How 5G Registration works

 

Background

5G wireless technology is the most recent wireless technology by 3gpp. when the mobile handset tries to connect to a 5G core network, it goes through registration procedure to gain access to the network. As per LTE/4G technology, registration and default PDN connection(IP allocation) was happening simultaneously when mobile is switch on. But it was not the case in older wireless technologies like 3G or 2G. And the same prospective is going to come back in 5G wireless technology again, due to user-plane and Control-plane Separation.

When UE initiates Registration ?

1. Initial registration during the switch on of the device.
2. If there is a new tracking area outside the UE’s current registration area.
3. When UE need to update its capability or protocol parameters  that are negotiated during previous Registration.
4. Periodic Registration update, if configured.
5. Emergency Registration.

What UE does during registration ?

• During initial registration UE updates its location to 5G Core Network.
• During initial registration UE may use SUPI or 5G GUTI for the registration.
• UE may provide its PEI(IMEI + IMEIsv) to AMF on demand.

UE Registration States

Registration FLow

Registration Request Contents (Important parameters only)

1.Registration Type-> Initial reg, Mobility Reg, Periodic Reg or Emergency reg 2.User Identity ->5G-GUTI, SUPI, PEI or 5G-GUTI derived from 4G GUTI for inter-working. 3.List of Visited TAI -> last visited TAI list 4.Security Capability-> Integrity and Encryption algos UE Supports. 5.Requested NSSAI -> Any preferred NSSAI on which UE want to camp on. 6.Default NSSAI indication -> If AMF can select default NSSAI if Requested NSSAI is not available. 7.UE Capabilities -> UE Radio capabilities and MM Capabilities. 8.PDU Session Status -> Previously connected PDU sessions under previous PLMN (3Gpp or Non 3Gpp) 9.List of PDU session need to be activated->  If there is some pending UL traffic for a PDU session then UE include that PDU session in the list of PDU sessions.

• gNB select a AMF based on the 5G-GUTI(SUCI) or existing N2connection for the UE. Else it selects preferred AMF based on NSSAI, if SUPI or PEI are included.
• After AMF selection, NG-RAN(gNB) forwards Registration request to MME piggybacking on NGAP Initial UE message.
• If SUCI is not provided in Registration request or retrieved from old AMF then AMF sends Identity request message to get SUCI from UE.
• Then AMF initiates Authentication procedure with AUSF and UDM.
• If PEI check is enforced by operator then AMF may send an Identity Request to UE if PEI is not obtained from old AMF.
• If AMF has changed from last registration or SUPI is provided as identity then AMF will select appropriate UDM and setup a UDM registration for the UE session.
• AMF retrieves Mobility Subscription data, SMF selection data, UE context in SMF data by using Nudm_SDM_Get.
• AMF select PCF and communicates with PCF for Policy association for the UE.
• In case of Emergency Registration and registration type is Mobility Registration, AMF communicates with SMF to activate or Re-activate PDU sessions requested in “List of PDU sessions need to be activated” IE.
• Once AMF finishes the Registration, It sends back Registration Accept message to UE.
• In response to Registration Accept message UE sends back Registration Complete message.