Emergency Session Creation in LTE Call Flow

Emergency Session Creation in LTE


StepDescription
1

When an emergency service is required and an emergency PDU session is not already established, the UE initiates the UE-requested PDU session establishment procedure with a request type indicating, "Emergency Request" in LTE.

2

The MME selects an APN or DNN for the emergency PDN creation, and sends a ‘Create Session Request’ to the P-GW via the S-GW.

3

The DNN profile lookup at P-GW is based on the subscriber policy or DNN policy. These policies are associated in the SMF profile. The subscriber policy has higher precedence over DNN policy when both the configurations are present.

4

The DNN policy can have the DNN profile configuration for each of the UE-requested APN or DNN received in the “Create Session Request” from the MME or S-GW.

5

When a new configuration ‘authorization local’ under the selected DNN profile is present:

  • P-GW skips the UDM interaction for fetch subscription and uses the values received in the ‘Create Session Request’ message from the MME.

  • P-GW skips the UDM interaction to ‘Subscribe-for-Notification’ from the UDM.

6

When the ‘Secondary Authentication Radius’ under the selected DNN profile is not present, the PGW-C rejects the RADIUS-based secondary authentication.

7

When ‘failure handling’ for charging interaction is set as ‘action continue’:

  • P-GW continues the call if converged charging is not configured.

  • P-GW falls back to offline charging and continues the call.

8

During handover from 4G to 5G using N26, if the emergency PDN gets handed over, the SMF checks the DNN profile and if ‘authentication local’ is present, it skips the UDM interactions for registration and deregistration.