UL). For each band, it is possible to allocate 19 subcarriers of 200 kHz, including a guard band. Each subcarrier supports 8 data or voice channels. A Wireless Local Area Network (WLAN) technology such as Wi-Fi represents the most common deployment on-board, and it is accepted that the replication of Wi-Fi APs within the train is the best approach to connect trains with a client interface. The delivery of broadband Internet access to trains has been previously analyzed in the literature and some authors have presented surveys that compare different technologies for such a purpose (e.g., IEEE 802.11, TETRA, satellite) [25]. Due to the rapid changes in technology, it is clear that railways will have to evolve to keep up with their pace. With such an aim in mind, in recent years operators have included in their systems different emerging technologies. For example, WiMAX was tested for train-to-ground deployments in order to provide Internet services to the passengers [26]. Other WLAN-based networks have been evaluated to deliver train operation traffic but, until the development of the IEEE 802.11 ac standard, there was a lack of essential Quality of Service (QoS) features related to traffic policy enforcement, end-to-end resource management or traffic admission. Likewise, new technologies like Wireless Gigabit (WiGig) or Light-Fidelity (Li-Fi) will have to be considered in the medium-term [20]. On the one hand, WiGig (IEEE 802.11 ad), promoted by the Wi-Fi Alliance, operates at the unlicensed 60 GHz band (in Europe 9 GHz of bandwidth from 57 to 66 GHz).