Mesh network switching Uplink without data loss
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Hello Everybody
As manufacturer of cable cars and the automation we often use wifi to get network access in the (moving) gondola
now we have the first cable car with no sight connection from one station to the other, so the chart must switch its AP on the flight. since safety protocols are transmitted over this network the connection might not be lost for longer than 199ms.
we did some tests with the UniFi AC and TP-Link Omada Mesh without luck, the switchover is not done before it looses the connection to one AP completely (even the ping gets higher than 199 ms causing the PLC to issue a emergency stop)
other tests using 2 independent WLANs with the RSTP feature on the managed network switch failed too since it takes up to 600ms to switch over (here as well the switch over is not done before it looses the connection completely)
are there maybe some 802.1k wifi - ethernet bridges available on the market?
ofcourse there are some very expensive solutions in industrial Ethernet, but spending about 2000$ just for a simple wifi device with less than 100mbit makes not much sense for us.
any advice would be highly appreciated
Kevin
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Welcome @Swissrope
Disclaimer -- I have no affiliation with TP-Link. But I do have a good deal of laboratory robotice engineering and safety experience (in the USA).
I'm not entirely sure I follow what you are trying to do. I would never use/trust a Wifi solution as part of a safety solution. Are you just logging location/telemetry data for each car/gondola? I would think an industrial Cellular/GPS solution or private wireless (non-wifi) solution would be a more appropriate choice. Are there multiple cars per cable? What determines the 199ms E-stop timing spec? How many meters is this (based on your velocity)?
Regardless, MESH does not enable a client NIC (aka STA) to associate with more than one AP (aka EAP) at a time. So, if you really want to do it this way, the computer in the Gondola would need to have two wireless network cards (for example). Each card will have a directional antenna. One antenna pointed towards the next EAP and one antenna pointed towards the last EAP. Then your application software (or perhaps a load balancing / redundency enabled router) will need to periodically test each NIC's connection and decide which path to take. OR repeatedly transmit on both, and then poll the receving application to make sure the message was received. If not, re-transmit.
But to do so in the time frame you've specified seems unlikely. Fast roaming supposely eliminated the need for the AES2 handshake. Regardless, I would expect you wouldn't want to disable encryption.
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Thank you for your reply...
the length of the cablecar is about 100 meters (300ft) with just one gondola, but it goes over a pylon (the slope changes from 30° to 70°) so i cannot just use 1 wireless connection and it looses the connection when moving behind the horizon.
both stations are connected with glass fibre together. We use the network to send commands from one station for example to open the door, if someone pushes a button....
we developed a device that gathers all these I/Os and forwards them over Modbus TCP to the PLC wich is located in the top station near the winch. that device also sends a heartbeat over the network. if this heartbeat is loss for longer than 200ms the PLC will issue a E-Stop because it can not longer be sure that the safety circuit on the gondola is still ok (i.E. E-Stop button, Safety brake released...)
the gondola drives with 0.8 m/s so 200ms equals 0.16 meters (about 1/2 ft)
The IO device contains redundant ATmega microcontrollers with seperate NIC, there is a POE switch on the gondola where we wanted to connect the Wireles device(s) and IO device.
is there maybe a software solution for a device (i.E raspberry pi or similar) with two Wireless NIC and 1 ethernet which then forwards the traffic and decides what Wireless NIC to choose?
what do you mean with "private wireless"
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@Swissrope, in my opinion using public RLAN such as WiFi networks open to the public to control critical infrastructure is a very bad idea. Consider use of industrial solutions such as radios using frequency bands reserved for commercial use. You should also consider to use redundant systems for this task, that's two (or more) independent systems, but not a software solution over a single network connection.
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to have a redundant network, we need then a device that splitts up the data from one RJ45 to two and on the other end one that decides wich packet to take and wich to block to prevent loopback issues....
Industrial radio is another thing, but there are not much solutions to forward safety signals according to the needed safety standard. (btw. most of them do also just rely on 5Ghz Wifi)
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Hi @Swissrope,
If I understood you correcty, you have two stations (let's say top and bottom) and one cable support pilon in-between. Additionally, you mentioned that you have fiber optic cable connecting the two stations.
I realize this would be a substantial, added, installation cost. But what if your wireless AP was on top of the cable support pilon (with power and Ethernet fiber) with directional antenna pointed towards each station? I assume you might also need additional AP's in each station. I'm thinking that the AP on the pylon might be a different SSID to which one of the two NICs in your gondola PLC were connected. This NIC might have two dirrectional antennas outside the car 180 degrees from each other.
