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The Common Alerting Protocol (CAP) is an XML-based data format for exchanging public warnings and emergencies between alerting technologies. CAP allows a warning message to be consistently disseminated simultaneously over many warning systems to many applications. CAP increases warning effectiveness and simplifies the task of activating a warning for responsible officials.
Individuals can receive standardized alerts from many sources and configure their applications to process and respond to the alerts as desired. Alerts from the Department of Homeland Security, the Department of the Interior's United States Geological Survey, and the Department of Commerce's National Oceanic and Atmospheric Administration (NOAA), and state and local government agencies can all be received in the same format, by the same application. That application can, for example, sound different alarms based on the information received.
By normalizing alert data across threats, jurisdictions, and warning systems, CAP also can be used to detect trends and patterns in warning activity, such as trends that might indicate an undetected hazard or hostile act. From a procedural perspective, CAP reinforces a research-based template for effective warning message content and structure.
The CAP data structure is backward-compatible with existing alert formats including the Specific Area Message Encoding (SAME) used in Weatheradio and the broadcast Emergency Alert System as well as new technology such as the Commercial Mobile Alert System (CMAS), while adding capabilities including:
- Flexible geographic targeting using latitude/longitude “boxes” and other geospatial representations in three dimensions;
- Multilingual and multi-audience messaging;
- Phased and delayed effective times and expirations;
- Enhanced message update and cancellation features;
- Template support for framing complete and effective warning messages;
- Digital encryption and signature capability;
- Facility for digital images, audio, and video.
The National Science and Technology Council (NSTC) report on “Effective Disaster Warnings” PDF (November 2000) recommended that “a standard method should be developed to collect and relay instantaneously and automatically all types of hazard warnings and reports locally, regionally and nationally for input into a wide variety of dissemination systems”.
In 2001 an international, independent group of over 120 emergency managers began specifying and prototyping the Common Alerting Protocol data structure based on the recommendations of the NSTC report. The project was embraced by the non-profit Partnership for Public Warning and a number of international warning system vendors. A series of field trials and long-term demonstration projects during 2002-03 led to the submission of a draft CAP specification to the OASIS standards process for formalization.
The CAP 1.0 specification was approved by OASIS in April 2004. Based on experience with CAP 1.0, the OASIS Emergency Management Technical Committee adopted an updated CAP 1.1 specification in October 2005. At a meeting in Geneva in October 2006 the CAP 1.1 specification was taken under consideration by the International Telecommunications Union for adoption as an ITU recommendation.
Current implementations 
United States 
According to a CAP 1.0 Fact Sheet, CAP implementations have been demonstrated by agencies and companies including: United States Department of Homeland Security; National Weather Service; United States Geological Survey; California Office of Emergency Services; Virginia Department of Transportation; GeoDecisions, Inc.; Blue292; Warning Systems, Inc.; Comlabs, Inc.; mobileFoundations; Ship Analytics; MyStateUSA; IEM, Inc.; Hormann America, Inc.; Oregon RAINS; Alerting Solutions, Inc. and others.
In early 2005, the U.S. Department of Homeland Security (DHS), in partnership with the Association of Public Television Stations, demonstrated CAP-based "digital EAS" broadcasts over public television digital TV transmitters and satellite links in the Washington, D.C. area and nationwide.
CAP is the foundation technology for the planned "Integrated Public Alert and Warning System," an all-hazard, all-media national warning architecture being developed by DHS, the National Weather Service within NOAA, and the Federal Communications Commission.
In Canada, a working group composed of public alerting practitioners and government agencies has developed a CAP Canadian Profile (CAP-CP) based on CAP but specialized to address the needs of Canadian public alerting stakeholders, such as bilingualism, geocoding for Canada, managed lists of locations and events, etc. The Canadian government has adopted CAP-CP for its National Public Alerting System (NPAS) project. The CAP‑CP working group, along with stakeholders and projects such as the Canadian Association for Public Alerting and Notification (CAPAN) and Netalerts' Sarnia Lambton trial, are now working with and refining CAP‑CP for national application in Canada.
CAP has been implemented for a small-scale, grassroots hazard information system in Sri Lanka following the 2004 Indian Ocean Tsunami. This implementation was part of the "HazInfo Project" funded by Canada's International Development Research Centre.
The Australian Government Standard for Common Alerting Protocol (CAP-AU-STD, 2012) was developed by a CAP-AU-STD stakeholder group comprising federal agencies Emergency Management Australia, the Bureau of Meteorology, GeoScience Australia, Department of Agriculture, Fisheries and Forestry and the Department of Health, as well as a number of State Government authorities and emergency services agencies. The project was co-ordinated by the Australian Government Attorney-General's Department (Australian Emergency Management).
In 2007, the International Telecommunication Union, Telecommunication Standardization Sector (ITU-T) adopted the Common Alerting Protocol as Recommendation X.1303. The recommendation annex contains an authoritative ASN.1 module translation of the CAP XML schema that may be useful for some implementations. Rec. X.1303 is within the remit of ITU‑T Study Group 17 (Security), Rapporteur Group on Cybersecurity (Q.4/17)  for purposes of further evolution of the standard.
- DEAS and Department of Homeland Security
- U.S. Federal Emergency Management Agency "Integrated Public Alert and Warning System" (IPAWS) fact sheet.
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