[chbot] Bluetooth tags

Thu Oct 1 07:51:08 BST 2020

I once did some statistical modelling on RF throughput and the results were
not what you would have originally thought.  The chance of collisions is
not linear and it is easy to get to a point where more is less.

With BLE three (IIRC) channels are reserved for advertising (which is what
beaconing is) and other data is hopped through other channels.

This has the advantage of leaving advertising channels relatively clear
(unless 100 covid cards turn up :-).

Note too that BLE is nothing like BT Classic and they do not interfere
except in the general sense of occupying the same RF spectrum. It is quite
crazy giving them the same name - very confusing.

BLE is much faster at setting up connections. A few year I did a joystick
remote using BLE using the low level transport - not the GATT stuff. The
connection between the remote and the device was negotiated within 70
milliseconds. This meant we basically negotiated the connect and then
dropped it again if the joystick was idle for more than half a second. THe
fast set up time made it completely invisible to the user with no lag at
all..

You could never do anything like that with regular BT.

On Thu, Oct 1, 2020 at 6:43 PM Helmut Walle <helmut.walle at gmail.com> wrote:

> All very good points. This raises one further question: how well would
> this work in high-noise environments? I am thinking of the typical office,
> where BT performance between 9am and 5pm is often somewhat less than great,
> but if you work early before everyone else arrives with all these wireless
> devices in their pockets, or late after everybody else has gone home, BT
> performance noticeably picks up... I like the suggestion of running in
> energy-saving mode to achieve both long battery life and a reduction of
> interference. But would that work equally well in environments that are
> fraught with a lot of noise from other (non-covid tracing) sources in the
> same band?
>
> Kind regards,
>
> Helmut.
> On 1/10/2020 17:39, Charles Manning wrote:
>
> That's an interesting data point. I suppose a monte carlo statistical
> model would give consistent results.
>
> Here your scanning is being done by a PC which, I assume, is scanning hard
> all the time. On most devices that's going to be chomping the power. eg,
> Silabs SOC is around 12mW in RX IIRC which would kill a CR2032 in about 50
> hours. so clearly hard scanning is not a winner.
>
> If you're prepared to take a much more relaxed statistical approach and,
> say, only worry about people being close for more than a couple of minutes
> then you can come up with something a bit more reasonable:
> * Set the TX power lower so that devices more than, say, 5 metres away do
> not interfere. That makes it unlikely that you will have more than 5 or so
> devices close enough. That reduces collisions.
> * Reduce beacon interval to 5 seconds. That reduces collisions.
> * Goal is now to scan, say, up to 20 devices in, say, 1 minute.
> * There is now way less interference which cuts down on the collisions so
> more beacons get through.
> * Run radio at 2% duty cycle, thus stretching out power to many months.
>
> On Thu, Oct 1, 2020 at 12:53 PM Stephen Irons <stephen at irons.nz> wrote:
>
>> Some weeks ago, we spoke about Covid tracking and using Bluetooth tags. I
>> wondered about what would happen if there were many tags in a close
>> proximity.
>>
>> I had the opportunity to set up a test, and now have some actual data.
>>
>>    - 104 devices operating as BLE beacons, transmitting an
>>    iBeacon-format signal every 1.5 s
>>    - CR2032 battery, plastic housing
>>    - all bundled together in a plastic bag
>>    - my PC with a USB BT adaptor acting as a monitor
>>    - using Python 'beacontools' to monitor beacons
>>    - with a filter to receive only the BT address prefix that I am
>>    interested in
>>
>> *Results*
>>
>>    - In 20 scans, the monitor has heard all tags, every time.
>>    - It takes ~100 ms for the system to report the first tag.
>>    - It takes 8--10 seconds for the system to report up all 104 tags.
>>    - In 60 s of scanning, each device is heard an average of 22.8 times.
>>    - Each device transmit 60/1.5 = 40 times, so we hear just over 50% of
>>    the transmissions.
>>
>> I know that there are other BLE beacons in the area, as well as many WiFi
>> networks, BT phones, headphones, TVs, etc in the vicinity.
>>
>> The system works better than I imagined it would...
>>
>> Stephen Irons
>>
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