The world of mobile networks is constantly evolving, and 5G represents a massive leap forward. Central to this new generation is a redesigned architecture with specific communication pathways or interfaces. One of the most crucial is the 5G N1 Connection, often referred to as the N1 interface. Understanding N1 is key to grasping how your 5G device actually talks to the core network.
This guide will dive deep into 5G N1 Connections, explaining what they are, how they work, and how they compare to other important interfaces like N2 and the older S1. We’ll cover 5G UE to AMF communication, the roles of the 5G RAN and 5G Core, and the difference between N1 and S1 Mode. Let’s break down this essential part of the 5G puzzle.
Introduction: Understanding the 5G Ecosystem and Interface Importance
Before zooming in on N1, let’s set the stage. The 5G System (5GS), as defined by the 3GPP, isn’t just about faster speeds. It involves:
- User Equipment (UE): Your 5G smartphone, tablet, or IoT device.
- Next Generation Radio Access Network (NG-RAN): The 5G base stations (called gNBs) provide the radio connection.
- 5G Core (5GC): The brain of the network, built on a flexible Service-Based Architecture (SBA).
A key design principle in 5G is separating the Control Plane (CP – signaling and control messages) from the User Plane (UP – actual user data traffic). Interfaces define how these different parts talk to each other. The 5G N1 Connection is a vital Control Plane interface.
What is the 5G N1 Connection Interface?
The N1 interface is the primary signaling pathway directly linking your 5G device (UE) to a specific part of the 5G Core network called the Access and Mobility Management Function (AMF). Think of the AMF as the main entry point and mobility manager for your device in the 5GC.
Definition, Endpoints (UE <-> AMF), and Location
- Definition: N1 is the logical reference point for Control Plane signaling between the UE and the AMF.
- Endpoints:
- User Equipment (UE)
- Access and Mobility Management Function (AMF) in the 5G Core.
- Location: It’s a logical path. This means the N1 messages actually travel through the Radio Access Network (NG-RAN) or even a non-3GPP access (like Wi-Fi connected to the 5GC). The RAN simply transports these messages without reading them – it’s transparent to N1 signaling.
Core Functions: Transporting NAS Signaling
The main job of the 5G N1 Connection is to carry Non-Access Stratum (NAS) signaling messages. NAS protocols handle communication between the UE and the Core Network, independent of the specific radio technology being used (5G NR, Wi-Fi, etc.).
NAS signaling over N1 covers two main areas:
- Mobility Management (MM): Deals with connecting to the network, moving around (mobility), security, and managing the connection state (like idle or active). The AMF directly handles these messages.
- Session Management (SM): Deals with setting up, modifying, and tearing down data connections (PDU Sessions) that let you access the internet or other services. While these messages travel over N1 to the AMF, the AMF relays them to another function called the Session Management Function (SMF) for processing.
N1 Protocol Structure: NAS-MM and NAS-SM
The NAS protocol used over N1 (defined in 3GPP TS 24.501) is split into:
- NAS-MM (5G Mobility Management): Handles registration, connection states (CM-IDLE/CM-CONNECTED), location updates, authentication, and security activation.
- NAS-SM (5G Session Management): Handles PDU Session procedures (establishment, modification, release). These messages are essentially “carried” inside NAS-MM messages over N1 to the AMF, which then forwards them to the relevant SMF.
This split allows the AMF to focus on access and mobility, while the SMF specializes in managing data sessions and interacting with the User Plane Function (UPF), which handles the actual data traffic.
The Complementary N2 Interface: Connecting 5G RAN and 5G Core
While N1 connects the UE to the Core, another interface, N2, connects the Radio Access Network to the Core.
Definition, Endpoints (NG-RAN <-> AMF), and Location
- Definition: N2 is the reference point connecting the NG-RAN node (e.g., gNB) to the AMF in the 5GC.
- Endpoints:
- NG-RAN node (like a gNB)
- Access and Mobility Management Function (AMF)
- Location: It’s the direct control link between the base station your device is connected to and the core network’s AMF.
