Sensor Area Networks (SANs)

[davidbrady]

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...