Totally OT - Covid App Operation

Hi Guys

Does the Covid app work over mobile data, or is proximity to other devices detected via the phone location on the telephone network. Just asking cos I am on pay-as-you-go, using Wi-Fi hotspots only.

Thanks

Phil

Telephone network location isn't much more than a vague handy wavy the phone is over there, somewhere. Even if triangulated from several cells. No where near the handful of metres accuracy required for the Covid app.

Correct.

That's how the orginal app was going to work, with the alerts based on proximty/duration sent from centralised system. The app now uses the Apple/Google decentralised API. Not exactly sure how that works, I think when someone enters into the app they have had a +ve test that phones (Covid) ID is uploaded and then sent to all other phones and those phones then check their list of phone IDs they have been close to for long enough to raise an alert.

I would assume that any data link conversations are IP based and thus it doesn't matter if that link is over mobile data or WiFi.

I'm curious as to how much data the Covid app does use. I've not bothered with it. Little point as there have probably only been a handfull occasions I'll have been close enough, for long enough, to anyone (with the app on their phone or not) for the app to raise an alert.

Triangulation from multiple wifi points can be pretty good (assuming you're nearby several APs) not that covid app uses location.

basically the same, except the phones do their own "matching" rather than upload everything and letting the NHS do the matching, but it still downloads a list of "infected" phone IDs just from google/apple instead of from the NHS.

I think the biggest difference between the NHSx version and the apple/google version wasn't the centralised vs decentralised approach, both use anonymous random mutating IDs anyway, but that apple don't allow apps to run in the background and sniff bluetooth details, so iphones couldn't really see other nearby phones unless you constantly "re-launched" the app

I think it uploads 14 days worth of every phone it's seen when you tell the app you *might* be ill

yes. I've never heard of an unscented kalman smoother, but apparently that's the latest and greatest algorithm the app will now use to let them detect more matches.

I would say so.

The API talks about "These files can grow so big that they become prohibitive to download. This is especially true for devices that don't have Wi-Fi access. You can break up files into batches to keep the file size below 16MB?about 750K keys."

Me neither.

several

And the location of the APs is known reasonably well. Seem to remember some fuss over Google logging rather more data about WiFi AP's that it really was allowed to via its streetmap cars.

Doesn't need to.

Ah that would make sense as the system then only needs to send data to *those* phones, not *every* phone on the system.

Ouch, though one assumes phone IDs older than 14 days are purged and most phones won't see anything like 750k device in 14 days. 21 bytes/phone ID, 100 phones/day logged is about 2 k/day complete download(*) every hour 50 k ish or 1.4 M/month. Not a great deal. Wonder if that very rough guesstimate is born out in reality?

(*) But no need to download the whole thing with tools like rsync available.

But there's not been any fuss about android phones with GPS turned on, reporting where they are and what access points they can see ...

That file size was for the daily download of "possibly infected" phones, not the upload of other phones it has been near, but the keys aren't one:one with phones since keys change every 15(?) minutes.

It uses bluetooth, aand handshaking with anywhere else is done via wifi or mobile data. Its up to you in the phone to disable mobile data for the app it asked me about both these things when I installed it. Brian

It only reveals identities if the user agrees it should do as well. I know this is good practice from the paranoia point of view, but its not for the data usage side to find infected people or should I say, possibly infected people Many are not inputting a positive test or leaving the device at home and going shopping this is to be discouraged and such people are at best selfish, at worst, probably guilty of manslaughter if somebody they infected died, but that would be impossible to prove since we are infectious before we get a test! Grin Brian

Thanks Guys

As I have no mobile data allowance and any contact will be outdoor away from Wi-Fi it will have to use blue tooth.

Phil

I thought I read something about the network providers letting data related to the app to be free of charge?

(Obviously, still requires access to a suitable mast!)