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5G NR end-to-end UE initial registration

Overall References: 3GPP TS 23.502:4.2.2, 4.3.2, 38.508-1:4.5.2, 4.5A.2, and 38.401:8.1, 8.9.1, 8.9.2.

You may further compare it with LTE - end-to-end UE initial attach for similarities and differences.

If you are interested only in messages, it is available here.


 USIM   

 UE   

 gNB_DU   

 gNB_CU_CP   

 gNB_CU_UP   

 AMF   

 SMF   

 UPF   

 AUSF   

 IP_NET   

Protocol/Interface presentation
UICC
Uu
F1
E1
NG-C
NG-U
IP

IP
Reference point presentation

N1
N2
N3
N11
N12

N4
UE powers on, modem activation.
27.007MN-AT:8.2 "Set phone functionality +CFUN", 27.007MN-AT:7.3 "PLMN selection +COPS"

USIM activation and fetching PLMN data
31.102USIM:4 "Contents of the Files", 31.102USIM:5 "Application protocol"

Band selection,
Frequency scanning,
Cell search,
PLMN selection,
Initial cell selection,
Synchronisation (SSB/MIB) and SIB1 reception
38.304IDLE MODE:5.1 "PLMN Selection", 38.304IDLE MODE:5.2 "Cell Selection", 23.122NAS:4.4 "PLMN selection process", 38.331RRC:5.2.2 "System Information Acquisition", 38.213PHY:4 "Synchronisation procedures", 38.211PHY:7.4.3 "SS/PBCH block", 38.213PHY:13 "UE procedure for monitoring Type0-PDCCH CSS sets"

PHY Identity
SI RNTI
Downlink
PBCH mapped to
BCH mapped to
BCCH
38.202PHY:6.1 "Uplink", 38.202PHY:6.2 "Downlink", 38.300RAN:6.2.3 "Mapping to Transport Channels"

5GSM-PDU-SESSION-INACTIVE
5GMM-DEREGISTERED
RRC-IDLE
NAS(5GMM) requests RRC Connection
23.502NAS:4.2.2.2 "Registration procedures", 24.501NAS:5.3.1 "Establisment of N1", 38.331RRC:5.3.3 "RRC connection establishment"
5GMM-REGISTERED-INITIATED
RRC-IDLE

Uplink
PRACH mapped to
RACH

Select RA-RNTI
Random Access Preamble
(msg1)
over RACH
38.321MAC:5.1 "Random Access procedure", 38.213PHY:8 "Random Access procedure"


Downlink
PDSCH mapped to
DL-SCH mapped to
CCCH (SRB0)
PHY Identity
RA RNTI
PDCCH monitoring with RA-RNTI
DL DCI (format 1) over PDCCH
Random Access Response
(msg2)
over PDSCH
38.213PHY:10.1 "UE procedure for determining physical downlink control channel assignment", 38.214PHY:5.1 "UE procedure for receiving PDSCH"
Got Temporary C-RNTI, UL grant, TA


RRC Setup Request
(msg3)
over PUSCH
38.321MAC:5.4 "UL-SCH data transfer", 38.214PHY:6.1 "UE procedure for transmitting PUSCH"


INITIAL UL RRC MESSAGE TRANSFER
[RRC Setup Request]
38.473F1:8.4.1 "Initial UL RRC Message Transfer"
DL RRC MESSAGE TRANSFER
[RRC Setup]

38.473F1:8.4.2 "DL RRC Message Transfer"

PHY Identity
Temporary C-RNTI
PDCCH monitoring with Temporary C-RNTI
DL DCI (format 1) over PDCCH
RRC Setup
(msg4)
over PDSCH
38.321MAC:5.3 "DL-SCH data transfer"
Got SRB1 configuration

