5G PDU Session Establishment

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prasanna sahu

Technical Product Manager at Viavi Solutions

Telecom Professional

Latest posts by prasanna sahu (see all)

• URSP Flow in a 5G Network - September 25, 2024
• Cell Broadcast Service or Public Warning System in 5G - July 10, 2020
• 5G Security (5G AKA Authentication) - January 20, 2020

PDU Session Establishment Concepts

• In 5G, PDU session Establishment is parallel procedure of PDN connection procedure in 4G.
• This Procedure can be UE requested or NW initiated(In case of emergency call with mobility registration).
• This procedure is required in case of
  1. UE requested PDU session establishment.
  2. UE initiated PDU session HO between 3GPP and Non-3Gpp.
  3. UE initiated HO from EPS to 5GS.
• In roaming scenario AMF determines, if the PDU session will be established at LBO(Local Breakout) or Home PLMN network slice.

PDU Session Management State

5G UE PDU Session Establishment

Steps for PDU Session Establishment

• UE includes following information while sending PDU session establishment.
• PDU Session Establishment Request is carried over NAS UL Transport
  1. Serving NSSAI-> This is UE preferred network slice or NSSAI where UE was registered before.
  2. DNN -> Data Network Name is same as APN in EPS. This is the data service name UE want to access.
  3. PDU Session ID : This is an unique identifier generated by UE. Can’t be same as any existing PDU session.
  4. Request Type-> This can be “Initial Request”, “Existing Session” or “PDU session Handover”
  5. 5GSM Capability-> This is UE’s session management capabilitys.
  6. PCO-> Protocol Configuration Option, same as EPS, and used to request various NW parameter.
  7. SM PDU DN Request Container -> This include Authorization information to access DN.

• Based on request Type AMF Determines if it is a new PDU session or associated to any existing PDU session.
• If NAS message doesn’t contain S-NSSAI. Then AMF selects default NSSAI.
• If NAS message contains S-NSSAI, but doesn’t contain DNN then AMF selects default DNN for that NSSAI, if UE has subscription to that DNN else a local DNN is selected.
• If Request type is Initial Request, or HO from EPS or non-3GPP then AMF stores a mapping of S-NSSAI, DNN, PDU-Session ID, SMF-ID, Access Type.
• When AMF doesn’t have an association with SMF for the PDU session ID provided by UE, AMF sends Nsmf_PDUSession_CreateSMContextRequest. Else Nsmf_PDUSession_UpdateSMContextRequest is sent to SMF.
• Based on the data provided by UE SMF communicates with UDM and PCF to get relevant information for PDU session creation.
• If request type is initial request, SMF initiates a N4 Session Establishment Request with selected UPF, Else it sends a N4 Session Modification Request.
• UPF Acknowledge the request N4 Session Establishment/Modification Response.
• Using N4 session establishment SMF gets the GTP tunnel info from UPF.
• After Successful creation of Tunnel end point, SMF sends Namf_Communication_N1N2MessageTransfer with Tunnel Details for N2 message and PDU session details in N1 Container.
• Upon Reception of above message AMF Sends a NGAP PDU session Setup Request along with N2 parameter from SMF in above message with parameters, PDU Session ID, QFIs, QoS Profile, CN tunnel Info, PDU Session type, Session AMBR.
• Inside N2 message above, AMF piggyback N1 PDU Session Establishment Accept with NAS Header with session parameters like QoS Rules, UE IP address.
• Then NG RAN(gNB) Setups the GTP Tunnel based on the N2 information received from AMF and setup the Tunnel End point (or Bearer).
• And gNB forwarded N1 message to UE for setting of PDU session.
• gNB after setting up the tunnel, it sends back N2 PDU session setup response to AMF.
• Then AMF Updates SMF about successful tunnel setup by sending Nsmf_PDUSession_UpdateSMContext_Request and receives response from SMF.