Core Functions: NGAP Signaling
N2 uses the NG Application Protocol (NGAP) over a reliable transport layer (SCTP/IP). NGAP handles control signaling between the RAN and the AMF. This includes:
- UE-associated services: Managing the UE’s context in the RAN, setting up radio resources for PDU sessions, handling handovers within the RAN, and, crucially, transporting NAS messages.
- Non-UE associated services: Managing the N2 link itself, exchanging configuration updates, reporting errors, and coordinating load balancing between AMFs.
N2’s Role in Transporting N1 NAS Messages
This is critical: the logical N1 path relies physically on the N2 interface.
- Uplink (UE to AMF):
- UE creates a NAS message (e.g., Registration Request).
- UE sends it over the air to the NG-RAN (gNB).
- The gNB receives the NAS message.
- The gNB wraps this NAS message inside an NGAP message (e.g.,
INITIAL UE MESSAGEorUPLINK NAS TRANSPORT). - The gNB sends the NGAP message (containing the NAS message) over the N2 interface to the AMF.
- Downlink (AMF to UE): The process is reversed, with the AMF wrapping the NAS message in an NGAP
DOWNLINK NAS TRANSPORTmessage sent over N2 to the gNB, which then extracts and sends the NAS message to the UE.
So, N2 provides the actual transport vehicle across the RAN-Core boundary for the N1 messages.
N1 vs. N2 Interface: Key Differences and Interaction
Understanding the differences between N1 and N2 is key to understanding 5G control plane operations.
Highlighting the Distinctions
- Who they connect: N1 connects UE <-> AMF (logically). N2 connects NG-RAN <-> AMF (physically/logically).
- What protocol they use: N1 uses NAS. N2 uses NGAP.
- What they signal: N1 carries UE-Core signaling (registration, PDU sessions, UE mobility state). N2 carries RAN-Core signaling (RAN resource control, handovers, N2 management, and transport for N1 NAS).
- How the RAN handles them: N1 messages are transported transparently by the RAN. N2 messages are processed by both the RAN and the AMF.
How N1 and N2 Work Together
They are distinct but completely interdependent. N1 defines the end-to-end conversation between your phone and the core network manager (AMF). N2 provides the essential link between the radio network you’re connected to and that core network manager, acting as the carrier for N1 messages across that gap. You can’t have N1 communication in a typical 5G setup without N2 providing the path.
Comparison Table: N1 Interface vs. N2 Interface
| Feature | 5G N1 Connection / Interface | N2 Interface |
|---|---|---|
| Endpoints | UE <-> AMF | NG-RAN node (e.g., gNB) <-> AMF |
| Protocol | NAS (NAS-MM, NAS-SM) | NGAP (NG Application Protocol) |
| Transport | Via Access Network (using N2/NGAP path) | SCTP/IP |
| Primary Use | UE Mobility & Session Management Signaling | RAN-Core Control Plane Signaling |
| Key Information | Reg/Auth/Sec, Conn Mgt, Mobility, PDU Mgt | UE Context (RAN), PDU Resrc (RAN), NAS Transport, Handovers, N2 Mgt |
| RAN Handling | Transparent (Transported) | Terminated/Processed |
| Reference Point Type | Logical UE-Core | Physical/Logical RAN-Core |
N1 Mode vs. S1 Mode: Understanding 5G Deployment Architectures
The terms “N1 mode” and “S1 mode” relate to the type of core network your 5G device is connected to.
5G Standalone (SA) and N1 Mode Operation
- Architecture: 5G SA (Standalone) means the NG-RAN connects directly to the 5G Core (5GC). This is the target 5G architecture.
- N1 Mode: A UE operates in N1 mode when it successfully registers with the 5GC via an NG-RAN (or non-3GPP access integrated with 5GC).
- Interfaces Used: N1 (UE-AMF), N2 (NG-RAN-AMF), N3 (NG-RAN-UPF for user data).
- Benefits: Unlocks full 5G capabilities like network slicing, advanced QoS, low latency, edge computing – all managed by native 5GC functions (AMF, SMF, UPF, etc.).
5G Non-Standalone (NSA)/LTE and S1 Mode Operation
- Architecture: 5G NSA (Non-Standalone), particularly EN-DC (Option 3), is an earlier deployment model. Here, a 5G NR base station (en-gNB) is added to an existing 4G LTE network which still uses the 4G Evolved Packet Core (EPC). The LTE base station (eNB) acts as the master.
