The emergency call service is one of the important GSM-R services. The success rate and delay of setting up emergency calls are important counters for project acceptance. The priority of emergency calls is 0. The following figure shows the fast setup process of emergency calls.
To quickly set up a group call or broadcast, the MS can initiate a call using the Immediate Setup process. In the process, the MS sends the Immediate Setup message to the network side before the MM connection is established. The message contains information such as the service type and group call reference. This way, the MM information is not necessary and therefore the group call is quickly set up. The figure on this slide shows the detailed procedure. After receiving the Immediate Setup message, the BSC performs the following operations:
1.The BSC decodes information in the Immediate Setup message, such as the classmark, MS identity, and group call reference.
2.The BSC obtains information about the MS, such as the supported frequency band and power level based on the decoded classmark.
3. The BSC prepares for the user tracing and group call tracing by decoding the MS identity and group call reference.
4. The BSC converts the Immediate Setup message to a full layer-3 message and sends it to the MSC.
The following figure shows the signaling tracing of a single MS that initiates the emergency call
