■■ Credible emergency call testing mechanisms mustbe provided.
■■ It should be possible to invoke relay services for the hearing or speech
disabled (see Chapter 17, “Accessibility for the Disabled”).Location Information
As mentioned, location information is a critical ingredient for any emergency
call. At the same time, location information is very sensitive and must be care-
fully protected. The protection of location information has been defined in the
IETF working group on Geographic Location and Privacy (geopriv) [3].
Types of Location Information
There are four types of location information as shown in Table 16.1.
Sources of Location Information
SIP UA can derive location information (LI) from a number of sources, and
should indicate the source of LI that may be used for call-routing decisions.
There are a variety of sources available to the SIP UA for location information,
such as the following:
■■ Manually entered LI
■■ Wire databases for the Ethernet switch, line identification dabase for
DSL, or cable service provider
■■ GPS (available only when there is a clear view of a large part of the sky)
■■ Third party:
■■ Wireless triangulation in mobile networks
■■ Location radio beacons announcing the locationUnless the SIP UA has direct access to LI, it must obtain the Location Object
(LO) using DHCP.
DNS-Based Location Information
As mentioned, determining the right PSAP to route an emergency call to is dif-
ficult for several reasons.
These difficulties can be circumvented with central databases that map
PSAPs to the civic address and to the data in the MSAG. Such a centralized
approach is used in the PSTN and has advantages and disadvantages common
to all centralized approaches. The centralized approach is described later in
this chapter in the section, “Using the PSTN for VoIP Emergency Calls.”
Emergency and Preemption Communication Services 275