Initial Attach on E-UTRAN or EPS Procedure
The following figure shows the call flow derived from 3GPP reference for initial attach on E-UTRAN or EPS.

Step | Description |
---|---|
1 | UE sends Attach Request to the MME through eNodeB. |
2 | The MME determines that the UE is capable and subscribed for handoff to NR. It selects an SMF node as the P-GW for this PDU session. |
3 | The MME sends Create Session Request to the selected S-GW and includes the selected SMF address in it. |
4 | The S-GW initiates Create Session Request towards the SMF. |
5 | The SMF extracts the PDU Session ID sent by the UE in the Protocol Configuration Option (PCO) 001AH (PDU session ID) and saves it. It then performs a Unified Data Management (UDM) registration and sends P-GW Fully Qualified Domain Name (FQDN) to the UDM. After registration, the SMF initiates subscription fetch from the UDM. |
6 |
The SMF sends Npcf_SMPolicyControl_Create to the PCF to initiate SM policy Association Establishment. The SMF includes the information elements received in Create Session Request message into the Npcf_SMPolicyControl_Create Service as follows:
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7 | The SMF receives Policy Charging and Control (PCC) Rules and PDU Session Policy Information, 5G QoS information in PCC Rule and in PDU Session Policy Information which are mapped into EPS QoS information. The SMF creates Traffic Flow Template (TFT) from the Service Data Filters (SDFs) received in PCC rules and associates them with the corresponding default and dedicated bearers. |
8 | Based on the charging policies received from PCF, the SMF initiates Nchf_ConvergedCharging_Create operation towards the CHF. This procedure is similar to a 5G session and is based on the charging rules received from the PCF. |
9 | The SMF performs UPF+PGW-U selection and N4 Session Establishment. Since this is a 4G session connecting the SMF, a separate CN tunnel is created for each bearer and QoS Flow ID (QFI) is not sent in the QoS Enforcement Rule (QER) and Packet Detection Rule (PDR). |
10 |
The SMF sends Create Session Response to S-GW and includes the bearer information and Tunnel Endpoint Identifier (TEID) for the default bearer. The SMF also includes the 5G QoS parameters in PCO options 001CH (QoS rules), 001DH (Session-AMBR), 001EH (PDU session address lifetime) and 001FH (QoS flow descriptions) to the UE. |
11 | The S-GW sends Create Session Response to the MME. |
12 | The MME sends Initial Context Setup Request to the eNodeB with N1 Attach Accept message. |
13 | The eNodeB and UE perform Radio Resource Control (RRC) configuration. |
14 | The UE sends Direct transfer message to the eNodeB. |
15 | The eNodeB sends Attach Complete message in Initial Context Setup Response to the MME along with the TEID of eNodeB. |
16 | The MME sends a Modify Bearer Request to the S-GW with eNodeB TEID. |
17 | The SMF sends the Modify Bearer Response to the S-GW. |
18 | The S-GW sends Modify Bearer Response to the MME. |