Network Architecture Diagram
Functional Split of NW entities
Above Network functions performed by different 5G Access Network entities
Protocol Stack on different interfaces
UU User-plane : Radio interface user-plane carries, user Application traffic, which follows UU-U (Above)protocol stack. User-data is transparent to access nodes like gNB.
UU Control-plane : in control plane RRC, PDCP, RLC, MAC, PHY get terminated at gNB, but NAS termination point is AMF, so gNB transparently passes the NAS messages to AMF.
NG Control-plane(N2): this protocol stack is between gNB and AMF. This stack helps providing access to UE to core network and transports NAS messages. Also helps establishment of user-plane tunnels for UE.
NG User-plane(N3) : This is the protocol stack between gNB and UPF. This stack carries user data over GTP-U tunnel established during session establishment over N2.
Xn user and control plane : Xn interface is between 2 gNBs or gNB-Ng-eNB. This does necessary signalling and data during mobility.
N4 Interface: This interface is between SMF and UPF, Used for UPF selection and setup of U-plane tunnels and enables PFDs on user plane entities.
Service based interfaces: Most of the core Network entities with Service based end points and they host services performed by them and exposed these services by REST APIs. Details are in below diagram.
Service Based Architecture
In Service based Architecture, Network functions(Ex AMF) opens up services using service based access point(Namf) and other authorized network functions(Service Consumers) access this service through service these service access points. These interfaces are driven by REST APIs(HTTP)
There can be single or dual registration mode UE supports to register with 5GC and EPC
Single Registration Mode:
- In single registration mode there is only one active mobility state at any given time.
- UE can be either in 5GC NAS mode or EPC NAS mode.
- UE maps EPC-GUTI to 5G-GUTI during mobility between EPC and 5GC.
- UE keeps 5G context for re-use when moving from 5GC to EPC
Dual Registration mode:
- In Dual registration mode UE keeps independent registration for 5GC and EPC.
- In this mode UE maintains 5G-GUTI and EPC-GUTI independently.
- UE can perform 5GC or EPC re-registration/TAU using corresponding GUTIs.
Inter-working architecture without N26 interface
5G to EPS mobility without N26 interface flow
Message flow steps
- Initially UE is registered to 5GC and establishes the PDU session.
- In order to move to EPS system, UE triggers Attach request(to preserver IP address) to EPC.
- Alternatively UE sends a Tracking area Update which fails and then UE initiates an Attach procedure. In this case there is no IP address preservation.
- Single Registered mode UEs provide a EPS-GUTI derived from 5GC-GUTI if available else uses IMSI.
- Dual Registered mode UEs provide 4G GUTI(if allocated earlier) with attach request.
- In PDN connectivity request, UE sends Request type as “Handover” to indicate that it is moving from 5GS to EPS.
- If Request Type is Handover and previous node is 5G AMF then MME is configured to send a location update request to HSS/UDM to tell them not to cancel Old AMF registration, as UE may move back to 5GS again.
- After successful Attach and PDN connection PGW-C/SMF allocates same IP address as the 5GS.
- And PGW-C/SMF initiates UE De-Registration and PDU Session release from 5GS.
EPS to 5G mobility without N26 interface flow
Message flow Steps
- Initially UE is Attached to EPC and establishes the PDN Connection.
- In order to move to 5G system, UE triggers Registration and PDU session procedure in 5GS.
- In single registration mode, UE provides Registration Type as “mobility Registration update” and provides 5G-GUTI derived from 4G-GUTI.
- UE in Dual Registration mode Provides Registration type as “initial registration” along with a native 5G-GUTI or SUPI.
- UE will include, NSSAI from current serving PLMN.
- AMF supports interworking with EPC, so it treats this as initial registration and skips PDU session status synchronization with SMF.
- If UE is moving in step 1, then AMF updates UE location in HSS/UDM and tells them not to cancel the MME registration for the UE if any by setting ULR-Flags.
- During PDU session establishment with 5GS, UE indicates “Existing PDU session” as Request Type to preserve the IP address.
- After Successful Registration and PDU session establishment, PGW-C triggers the PDN connection release and detach of the UE from EPS.
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 ?
- Initial registration during the switch on of the device.
- If there is a new tracking area outside the UE’s current registration area.
- When UE need to update its capability or protocol parameters that are negotiated during previous Registration.
- Periodic Registration update, if configured.
- 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 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.