5G NSA Core Architecture

5G NSA Core Architecture Details

5G NSA (Non-Standalone) Core Architecture Details

Section 1: NSA Core Architecture Overview

Core Principle: Leveraging the EPC

Explains the fundamental reliance on the 4G Evolved Packet Core (EPC) in NSA mode.

The 5G NSA (Non-Standalone) architecture, specifically Option 3, 3a, and 3x (EN-DC: E-UTRA NR Dual Connectivity), utilizes the existing 4G Evolved Packet Core (EPC) as its core network.
There is no 5G Core (5GC) involved in this initial phase of 5G deployment.
The primary goal is to introduce 5G NR radio capabilities for enhanced data rates (eMBB) while relying on the mature and widely deployed EPC for control plane functions.

Key Architectural Aspects:
- LTE eNodeB (eNB): Acts as the Master Node (MN). It handles the control plane connection to the EPC (MME) and manages the overall UE connection.
- 5G gNodeB (gNB): Acts as the Secondary Node (SN). It provides additional user plane resources over the NR interface.
- Dual Connectivity (EN-DC): The UE maintains simultaneous connections to both the LTE eNB and the 5G gNB.

[Conceptual Diagram: UE connected via LTE Uu to eNB (MN) and via NR Uu to gNB (SN).
eNB connected to MME (S1-MME) and S-GW (S1-U).
X2 interface between eNB and gNB.
User plane paths:
Option 3/3a: UE -> gNB -> X2-U -> eNB -> S1-U -> S-GW.
Option 3x: UE -> gNB -> S1-U_direct -> S-GW (if S-GW upgraded)]

Section 2: Functional Roles of EPC Nodes in NSA

Mobility Management Entity (MME)

Role of the MME in the NSA architecture.

The MME remains the primary control plane anchor in NSA.
Responsibilities:
- UE Authentication and Authorization: Via HSS.
- Mobility Management: Tracking Area Updates, Handovers (LTE part).
- Session Management (initiation/modification requests to S-GW/P-GW): Establishes bearers through S-GW and P-GW.
- NAS Signaling Termination: Handles NAS messages from the UE over the LTE link.
- Manages S1-MME interface with the Master eNB.
- Indicates UE's 5G NSA capability to the eNB.
- Orchestrates the addition/modification/release of the gNB as a Secondary Node via the Master eNB.

Serving Gateway (S-GW)

Role of the S-GW in the NSA user plane.

The S-GW is the primary user plane anchor within the EPC for NSA.
Responsibilities:
- Local Mobility Anchor for inter-eNB handovers (LTE).
- Packet Routing and Forwarding for the user plane.
- Manages S1-U interface with the Master eNB.
- Receives user plane data from both LTE eNB bearers and (indirectly or directly) 5G gNB bearers.
- Forwards uplink data towards the P-GW and downlink data towards the eNB.
- Charging data collection (for user plane).

PDN Gateway (P-GW)

Role of the P-GW in NSA connectivity and policy.

The P-GW provides connectivity to external Packet Data Networks (PDNs).
Responsibilities:
- IP Address Allocation for the UE.
- Policy Enforcement (PCEF): Enforces QoS rules received from PCRF.
- Packet Filtering, Inspection.
- Anchor point for mobility between 3GPP and non-3GPP access.
- Charging data collection and interface to charging systems.

Home Subscriber Server (HSS)

Role of the HSS in NSA for subscription and authentication.

The HSS remains the central database for subscriber information.
Responsibilities:
- Stores User Subscription Data: Including QoS profiles, access restrictions.
- Provides Authentication Vectors (AVs) to MME/AUSF (if AUSF is co-located or through an intermediate function) for UE authentication.
- Tracks user location at the MME level.
- Indicates if a subscriber is allowed to use 5G NSA capabilities.

Policy and Charging Rules Function (PCRF)

Role of the PCRF in NSA for dynamic policy and charging control.

The PCRF provides dynamic policy and charging control.
Responsibilities:
- Defines and provides PCC (Policy and Charging Control) rules to the P-GW (PCEF).
- Determines QoS parameters, gating control, charging keys based on subscription, service, and network conditions.
- Interacts with Application Functions (AF) for service-specific policy.

Section 3: CUPS in NSA Context

CUPS in the EPC

How CUPS principles apply to the EPC nodes (S-GW, P-GW, TDF) relevant to NSA.

