Multiple devices frequency saturation

[Aspenforest]

Greetings!

After reading about Flutter as much as possible from the data available,
I got one question... how many devices can operate within the vicinity of
each other, until the frequency spectrum used by the devices becomes
over saturated and prevents the devices from correctly identifying each other?

Is the frequency spectrum of the devices time-shared or frequency-shared?
If it is time-shared, at how many discrete broadcasting devices in close proximity
to each other will the network start significantly lagging, so it becomes impossible
to relay clear audio over the network without interrupts?

[Aspenforest]

(@ Taylor) As I read from your latest update on the Kickstarter page,
you mentioned that the devices jump between 50 channels and the
data transfer rate is around 10 to 20 kbps.

Considering these facts, does it mean that only 50 devices can work in close
proximity to each other?
Or, if they are constantly channel hopping, and you consider that not all devices
within the range of each other are constantly broadcasting, but rather sending
datagrams in groups of some arbitrary number, would it mean that more than 50
devices can operate within the broadcasting distance of one another?

How do the devices negotiate channel acquisition among each other?
Do the devices notify each other with the time they are going to be occupying
a channel before acquiring the said channel?
If no free channels are available, how long will a device wait until attempting
acquisition again?

As a side-question: Why is the device using insecure SHA-256 cryptography algorithm?

[taylor]

Thanks for the questions!

Devices on the same network will hop together, so it is only different networks that need to avoid eachother. Flutter hops across 50 different channels, but there is a space of about 1300 channels between 902-928MHz. Different networks will arbitrarily grab a channel schedule from that block, and will hop in a different pattern. So the chances of interference between any two networks, even if you had 50 different networks at once, is pretty low.

The actual scheme for channel acquisition is still in development, but it will be generated by the system based on the network ID and a random seed. Haven't pinned down the details yet. We do have to configure the boards to do listen-before-talk for the european market, so no board will try to transmit if the airwave is already occupied (it will try again in 20 or 40 milliseconds). That will help avoid collisions.

As far as crypto - SHA-256 is to my knowledge still recommended for use, but we are looking into using ECC crypto instead. Now that the official Arduino Wifi module has been announced using the ECC chip we have been looking at, we feel a little more comfortable going that direction. This will be determined in a few weeks, after we have finished wireless testing and are ramping up production.

Thanks!