Part 2: 5G NSA (Non-Standalone) Architecture

Learning Objective: Understand how 5G NSA leverages existing 4G EPC infrastructure with 5G NR radio for enhanced data speeds.

Table of Contents

• What is 5G NSA?
• EN-DC (E-UTRA-NR Dual Connectivity)
• NSA Architecture
• Master Node vs Secondary Node
• Bearer Types
• NSA Deployment Options
• Why NSA Exists

What is 5G NSA?

5G NSA (Non-Standalone) is a deployment mode where:

• Control plane uses 4G LTE (via MME)
• User plane can use both 4G LTE + 5G NR simultaneously

This allows operators to deploy 5G radio (gNB) without building a full 5G core network.

NSA vs SA Comparison

EN-DC (E-UTRA-NR Dual Connectivity)

EN-DC is the 3GPP standard (Option 3/3a/3x) that enables NSA by connecting:

• E-UTRA = LTE (eNodeB)
• NR = 5G New Radio (gNodeB)

How EN-DC Works

graph TB
    UE[📱 UE
Dual Connectivity]
    
    subgraph "Radio Access"
        eNB[📡 eNodeB
LTE Master Node
MeNB]
        gNB[📡 gNodeB
5G NR Secondary Node
SgNB]
    end
    
    subgraph "4G EPC Core"
        MME[🎛️ MME]
        SGWU[📦 SGW-U]
        PGWU[📦 PGW-U/UPF]
    end
    
    Internet[🌐 Internet]
    
    UE <-->|LTE Radio
Control + Data| eNB
    UE <-.->|5G NR Radio
Data Only| gNB
    
    eNB <-->|S1-MME
Control Plane| MME
    eNB <-->|S1-U
User Plane| SGWU
    gNB <-.->|X2/Xn
Coordination| eNB
    gNB -.->|S1-U
User Plane| SGWU
    
    SGWU --> PGWU
    PGWU --> Internet
    
    style UE fill:#e1f5ff
    style eNB fill:#fff4e1
    style gNB fill:#ffe1e1
    style MME fill:#f0e1ff
    style SGWU fill:#ffe1f0
    style PGWU fill:#fff0e1

Key Points

• eNB is the master (handles all control plane signaling)
• gNB is the secondary (provides additional user plane capacity)
• UE maintains two radio connections simultaneously
• X2 interface coordinates between eNB and gNB

NSA Architecture

Option 3 (Most Common)

graph LR
    subgraph "UE"
        UE_LTE[LTE Stack]
        UE_NR[NR Stack]
    end
    
    subgraph "RAN"
        eNB[eNodeB
Master]
        gNB[gNodeB
Secondary]
    end
    
    subgraph "EPC"
        MME[MME]
        SGW[SGW]
        PGW[PGW/UPF]
    end
    
    UE_LTE -->|Control + Data| eNB
    UE_NR -->|Data Only| gNB
    
    eNB -->|S1-MME| MME
    eNB -->|S1-U| SGW
    gNB -->|X2| eNB
    gNB -.->|S1-U| SGW
    
    SGW --> PGW
    
    style eNB fill:#ffcccc
    style gNB fill:#ccffcc

Option 3 Characteristics:

• eNB terminates S1-MME (control to MME)
• Both eNB and gNB terminate S1-U (data to SGW)
• gNB connects to eNB via X2 for coordination

Option 3a

graph LR
    UE[UE] -->|LTE| eNB[eNodeB
Master]
    UE -->|NR| gNB[gNodeB
Secondary]
    
    eNB -->|S1-MME| MME
    eNB -->|S1-U| SGW[SGW]
    gNB -->|X2| eNB
    
    SGW --> PGW[PGW]
    
    style eNB fill:#ffcccc
    style gNB fill:#ccffcc

Option 3a Characteristics:

• Only eNB terminates S1-U
• gNB sends data to eNB, which forwards to SGW
• Simpler but lower throughput (bottleneck at eNB)

Option 3x

graph LR
    UE[UE] -->|LTE| eNB[eNodeB
Master]
    UE -->|NR| gNB[gNodeB
Secondary]
    
    eNB -->|S1-MME| MME
    eNB -.->|S1-U| SGW[SGW]
    gNB -->|X2| eNB
    gNB -->|S1-U| SGW
    
    SGW --> PGW[PGW]
    
    style eNB fill:#ffcccc
    style gNB fill:#ccffcc

Option 3x Characteristics:

• Only gNB terminates S1-U
• All user data goes through gNB (offloads eNB)
• Best for high-throughput scenarios

Master Node vs Secondary Node

Master eNodeB (MeNB)

Responsibilities:

• ✅ RRC connection management
• ✅ NAS signaling to MME
• ✅ Mobility decisions (handover)
• ✅ Bearer management
• ✅ Security (encryption keys)
• ✅ Coordinate with secondary gNB

Interfaces:

• S1-MME (to MME)
• S1-U (to SGW-U)
• X2 (to gNB)

Secondary gNodeB (SgNB)

Responsibilities:

• ✅ Provide additional radio capacity
• ✅ Forward user data (depending on option)
• ✅ Report measurements to MeNB
• ❌ No control plane signaling to core

