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5GC Textbook

A free, in-depth guide to the 5G Core Network

The goal of this site is NOT to summarize the specifications. It is a textbook whose top priority is helping you truly understand the 5G Core Network (5GC), explain it in your own words, and apply it in real-world work.

What's New

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Why we built this site

The 3GPP specifications (TS 23.501 / TS 23.502 / TS 24.501, etc.) are accurate, but extremely difficult for first-time readers. Terminology appears with no assumed background, and the "why it works this way" is rarely explained.

On the other hand, most introductory articles are easy to read but weakly grounded in the specifications, and using them as-is in practice can lead to misunderstandings.

This site aims to combine the best of both.

  • Clarity … Diagrams, tables, analogies, and short paragraphs. Readable even for beginners.
  • Accuracy … Based on public specifications from 3GPP/RFC/ETSI/GSMA and more, with references cited down to the TS number + section number. Anything not in the spec is explicitly marked as "implementation-dependent."

Every explanation is structured in the order Why → What → How.

What you'll be able to do after reading

What you'll be able to do Concrete example
Explain the 5GC Explain the roles of AMF/SMF/UPF/AUSF/UDM and more to others
Read 3GPP specs Know "where to find what" in TS 23.501/23.502/24.501
Analyze with Wireshark Read and interpret NGAP/NAS/PFCP/HTTP2 packets
Understand call flows Follow procedures like Registration and PDU Session Establishment
Troubleshoot failures Isolate which NF and which message failed
Design systems Grasp the key points of designing SBA/Slice/QoS from requirements
Understand vendor differences Distinguish "the spec" from "implementation-dependent" behavior

Who this site is for

This site is written for a broad range of readers:

  • Students … Those about to start learning telecom networks
  • New hires … Those who just entered the telecom industry
  • Telecom / network engineers … Those with general networking experience but new to the core
  • 4G EPC veterans … Those who want to bridge EPC knowledge into the 5GC
  • 5GC developers … Those implementing and integrating NFs
  • Operations/verification staff … Those doing troubleshooting and testing

To keep it readable for beginners, technical terms are always explained on the spot.

How to get started

Choose an entry point that matches your level.

For beginners (start from the big picture of 5GC)

We recommend proceeding in order from Level 1 (5GC Overview). Start with Mobile Network Overview (up to LTE), then continue to 5GC Overview and SBA to grasp the whole picture.

For experienced readers (4G EPC veterans / network engineers)

Check where you stand with the "Learning Roadmap" below, and if you prefer learning by doing, pick a procedure that interests you from each Level's topics, such as Registration or Handover.

If you just want to look up a specific term

To quickly look up an NF name, Interface name, or Protocol name, see the Dictionary. Individual NF pages like AMF and Interface pages like N2 are also extensive.

Learning roadmap (Level 1 → Level 5 + Level ∞)

A five-stage roadmap from Level 1 (5GC Overview) to Level 5 (Advanced/Latest), ordered so that prerequisite knowledge accumulates. On top of that, Level ∞ (Deep Dive) digs into the details that matter when reading traces, down to 3GPP clause level, and can be read at any point from Level 2 onward. Beginners should start from Level 1 in order; experienced readers can enter at whichever Level they need.

flowchart TD
    L1["Level 1
5GC Overview / SBA / NF"] --> L2["Level 2
Registration / Auth / PDU Session"]
    L2 --> L3["Level 3
Mobility / Handover / Interworking / QoS"]
    L3 --> L4["Level 4
VoNR / Emergency / Slice / RedCap"]
    L4 --> L5["Level 5
Charging / Policy / NWDAF / Roaming / Rel17-18"]

    L2 -.-> LM["Level ∞
Identity / PFCP / QoS / Policy / SBI deep dive"]

    L1 -.- L1t["Mobile Network / LTE / Interface"]
    L2 -.- L2t["Security / Session establishment"]
    L3 -.- L3t["Idle / Connected / N26 / QoS"]
    L4 -.- L4t["Network Slice / Voice / Fallback"]
    L5 -.- L5t["CAPIF / NEF / Analytics"]

  • Level 1 — 5GC Overview

    Understand the overall picture of mobile networks and the architecture (SBA) that 5GC is built on

    Prerequisite: None (this is the starting point)

  • Level 2 — Basic Procedures (Registration / Session)

    Be able to follow, via Call Flows, the basic procedures from UE registration to PDU Session establishment

    Prerequisite: Level 1

  • Level 3 — Mobility & QoS

    Understand the mechanisms that keep communication alive while the UE moves (Mobility/Handover), and the 5G QoS model

    Prerequisite: Level 2

  • Level 4 — Service / Device Types

    Understand the main services such as voice, emergency calls, and network slicing, plus device types such as RedCap

    Prerequisite: Level 2. In addition, for voice (VoNR/Emergency), understanding Level 3 (mobility) is recommended

  • Level 5 — Advanced / Latest Releases

    Survey advanced areas such as charging, policy, data analytics, and roaming, plus the latest Rel-17/18 features

    Prerequisite: Levels 1–4 (varies by topic)

  • Level ∞ — Deep Dive

    Dig into details that matter when reading traces, such as identifiers and AMF selection, down to 3GPP clause level

    Prerequisite: Level 2 or above (varies by topic)

Level 1 — 5GC Overview

How to work through this Level

Start with the 4G/LTE map (Mobile Network Overview), grasp the design philosophy of 5GC in 5GC Overview and SBA, then survey the building blocks in NF Overview and Interface / Protocol Overview. Individual NFs and reference points can be explored in their detail pages and the dictionary.

