This section describes hardware issues encountered during testing and evaluation.
Bluetooth Low Energy as a technology was introduced in Bluetooth 4.0 and extended in 4.2 and 5.0. It consists of advertisements across all three advert channels, and 7 or more (in later versions) data transport channels chosen through negotiation during a connect event.
Thus not all phones that support Bluetooth support Bluetooth Low Energy, although practically any phone released since 2010 support Bluetooth Low Energy.
If constant scanning and interaction occurs then mobile phones will run out of battery very quickly. They will also get uncomfortably hot.
The simplest workaround is to perform a scan for 1-3 seconds but with a gap of a few seconds between each scan-identify-estimate interaction.
In our testing this results in a battery drain of approx 1-2% per hour of use. This is a minor drain and much smaller than using WhatsApp for the same period of time. Any reasonably designed protocol should be able to achieve this low level of drain.
The below devices (and perhaps the entire series of devices) do not support Bluetooth Advertising. This means that protocols that are ‘advertising only’ such as P3PT and GAEN (Apple-Google) can never support information transfer from these devices or detection of these devices for the purpose of contact tracing.
This is a major issue as in our research approx ~14 of UK mobile phones currently in use do not support advertising. This is a much larger issue than the proportion of the population without a phone. This is a major percentage of the population and so a workaround should be used.
The way around this is realise that this device can still discover, read and write to bluetooth peripherals. This the best workaround is for this device to write its own information, and the RSSI it has seen from the device it is writing to, to the device that does support advertising.
Some hardware will rotate its mac address (network identity) on every interaction rather than every 15 minutes as standard. The effect of this is that it becomes a ‘new device’ every time it does this, meaning the remote device seeing it forgets this devices identity, causing it to ask for identity information again.
This can reduce detection and continuity measures in performance terms and also use a lot more battery, as well as taking up more bandwidth exchanging this information, increasing crowding issues.
The following devices are known to present this issue:-
Note: Samsung A70 seems fine. Samsung S series seem fine.
By using the ‘spare’ space in the advertising packet a rotating ephemeral identifier can be provided. By using this ID rather than just the mac address a device seeing the advertisement with a new mac address will still know the underlying identity of the device has not changed and so will not need to ‘read’ identity from the device.
It should be noted that the ephemeral spare space is approx 31 bytes and so is too short to exchange a cryptographically secure identifier that would prevent reverse engineering of identity. This means it’s still desired to read the full identity (in our protocol over 100 bytes), but the ephemeral identity does provide a means to minimise data exchange.
This reduces identity requests from once per 4 seconds, say, per device that discovers it to once every 15 minutes (on mac address and ephemeral identity rotation, as normal).