# Communication Transport Protocol Strategies for Rail Applications
## 2 The Railway Context
### Railway Applications and Users  
In the rail sector, applications are related to the users and typical use cases in the rail world. It comprises (but is not limited to) drivers, persons on platforms (e.g., dispatchers and traffic controllers), persons at level crossings, conductors on-board trains (responsible for operational and safety duties such as ticket collection, observing door closure, performing safety tasks in case of an emergency/accident), catering, security and train preparation staff on-board trains, and train and track-side maintenance staff. Within this sector, it is possible to identify the following service categories [10; 11]:  
* **Signaling**. This refers to the transmission of safety-related information regarding the movement of trains, needed for the operational work of train transport services. It refers to the regular and frequent message exchange between an application on-board a train and a corresponding ground application used to allow trains to move a distance with a speed based on track limitations and known braking capabilities of the specific train. Then, it is fundamental to know the current position of the train at all times and the characteristics of the rail environment where the train is moving. Typically, the train sends its position to the remote-control center (namely, the position report, PR) and the control center responsible of the safety of the train and of the on-board passengers sends back the authorization for movement along the rail line (namely, the movement authority, MA). Examples of this protocol are Computer-Based Train Control (CBTC) in the urban/metro line and the ERTMS/ETCS in the other rail environments. * **Critical voice**. This is related to communication between internal staff such as driver(s) and controller(s) or emergency calls. In the first case, it regards operation and movement of the train and can be one-to-one or a group call. In the second case, it informs driver(s) and controller(s) about possible hazards along the track (e.g., people near the rail line, obstacles on the track). Voice over IP (VoIP) with an Adaptive Multi-Rate Wide Band (AMR-WB) speech codec is implemented up to 23.85 kbit/s, plus transport and application protocol overhead. Due to its nature, latency and setup time should be low even at a high speed of mobility. * **Critical video**. This provides highly reliable and available real-time video streams supporting the train operations. It can be from the train to the ground and vice versa. Typical use cases include automatic obstacle detection for Automatic Train Operation (ATO), monitoring of doors inside trains and people on platforms. Two options are possible: uplink Closed-Circuit TV (CCTV) for surveillance and downlink Platform TV (PTV) for platform surveillance, thus different requirements may be available. * **Critical data**. This refers to generic applications important to the train operation and to prevent damage to people or to one o more elements of the infrastructure. Examples are downloading data needed for a journey to a single location, connecting a smart object controller on the wayside to the inter-locking systems and detecting possible train interruptions, above all, for freight trains (i.e., the vehicle integrity). * **Non-critical data**. This refers to non-critical services for the operation of the transport service. Possible cases are: i. predictive maintenance: transport vehicle sends to the remote control center the report of on-board sensors in order to give information on the vehicle's status to perform a predictive maintenance; ii. public announcements:along the track as well as close to the stop station, public transport staff can spread out public announcements generally related to non-periodic activities such as information on vehicle delays, changes in scheduling, and accidents along the way; iii. Passenger Information System. * **Passenger connectivity**.