[SOLVED] 20 XBee Series 1, would there be data collision/interleaving?

[massoumi]

Hello everyone,

We are using XBee Series 1 in a design where 20 dsPIC microcontrollers send a 65-character long packet continuously to the main computer, and the data is parsed on the computer. We're using the AT mode and just add our own tag (ID) at the beginning of each packet for parsing purposes (security is not a big issue).

We haven't finished the 20 units, so I tested the idea with 2 XBee's transmitting to the computer's XBee and monitored the results in X-CTU's terminal. No collision/interleaving so far!

Should I worry about future when we have 20 mobile units transmitting to the main computer's XBee? How does this network of XBee's take care of data collision/interleaving?

Also, I was wondering what are the benefits of switching to Series 2?

Sincerely,
Soren

 

[kalbun]

Hello Soren,

for Series 1, I know 802.15.4 and Digimesh protocols. Both are able to detect collisions and bad packets, and automatically issue retransmissions.

With 802.15.4 your 20 nodes should be in range with the collector node. This is not required with Digimesh, which can build up a path between any two nodes using intermediate nodes as repeaters (hops).

In both cases, there is a maximum amount of data the network can transport - 80 kbit/s for 802.15.4 (according to Digi. I have direct experience up to 40 kbit/s) and 30 kbit/s with Digimesh (but each hop decreases the throughput dramatically).

So, divide this value by 20 and this is roughly the maximum dataflow you can transmit from each node.

Switching to Series II allows you to use Zigbee (it can't run on Series I). IMHO the biggest advantage of Zigbee is channel scan, which reduces the risk of using an already occupied channel to build up the network. There is also an increased level of security. Throughput drops to 27 kbit/s (1st hand experience, two nodes in peer-to-peer configuration).

Hope all of this helps. Should you need more information, I suggest you to have a live chat with technical support at Digi. They are very helpful and extremely skilled.