Interfaces:

• X2 (to eNB)
• S1-U (to SGW-U, in Option 3/3x)

Bearer Types

In EN-DC, bearers can be split across LTE and NR:

MCG Bearer (Master Cell Group)

• Path: UE ↔ eNB ↔ SGW
• Use: Control plane, low-priority data

SCG Bearer (Secondary Cell Group)

• Path: UE ↔ gNB ↔ SGW (Option 3x)
• Use: High-throughput data

Split Bearer

• Path: UE ↔ eNB + gNB ↔ SGW
• Use: Aggregate LTE + NR bandwidth

graph TB
    UE[📱 UE]
    
    subgraph "MCG Bearer"
        eNB1[eNB]
        SGW1[SGW]
    end
    
    subgraph "SCG Bearer"
        gNB1[gNB]
        SGW2[SGW]
    end
    
    subgraph "Split Bearer"
        eNB2[eNB]
        gNB2[gNB]
        SGW3[SGW]
    end
    
    UE -->|LTE only| eNB1
    eNB1 --> SGW1
    
    UE -->|NR only| gNB1
    gNB1 --> SGW2
    
    UE -->|LTE + NR| eNB2
    UE -->|LTE + NR| gNB2
    eNB2 --> SGW3
    gNB2 --> SGW3

NSA Deployment Options

3GPP Options Summary

Why NSA Exists

Business Reasons

1. Faster 5G Launch: Reuse existing 4G core (no need to build 5G SA core)
2. Lower CAPEX: Only deploy 5G radios (gNBs)
3. Coverage: LTE provides wide coverage, NR provides capacity in hotspots
4. Marketing: Operators can claim "5G" without full SA deployment

Technical Reasons

1. Spectrum Efficiency: Use LTE for control, NR for data (saves NR spectrum)
2. Handover: LTE anchor ensures seamless mobility
3. Battery Life: UE doesn't need to maintain two full stacks (control only on LTE)

Limitations of NSA

• ❌ No network slicing
• ❌ No ultra-low latency (URLLC)
• ❌ No edge computing (MEC) integration
• ❌ Limited to 4G core features (no SBA, no NRF)

NSA in Open5GS

Open5GS supports NSA by:

1. Running 4G EPC components (MME, HSS, SGW, PGW/UPF)
2. Configuring UERANSIM to run both eNB + gNB
3. Enabling EN-DC in UE configuration

Configuration Highlights:

• MME handles all control plane
• UPF handles user plane from both eNB and gNB
• No 5G core components (AMF, SMF, NRF) needed

SgNB Addition Procedure (3GPP TS 37.340)

When a UE is eligible for EN-DC, the MeNB initiates the SgNB Addition procedure:

sequenceDiagram
    participant UE as 📱 UE
    participant MeNB as 📡 MeNB (eNB)
    participant SgNB as 📡 SgNB (gNB)
    participant MME as MME
    participant SGW as SGW
    
    Note over MeNB: UE measurements show
strong NR coverage
    
    MeNB->>SgNB: SgNB Addition Request
(X2-AP: UE capabilities, bearer config)
    SgNB->>MeNB: SgNB Addition Acknowledge
(X2-AP: SgNB radio resources)
    
    MeNB->>UE: RRC Connection Reconfiguration
(Add NR secondary cell group)
    UE->>MeNB: RRC Connection Reconfiguration Complete
    
    MeNB->>SgNB: SgNB Reconfiguration Complete
(X2-AP)
    
    Note over UE,SgNB: UE now has dual connectivity:
LTE (MeNB) + NR (SgNB)
    
    MeNB->>MME: E-RAB Modification Indication
(S1-AP: bearer split info)
    MME->>SGW: Bearer Modification
    SGW->>MeNB: Bearer Modification Response

Key Points

• MeNB decides when to add SgNB (based on UE measurements)
• X2-AP protocol coordinates between MeNB and SgNB
• UE receives RRC Reconfiguration to configure NR radio
• MME is informed of bearer split via E-RAB Modification

Real-World NSA Deployments

🔬 Exercises

1. Architecture Exercise: Draw the Option 3x data flow for a UE downloading a video. Show which data goes through eNB vs gNB.
2. Comparison Exercise: Why can't NSA support network slicing? What 5G core component is needed?
3. Business Exercise: An operator has 10,000 eNBs and wants to add 5G. Calculate the cost difference: deploy 10,000 gNBs only (NSA) vs 10,000 gNBs + new 5G core (SA).
4. Security Exercise: In NSA mode, is the IMSI still sent in cleartext? Why? (Hint: which core's auth is used?)

Summary

You now understand:

• ✅ 5G NSA uses 4G EPC core with 5G NR radio
• ✅ EN-DC enables dual connectivity (LTE + NR)
• ✅ eNB is master (control), gNB is secondary (data)
• ✅ Three main options: 3, 3a, 3x (Option 3 most common)
• ✅ SgNB Addition procedure from 3GPP TS 37.340
• ✅ NSA enables fast 5G deployment but lacks full 5G features
• ✅ Most global operators still use NSA (SA adoption is slow)

Next: Part 3: 5G SA Architecture →