Sensor Area Networks (SANs)

[davidbrady]

I think it's due to the way cellular networks work. Communication on a cell phone is always client side initiated (initiated by the cell phone). You can't get a phone call until you turn your phone on and let it identify itself and it's geographic location. Cell phones don't have IP addresses. They adopt the IP address of the nearest cellular gateway (GPRS). With no IP address and the phone able to be anywhere geographically, the server can't initiate, or at least that's the way the protocols are written. (Scalability issues come to mind). So when a ZigBee comes online it sends a message to a server saying "here I am". Not a broadcast - the ZigBee Gateway get's programmed with a server's URL. Over WiFi no keepalives are needed.

The $100 ZigBee Gateway is a box that speaks ZigBee (RF) on one side and ethernet on the other. On the ethernet side it sends and receives HTTP/XML packets to allow configuration, monitoring, and control, and it generally does what it needs to do to stay visible on the network. It'll cost $50 for an rPi and $25 for an XBee Series 2 to duplicate the Gateway and then you need to add all the protocol development time. For an extra $25 you get all this plus more. It's not a Digi thing; it's a ZigBee standards thing.

From the ZigBee Alliance:
  Understanding ZigBee Gateways Sept 2010.pdf (Size: 939.35 KB / Downloads: 1721)

It's neat and light weight Ned. All WaGu needs to do is format an XML form and send it to the Gateway using HTTP! The Gateway will respond with either flipping switches or reporting back monitoring conditions (temps, pressures, voltages, etc). There are still questions to be answered though.