Regardless, if you're "roling your own" it seems like you application safety software would need to incorporate the "redundency testing" between the various networks / AP's itself and likely this will require a longer "timeout period". I asked about the gondola velocity as typically, for practicality reasons, we need to equate timings with distance to evaluate what is realistic and dafe and what isn't.
It looks like Pilz makes a solution specifically for cable cars. Pilz and SICK are well recognized industrial safety system providers
https://www.pilz.com/en-INT/products/success-stories/articles/198994
-Jonathan
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JSchnee21, the LOOP21 products are mainly for non-time-critical tasks such as sending announcements from staff to passengers, importing music, controlling lights in the cabin, tracking/monitoring, security cams etc. and even Internet access for the passengers. For those tasks standard WiFi is first choice, but not so for real-time device controls.
For real-time device controls you would use telemetry/telecontrol systems such as produced by memotec using the VHF frequency band or by Fraunhofer Institute using GSM/UMTS/LTE.
Swissrope, by redundancy I don't mean just network redundancy, but system redundancy. If I had to design such a system, I would design a completely redundant system from sensors / controls over the processing units up to the network. Both systems would compare results, re-transmit if they differ, compare again and initiate an emergency stop if results still differ. I once developed such redundant moving systems for steelworks.
Yes, the 5 GHz non-RLAN band probably could be used. For example, in my country 5.8 GHz (BFWA band) is not accessible to the public, but only to commercial users. Using this frequency band would ensure at least that passenger's devices could not cause interferences or delay of communication, but still the non-deterministic timing remains an issue IMO.
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Thank you for your great support guys
We did use PILZ devices in one cablecar ended up repalcing them because of missing reliability, so we won't use them ever again.
is it possible to connect two antennas to one AP? if so, moving the AP to the pylon will be a possibility.... (at least for this application, but not a solution if we build a cablecar with multiple pylons in the future)
@R1D2 the controllers are completely redundant. they even have 2 IP adresses transmitting the data (with checksum) to independent PLCs, the two PLCs then compare the result. it complies with all safety standarts and is TUV certified ;-)
i just talked to one of our suppliers, he recomends this device:
https://www.moxa.com/en/products/industrial-network-infrastructure/wireless-ap-bridge-client/wlan-ap-bridge-client/awk-3131a-series
it seems like we can set up a Omada Mesh network with TP-Link EAP225-Outdoor and have the moxa device in the gondola.
just sad that once again, you pay about 600 € for a device with 100mbit network.... and it is not weatherproof even. So the official way is to mount the device inside the cabinet and have antenna cables with external antennas (yeah we all know that antenna cables suck due lost of db....)
can someone do a recommendation about what antennas we should use? (in station and cabin)
my eyes and ears are still wide open for a better solution... luckily we got some time to figure out the right solution...
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btw, thank you for the LOOP21 Hint, i'll give them a call, maybe it is a suitable solution, we'll see....
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Swissrope wrote
@R1D2 the controllers are completely redundant. they even have 2 IP adresses transmitting the data (with checksum) to independent PLCs, the two PLCs then compare the result. it complies with all safety standarts and is TUV certified ;-)
Sounds good.
i just talked to one of our suppliers, he recomends this device:
https://www.moxa.com/en/products/industrial-network-infrastructure/wireless-ap-bridge-client/wlan-ap-bridge-client/awk-3131a-series
The Moxa device uses DFS channels. What if high-priority users (commercial live broadcasting, satellite links, weather radars, military radars) force your AP to change channels and re-negotiate with clients? There will be a short break. What if the whole 5 GHz band gets jammed by military for days as it was done at this year's WEF meeting in Davos, Suisse, forcing nearby Pharos CPE backhauls to stop working at all in an area of 60+ km around the event location? Did you consider this?
it seems like we can set up a Omada Mesh network with TP-Link EAP225-Outdoor and have the moxa device in the gondola.
Keep in mind that Omada mesh uses a proprietary protocol, it's not an 802.11s based mesh network (just in case you want wirelessly mesh the Moxa with EAPs – that won't work). You can use STA-to-AP links, though, if the mesh is just for the backhaul between EAPs, but the mesh links are affected by DFS/TPC, too.
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