38.331RRC:5.3.3.4 "Reception of RRCSetup by UE"
RRC-CONNECTED

Downlink
PDSCH mapped to
DL-SCH mapped to
CCCH (SRB1)
Uplink
DCCH (SRB1) mapped to
UL-SCH mapped to
PUSCH
PHY Identity
C-RNTI
SR over PUCCH
(Request UL allocation)
38.213PHY:9.2 "UCI reporting in PUCCH"
PDCCH monitoring with C-RNTI
UL DCI (format 0) over PDCCH
RRC Setup Complete
[5GMM REGISTRATION REQUEST]
over PUSCH

38.331RRC:5.3.3.4 "Reception of RRCSetup by UE"
5GMM Identity
(old) 5G-GUTI or SUCI (IMSI or NSI) or PEI
24.501NAS:5.3.2 "Permanent identifiers", 24.501NAS:5.3.3 "Temporary identities", 23.003NAS:2.2B "SUCI", 23.003NAS:6.4 "PEI"

Above Access Stratum "machine", consisting of PDCCH, PDSCH, PUSCH, and PUCCH continue to operate and it is not shown in the rest of the message sequence for simplicity.

UL RRC MESSAGE TRANSFER
[RRC Setup Complete]

38.473F1:8.4.3 "UL RRC Message Transfer"

INITIAL UE MESSAGE
[5GMM REGISTRATION REQUEST]

38.413NG:8.6.1 "Initial UE message"

AMF may do (optional) identity procedure before triggering authentication procedure.It is not shown here.
24.501NAS:5.4.3 "Identification procedure"

Nausf UEAuthentication Authenticate Request
(SUCI)
AUSF and UDM interaction is not covered here.
33.501NGC:6.1.2 "Initiation of authentication and selection of authentication method"
Nausf UEAuthentication Authenticate Response

33.501NGC:6.1.3.2 "Authentication procedure for 5G AKA"
DL NAS Transport
[5GMM AUTHENTICATION REQUEST]
24.501NAS:5.4.1 "Primary authentication and key agreement procedure", 38.413NG:8.6.2 "DL NAS transport"
DL RRC MESSAGE TRANSFER
[RRC DL Information Transfer]
RRC DL Information Transfer
[5GMM AUTHENTICATION REQUEST]

38.331RRC:5.7.1 "DL information transfer"
UICC Authenticate
31.102USIM:5.2.1 "Authentication algorithms computation"
RRC UL Information Transfer
[5GMM AUTHENTICATION RESPONSE]
38.331RRC:5.7.2 "UL information transfer"
UL RRC MESSAGE TRANSFER
[RRC UL Information Transfer]

UL NAS Transport
[5GMM AUTHENTICATION RESPONSE]
38.413NG:8.6.3 "UL NAS transport"

Nausf UEAuthentication Authenticate Request
(SRES)

Nausf UEAuthentication Authenticate Response
As seen above, F1AP provides transport for RRC messages.In turn, RRC, F1AP, and NGAP provides transport for NAS messages.Transport messages are not shown in the rest of the message sequence for simplicity.

5GMM SECURITY MODE COMMAND
24.501NAS:5.4.2 "Security mode control procedure"
5GMM SECURITY MODE COMPLETE
AMF and UDM interaction is not covered here (these interactions are similar to interactions between MME and HSS in LTE e.g. location update).
29.503UDM "UDM services"

INITIAL CONTEXT SETUP REQUEST
[5GMM REGISTRATION ACCEPT]
38.413NG:8.3.1 "Initial Context Setup"
UE CONTEXT SETUP REQUEST
[RRC Security Mode Command]

38.473F1:8.3.1 "UE Context Setup", 38.331RRC:5.3.4 "Initial AS security activation"
RRC Security Mode Command
UE CONTEXT SETUP RESPONSE

RRC Security Mode Complete

UE Capability Enquiry
38.331RRC:5.6.1 "UE capability transfer"
UE Capability Information

RRC Reconfiguration
[5GMM REGISTRATION ACCEPT]

