The Common Alerting Protocol (CAP) is the standard XML format emergency alerts travel in, so that one alert can reach sirens, phones, radio and screens without being rewritten for each. For digital signage, CAP is how a screen can show an official alert the moment it is issued, instead of waiting for someone to type it into a dashboard.
What a CAP alert contains
CAP is an OASIS standard, currently version 1.2. Each alert carries:
- who sent it and when
- its urgency, severity and certainty
- the area it applies to
- a headline, a description and instructions
- when it expires
In the United States, FEMA's IPAWS distributes official alerts as digitally signed CAP messages, and most mass notification systems can send and receive CAP too.
How alerts reach screens
| Route | How it works |
|---|---|
| A CAP feed | The signage system reads the feed and acts on alerts for its area |
| A mass notification system | The organization's alerting system sends one alert to every channel, screens included |
| An API call | Whatever raises the alert tells the signage system to switch to an emergency message |
What the signage side has to do
- Take over the right screens at once, ahead of everything else scheduled.
- Show the alert large enough to read across a room.
- Keep showing it if the network drops after it arrives.
- Clear when the alert is cancelled or expires, without anyone remembering to undo it.
- Leave a record that it played, for the drill log and the incident review.
What to ask a signage vendor
- Can it read CAP directly, or does it need another system in between?
- How quickly does a takeover reach every screen?
- What happens to screens that are offline when the alert is sent?
- How is a takeover cleared?
- Is there a record of what played, and when?
Where Screenly stands
Screenly has no built-in CAP reader. Its takeover is the priority playlist: while its schedule is active it replaces everything else on the screens you choose, and it hands back on its own when the schedule ends. Everything in the dashboard is in the API, so the system that raises the alert can switch it on. ScreenlyOS devices play from local storage if the network drops, and proof of play records what played, on request.
