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Consider IEEE 802.15.4 Protocol superframe structure (Image Src: Google)

IEEE 802.15.4 Superframe Structure

In this structure Contention Access Period(CAP) is always followed by Contention Free Period(CFP).

So is there any particular reason behind keeping CAP first and then CFP? Could it be other way around?

Thank you.

abhi195
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It can't really be the other way around because that is what is in the standard. Obviously, you are free to implement your own use of the radio but then I guess it wouldn't be 802.15.4!

The designers of the standard probably had good reason to place the CAP before the CFP (and if you are really interested I imagine it will be documented somewhere in the IEEE meeting minutes etc). My guess is that I think it would have these following benefits:

  • devices have to wake up their receiver to listen for the beacon frame, and thus if they have any ad-hoc comms to perform (like collecting a pending message or negotiating a connection etc) they can do it straight away and then go to sleep for the rest of the superframe
  • having the CAP first allows any devices that do not have a GTS to power down their radio for as long as possible
  • having the CAP first provides time for devices to negotiate a GTS before the CFP starts, thus reducing the latency to their first GTS (i.e. it would be possible to hear a beacon, associate, and obtain a GTS prior to the very next CFP)
Doddie
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