273Users and government regulators expect VoIP service providers to support
emergency calls as good or better than the PSTN. Emergency calls need to con-
vey the exact location of the caller so that assistance can be dispatched to
where it is required. Note that this is in contrast to the initial deployments of
mobile telephony, where even as of this writing, mobile phone services do not
always support emergency calling with location information.
Making emergency calls over the Internet is not a trivial task for a variety of
reasons, all Internet-related, but also because of other administrative issues,
some of which will be mentioned here.
Internet-related difficulties stem from the very fact that users can be located
anywhere, and the IP address bears no relation to their location. There are var-
ious mobility scenarios on the Internet (see Chapter 15); the most obvious is
users of VPN tunneling. An employee of a company located in Ireland may be
traveling in Australia and connect from a Wi-Fi hot spot to the home network
in Dublin using VPN. The SIP UA will have an internal enterpise IP address
obtained from the VPN system. If an emergency call, say, for medical pur-
poses, is placed by the user, the call will appear on the Internet as coming from
the home network in Dublin, and sending an ambulance to the office in Dublin
would not be helpful.
Emergency calls are routed to a Public Safety Access Point (PSAP), where an
agent takes the call and determines where to route it further to provide the
required assistance (such as police, fire, ambulance, mountain rescue, and so