Low bandwith distributed network via GPRS

[peter_smith]

Hi!

I'm currently designing a distributed network which will consist of about 7 nodes, 3 of which are the water plants of small villages, and the others are wells on the field.

Each node has about 8-10 digital inputs and max. 2 analog input channels.

The system's primary goal is to acquire the state (running/ready/error) of some devices (pumps,freq. converters), display them at a central location and report any errors via SMS.

Local control of the plants is already implemented, as they are complete WILO systems.

Only a minimal control is needed (for each plants to command it's wells if the water level is low or high in the basin).

So I think there should be no need for a lot of data transfer as digital signals should change only few times a day, and it is enough to read even analog signals once per minute.

The project is quite limited financially, so it is not possible to put a cFP controller on each points.

My current plan is to use a central cFP or cRIO controller at the largest plant and place ethernet expansion modules (such as cFP-1804 or NI 9144) on all other (6) points.

The problem is, that the only possible way of communication is via GPRS IP network, so the available bandwith is about 50-60 kbit/s, however the devices will "see" a 100Mbit connection to the routers.

Does anyone have any experience with such a bandwith limited ethernet network?

I'm not sure if the FP Read and Write VIs and/or Shared variables on the central controller bound to expansion modules' channels are using PSP protocol. If so, I assume the required bandwith could be lowered by changing deadband of analog channels. Am I right with this?

The other problem I face with is the possibility of connection errors. Is it possible to setup expansion modules to periodically check a gateway or any network device, and set their outputs to a predefined state?

I also would like to ask if it is possible to use cRIO expansion chassis (NI 9144, which uses EtherCAT) in such a topology, where I have to route packets through a slow GPRS network?

Thank you in advance for any help!

Peter Kovacs