epdg call flow

The Enhanced Packet Data Gateway (ePDG) is a key component in the evolved Packet System (EPS) architecture of 3GPP LTE (Long-Term Evolution) networks. It plays a crucial role in supporting the seamless handover of IP sessions for user equipment (UE) between the mobile network and non-3GPP access networks, such as Wi-Fi.

Here is a technical explanation of the ePDG call flow:

1. UE Attachment to the ePDG:
   • The User Equipment (UE) initiates the attachment procedure by sending an Attach Request to the ePDG.
   • The Attach Request contains UE identity, capabilities, and other relevant information.
   • The ePDG validates the UE's credentials and allocates resources for the UE.
2. Authentication and Security Setup:
   • The ePDG communicates with the Home Subscriber Server (HSS) to authenticate the UE.
   • The HSS generates authentication vectors, which are sent to the ePDG.
   • The ePDG uses these vectors to authenticate the UE.
   • Security associations are established between the UE and the ePDG using IKEv2 (Internet Key Exchange version 2) protocol, ensuring secure communication.
3. IP Connectivity Establishment:
   • Once authenticated, the ePDG allocates an IP address to the UE.
   • The ePDG may act as a DHCP server or interact with a separate DHCP server to assign an IP address to the UE.
4. Bearer Establishment:
   • The ePDG establishes a default bearer or dedicated bearers for the UE based on the QoS (Quality of Service) requirements.
   • Bearer establishment involves signaling with the Mobility Management Entity (MME) and the Serving Gateway (SGW) in the LTE core network.
5. P-CSCF Discovery:
   • The UE needs to discover the Proxy Call Session Control Function (P-CSCF) for IMS (IP Multimedia Subsystem) services.
   • The ePDG provides the P-CSCF information to the UE, which the UE uses for IMS registration.
6. IMS Registration:
   • The UE registers with the IMS using the obtained P-CSCF information.
   • The registration process involves interaction with the IMS components, including the Home Subscriber Server (HSS) and the Proxy Call Session Control Function (P-CSCF).
7. Data Transfer:
   • With the IP connectivity and bearers established, the UE can now transfer data through the ePDG.
   • The ePDG acts as a gateway, forwarding user data between the UE and the external network (e.g., the Internet) over the non-3GPP access.
8. Mobility and Handover:
   • If the UE moves between different access networks (e.g., from LTE to Wi-Fi or vice versa), the handover process involves signaling between the ePDG, MME, SGW, and other network elements to ensure a seamless transition of the IP session.
9. UE Detachment:
   • When the UE no longer needs the IP connectivity through the ePDG (e.g., when disconnecting from the Wi-Fi network), a Detach procedure is initiated.

This call flow represents a simplified overview, and the actual implementation details may vary based on network configurations and specific deployment scenarios. Additionally, the ePDG call flow may interact with other elements, such as the Evolved Packet Core (EPC), in the LTE network architecture.