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Old Dec 04, 2009, 01:17 PM
shifteroo is offline
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Question

2.4 Ghz Technology based on..?


Just out of interest, with all the new 2.4Ghz RC systems, are they loosely based on any 802 protocols, (802.11 WLAN / 802.15 WPAN) or are the systems completely proprietary to each manufacturer?

I'm mostly interested in the hardware behind it, and have been assuming it might be built on top of 802.15.4 lower layers as they'd be perfect for the task i.e. low bandwidth (not much needed to send a few servo position bytes) and low-power consumption and can have very good range.

Don't imagine Spektrum and Futaba tell any old tom dick or harry about the specifics of their designs but if anyone can clear this up for me I'd appreciate it,

Cheers,
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Old Dec 04, 2009, 01:22 PM
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Hi!

There are some systems based on the ZigBit protocol (xps, Jeti), but most of the system have nothing in common, be it bandwidth, data rate, FHSS or not, number of channels used simultaneously, etc.
Old Dec 04, 2009, 03:03 PM
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Most of the other 2.4ghz systems (other than Jeti & XPS) are one-way transmissions...so they don't need the overhead of a link layer/protocol stack. So, they've basically written their own simplex protocols and defined their own signal properties (w.r.t. spread spectrum, channels, and bandwidths) within the limitations of the chipsets their each using.

D.
Old Dec 04, 2009, 06:13 PM
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XPS use 802.15.4 radios, but with their own custom protocol. Spektrum use Cypress wireless USB chipsets, with their own custom protocol and much more power on the transmitters. Futaba went full-custom, and built their own chips. The others I don't know, but it wouldn't be surprising if many of them are using radio chips made for 802.15.4.
Old Dec 05, 2009, 03:47 AM
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Jeti uses the premade ZigAmp transmitter module in their modules.
Old Dec 05, 2009, 03:56 PM
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"XPS use 802.15.4 radios, but with their own custom protocol."

Andrew, have you personally confirmed that there is anything unique going on there - not pushing you just interested.
Old Dec 05, 2009, 05:39 PM
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An Introduction to Spread-Spectrum Communications

http://www.maxim-ic.com/appnotes.cfm/an_pk/1890
Old Dec 05, 2009, 05:46 PM
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Quote:
Originally Posted by Andrew McGregor View Post
XPS use 802.15.4 radios, but with their own custom protocol.
Bzzt. Wrong.

Protocol analysis shows that the XPS systems used stock standard protocols. The latest versions may move away from that but the originals were not a custom protocol at all.
Old Dec 05, 2009, 05:49 PM
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Jim Drew is on record on this site saying they did something to the firmware.

Now maybe that was just tweaking for latency, rather than protocol changes, but they did do something; their modules are XBee hardware, but with firmware changes.

Or at least, that's what Jim said.
Old Dec 05, 2009, 06:01 PM
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We definitely have never used the stock 802.15.4 protocols. In fact, the XBEE modules have to be put into pure 802.15.4 compatibility mode if you want to use them on a normal 802.15.4 network, and we don't do that. Also, our backoff exponent and other features are not the same as true 802.15.4.

802.15.4 protocol has been used in everything from water control systems to hospital inter-communications. This technology predates 2.4GHz for R/C, so it has a long history of success in numerous other industries.
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Old Dec 05, 2009, 06:50 PM
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Ah, here's Jim

Yep, protocols are more than just frame formats.
Old Dec 06, 2009, 05:19 PM
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Hmmmm


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