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We are working on a project to create a V2V ad-hoc network between vehicles. Basically we are deploying Raspberry Pis in vehicles and we are using Xbee modules and Zigbee protocol in order to exchange data between vehicles if they are near from each other.

Our ad-hoc network is completely dynamic and decentralized (We cannot have any static nodes in the network). Our problem is that the topology of the mesh network created by the Zigbee protocol requires a coordinator to be always in the network and the network will fail in case this coordinator failed down.

It seems that using Zigbee protocol requires knowing the topology of the network beforehand.

We do not know whether it is feasible to create our dynamic network using the Zigbee protocol without knowing the topology of the network beforehand and without the requirement of the coordinator. Can we have more than 1 coordinator in the network to overcome this problem? Thanks in advance.

user1920191
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2 Answers2

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Is there a particular reason you are looking to use Zigbee? if you are after a truely decentralised network you would be better off using either a different protocol (one that has no need for a coordinator) or by defining your own using basic RF modems (which is a lot more complicated)

however XBee modules are configurable by AT commands, with a bit of work you could probably set them up to periodically change mode (when it is running as coordinator) to check for other coordinators and if it finds one then stay as a standard node, and if the coordinator drops out (stops replying) then steps up to be coordinator.

this approach would require you to solve a few issues though:

  1. how do the remaining modules in the network decide which becomes coordinator
  2. how often would the coordinator scan for other coordinators in order to effect a reasonable response time, yet not disrupt data flow

i would suggest that you provide a bit more information relevant to the question such as:

  1. how much data is being transferred?
  2. how often is this data being transferred?
  3. how are you planning to define where the data gets sent (addresses? or multicast to everywhere?)

hopefully this helps you in the right direction, but in the mean time i'd suggest you look at the XBee datasheets for the AT commands and what is possible (has been a while since i used them)

James

James Kent
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    I do have the same problem actually, but there is no way to change the status of the role of the node from the AT commands. AT commands let you to control the channel, network ID and other configuration data, BUT you can't change the mode of the node from the AT commands. In order to change the mode of the network, I guess that you need to change the firmware installed on the zigbee device itself. Do you know or tried a way that u can change the rule of the node from router to coordinator using AT commands?? – I.el-sayed Apr 15 '14 at 13:21
  • Unfortunately this isnt something i have ever tried to do, and if as you say they cannot change mode, you could look at working out some kind of handoff protocol where every single node has a coordinator and a standard module and every node seeks to connect to a common coordinator, although the practicality of that could be very difficult. and after that you could look at using another type of RF modem, other than a zigbee based one, but that would still require you to do alot of your own protocol development. – James Kent Apr 15 '14 at 13:27
  • what kind of transmission are you trying to achieve? multicast (from each node to all nodes) or point to point within the mesh? – James Kent Apr 15 '14 at 13:36
  • We are trying to multicast to all the reachable nodes. – user1920191 Apr 15 '14 at 13:46
  • if you are doing multicast then why not use plain RF modems instead? as then every modem in range will receive it? and none of them need be a coordinator? through the use of simple acknowledgement signals you could let the sender know which nodes recieved it – James Kent Apr 15 '14 at 14:24
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Digi sells XBee modules that implement protocols other than ZigBee. Both the plain 802.15.4 module and their proprietary DigiMesh module would be possible candidates for your project if you don't need to be ZigBee-compliant.

I think that you could send broadcast messages with 802.15.4.

With DigiMesh, all nodes are of the same node type. But I do not know how well it will handle having networks come together and fragment on a regular basis. You could contact Digi's technical support or sales support teams to see if they can provide any guidance.

tomlogic
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