on). Determining the right PSAP is not trivial either, because the service
boundaries for the various response centers for police, fire, ambulance, and so
on may differ for various local administrative reasons. They vary by local
jurisdiction, and they also vary from country to country. It is estimated there
are about 6,000 PSAPs in North America alone and possibly three times as
many in the rest of world [1]. Accurate dispatch of assistance to the emergency
caller requires mapping the civic or geographic address to the exact street
address where the caller will be found. This mapping is done using the Master
Street Address Guide (MSAG). The organization of PSAPs and responders
(such as fire, police, ambulance, and so on) is done differently in various parts
of the world.Requirements
The IETF working group on Emergency Context Resolution Using Internet
Technology (ECRIT) has oultined the requirements for emergency calling [2],
and we will provide here a short overview of these requirements.
■■ The caller may nothave a VoIP service provider, since a user, a resi-
dence, or a small business may have their own domain name and SIP
endpoints for their domain (such as SIP UAs or their own SIP proxy).
Larger enterprises or university campuses may also provide VoIP for
their users. A VoIP service provider must,therefore, not be assumed.
■■ PSAP information mustbe available even when no VoIP service
provider is used.
■■ Internet emergency protocols and data formats mustsupport consistent
international deployment.
■■ Deployment of emergency calling over the Internet must notdepend on
any central authority.
■■ Internet multimedia, including IM and Text over IP (ToIP), mustbe sup-
ported for better assistance. It should be possible to convey telemetry
data, such as that from crash sensors or medical vital signs from
patients under remote supervision.
■■ Emergency calls that can be routed within a region must notbe routed
over long paths that are outside the region, and must nothave any out-
of-region dependencies, such as remote SIP proxies, or gateways to the
PSTN.
■■ The ECRIT mapping protocol mustreturn a URI that can be understood
by any “legacy” SIP UA.
■■ Callback information mustbe provided to enable emergency assistance
to call back the user.274 Chapter 16