Comment by tjohns | Hacker News Reader

Comment by tjohns | Hacker News Reader

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At least in the US, the 911 infrastructure is dated.

In older systems, your caller ID is sent using in-band DTMF tones, which are decoded by the dispatch computer.

On newer E-911 systems they get some additional digital address data from the telephone network, but the record format wasn't designed with VoIP or cellular in mind. So in those cases, the telephone network sends a virtual number and the dispatch computer does a seperate out-of-band lookup with the VoIP/cellular company using that number as a key to get your location.

The whole emergency calling system is layers upon layers of hacks. While they can bolt additional functionality on if they're creative, it's more likely a given feature is _not_ implemented. There's a good chance that by the time the call gets to dispatchers, the IMEI/IMSI isn't displayed anywhere and they just see a random virtual number.

Gosh, yes it's a bit of a mess...

The PSAP (E911 end point) likely will receive an MDN/MSISDN (10 digit number you dial for NANP networks) - this is so they can call back if the call is dropped.

E911 is a special service, so in the case of deactivated/missing SIM cards, the carrier assigns a temporary MSISDN for duration of the call when the UE exits E911 mode - there are actually of regulatory and carrier requirements around E911 mode.

E911 Phase 2 required not only the DN, but also if possible the location of the device - whether thru Cell Base Station Triangulation (if possible) or GNSS with a LAT/LONG based on coarse/fine location info.

In any case, as @tjohns mentions, IMSI/IMEI are not typically used outside of the servicing network.

Huh. That's a think I've had vague notions of, but you've made pretty explicit.

Technology under pressure looks a lot like biology.

I've had this thought before while studying biology in college while working on legacy code as an intern. There's definitely a "designed by natural selection" feel to some old legacy systems.

Good point, and implementing a "this IMEI calls 911 to ask for the time of day every hour" block on the side of the networks seems risky as well, so I get how it might not be that helpful.

But then I wonder why having an IMSI (as far as I understand, the SIM can be deactivated, foreign etc., i.e. it doesn't need to actually register to the network) improves this in Germany?

Maybe German authorities just hope that having to insert a SIM might deter people, since SIMs are perceived as being personally identifiable more than just phones without a SIM?

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