The document describes the 5G registration process between a UE and an AMF. It involves the following key steps: 1. The UE sends a registration request to the AMF via the (R)AN. 2. The AMF authenticates the UE and retrieves subscription data. If a new AMF is selected, it retrieves the UE context from the old AMF. 3. If registration is successful, the AMF sends a registration accept message to the UE to complete the process. It also notifies other network functions like SMFs and PCF.

1. Koorosh Hoveyda
5GC / EPC Consultant , Summer 2021

3. A. The UE sends an NAS Registration Request message to AMF via
the (R)AN.
B. In case a new AMF is selected (e.g., because the UE registers in an
area not served by the old AMF), and the UE provided a GUAMI
containing the identity of the old AMF, the new AMF retrieves the
UE context from the old AMF
C. Authentication is carried out, either using 5G AKA or EAP-AKA
D. In case a new AMF has been selected, the new AMF indicates to
the old AMF that it is now taking over as serving AMF for the UE.
E. The AMF registers as serving AMF for the UE in the specific access
technology using Nudm_UECM service. The AMF also requests
subscription data and subscribes to subscription data updates using
Nudm_SDM service.
5G Call Flow:
Registration

4. F. the AMF initiates establishment of the AM policy association with
PCF and retrieves the AM policies
G. If the UE has indicated that it wants to activate User Plane
connection for existing PDU Sessions, the AMF invokes the
Nsmf_PDUSession_UpdateSMContext service operation for those
PDU Sessions. If there is a mismatch in the UE and AMF PDU Session
state, the AMF invokes the Nsmf_PDUSession_ReleaseSMContext
service operation to notify the affected SMFs about those PDU
Sessions.
H. If the Registration procedure is successful so far, the AMF
provides a NAS Registration Accept to the UE. In some cases, the UE
sends a NAS Registration Complete message to the AMF.
I. In case UE policies (ANDSP and/or URSP) are deployed, the AMF
initiates establishment of the UE policy association with PCF. This
allows the PCF to provide UE policies to the UE
5G Call Flow:
Registration

5. Note:
The AMF may also check the ME
identity with an Equipment Identity
Register (EIR). typically between steps
D and E. The EIR can be used to
blacklist, for example, stolen UEs.
Depending on the response from the
EIR, the AMF may continue the
attachment procedure or reject the UE
5G Call Flow:
Registration

6. 5G Call Flow:
Deregistration
A. The UE sends an NAS Deregistration Request message to AMF
via the (R)AN.
B. The AMF notifies each SMF with active SM context that the
corresponding SM context is released. The SMF then in turn
notifies other NFs about the release of the
C. The AMF releases the AM policy association with PCF (if any).
D. The AMF releases the UE policy association with PCF (if any).
E. The AMF sends a Deregistration Accept message to the UE,
unless the UE indicated switch-off, i.e., the UE in that case does
not wait for any acceptance from the network.
F. Finally, the AMF instructs (R)AN to release the N2 UE context. If
there is still a (R) AN-level association between UE and (R)AN, the
(R)AN may request the UE to release it.

7. A. When the UE wants to initiate the Service Request
procedure, it sends a NAS Service Request message to AMF
via the (R)AN. If the UE wants to establish User Plane
connections for one or more existing PDU Sessions, the UE
includes information to AMF about the PDU Session IDs for
these PDU Sessions.
B. The network may optionally reauthenticate the UE
during this procedure.
C. If the UE indicated in step A that it wants to establish
User Plane connection for one or more PDU Sessions, the
AMF notifies each SMF serving the corresponding PDU
Session. In the simplest case the SMF can reply to this
message with the UPF tunnel endpoint identifiers. In other
cases, e.g., if the UE location is outside the serving area of
the current UPF, the SMF may have to select a new I-UPF
and then this step C becomes more involved, with
signaling towards the old I-UPF (if any), new I-UPF and the
anchor (PSA) UPF. For simplicity we have not shown this
additional N4 signaling.
5G Call Flow:
Service Request
UE Triggered

8. D. If the procedure was triggered by the UE, the AMF sends
a NAS Service Accept to the UE. If User Plane is to be
established, the AMF also forwards the PDU Session
information to the (R)AN, including the UPF tunnel
endpoint identifiers. The (R)AN configures the User Plane
connection towards the UE. The exact details of how this is
done depends on the AN technology. For 3GPP RAN, this
may, e.g., be done via RRC reconfiguration. Once this has
been done, up-link traffic can start to be sent. The (R)AN
then replies with the (R)AN tunnel endpoint identifier(s) to
the AMF.
E. The AMF now need to notify each SMF again, to provide
the result of the User Plane establishment and the (R)AN
tunnel endpoint identifier(s). If the PCF has subscribed to
UE location information, the SMF then notifies the PCF
about the new UE location. The SMF also provides the
(R)AN tunnel endpoint identifier(s) to the UPF so that
down-link traffic can be sent towards (R)AN.
5G Call Flow:
Service Request
UE Triggered

9. 5G Call Flow:
Service Request
Network Triggered
A. If the UPF receives down-link data and has been instructed by SMF to
buffer the packets, the UPF does the buffering and notifies SMF about the
received data.
B. The SMF sends PDU Session information (UPF tunnel endpoint
information and QoS information) to AMF for forwarding to (R)AN.
C. If the UE is in CM-IDLE state, the AMF needs to page the UE. The AMF
stores the PDU Session information and sends a paging request to NG-RAN.
The NG-RAN then pages the UE. When the UE receives the page, the UE
sends a Service Request message to the network. That message and the
rest of the procedure then follows the UE-triggered Service Request
described in Section 15.3.2.
D. If the UE is in CM-CONNECTED state, there is no need to page the UE.
Instead the AMF simply forwards the PDU Session information received
from SMF to the (R)AN so that (R)AN can proceed with setting up the User
Plane. This step (A) (B) (C) (D) Fig. 15.6 Network-triggered Service Request.
Selected call flows 403 and the rest of the procedure also follows the UE-
triggered Service Request described in Section 15.3.2, but here only the
steps (C) and (D) are needed in the UE-triggered Service Request.

