5G PDU Session Establishment

5G PDU Session Establishment

prasanna sahu
Latest posts by prasanna sahu (see all)
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.
prasanna sahu

Telecom Professional


Debashish Deka Posted on12:07 pm - March 3, 2019

How SMF encodes N2 SM information? AMF-service based document (29.518) specifies: it will be encoded in binary. Which NGAP IE corresponds to N2 SM information?

23.501 says: N2 SM information will be transparently transferred to RAN or UE by AMF.
In various sections of 23.502, it has been made clear than N2 SM information will be part of NGAP Message.

There are two NAS PDU in “PDU session resource setup request” NGAP message. Which one corresponds to N1 SM container?. What about the other one?

Thank You !

    prasanna Posted on3:09 pm - July 10, 2019

    Hi Debashis,
    Yes, SMF will encode N1/N2 SM information with DL tunnel details and send it to AMF so that AMF will pass on that information to gNB and UE. AMF will not do any manipulation on to this IE, it will transparently pass this info to UE/gNB on NGAP Initial Context Setup Request and NAS PDU session establishment accept.


      Debashish Deka Posted on6:18 am - November 29, 2019

      The mappings are there in [ N2 Information Content, 29.518]

susil Posted on1:02 pm - March 4, 2019

good information prasanna

Vishwajeet Posted on1:04 pm - May 22, 2019

can anyone mail me the full documents of 5G.

Nikita Posted on12:06 pm - July 9, 2019

How multiple PDU sessions differs from one another?

ddeka Posted on9:55 am - August 23, 2019

Go to https://www.3gpp.org/specifications/specification-numbering and download your required specification

Rick Posted on6:47 am - October 23, 2019

In LTE, EPS bearer ID is mapped on EPS bearer ID.
What is mapped on the NSAPI in 5G?

    Rick Posted on6:50 am - October 23, 2019

    Correction :
    In LTE, EPS bearer ID is mapped on NSAPI.
    What is mapped on the NSAPI in 5G?

iahim Posted on10:08 am - November 29, 2019

E2E Network Slicing within One PLMN can be deployed with which 5g architecture option?
a)SA Option 1
b)SA option 2
c) NSA Option 3a
d) NSA Option 3x
Can anybody help me with this question?
Thank you

    prasanna Posted on5:32 pm - January 3, 2020

    In Option 2 and 7/7a/7x

Susil kumar gupta Posted on10:18 am - November 29, 2019

hello prasanna – like LTE support 11 bearer initially and later part support max 15 bearer do you came across any information for the 5G – what is the max number of bearer possible for a UE it is still 15 or some thing else

verma Posted on2:05 pm - April 13, 2020

Unlike to 4G, in 5G we have separate attach and PDU session request, can anyone let me know why this is its so!!

yuzf Posted on11:06 am - June 4, 2020

“PDU session resource setup request” NGAP message。when the Payload container type IE is set to “N1 SM information”,What documents I can see to parse this “N1 SM information”

navneet singh Posted on7:03 pm - June 23, 2021

AS per TS 24.501, PDU session establishment message is not having IE SNSSAI, then how UE send information to AMF for select slice for PDU session establishment other than SNSSAI used for registration.

Sri Posted on11:50 am - July 25, 2021

Consider that is new UE…no sessions are involved..so aftr registration Accept message , when PDU session will be trigger…which IE will inform trigger the session establishment?? In 5g?

Himanshu Bansal Posted on4:12 pm - August 3, 2021

Thanks a lot Prasanna for the detailed call flow.

I have two more queries : –

1) RAT- Type used in 5G Core
2) Which message transaction carries this information during Registration or PDU Session Establishment

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