Software Subsystems and Communications Protocols
GEO devices, Unipolar I/O, and SEAR2s collect information about the crossing and surrounding region, either by monitoring voltages on wires or by passing messages among themselves. Logic software massages the real-world information and decides what signal aspects to show locomotives and decides what information to indicate to crossing predictors which assist the predictors in determining whether crossing gate arms should be lowered or raised, lights flashed, and bells rung.
Though the communications infrastructure (consisting of WAGs, radios, Ethernet bridges and such) are non-vital, messages conveying vital information is passed between vital devices using the communications infrastructure, messages which are encapsulated in Advanced Train Control System (ATCS) messages and carry 32-bit Cyclic Redundancy Check (CRC) values which are used to verify with extreme accuracy that messages have not been corrupted.
Vital messages also carry timestamps and sequence information such that in the event of unexpected routing delays or routing loops, obsolete messages will be ignored and duplicate messages will be eliminated.
Vital systems establish communications sessions among themselves which are constantly maintained so that in the event of interference (incidental or deliberate) sessions will drop and the crossing will go to the least permissive state.
The vital systems communicate among each other via twisted-pair LAN called Echelon which operate at around 1.2 megabit per second. The WAGs take the Echelon-encoded ATCS messages and may route them out their 10-megabit Ethernet interface, causing the messages to be transmitted to other WAGs via spread spectrum radios where they are converted back in to Echelon messages.
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