[mgsouth]

For folks jumping on saying "that's not a carrier thing". All comms are a carrier thing. Whether it's ETWS, SMS, or IP, it's going through the carrier, they process it, and they do extensive traffic management. Carriers absolutely can and will inspect, proxy, aggregate, and do anything else that will tease out another few % of "free" capacity.

[Edit:] All too real scenario: Carrier knows about particular IP addresses and ports used by alert service. Carrier makes provision for separate path for it. Carrier also tries to shave said provisioning to the bone, calculates a worst-case, and adds 5% capacity. Which doesn't get updated when that particular app gets a 6% boost in subscriptions. Back in the old days the traffic management folks would be on top if it, but that's all been outsourced...

[sib]

In this case, there is a separate service that Google developed for early warning.

(Source, worked at Google in Android team.)

[numpad0]

PWS is tower based broadcast. Everyone within range of a tower gets the alert. Data source is supposed to be local government weather authority, I think USGS and NOAA in US. Or the Meteorological Agency in Japan.

You can do a location-based two way warning system and there are such services, but it's going to be laggy and won't scale to 100M+ simultaneous subscribers. One-way broadcast scales to the planet if wanted.