10. A. The UE sends a 5GSM NAS PDU Session Establishment message
to the AMF, including PDU Session Id, DNN-requested S-NSSAI,
PDU Session type, etc. The AMF processes the NAS security. If the
PDU Session Establishment is a request for a new PDU Session, the
AMF selects a new SMF. The AMF may use the NRF to discover
available SMFs serving the specific DNN and S-NSSAI. If the PDU
Session Establishment is a request for handing over an existing
PDU Session, the AMF uses its UE context to determine what SMF
is serving the PDU Session Id.
B. The AMF then forwards the 5GSM container (containing the
PDU Session Establishment message) to the SMF. The SMF
retrieves the Session Management related UE subscription data
from UDM and also subscribes to subscription data updates from
UDM
C. In case secondary authentication is applied, it is performed now
D. The SMF then selects a PCF and initiates SM policy session
establishment to retrieve the initial set of PCC rules. The SMF also
selects a UE IP address and UPF and initiates N4 session
establishment towards that UPF
5G Call Flow:
PDU Session
Establishment

11. E. The SMF then sends a 5GSM NAS PDU Session
Establishment accept message towards the UE as well as the
UPF GTP-U tunneling endpoint information and QoS
information towards (R)AN. This message is sent via the AMF
G. The AMF forwards the PDU Session information received
from (R)AN to the SMF so that SMF can provide the (R)AN
GTP-U tunnel endpoint to the UPF for downlink forwarding.
Now also the down-link data path is ready for use.
H. If all works well, the SMF then registers itself in UDM, as
serving the PDU Session Id in UDM.
F. The AMF creates and sends the N2 message containing the
NAS message (PDU Session Establishment accept) as well as
the PDU Session information for (R)AN, i.e., GTP-U tunneling
information and QoS information. The (R)AN establishes the
required resources towards the UE and replies to the AMF
with information about the (R)AN GTP-U tunnel endpoint.
When this is done, the up-link data path is ready for use
5G Call Flow:
PDU Session
Establishment

12. 5G Call Flow:
UE Configuration
Update
A. The AMF decides, e.g., based on updated subscription data, that
UE configuration needs to be changed or a re-registration is
needed.
B. The AMF sends a UE Configuration Update command that may
include the updated configuration. It may also include indications
whether or not UE shall send an acknowledgement (UE
Configuration Update Complete) or whether or not Registration
procedure shall be performed.
C. The UE then applies the new parameters. Depending on
updated parameters, the AMF also may need to notify other
entities. For example, if 5G-GUTI is changed, AMF needs to inform
(R)AN about the new temporary identity. If the update was due to
a Network Slicing Subscription Change Indication, then UDM is
informed that UE has received the update
D. Depending on the updated parameters, the AMF may release
the AN association (e.g., Allowed NSSAI or Configured NSSAI are
updated in a way that affects existing connectivity to Network
Slices). The UE may also initiate a re-registration, e.g., to allow the
UE to be connected to a new set of Network Slices.

13. 5G Call Flow:
Handover from
5GS to EPS
A. The NG-RAN decides to trigger a handover to EPS and
sends a Handover Required message to AMF. Based on the
target “RAN” identifier the AMF determines it is a
handover to E-UTRAN, and the AMF then request PGW-
C+SMF to provide SM Context that is needed for AMF to
transfer SM context to MME.
B. The AMF then selects a target MME (based on
information about target E-UTRAN provided by NG-RAN).
The AMF then acts as a source MME and sends a
Relocation Request to the MME. The MME will initiate
session setup and handover in target E-UTRAN access,
basically following the procedure for S1-based handover.
If indirect forwarding applies (i.e., user data forwarding
from source NG-RAN to target E-UTRAN via the core
network), the MME initiates setup of forwarding tunnels
on the EPC side.
C. If indirect forwarding applies, the AMF triggers SMF to
setup forwarding tunnels also on 5GC side.

14. 5G Call Flow:
Handover from
5GS to EPS
D. The source side now issues the Handover Command
to the UE, and UE moves to target cell.
E. The data path for indirect forwarding (if applicable) is
established and down-link data can be forwarded. Also,
the up-link data path is available over the target access.
F. The target E-UTRAN then sends a Handover Notify to
target MME, and by this the target E-UTRAN considers
the handover successful. The MME now notifies the
source side (AMF) that the handover is complete.
G. Finally, the UE initiates a Tracking Area Update
procedure (as done according to normal S1-based
handover). The network also triggers establishment of
dedicated bearers that were not established during the
actual handover procedure. The network also releases
the indirect forwarding tunnels when they are no
longer needed.

15. Stay In Touch
k.hoveyda@gmail.com