Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 16.1 Emergency call routing on the Internet

Once the location and the PSAP URI are available, three call routes are
possible:

1. The emergency caller routes the call directly to the PSAP. This is the
   most reliable alternative, since there are no other dependencies of possi-
   bly remotely located SIP proxies owned by a VoIP service provider.


2. The emergency call is routed to the PSAP via the ECRS SIP proxy of the
   VoIP service proivder.


3. There is no Internet-connected PSAP available for the location of the
   user. The ECRS will route the emergency call as a last resort to the
   PSTN gateway to connect to the PSAP.
   Routing a VoIP emergency call to the PSTN implies additional complexity,
   some of which is presented later in this chapter in the section, “Using the
   PSTN for VoIP Emergency Calls.”

Security for Emergency Call Services

Attacks on the PSTN to exploit emergency services have been known to hap-
pen, mainly to steal service. Security is a much larger issue when routing
emergency calls on the Internet. The possible threats have been analyzed in [6]
and the requirements for secure SIP call routing are also proposed.

Directory of

PSAPs SIP ECRS

2 3

2

1

3

3

3

Location Information

Routes to the PSAP:

1. Direct


2. Internet Emergency Call Routing Support (ECRS)


3. PSTN gateway and the PSTN

PSTN

Gateway

Emergency

Caller

PSAP

PSTN

Location

Server

Emergency and Preemption Communication Services 279