Which RF to use for short range bidirectional data transfer?

Projects General Guidance
1

I'm new to the electrical side of computing so bare with me. I'm working on a small arduino side project and would like to build a bump feature where any device can touch/bump another device and the two devices will become aware of each other by passing a small amount of data between each other.

Example:

Device 1 with Id 123
Device 2 with Id 456
Device 3 with Id 789

Usage:
Device X(Host Id, Shared Id)
Device 1 (123, 456) <--> Device 2 (456, 123) | Device 3(789, null)
Device 2 (456, 789) <--> Device 3(789, 456) | Device 1(123, null)

If device 1 touches device 2 then they both tell each other their ids. The same with device 2 touching device 3 or any other permutation. The caveat is that I do not want the device that is not part of the physical touch to be aware that the meeting has taken place. Nor do I want an explicit handshake protocol to commence. It should be conceivable to have 50 of these devices that have never interacted before and be able to bump them together for an accurate data swap.

Protocols I've looked into:
-RFID: Seems that there can only be one tag/"id" in view. Which is sort of a deal breaker.
-XBEE: Seems to require a handshake as well as would broadcast to all nodes
-IR: Seems like it would make the most sense but I'm a little worried about sensor alignment
-NFC: I believe this has the same problem as RFID where only one tag can be in view at a time.

Resulting Question:
In short, is IR the best solution for this problem or is there something I have overlooked? Thanks!

2

Give each device a bar code label and a bar code reader that is fitted with a lens so that it can only focus an image at short distances. In other words, make it short sighted.
Devices too far from each other will just see a blur.

3

Is there no restriction on the orientation when bumping? If you have electrical contacts, you do have to touch two contacts simultaneously, but you can send all the data you want in a millisecond while they're touching.

A camera or IR system will need lenses on every side that is bumpable. 6 sides in the case of a cube.

I'm thinking RFID is what you want. Think about the wireless keys that you see in a lot of offices - the tag/card needs to basically touch the reader at the door. Those systems have problems if you have two cards in a wallet and you try to touch the wallet to the reader. But if you have some reasonable separation between the two RFIDs, like an inch, then it should be do-able.

How many devices are touching simultaneously? Are you trying to bump-on a 4th cube to a cube that's already touching 3 others? Or can you just pick up pairs and touch them, with no other cubes within a few inches?

4

Most promising looks to me a combination of multiple bumpers with related IR transmitters. Make sure that each touch activates the bumper switches of both devices, so that each bumper will activate the right IR channel. Focus and reduce the IR sender current, just enough for covering adjacent devices in that direction.