Step 1: UE switched ON – Cell selection
Step 2: UE to eNB – RRC connection request
Step 3: eNB to UE – RRC connection set-up
Step 4: UE to eNB – RRC Connection complete+ Attach request + PDN conn request
Step 5: eNB to MME – Initial UE message + NAS-PDU ( Attach request + PDN conn request )
Step 6: MME to HSS – Authentication request ( Acquisition of Authentication vector )
Step 7: HSS to MME – Authentication information Answer ( RAND, AUTN, XRES, Kasme )
Step 8: MME to UE – Authentication request ( RAND, AUTN )
Step 9: UE to MME – Authentication response ( RES )
Step 10: MME – Authenticates UE if RES = XRES ( UE authenticated )
MME will generates Security keys
Step 11: MME to UE – Security Mode command
UE will generates NAS keys
Step 12: UE to MME – Security mode Complete.
Step 13: MME to HSS – Location update request
HSS will registers UE IMSI , MME ID.
Step 14: HSS to MME – Update location answer ( Subscriber profile + APN + P-GW + QoS )
MME stores APN & subscriber profile
MME will Assigns EPS bearer ID & P-GW selection by HSS or MME
Step 15: MME to S-GW – Create session request
Step 16: S-GW to P-GW – Create session request
P-GW will assigns IP address to UE
Step 17: P-GW to PCRF – Notification of EPS session set-up
PCRF will determines PCC ( Policy charging & control ) policies for the EPS session to established
Step 18: PCRF to P-GW – Acknowledging EPS Session Establishment
P-GW will perform olicy Enforcement (Install Policy Rule)
Step 19: P-GW to S-GW – Create session response
Step 20: S-GW to MME – Create session Response
MME – Generates Various parameters ( UE AMBR )
Step 21: MME – eNB – Initial context set-up request ( Attach accept in NAS-PDU )
eNB – Receives S-GW address and generates keys
Step 22: eNB to UE – Security mode command
UE generates Kenb and AS keys
Step 23: UE to eNB – Security mode complete
Step 24: eNB to UE – RRC connection reconfiguration + Attach Accept
UE – DRB is created between UE and eNB
Step 25: UE to MME – Attach complete ( via eNB )
Step 26: eNB to MME – Initial context set-up response
Step 27: MME to S-GW – – Modify bearer request + eNB address + EPS bearer ID
Step 28: S-GW to MME – Modify bearer response
S1 bearer is established between eNB and S-GW. EPS default bearer is UP and user can exchange data.