- S1 Mode: A UE operates in S1 mode when it is attached to the 4G EPC, even if it’s using 5G radio resources for faster data speeds.
- Interfaces Used: Primarily the S1 interface (connecting the LTE eNB to the EPC’s MME for control and SGW for user data). The X2 interface connects the LTE eNB and 5G en-gNB. N1 and N2 are generally NOT used in this mode for connecting to the core.
- Benefits/Limitations: Allows faster deployment of 5G data speeds by leveraging the existing 4G core. However, it cannot support the advanced features of the 5GC because the core network is still the 4G EPC (using MME, SGW, PGW functions).
Contrasting Core Networks and Interfaces
- N1 Mode: Relies on 5GC and NG interfaces (N1, N2, N3). Enables full 5G features.
- S1 Mode: Relies on 4G EPC and S1/X2 interfaces. Primarily provides a 5G speed boost over LTE infrastructure.
Comparison Table: N1 Mode (SA) vs. S1 Mode (NSA/LTE)
| Feature | N1 Mode (Typical: 5G SA) | S1 Mode (Typical: 5G NSA Option 3) |
|---|---|---|
| Core Network | 5G Core (5GC) | Evolved Packet Core (EPC) |
| Key CN Functions | AMF, SMF, UPF, PCF, etc. | MME, SGW, PGW, HSS, etc. |
| Primary RAN I/F | NG Interfaces (N1, N2, N3) | S1 Interface (S1-MME, S1-U), X2 |
| RAN Type(s) | NG-RAN (gNB), Non-3GPP Access | E-UTRAN (eNB Master), NG-RAN (en-gNB Secondary) |
| UE Attach Point | 5GC (AMF) | EPC (MME) |
| Key Features | Full 5G Suite (Slicing, QoS, etc.) | Primarily eMBB speed boost |
| Performance | Optimized 5G Latency/Throughput | Sub-optimal (EPC limitations) |
| Deployment Need | Full 5GC Deployment | Leverage Existing EPC |
How 5G UE Communicates with AMF via N1 Connection
Let’s revisit how the 5G UE to AMF communication happens using the N1 interface.
The End-to-End N1 NAS Signaling Path
- UE generates NAS message.
- UE sends NAS message over the air (using radio protocols like RRC) to the NG-RAN (gNB).
- NG-RAN wraps NAS message in an NGAP message.
- NG-RAN sends NGAP message over N2 interface to the AMF.
- AMF receives NGAP message, unwraps the NAS message, and processes it. (Reverse path for downlink messages from AMF to UE)
Transparency Through the 5G RAN
As mentioned, the 5G RAN (gNB) acts like a mail carrier for N1 NAS messages. It doesn’t open or read the NAS “letter”; it just puts it in the right “envelope” (NGAP) and sends it over the right “road” (N2) to the AMF. This separation keeps radio (Access Stratum – AS) concerns distinct from core network (Non-Access Stratum – NAS) concerns, allowing them to evolve independently.
AMF: The N1 Termination Point in the 5G Core
The AMF is where the 5G N1 Connection logically ends within the 5G Core.
- It receives all uplink N1 NAS messages (via N2/NGAP).
- It processes NAS-MM messages directly (registration, mobility, connection state).
- It identifies the target SMF for NAS-SM messages (based on PDU Session ID) and relays them (usually over the N11 interface between AMF and SMF).
- It manages the security context for N1 signaling (authentication, ciphering, integrity protection).
N1 Connections Key procedure: NAS Signaling
The N1 interface is the stage for critical NAS procedures that manage your phone’s life in the 5G network. Here are some major ones (defined in 3GPP TS 24.501):
Registration Management
This is how a UE joins the 5G network. Initiated by the UE sending a REGISTRATION REQUEST over N1.
- Types:
- Initial Registration: When powering on or entering a new network.
- Mobility Registration Update: When moving to a new tracking area while idle or connected.
- Periodic Registration Update: A timer-based keep-alive to show the UE is still present.
