I've been working on a product using WiFi. The reason I picked WiFi, is cause I essentially have 20-30 nodes communicating to a main node via a router. This all works very well, and its easy as its simple TCP/IP requests.
However,the nodes sometimes get submerged in water and when they do, the node quickly loses connection, and takes a long time to connect back to the network (5-10). Additionally, the devices communicate via HTTP requests, and the latency of the communication isn't the greatest.
I've recently started exploring Zigbee as an alternative. However, I can't find any information around node reconnect times, power usage, and communication latency. Does anybody have any information they would like to share? Also, does it automatically work in a star network?
I'm not particularly married to Zigbee either. If there are other modules/protocols/frequencies I should consider – I'd love to hear them. I essentially want something with which I'd be able to replace my WiFi communications stack quickly, and isn't a huge power hog.
If its any help, I'm using the Atmega1284P as my microcontroller.
Best Answer
ZigBee (and more generally 802.15.4) targets different uses compared to WiFi (control vs data streaming), thus has lower data rate (250 kbps max) but also much lower power consumption (few tens of mW transmitting and receiving).
For sure you can make star networks, but also more complex tree and mesh networks. In fact, meshing is one of the main selling points of ZigBee.
But concerning reconnect times, it depends on how good the reception is. If you use standard power modules, they are limited to 1/100 of the transmission power of 802.11 thus you might need to use high power modules or use routers to relay messages. You may need to tune the frequency of the reconnect attempts since nodes are typically configured to operate in a duty-cycled manner, waiting for a certain time between transmissions in order to reduce power demand.
Latency is generally low, as the collision avoidance scheme is lighter (read "less sophisticate") than the RTS/CTS used in WiFi. But if you have to send large amounts of data, the throughput will limit your responsiveness as well.
If you want to retain IP networking, consider using either 6LoWPAN on bare 802.15.4 or ZigBee IP.
And if you want to stick to Atmel you can also consider replacing the whole microcontroller with a one-chip MCU with transceiver. Link