Control and User Plane Separation (CUPS) is a feature introduced in later EPC releases (Rel-14 onwards) that can be beneficial for NSA deployments, even though NSA itself doesn't mandate CUPS.
If the EPC S-GW and P-GW are CUPS-enabled:
- S-GW-C / P-GW-C (Control Plane Functions): Handle signaling, session management, policy interaction.
- S-GW-U / P-GW-U (User Plane Functions): Handle packet forwarding, QoS marking, traffic inspection.

Interface between Control and User Plane parts:
- Sx interface (based on PFCP - Packet Forwarding Control Protocol): Used between S-GW-C and S-GW-U, and P-GW-C and P-GW-U. This is the same protocol used for N4 in 5GC.

Benefits of CUPS-enabled EPC for NSA:
- Independent Scaling: S-GW-U/P-GW-U can be scaled independently of S-GW-C/P-GW-C to handle increased user plane traffic from 5G NR.
- Flexible Placement: User plane functions (S-GW-U/P-GW-U) can be placed closer to the network edge (e.g., near gNBs) to reduce latency for 5G traffic, even if the control plane remains centralized.
- Optimized Resource Utilization.

CUPS is an EPC evolution, not an NSA-specific feature, but it enhances EPC’s ability to support 5G NSA traffic demands.

Section 4: Upgrades Needed for NSA

RAN Upgrades

Modifications required in the Radio Access Network.

LTE eNodeB (eNB) Software/Hardware Upgrade:
- Support for Dual Connectivity (EN-DC) Master Node functionality.
- Implementation of the X2 interface (X2-C and X2-U) for communication with the 5G gNB (Secondary Node).
- Ability to understand UE's 5G NR capabilities.
- RRC modifications to manage the gNB as a secondary cell group (SCG).
- Capability to forward/receive user data to/from the gNB over X2-U.

New 5G gNodeB (gNB) Deployment:
- Full gNB hardware and software supporting NR radio interface.
- Implementation of X2 interface for Secondary Node functionality.
- For Option 3x: Implementation of a direct S1-U interface to the S-GW (bypassing eNB for gNB user plane).

EPC Core Network Upgrades

Modifications required in the Evolved Packet Core.

MME (Mobility Management Entity) Software Upgrade:
- Support for recognizing and handling UEs with 5G NR (NSA) capabilities.
- Ability to signal "NR Restriction" or "NR Allowed" to the eNB.
- Procedures to manage the setup/modification/release of the gNB as a secondary RAN node via the eNB.
- Potentially enhanced capacity to handle increased signaling from dual-connected UEs.
- Support for DCNR (Dual Connectivity with NR) feature if UE reports it (allows MME to know if UE is using NR resources).

S-GW (Serving Gateway) Potential Upgrades:
- Increased User Plane Capacity: To handle aggregated data from both LTE and NR legs.
- For Option 3x: Software upgrade to support a direct S1-U interface from the 5G gNB. This means S-GW must be able to terminate GTP-U tunnels from gNBs.
- If CUPS is implemented, this involves deploying S-GW-C and S-GW-U.

P-GW (PDN Gateway) Potential Upgrades:
- Increased User Plane Capacity.
- If CUPS is implemented, this involves deploying P-GW-C and P-GW-U.
- Ensure QoS mechanisms can handle the potentially higher throughput and diverse requirements of 5G services (even if limited in NSA).

HSS (Home Subscriber Server) Update:
- Ability to store and provide "5G NSA Subscriber" indication or specific subscription parameters related to NR access.
- May need to support new APN or QoS profiles related to 5G services.

PCRF (Policy and Charging Rules Function) Update:
- Potentially new policy rules for 5G services (e.g., higher data rate allowances, specific QoS for NR bearers).
- May need to interact with new AFs relevant to 5G use cases.

Transport Network Upgrades

Considerations for the underlying transport network.

- Increased Backhaul/Midhaul Capacity: To support higher data rates from aggregated LTE and NR.
- Lower Latency Transport: For X2 interface between eNB and gNB, and for user plane paths.
- Synchronization: Ensure precise time and frequency synchronization for both eNBs and gNBs, crucial for TDD NR and inter-RAT operations.

OSS/BSS Upgrades

Impact on Operations and Business Support Systems.

- Performance Monitoring: Systems need to monitor both LTE and NR performance metrics.
- Fault Management: Ability to correlate faults across LTE and NR domains.
- Configuration Management: For new gNBs and modified eNBs.
- Billing & Charging Systems: May need updates to differentiate or rate 5G NSA traffic, handle new charging triggers or QoS parameters.

Incremental Approach: NSA upgrades are often incremental, building upon the existing EPC infrastructure, which is a key advantage for rapid service launch.

5G Fundamentals: Architecture, Protocols, and Use Case

Curriculum