2.1 Emergency Calls
The support for emergency calls, that is, calls to a special emergency center, is a mandatory functionality in all cellular wireless networks. Strictly speaking, User Equipment (UE) support of emergency calls is only required under the condition that the UE supports also speech calls. So a data card UE with no speech call support does not need to support emergency calls. In principle, emergency services can use other means for communication as well, for example, text messages for disabled persons or video calls.
Emergency calls will be routed to a call center called PSAP (Public Safety Answering Point). In general, a call is routed to a particular PSAP based on the dialed number (such as 911 or 112) and location of the user in accordance with national regulations. There are different emergency call centers for police, ambulance, fire brigade, marine guard, and mountain rescue depending on the type of emergency. Emergency call centers may be connected to the Public Switched Telephone Network (PSTN), Circuit-Switched (CS) domain, Packet Switched (PS) domain, or any other packet network. For historical reasons they are still connected to the PSTN but a transition to IP-based PSAPs is underway in some countries. An emergency call over the Long-Term Evolution (LTE) Internet Protocol (IP)-based network utilizes IP Multimedia Subsystem (IMS) as application over the top. Both LTE and IMS required some adaptations to fulfill regulatory ...