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Topic: Multipoint xbee Network (Read 309 times) previous topic - next topic

gdub61

Hi everyone,

I am building a collision avoidance system for many vehicles (up to 50) and was planning on using GPS modules and xbees.

I need a network where everyone on the network can talk to everyone else. ie:- All vehicles transmit their vehicle ID  and GPS coordinates and if any vehicle comes within a certain distance of any other vehicle, the ID and location is printed on a screen.

I am using Venus GPS modules and MEGA2560 boards with a 7" LCD touch screen and was planning on using xbee pro 900 modems.

What I am unsure of is what type of network to use to achieve this. Any help would be much appreciated. Cheers.
 

PaulS

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and if any vehicle comes within a certain distance of any other vehicle, the ID and location is printed on a screen.

How does that contribute to collision avoidance?

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and was planning on using xbee pro 900 modems.

What range do you need?

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What I am unsure of is what type of network to use

Your only two choices are point-to-point or mesh network. Obviously, point-to-point is out, because you have too many points.

You will need a coordinator - the center of the mesh. All other devices send data to the coordinator. It then sends data to all the other devices.

If the amount of data being sent is small, this might even work. This, of course, means that you need to minimize the data sent to the coordinator, and the amount of data sent to the router/end devices. That, then, means not sending ASCII data.

Jack Christensen


You will need a coordinator - the center of the mesh. All other devices send data to the coordinator. It then sends data to all the other devices.


With XBee ZB modules, the coordinator need not be the center of the mesh in any particular sense, and also it need not be a data concentrator. But it is needed to initially start the network and allow other modules to join. However, routers also can allow other modules to join, so once one or more routers have joined, this is not the exclusive domain of the coordinator. In fact, the coordinator can even be removed at this point.

If using XBee ZB modules, indeed each network needs exactly one coordinator. I assume the population of vehicles is dynamic. Vehicles will come and go, join and leave the network, so this begs the question which vehicle has the coordinator. Even if this only affects initial network formation, at some point we must expect all vehicles to leave the network, so the question then is how does the network get re-established?

I haven't used Digi's proprietary DigiMesh modules, but I believe the concept of the coordinator node is absent with that protocol.

But I have my doubts, even disregarding the coordinator concerns. I just do not think that XBee modules are meant to move around frequently. Route discovery can cause excessive overhead on the network. As vehicles move around, there will be frequent changes in the routing tables that are maintained by every node in the mesh. Each node will be kept busy trying to determine its nearest neighbors, link quality, etc.

Broadcast transmissions might seem to make sense in a scenario like this but in fact they are to be used sparingly even in stationary networks due to the network overhead they cause.

A system like this needs some fairly impressive intelligence, plus fast and reliable RF networking. I know a little about XBee modules, but I sure wouldn't be choosing hardware until I had a very good handle on the requirements. I'd be doing my homework and a huge amount of research. A fair amount of prior art exists in this area, including some impressive demonstrations, so that is obviously one tree to bark up.
MCP79411/12 RTC ... "One Million Ohms" ATtiny kit ... available at http://www.tindie.com/stores/JChristensen/

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