I'm getting started with XBEE S2 modules and I have a doubt I still couldn't get a clarification, hope you guys can help.

I want to have 2 end devices transmitting data to one coordinator, all of them working in API mode (escaped characters). The data should be sent "as fast as possible" so I'm afraid I will get many collisions. How XBEE deals with that?

On the the RX packets I can get the source address but I need to read many bytes before I know it. What happens if while I'm reading bytes from a packet the other end device starts to send another packet? If XBEE implicitly uses collision avoidance, are that "per byte" or "per packet"?



2 Answers 2


The radio being used by these modules (MAC & PHY layer) is defined by the IEEE 802.15.4 standard which specifies the use of Carrier sense multiple access with collision avoidance or abbreviated as CSMA/CA.

A simple summary, a device will do a quick scan of the channel that its PAN network is on and if it senses that it is being utilized it will wait and retry using a random exponential backoff scheme. Using this method, you can think of the channel as a shared resource that is "taken" by a device to do a burst transmission and then released for others to use.

Further questions about reliable transport can be answered by reading about it in the Zigbee spec or IEEE 802.15.4 spec.

  • \$\begingroup\$ Thanks for the clarification! But... is the collision avoidance "per byte" or "per packet". I guess "per byte" doesn't make much sense but I would like to make sure of that. \$\endgroup\$
    – cidadao
    Commented Aug 4, 2012 at 0:29
  • \$\begingroup\$ @msr Definitely per packet. Also note that collision avoidance isn't 100% successful - you still need to have an acknowledgement channel and retransmissions to make it a reliable transport \$\endgroup\$
    – vicatcu
    Commented Aug 4, 2012 at 2:05

I don't know the specifics of XBee, but I believe it's a pretty heavyweight protocol. As such I would wager it employs both collision avoidance (at the medium access control level) and automatic retransmissions (akin to TCP) and does so at the packet level.


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