How to have a couple hundreds of nrf51822 using S130 advertise at the same time

RE: How to have a couple hundreds of nrf51822 using S130 advertise at the same time

Philippe Laulheret — Fri, 10 Oct 2014 18:24:36 GMT

Thanks! That's encouraging.

RE: How to have a couple hundreds of nrf51822 using S130 advertise at the same time

John — Fri, 10 Oct 2014 18:14:23 GMT

Hi Philippe,

For the first 30 seconds after the device powers up or comes out of reset we advertise every 20mS. After 30 seconds we slow the advertising rate to 1285mS.

RE: How to have a couple hundreds of nrf51822 using S130 advertise at the same time

Philippe Laulheret — Fri, 10 Oct 2014 17:59:46 GMT

Thanks John for sharing your experience, do you remember what advertisement interval you were using (as the more packet you send the more likely you'll get collision)?

RE: How to have a couple hundreds of nrf51822 using S130 advertise at the same time

Philippe Laulheret — Fri, 10 Oct 2014 17:57:22 GMT

Thank Mik for your insight. I believe you have a factor 1/1000 off (as the advertisement duration is in us rather than ms). As a back of the napkin approximation it's a good start I guess, there are a few extra details to add for a more accurate model (the three channels for adv, the 3 messages per adv interval, the (uniformly?) random duration between two messages, etc.). Unfortunately probabilities have never been my forte !

RE: How to have a couple hundreds of nrf51822 using S130 advertise at the same time

John — Fri, 10 Oct 2014 13:30:24 GMT

I am not an expert with the BLE specification, but I believe there are random back off times in the case of a collision.

During our first product development we wanted to stress test our mobile app to make sure it could keep up with lots of our devices in a small space all advertising at the same time. We ran that up to 80-100 devices without seeing a problem with collisions preventing us from seeing the advertising packets and connecting to devices.

RE: How to have a couple hundreds of nrf51822 using S130 advertise at the same time

Michael Lamming — Thu, 09 Oct 2014 20:24:09 GMT

Phillippe,

I needed to figure out the same thing. It's one of those nasty exponential effects. To first approximation it's ethernet math from Metcalfe and Boggs. Being a bear of little brain, my back-of-the-envelope estimates, based on an overly simplistic model (please correct me) were as follows:

Assume a single adv. channel rather than 3. Assume an advertising packet fills the ether for 300uS every 10 seconds-ish. Assume that there is no collusion between nodes, and no other traffic.

Then the probability of node A colliding with node B is 300/(10.24*1000) ~ 3%.

The probability that A does not collide with B is (1 -(300/(10.24*1000))) ~ 97%

If there are 20 nodes, then the probability that nobody collides with A is (1 -(300/(10.24*1000))) ^ 19 ~ 57% So you were probably experiencing no worse than a 50/50 chance of a collision, which seems like an engineers fighting chance.

If there are 200 nodes then it gets a lot worse: ~ 0.3% (Snowball's chance in hell)

The probability that all 200 nodes don't collide is some binomial theorem calculation which makes it all a lot worse still.

And then of course, every collision results in a re-transmit and you get a packet storm, so everyone has to back off and wait.

I'm sure that the BLE designers figured the model out, and have much better answers than these.