– How does the Emergency Alert system work including ‘Cell Level Broadcast’ from Base Station to Handset

Question

– How does the Emergency Alert system work including ‘Cell Level Broadcast’ from Base Station to Handset

In: 7

Answer 1

Every phone on a cell network is always listening to a paging channel. The main CPU may sleep for a long time but the modem wakes up every second, decodes the paging channel, and goes to sleep in 10-20 milliseconds if nothing relevant to it was broadcast.

That is how incoming calls and push notifications work. Cell tower broadcasts on the paging channel phone ids which should wake up and check for incoming calls and data. To wake up all phones cell tower broadcasts on the paging channel special id which mean every phone should wake up and decode special cell broadcast data in the data channel. To prevent overwhelming the tower unlike incoming calls and push notifications cell broadcast data does not require each cell phone to request any data from the network. All phones operate in receive-only mode. If a phone fails to receive the broadcast it cannot request it. Cell towers can be configured to repeat the same broadcast several times so that phones have multiple opportunities to receive the data. Each message has an id so the phones that already received it once successfully can ignore retransmissions.

Authorities that wish to broadcast a message coordinate the list of areas with the carriers ahead of time. For example a US state may provide a list of counties and a list of major cities with canonical spelling. The carriers compile a list of towers in each area. When it is time to broadcast a message across a list of areas carrier merges the lists of towers in the requested areas and tells each tower to broadcast the message.

In many cases especially in rural areas cell tower coverage does not match the true boundaries of areas. About 7-10 years ago to make notifications match the true boundaries the FCC requested an enhancement. Now a broadcast message can have an area defined as a polygon (list of latitude, longitude points). Once phone decodes the broadcast it checks if it is actually in the area. If it is not it goes to sleep.