- Posted 3 years ago

WiFi for Train Communications

  WiFi for Train Communications   WiFi can be used to communicate with moving trains in a metro system. Ordinary WiFi like in my home? Could be.  IEEE802.11g 2.4GHz band, like you use in your home.  But the 5.8-5.9GHz band using 802.11a or 802.11n has certain advantages. There is even an 802.11p available but I think that is reserved for trucking.   http://standards.ieee.org/about/get/802/802.11.html By using WiFi in the 5.8-5.9GHz band you can go to higher power EIRP.  http://en.wikipedia.org/wiki/U-NII  shows this spectrum for 802.11a in the United States.  Most countries, Canada, Europe, Saudi Arabia, etc. will more-or-less follow this spectrum;  perhaps with blocks reserved or different maximum powers. When you want your WiFi access points spread out along the guideway, power determines how close or how far apart your wayside access points can be located.  According to  http://en.wikipedia.org/wiki/U-NII  you can go up to 4Watts=36dBm in the USA. Trains move, fast.  A higher power allows you to space out your Wayside Access points.  100 or 200m being typical.  If the train had to perform a complete logon for every change of access point more time would be spent authenticating than transferring data.  A wireless access controller can reduce time wasted.    http://www.youtube.com/watch?v=0n-uIusTpU8 Finally, everybody has a device that can affect the 2.4GHz range. Laptops, smartphones.  By going to the 5.8-5.9GHz band a cyber-intruder needs to have a wifi device that can work in that "exotic" range. What is in the data? Train position, train speed, commands to stop, commands to go, status and health of train electronics and mechanical equipment, perhaps even who is logged in as driver.  Each manufacturer:  Thales, AnsaldoSTS, Siemens, Bombardier, to name just a few, will have their own proprietary messaging and their own way of doing things for CBTC communications based train control.  But they all have the same goal of moving trains, or stopping trains, without the nuisance of the trains hitting each other. Any WiFi link carrying ATC or CBTC data must be secured.  WPA encryption is only the beginning.  Authentication with passkey and MAC address control are needed, as well.  By writing the CBTC protocol to conform to IEC EN 50159 the actual data messages will be encrypted, again, authenticated and they must make sense to the CBTC servers and clients or they will be rejected.  Any lost messages must be detected and re-transmitted. Just like your home WiFi the SSID beacon can be suppressed.  The train knows what network to connect to and it's not like railway tracks move around a lot.   What else is special about WiFi for Metro Trains Rail alignments are long and skinny. By using a directional antenna you amplify the signal along the track and reduce the waste of power going to the sides. 12-15-18 dBi are common antenna gains. EIRP is the total power put into the antenna, directional gain means the receiver will receive a greater portion of that power. On board the train are many systems using open protocols. To talk to doors, HVAC, break systems you will commonly encounter CANBus, MVB or TWN standards. A voice circuit is needed for a manual operator, a SoftPhone using a VoIP client can connect through the WiFi network along with the ATC, ATS and ATO protocols.   image from researchgate.net