Trenitalia did some research efforts to develop Ethernet based networks suited for the on board train harsh environment and identified the main technical challenges to deal with such a sophisticated mobile local area network:
• fault tolerant local area network
• topology constraints due to train architecture
• data cabling requirements due to very strict fire-flames rules
• data connector housing requirements for on-board in-vehicle and between-vehicles links
• data connector requirements for on-board railway operations (mechanical vibrations and wide temperature range)
• strict latency and jitter application level response time and synchronization issues
After some prototype testing, we were able to identify the train network technical requirements: a fully switched Ethernet network with standard IEEE 802.1Q (Vlan and QoS) features. We developed and tuned two building blocks to address the main technical challenge: develop a fault tolerant network. Train architecture topology constraints were specially addressed with a “no single point of failure and no single vehicle point of failure” network meshed architecture.
Two very advanced technologies were borrowed directly from the latest high availability data centers designs: the logical switch and the logical link. From the Ethernet point of view, the logical switch (aka virtual switch) is a single logical device physically made by two or more different equipments sharing their switching tables. Logical switches support “cross-stack” logical links.
Logical links are made with two or more parallel physical connections between the same two devices. Logical links are also referred as trunk or aggregated links, with the latter automatically managed by the IEEE802.3ad-2000 LACP protocol.
A full scale train network topology with a series of 13 logical switches, connected one after the other, was build using only logical links between devices. Ethernet topology requirements (only star or extended star are allowed) were addressed with fine tuning of MSTP protocol between logical links.
Trenitalia was finally able to demonstrate a working fault-tolerant full train network. Very high data rates streaming flows were used to test the network, even with the use of very specialized industry standard deterministic traffic generators as the Ixia IxChariot tools.
A highly specialized cabling scheme for vehicles was developed with the use of two identical jumpers between coaches.
Low latency and jitter requirements as well as network bandwidth management led to a fully switched train network with standard IEEE 802.1Q (Vlan and QoS) features. The goal was achieved with a specialized high performance solution: a dual-Gigabit Ethernet Fault Tolerant Train Network proposed by Trenitalia as a Gigabit Ethernet standard solution in July 2008 to IEC TC9 WG43 and WG46.
The availability of a standard and high performance fault tolerant network will become the preferred data exchange infrastructure to deploy new services. It will be able to transfer either dedicated railway data (i.e TCMS) or new passenger multimedia services.