Zigbee / 802.15.4 Datarate & Latency

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Hi Freaks,

I am mulling over possible solutions to a project that I have been thinking about.

I need to have reliable bi-directional radio communication between two MCUs. The link should be reliable at a distance of 500m line of sight, and should achieve an end to end speed of at least 4,800bps and preferably 9,600bps. Cost is of a concern (less than £20 in single unit qty)

I was thinking of zigbee or 801.15.4. I would like to here of people's experiences with various modules.

As the communication needs to be bi-directional and the data is both directions is likely to be pretty constant I would like to know how the system copes with this, and what sort of latencies I can expect between the data transmission from one MCU to it being received by the other as this is to form the basis of a real time control system.

Can any one help shed some light?

Thanks

Tim

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> I was thinking of zigbee or 801.15.4. I would like to here of
> people's experiences with various modules.

You'd only need ZigBee if you intend to add routing. Your question
doesn't sound like that, so plain 802.15.4 (node-to-node
communication) would suffice.

> The link should be reliable at a distance of 500m line of sight, ...

That might be an issue, in particular given the price range quoted.
Also, you are quoting a price in GBP, so I assume you are bound to the
European regulations which limit the power to 10 mW.

We once extrapolated the range of some early AT86RF230 setup to be
about 1000 m line of sight (based on the measured RSSI in 250 m
distance, and the known Rx sensitivity), but your 500 m requirement
would be just a few dB over the sensitivity, and already well below
the noise level. (802.15.4, similar to GPS, can decode signals below
the noise level.) I would not really assume this to be "reliable".

> As the communication needs to be bi-directional and the data is both
> directions is likely to be pretty constant I would like to know how
> the system copes with this,

Normally, channel access is controlled by CSMA/CA in IEEE 802.15.4,
i.e. any transmission is required to be preceded by listening into
the channel. When you use the CCA-CS algorithm (carrier sense) rather
than CCA-ED (energy detection), you should be able to notice a channel
busy even by a very distant 802.15.4 transmitter. When being at the
range boundary, I'd expect that to fail quite frequently though.

> and what sort of latencies I can expect between the data
> transmission from one MCU to it being received by the other as this
> is to form the basis of a real time control system.

Look into the 802.15.4 documents, then you can calculate how long a
CSMA/CA transmission could take (including all retransmissions),
including all the random backoffs upon detecting a busy channel.

You should be aware of one thing: with radio communications, in
particular within an ISM band, there are *no* guarantees, whatsoever.
(Unless you live in a shielded box, of course. :) There are
statistics, estimations, likelihoods -- but no guarantees. Your
neighbour opening the door of his microwave oven, shortcutting the
security switch, and then firing 500 W @ 2.4 GHz in a random way into
the environment, will always be able to DoS your network (if not even
kill your receivers).

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.

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I agree with dl8dtl.

Zigbee is really not designed for long haul streaming data, but rather a relatively close group of sensors or the like. Yes, it can be used for streaming data, but it gets interesting doing that.
===
Have you checked out Maxstream? They have Zigbee modules, and other RF modules.
The serial modules I have used in robotics, and have tested the 9Xcite modules to over 1km. This was at 9.6kb, but they can be set to 38kb as well. These are a frequency hopping radio in the 900mhz band. They are also very reliable, as they use an internal protocol to carry the payload.

You can get a dev kit for I seem to remember $149 for the 9Xcite. The longer range models are more.

I also seem to remember the OEM modules for these being around your price range...

Best,
Kent

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> These are a frequency hopping radio in the 900mhz band.

900 MHz is not available in Europe. We've only got 868 MHz, which
is not an ISM band: it's very narrow (so forget about frequency
hopping), and power-limited, but has a good reputation for being
kept free of too much interference noise.

Jörg Wunsch

Please don't send me PMs, use email if you want to approach me personally.