Friends -
As I went back to the books on writing use cases, I realized that
design use cases are very useful for describing processes, but not
quite so handy for describing events, which is really what individual
messages are.
So what I'm offering here for your discussion aren't so much use
cases (except perhaps in the dreaded "business use case" sense!) as
they are just illustrations of some potential real-world applications.
- Art
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Use Scenarios - Common Alerting Protocol Message Format
draft 5/11/2003
The following scenarios illustrate a few possible use cases for the
Common Alerting Protocol message format. [Note that at this point we
are discussing only the message itself, not the telecommunications or
applications architecture(s) that support(s) it.]
I - Manual Origination
The Incident Commander at an industrial fire with potential of a
major explosion decides to issue a public alert with three
components: a) An evacuation of the area within half a mile of the
fire; b) a shelter-in-place instruction for people in a polygon
roughly describing a downwind dispersion "plume" extending several
miles downwind and half a mile upwind; and c) a request for all media
and civilian aircraft to remain above 2500 feet above ground level
when within half a mile of the incident.
Using a portable computer and a web page (and a pop-up drawing tool
to enter the polygon) the Incident Commander issues the alert as a
CAP message to a local alerting network.
II - Automated Origination by Sensor System
Along a popular Northwest beach a series of automatic tsunami sirens
have been installed. In the base of each siren is an accelerometer
and a computer with a GPS receiver. All of these devices are linked
by a wireless local area network.
When an individual unit detects what appears to be a P-wave/S-wave
sequence that might indicate a near-shore earthquake, it generates a
CAP message containing its location, the precise time of the P-wave's
arrival, and the estimated distance of the epicenter based on the
P-wave/S-wave interval.
Each computer in the network correlates all the messages received
from the other instruments (which form what amounts to a phased
array) and, if the pattern of messages suggests a near-shore quake
(and not just a truck driving down the coastal highway) then each
siren activates. In addition, one of the instruments assembles a
"summary" CAP message describing the event and feeds it to regional
and national seismic networks.
(Based on an idea shared by David Oppenheimer at the USGS in Menlo Park.)
III - Aggregation and Correlation on Real-time Map
At the State Operations Center a computerized map of the state
depicts, in real time, all current and recent warning activity
throughout the state. All major warning systems in the state... the
Emergency Alert System, siren systems, telephone alerting and other
systems... have been equipped to report the details of their
activation in the form of a CAP message. (Since many of them are now
activated by way of CAP messages, this is frequently just a matter of
forwarding the activation message to the state center.)
Using this visualization tool, state officials can monitor for
emerging patterns of local warning activity and correlate it with
other real time data (e.g., telephone central office traffic loads,
9-1-1 traffic volume, seismic data, automatic vehicular crash
notifications, etc.)
IV - Integrated Public Alerting
As part of an integrated warning system funded by local industry, all
warning systems in a community can be activated simultaneously by the
issuance by authorized authority of a single CAP message.
Each system converts the CAP message data into the form suitable for
its technology (text captioning on TV, synthesized voice on radio and
telephone, activation of the appropriate signal on sirens, etc.)
Systems that can target their messages to particular geographic areas
implement the targeting specified in the CAP message with as little
"spill" as their technology permits.
In this way, not only is the reliability and reach of the overall
warning system maximized, but citizens also get corroboration of the
alert through multiple channels, which increases the chance of the
warning being acted upon.
V - Repudiating A False Alarm
Inadvertently (or perhaps not, but that will be the subject of
investigation later on) the integrated alerting network has been
activated with an inaccurate warning message.
This activation comes to officials' attention immediately through
their own monitoring facilities (e.g., III above). Having determined
that the alert is, in fact, inappropriate, the officials issue a
cancellation message that refers directly to the erroneous prior
alert. Alerting systems that are still in the process of delivering
the alert (e.g., telephone dialing systems) stop doing so. Broadcast
systems deliver the cancellation message. Other systems (e.g.,
highway signs) simply reset to their normal state.
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