38.331RRC:5.3.5 "RRC reconfiguration"
Got updated cell configuration (spCellConfigDedicated) (possibly based on UE capability)
RRC Reconfiguration Complete

INITIAL CONTEXT SETUP RESPONSE
5GMM-REGISTERED
5GMM Identity
new 5G-GUTI
or keep the same as old one


5GMM REGISTRATION COMPLETE
5GSM triggers PDU Session Establisment
24.501NAS:6.4.1 "UE-requested PDU session establishment procedure"

5GMM UL NAS TRANSPORT
[5GSM PDU SESSION ESTABLISHMENT REQUEST]
24.501NAS:5.4.5.2 "UE-initiated NAS transport procedure"
Nsmf PDUSession CreateSMContext Request
[5GSM PDU SESSION ESTABLISHMENT REQUEST]
29.502N11:5.2.2.2 "Create SM Context service operation"
Nsmf PDUSession CreateSMContext Response
N4 Session Establishment Request
23.502NGC:4.4.1.2 "N4 Session Establishment procedure"
UPF's interaction with PDN (usually IP based) is not covered here.
PDU Session to NG-U/GTP-U Tunnel mapping from UPF side
38.300RAN:12 "QoS"

N4 Session Establishment Response


Namf Communication N1N2MessageTransfer
[5GSM PDU SESSION ESTABLISHMENT ACCEPT]
PDU SESSION RESOURCE SETUP REQUEST
[5GSM PDU SESSION ESTABLISHMENT ACCEPT]
38.413NGC:8.2.1 "PDU Session Resource Setup"
BEARER CONTEXT SETUP REQUEST
38.463E1:8.3.1 "Bearer Context Setup"
BEARER CONTEXT SETUP RESPONSE
PDU Session to NG-U/GTP-U Tunnel mapping from gNB CU UP side
PDU Session to DRB and DRB to QoS flow (5QI) mapping
One PDU Session may have more than one DRB and one DRB may have more than one QoS flow
38.300RAN:12 "QoS"
UE CONTEXT MODIFICATION REQUEST
[RRC Reconfiguration
[5GSM PDU SESSION ESTABLISHMENT ACCEPT]]
38.473F1:8.3.4 "UE Context Modification (gNB-CU initiated)"
RRC Reconfiguration
[5GSM PDU SESSION ESTABLISHMENT ACCEPT]

Got SRB2 and PDU session + DRB configuration

Downlink
PDSCH mapped to
DL-SCH mapped to
CCCH (SRB2)
Uplink
DCCH (SRB2) mapped to
UL-SCH mapped to
PUSCH

Downlink
PDSCH mapped to
DL-SCH mapped to
DTCH (DRB)
Uplink
DTCH (DRB) mapped to
UL-SCH mapped to
PUSCH


Got information about PDU Session and QoS flows
23.501NAS:5.7 "QoS model"
5GSM-PDU-SESSION-ACTIVE

RRC Reconfiguration Complete
UE CONTEXT MODIFICATION RESPONSE
BEARER CONTEXT MODIFICATION REQUEST
38.463E1:8.3.2 "Bearer Context Modification (gNB-CU-CP initiated)"
BEARER CONTEXT MODIFICATION RESPONSE
PDU SESSION RESOURCE SETUP RESPONSE
Nsmf PDUSession UpdateSMContext Request
29.502N11:5.2.2.3 "Update SM Context service operation"

N4 Session Modification Request
23.502NGC:4.4.1.3 "N4 Session Modification procedure"

UPF now knows NG-U/GTP-U tunnel ID from gNB CU UP side
N4 Session Modification Response
Nsmf PDUSession UpdateSMContext Response

DL data starts from UPF
DL data transfer


IP packet

NG-U over GTP-U
F1-U over GTP-U

PDSCH

UL data transfer
PUSCH
F1-U over GTP-U
NG-U over GTP-U

IP packet



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