\section{Introduction}
\label{sec:intro}

Providing emergency communications, exemplified by the Enhanced 911
(E911) service available in the United States, encompasses a number of
steps and requirements \cite{Nena:911,Telc9705:E911}:

\begin{changebar}
\begin{itemize}
\item The caller needing emergency assistance initiates a call to the
appropriate Public Safety Answering Point (PSAP), typically via
nationwide uniform number (911 in the United States, 000 in Australia,
112 in the European Community, with separate numbers in some countries
for police, ambulance or fire). This functionality, without the
additional features below, is called ``basic 911' in the United States.

\item The call is routed by the local switch to an E911 tandem switch. 
The E911 tandem switch consults a Selective Routing Data Base (SRDB)
that maps the calling number to a PSAP.

\item At a PSAP, emergency operators determine the nature of the
emergency and contact the appropriate agency.  A single PSAP is usually
responsible for an area covering several independent police and fire
departments.  This area is called an {\em Emergency Service Zone} (ESZ). 
It is identified by an Emergency Service Number (ESN), a number
consisting of three to five digits that represents a unique combination
of emergency service agencies such as law enforcement, fire and
emergency medical service.  The Master Street Address Guide (MSAG) is a
database of street names and house number ranges within a community
defining emergency service zones and the associated Emergency Service
Numbers (ESNs).

Note that the appropriate PSAP may not be the one geographically closest
to the caller, due to political, geographic and administrative
boundaries.  Also, service areas of different emergency service agencies
may not be the same.  For example, a whole city may be served by the
same law enforcement agency, but may have a number of different
emergency medical service units serving different boroughs or other
divisions.

Calls to the first PSAP, the so-called primary PSAP, may be relayed to a
secondary PSAP.

\item Wireless phones in the United States must be able to reach 911
even if the phone is not a registered subscriber.

\item Since it is possible that the caller is too confused, frightened
or young to properly identify the location of the emergency, modern 911
systems [not sure about Europe] require all local telephone operators to
maintain a database, called the ALI (Automatic Location Identification)
database, containing current subscriber addresses, telephone numbers and
supplementary emergency information for landline telephones. 

For wireless service, operators must be able to identify
the location of the user within 50\,m (for 67\% of calls).

The PSAP obtains the caller's phone number, name and address.  This
database lookup is called Automatic Location Identification (ALI).  The
caller's phone number is delivered to the PSAP regardless of any
restrictions on caller ID delivery (CLID).

The location and routing features above constitute ``enhanced 911'' in
the United States.

\item Certain services, such as call-waiting or three-party calls, are
restricted during (basic) 911 calls.  For example, the caller cannot
hang up the call and place another call.

``If the PSAP attendant gets disconnected from the caller, the PSAP
attendant may make an outgoing call using the information provided to
construct the 10-digit ANI for dialing.

It should be noted that certain features traditionally provided with
Basic 9-1-1 service are not provided with E9-1-1 service; these features
are emergency ringback, end-to-end called-party hold, and switch-hook
status display.  Other E9-1-1 features replace the intent of these
features and make them unnecessary.'' \cite{Telc9705:E911}

\end{itemize}
\end{changebar}

Internet telephony offers the opportunity not only to supply current
emergency services, but also to add functionality.  For example,
multimedia communications can be used to provide video images or
biometrics.  A two-way video call allow an emergency medical technician
to instruct a person at the scene of the emergency in emergency
procedures and provide corrective feedback to those performing first
aid.

The call setup can provide additional medical background, without having
to store the information in a central database.  Due to the faster call
setup times and the ease of redirection, calls can be forwarded and
transferred to emergency personnel en route to the caller or a nearby
PSAP, in case the primary PSAP serving the caller's location is
overloaded.  Unlike current PSAPs, which require dedicated equipment, an
Internet-based PSAP could be set up anywhere Internet access is
available, providing easy relocation should the PSAP be unavailable
during natural disasters.  The ability to indicate language capabilities
of the caller can help route the call to an operator, without the
additional delay of having a general operator try to ascertain the
language of the caller.

Multiple media, including audio, video and text chat, also offer
improved capabilities to people with disabilities, making it more
likely, for example, that a deaf person visiting somewhere can find a
multimedia-capable device from which at least text (instant message)
communication is possible.

Also, while 911 service is available to about 85\% of the U.S. 
population, other countries have very different numbering schemes (e.g.,
110 and 112 in Germany and other European countries).  Also, many
corporate and hotel PBX systems do not support 911.  Internet telephony
may be able to make the service universally available.

However, many of the assumptions underlying the current 911 service do
not hold for Internet telephony. 

It appears likely that users of Internet telephony services, wired or
wireless, expect similar behavior as in existing networks.  Indeed, even
if the call is terminated at the PSAP via the PSTN, difficulties can
arise if the caller is using Internet telephony, as the fire department
may well be summoned to the telephone company building housing the
VoIP-PSTN gateway.

Simply replicating the existing phone-only emergency call system seems
ill-advised.  In the future, users are not likely to perceive Internet
telephony as a separate service, but simply as another communications
service along with email, chat and maybe even distributed games.  Thus,
it is desirable to allow a user to summon emergency assistance from any
communication application, regardless of the underlying protocols.  A
general infrastructure also makes it easier for users with disabilities
to summon help \cite{draft-rosenberg-sip-hearingimpaired}.

Emergency numbers in an Internet context require components similar to
those listed above.  We first describe alternatives for reaching an
emergency operator in Section~\ref{sec:reaching} and then discuss how
the emergency operator can locate the caller in
Section~\ref{sec:locating}.

