Initial Attach on E-UTRAN via MME and S-GW

Initial attach on E-UTRAN/EPS follows the procedure defined in 3GPP TS 23.401, Section 5.3.2.1.

The following diagram shows the call flow derived from 3GPP reference for initial attach on E-UTRAN/EPS.

Initial Attach on E-UTRAN via 5G Core Call Flow
StepDescription

1

At Step 9, SMF+PGW-C perform a UPF selection and perform N4 Session Establishment procedure. Since this session is a 4G session connecting to SMF+PGW-C, separate CN tunnel is created for each bearer and QFI is not sent in the QER and PDR, correlation ID might be present.

2

At Step 18, SMF+PGW-C performs N4 Session Modification to update the eNodeB TEID on the data path to the UPF.

The 3GPP specifications provide mechanisms to achieve mobility of a UE from LTE to 5G NR and vice versa. This mobility is achieved in two different architectures – with and without N26 interface between AMF and MME.