- Emergency Registration: For accessing emergency services.
- Process: Involves exchanging messages like
REGISTRATION REQUEST, Authentication messages,SECURITY MODE COMMAND/COMPLETE, and finallyREGISTRATION ACCEPTorREJECT. The Request includes vital info like UE identity (GUTI or SUCI), capabilities, requested network slices (NSSAI), and security context identifiers (ngKSI).
Deregistration Procedures
How a UE leaves the network.
- UE-initiated: UE sends
DEREGISTRATION REQUEST(e.g., when powering off). - Network-initiated: AMF sends
DEREGISTRATION REQUEST(e.g., subscription issue).
Service Request Procedure
Used mainly to transition the UE from power-saving CM-IDLE state to active CM-CONNECTED state, or to activate the user plane for data transfer.
- Triggers: UE needs to send data/signaling, UE responds to paging from the network.
- Messages: UE sends
SERVICE REQUEST, AMF responds withSERVICE ACCEPTorSERVICE REJECT. This procedure ensures resources are used efficiently.
PDU Session Management
These NAS-SM procedures manage your data connections. They are relayed by the AMF to/from the SMF.
- PDU Session Establishment: UE sends
PDU SESSION ESTABLISHMENT REQUESTto set up a data link to a network (like the internet). Includes requested DNN (APN), PDU Session Type (IPv4/v6 etc.), requested Slice info (S-NSSAI). SMF responds withACCEPT(providing IP address, QoS rules) orREJECT. - PDU Session Modification: Changes parameters (like QoS) of an existing session.
- PDU Session Release: Tears down an existing data session.
Summary Table: Major N1 NAS Procedures
| NAS Procedure | Initiator(s) | Purpose | Key N1 NAS Messages | Core Function(s) Involved |
|---|---|---|---|---|
| Registration | UE | Attach, authenticate, security setup, location update | REG REQUEST, AUTH REQ/RESP, SEC MODE CMD/CMPLT, REG ACCEPT/CMPLT/REJECT | AMF, AUSF, UDM |
| Deregistration | UE, AMF, UDM | Detach from 5GC | DEREG REQUEST (UE<->AMF), DEREG ACCEPT (AMF->UE) | AMF, UDM |
| Service Request | UE, Network | Transition IDLE->CONNECTED, Activate User Plane | SERVICE REQUEST, SERVICE ACCEPT/REJECT | AMF, SMF (for UP) |
| PDU Session Establish | UE | Create data connection, get IP, setup QoS | PDU SESS ESTAB REQUEST/ACCEPT/REJECT | AMF (Relay), SMF, UPF |
| PDU Session Modify | UE, SMF | Change session parameters (e.g., QoS) | PDU SESS MOD REQUEST/COMMAND/COMPLETE/REJECT | AMF (Relay), SMF, UPF |
| PDU Session Release | UE, SMF | Terminate data connection | PDU SESS REL REQUEST/COMMAND/COMPLETE/REJECT | AMF (Relay), SMF, UPF |
Conclusion:
The 5G N1 Connection is far more than just a technical label; it’s the essential communication channel between your 5G device and the core network’s control center (the AMF). It’s the pathway for NAS signaling, enabling everything from initial network entry and security setup (5G UE to AMF) to managing mobility, controlling data sessions, and requesting advanced 5G services like network slicing.
While logically direct, N1 relies on the N2 interface for transport across the 5G RAN. Understanding the distinction between N1 vs. N2 Interface and the different operational modes (N1 Mode for SA vs. S1 Mode for NSA/LTE) is crucial for comprehending how the 5G RAN and 5G Core interact in various deployment scenarios.
Ultimately, the N1 interface and the NAS procedures it carries are fundamental to delivering the security, flexibility, and diverse service capabilities promised by the 5G system. It embodies the core principles of modern network design, making it a cornerstone of the 5G architecture.
Is a freelance tech writer based in the East Continent, is quite fascinated by modern-day gadgets, smartphones, and all the hype and buzz about modern technology on the Internet. Besides this a part-time photographer and love to travel and explore. Follow me on. Twitter, Facebook Or Simply Contact Here. Or Email: [email protected]
