I'm trying to devise some type of simple communication protocol between a single interrogator device and several corresponding transponders. The idea is that the transponders will be in "Sleep Mode". The interrogator will periodically wake up the transponders in the area and the transponders will each transmit some data to the interrogator for collection.

I'm having trouble figuring out how to resolve the collision that occurs when two or more transponders transmit their data at the same time and thus, the reader receives some mixed up, incorrect data. Or, errors that may occur due to the noisy environment. I figured adding some type of checksum to the data packet would be useful in detecting errors. That way, the reader can check if what it received is correct or not.

The premise is that if the reader receives something and detects it has being incorrect, it can tell the transponder that sent it to resend it in hopes of receiving it error free. My problem is that I do not know how to make the reader able to tell the correct transponder to resend. How can I make this possible? Is there perhaps an easier way? I read about the different TDMA algorithms and ALOHA based schemes but they seem to be way out of my league. Any pointers or tips?

Thanks and I apologize if I may seem a bit vague but will further elaborate if need be.

 

Could you have the interrogator just wake up each transponder one by one rather than a group of transponders? Wouldn't that also easily solve all your resend problems? Just wake up the offending transponder again.

I don't see why you HAVE to wake up a group of transponders all at once (and then have them arbitrate amongst themselves who sends first, etc!) since they will still fundamentally transmit one by one.

 

Try CAN(Controller Area Network). It has an extremely robust arbitration mechanism for simutaneous transmitters. The best part is that it is nondestructive.

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We never have time to do it right; but we always have time to do it over.

 

Heh. That would work in my case since I only have a couple transponders but for a large number of transponders, it may not be sufficient. Thanks for the suggestion though!

 

WHy would it not be sufficient? You get 100% bandwidth utilization. WIth the methods that require collision resolution you get about 50% or so utilization of the bandwidth.

 

Papabravo, do you have a link or any more info for the wireless version of CAN?

Another possibility is to use some sort of 802.15.4 technology like Zigbee or Bluetooth. There are a lot of micro-controllers now with built-in Zigbee support (for instance, see http://search.digikey.com/scripts/Dk...ame=MC13211-ND
) The Zigbee protocol stack will handle all collisions, acks and retries, so you can concentrate on just the communication. You can also implement a mesh network so nodes don't have to be within their own transmitter range to talk to each other.

The only other suggestion I have is to use some sort of multiple access method like TDMA (time division), FDMA (frequency division) or CDMA (code division).

 

Suppose there is more than one reader. Each reader would have to wake up each transponder individually while the others remain asleep. The transponders will not remain in one spot forever. The idea is that different transponders with different IDs will be able to be collected by different readers depending on their location. How would the reader know which transponder to wake up first and so on?

 

Whoa...
There is a lot of obfuscation going on here.
We only know a part of the problem.
It would help to know:

How many readers are there?
How many transponers are there?
What is their geographical distribution?
If these things move about a bit, how far do they move, and how quickly?

JimB

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Experience is directly proportional to the value of the equipment ruined.

 

No, I'm not familiar with any wireless versions of CAN. In fact I think non destructive arbitration in a wireless environment is a difficult feat, if it can be accomplished at all.

__________________
We never have time to do it right; but we always have time to do it over.

 

the readers will be placed about 30 meters from each other. We have only 3 transponders but are considering the effect of more. The transponders will enter the range of a reader, and then stay for at least 10 minutes.