Sensor Area Networks (SANs)

[nedb]

(06-04-2013 21:16)davidmbrady Wrote:  Just a quick update on what I've recently discovered.

First, the Digi WR21 Cellular Router that I bought for $375 (plus $10.00 shipping) doesn't contain a ZigBee Internet Gateway as I initially had thought. This means another small box is required, the Digi ConnectPort X2 which costs $108.

On the plus side, EmbeddedWorks, the same folks who offer affordable byte size chunks of GSM and CDMA bandwidth carry a comparable router for less money and they offer group buys.

Another interesting tidbit is that the discovery of ZigBee Internet Gateway (XIG) devices requires client side connection initiation. This is because we're transmitting over the cellular network. To keep an XIG active and connected periodic keepalive messages need to be transmitted. If not then the device looks like it's disconnected from the network and the server can't find it. I think TCP keepalives are used and the total packet overhead for a keepalive exchange is 114 bytes.

Well if you do the math you can see that a keepalive sent every 5 minutes will consume almost 1 MB of bandwidth per month! Ouch! No big deal really. We have two choices, buy more bandwidth and/or decrease the keepalive frequency.

Putting one knee in front of the other...

If it was my business plan instead of Digi's, I'd be giving away Zigbee features in everything in the product line. An extra $100 box? Noooo. I'd dump Zigbee on the market below cost, drive the competition down, buy 'em out, and generally make way for the ubiquitous.

Hmm. Client initiation... So when a Zigbee comes on the network, it waits for a broadcast? Then gets discovered, then goes dark and gets scratched off the server list. Why not just wait for another broadcast from the client?

Must be a very orderly network.

Maybe Digi modems aren't the best way to go. Why not let the zigbees and the arduino do their transmit/aggregate thing, run some statistics on the data periodically to provide near-real-time status in a web page on a monitor. Use the gateway to upload a formatted email to the WaGu server's account, where scripts parse the email and add the data tp the database. It analyzes and calls you on the phone: "The last fuel stop slimed your filters. Vacuum levels will shut the engine down in 2.87654 x 10 23 femtoseconds." OK that was silly, but not bad considering I've forgotten what the modem connection was for.

For sure, not the end-all solution. If it isn't going to work, I can forego the explanation of why not.

I still think the all-in-one Digi modem is pretty swift. But I don't expect to be getting satellite, in fact my flatscreen is broken. I've turned it into a cabinet door. I like it now