Notes on Interface / Protocol

N1 (UE⇔AMF, a logical reference point carrying NAS) and N2 are control plane; N3 (gNB–UPF) / N6 (UPF–DN) are user plane; N4 is between SMF and UPF (PFCP). Separately, SBI (Service-Based Interface, such as Namf/Nsmf/Nudm) lets NFs interact (the SBA / reference point model is TS 23.501 §4; the use of HTTP/2 for SBI is TS 29.500 §5.2). User plane packets are encapsulated in GTP-U and carried over N3 (TS 29.281, TS 23.501 §8.3).

Level 2 — Basic Procedures (Registration / Session)

How to work through this Level

First follow Registration up to the point where the UE registers with the network, then proceed to authentication, security, and PDU Session; the whole flow up to an open data path connects together. Each chapter has Call Flow diagrams, Wireshark examples, and comprehension checks. QoS (the 5G QoS model) is placed in Level 3.

Relationship between Authentication and Security

Authentication (5G-AKA / EAP-AKA') and NAS/AS Security (key hierarchy, ciphering, integrity protection) are separate topics, but in practice they run consecutively within the Registration procedure as Authentication → NAS Security Mode Control (TS 23.502 §4.2.2.2.2). Authentication may be skipped when a valid 5G security context already exists (to be confirmed). The subscriber identifier is not sent as SUPI directly, but as SUCI, concealed with a public key (TS 33.501 §6.12 / Annex C, to be confirmed).

Level 3 — Mobility & QoS

How to work through this Level

Start with Paging to grasp "reachable even while moving", then widen to Service Request / RRC Resume and the registration updates (Mobility / Periodic). Next cover Handover for "not dropped while moving", and it follows naturally into 4G-5G Interworking (N26 and EPS Fallback). Finally organize the 5G QoS model in QoS. Network slicing is covered in Level 4's Network Slicing.

What EPS Fallback is

The EPS Fallback procedure — temporarily moving the UE to 4G (EPS) for voice origination/termination — is specified in TS 23.502 §4.13.6.1 (sub-clause number to be confirmed). It is widely adopted as a way to complete voice calls in areas without VoNR, but whether it applies depends on operator policy and deployment (implementation-dependent).

Level 4 — Service / Device Types

How to work through this Level

The voice-related chapters (VoNR / Emergency / EPS Fallback / Emergency Fallback) are closely related, so reading them together helps. EPS Fallback drops normal voice to 4G when VoNR is unavailable, while Emergency Fallback moves an emergency call to 4G/E-UTRA triggered by the UE's emergency service request. SMS (carried over the control plane), Network Slicing (logical networks per use case), and RedCap (a device category) can be read independently.

VoNR / Emergency assume IMS

VoNR and Emergency (voice) assume IMS (IP Multimedia Subsystem), and voice is tied to QoS Flows (5QI=1: conversational voice, 5QI=5: IMS signalling; TS 23.501 §5.7.4 standardized 5QI table). In areas where VoNR is unavailable, EPS Fallback completes the voice call.

RedCap is a device category, not a service

RedCap is not a service provided by the 5GC like SMS or VoNR; it is a device category with reduced UE capability (Reduced Capability, Rel-17). It refers to device types that intentionally limit UE capabilities such as bandwidth and number of antennas, aimed at low-cost, low-power mid-tier IoT (UE capability details are in the RAN specs TS 38.306 / TR 38.875; see the RedCap chapter).

Level 5 — Advanced / Latest Releases

How to work through this Level

Advanced areas are fairly independent, so you can read whichever topic interests you. Charging and policy start at Charging / Policy, data analytics at NWDAF, inter-operator interworking at Roaming, external exposure at NEF / CAPIF, the latest features at Rel-17 & Rel-18, and positioning at Location Service.

Relationship between Analytics and NWDAF

Analytics and NWDAF may look like separate items, but NWDAF (Network Data Analytics Function) is the NF that performs Analytics, exposing the service-based interface Nnwdaf to provide analytics results to other NFs (TS 23.288). See the NWDAF chapter for details.

Level ∞ — Deep Dive

Identity / N2 Setup / Registration / Connection State

Session / UPF

QoS control

Policy / Exposure

SBI / Redundancy

Reference lookup

  • Quick term lookup → Dictionary (16 dictionaries incl. NF/Interface/Protocol)
  • Individual NF/Interface pages → AMF / N2, etc.