E-STN-SR (Emergency session transfer number for SRVCC)

Introduction:

E-STN-SR (Emergency Session Transfer Number for Single Radio Voice Call Continuity) is a feature used in LTE (Long-Term Evolution) networks that enables emergency calls to be transferred from LTE to 3G/2G networks in case of network unavailability or loss of LTE coverage. It is a vital feature for public safety, allowing emergency calls to be handled without interruption.

Background:

When a user makes an emergency call, the network automatically prioritizes the call and assigns it the highest priority level, known as emergency bearer. This ensures that the call is transmitted with the maximum available bandwidth, and all necessary resources are allocated for the call. However, in the case of LTE networks, there is a possibility of network unavailability or loss of LTE coverage due to various reasons, such as network congestion, natural disasters, or other unexpected events. In such cases, the call may be dropped or disconnected, which can have severe consequences.

To overcome this issue, the concept of SRVCC (Single Radio Voice Call Continuity) was introduced. It allows for the seamless handover of voice calls from LTE to 3G/2G networks without interruption. However, in the case of emergency calls, it may not be possible to perform the standard handover procedure due to the high priority level and the need for uninterrupted service.

To address this issue, E-STN-SR was introduced, which provides a dedicated emergency session transfer number that can be used to transfer the emergency call from LTE to 3G/2G networks without interruption. Let's take a closer look at how E-STN-SR works.

E-STN-SR Procedure:

When a user makes an emergency call, the LTE network automatically assigns an emergency bearer and establishes a connection with the IMS (IP Multimedia Subsystem) network. The IMS network is responsible for handling the voice and multimedia services over LTE. However, if the LTE network becomes unavailable, or the user moves out of LTE coverage, the emergency call needs to be transferred to a 3G/2G network to ensure uninterrupted service.

In such cases, the E-STN-SR procedure comes into play. The E-STN-SR procedure is triggered by the LTE network when it detects that the call needs to be transferred to a 3G/2G network. The LTE network then assigns a dedicated E-STN-SR number to the call and forwards it to the MSC (Mobile Switching Center) of the 3G/2G network. The MSC is responsible for routing the call to the appropriate PSAP (Public Safety Answering Point), which is the emergency service center responsible for handling emergency calls.

Once the MSC receives the call with the E-STN-SR number, it checks whether the PSAP is capable of handling emergency calls over 3G/2G networks. If the PSAP is capable, the MSC forwards the call to the PSAP, which handles the call as a regular emergency call.

In case the PSAP is not capable of handling emergency calls over 3G/2G networks, the call is routed to an alternate PSAP that can handle emergency calls over 3G/2G networks. The MSC uses location-based routing to determine the nearest alternate PSAP and forwards the call to it. The alternate PSAP then handles the call as a regular emergency call.

Once the call is transferred to the 3G/2G network, it is treated as a regular emergency call, and the user can continue to communicate with the PSAP without interruption.

Conclusion:

E-STN-SR is a vital feature for public safety, as it ensures that emergency calls are handled without interruption, even in the case of network unavailability or loss of LTE coverage. The E-STN-SR procedure provides a dedicated emergency session transfer number that can be used to seamlessly transfer emergency calls from LTE to 3G/2G networks without interruption. The procedure involves assigning a dedicated E-STN-SR number to the call, forwarding it to the MSC of the 3G/2G network, and routing it to the appropriate PSAP. If the PSAP is not capable of handling emergency calls over 3G/2G networks, the call is routed to an alternate PSAP.

Overall, E-STN-SR is a crucial feature for ensuring public safety, and it highlights the importance of seamless communication during emergency situations. As LTE networks continue to evolve and 5G networks become more prevalent, it will be interesting to see how E-STN-SR adapts and evolves to meet the changing needs of emergency communication.