[EAS] WEA - time out?

[Sean Donelan]

On Wed, 3 Oct 2018, Rich Parker wrote:
> I learned today that the WEA alerts are set to time out - either 15
> minutes, or 30 minutes (depending upon who you listen to).   It came
> up in the context of 'what if my phone is off' and someone said that
> when you turned your phone back on you would get the alert, if it
> hadn't timed out.

All CAP/IPAWS messages have an expiration date/time set by the sender. In 
this case, FEMA set the CAP message from 2:20pm EDT until 2:50pm EDT (30 
minutes. The maximum expiration time for CAP/IPAWS alerts is 24 hours.

NCMEC usually sets the experiation for Amber alerts to 24 hours.
NWS usually sets flash flood alerts to 6 hours, and tornado alerts to 45 
minutes.
Tests messages usually have 15 minutes to 60 minutes.

Participating WEA carriers repeat the cell broadcast periodically 
throughout the effective period of the alert. The precise repetition 
pattern varies by carrier. Its typically a initial burst of 3 to 5 
duplicate broadcasts two seconds apart, and then every 10 to 30 minutes 
until the end of the effective time of the alert, or the alert is 
cancelled through IPAWS.

Each phone handset is supposed to de-duplicate WEA alerts. Handsets should 
only show an alert once no matter how many times it receives duplicate cell 
broadcast.  The WEA alert message includes a mini-serial number to help 
detect duplicates, but too many alerts within a short time period can 
cause the serial number to roll-over.

There appears to be a bug in some handset software or tower management 
software that causes "stuck" duplicates. Some people get lots of duplicate 
messages, every time the cell broadcast repeats.