School of Specs The 5G system architectureIn depth

TS 23.501 · version 20.2.0 · 1180 clauses · In depth · 370 minutes

The 5G system architecture

TS 23.501 describes how a 5G network is put together: which functions exist, what each one is responsible for, and how they talk. It is 1180 clauses long and assumes you already know the field. This course walks through it in plain English, and every claim links to the clause it came from, so you can always read the original next to the explanation.

Start at chapter 1 Sit the final test Look at the document first

This course was written against TS 23.501 version 20.2.0, verified 2026-08-04.

Same subject, different depth: quick start · overview ·

The document, all of it

One block per clause of TS 23.501, in the order they appear, sized by how much text the clause holds. Lit blocks are taught in this course.

Open the whole of TS 23.501 as a page

The chapters

Part 1 Getting your bearings

  1. 1 Who writes this document, and why Who 3GPP is, which group owns TS 23.501, what a release is, and how the document changes meeting by meeting. 10 min
  2. 2 How to read a 3GPP specification The scope, the reference numbers, the definitions, the words that bind, and how to find one thing among 1180 clauses. 10 min

Part 2 The system in one piece

  1. 3 The shape of the 5G core What a network function is, why the core is built from services that call each other, and what a reference point really names. 12 min
  2. 4 What each network function does The job, the stored data and the callers of every network function the specification names, from AMF to EIF. 15 min
  3. 5 Services instead of interfaces What a network function service is, how a consumer calls one, and the four ways that call can be routed. 12 min
  4. 6 How one function finds another How core functions register themselves in a directory, ask it for candidates, and then apply their own rules to pick the one instance that will serve this subscriber. 14 min
  5. 7 Every name the system uses Every identifier a 5G system hands out — who the subscriber is, what the equipment is, which temporary name travels over the air, and which of them are privacy-sensitive. 13 min
  6. 8 What actually travels on the wire The layers that carry 5G signalling and user data — NGAP over SCTP on N2, NAS on N1, GTP-U over UDP and IP on N3 and N9 — and what each hop adds. 10 min

Part 3 What the system does

  1. 9 Registration, reachability and paging Why a device has to register at all, the two state machines that track it, and how the network finds it again once it has gone quiet. 15 min
  2. 10 Radio, Wi-Fi, cable — every way in What the 5G core still has to know about a radio network, and how Wi-Fi, wireline and the old 3GPP radios are made to look the same to it. 16 min
  3. 11 Using two accesses at once How one connection can run over the mobile radio and Wi-Fi at the same time, and how the network decides which packets go which way. 13 min
  4. 12 The connection to a data network What a PDU session is, the five kinds it can be, how long an address lives, and how the network keeps, moves or drops the session as you walk. 16 min
  5. 13 Where the packets actually go How the SMF programs a UPF over N4 with detection, forwarding, QoS and reporting rules, how a path through one or more UPFs is chosen and changed, and how big a packet may be. 13 min
  6. 14 What the network promises How 5G splits a connection into QoS Flows, describes each one with a small set of parameters, and enforces them at the phone, the radio and the core. 13 min
  7. 15 Rules, and paying for them What a policy is in the 5G core, which function decides it, how it reaches the phone, the AMF and the SMF, and where traffic is counted and paid for — with the two clauses that hand almost all the detail to other documents. 14 min
  8. 16 Who you are and who may listen What the architecture document actually settles about authentication, security contexts, identifier privacy and user-plane protection — and the much larger part it hands over to TS 33.501. 14 min
  9. 17 One network, many networks How one physical 5G network is cut into named slices, how a device asks for the ones it wants, and what the network does when it asks for one it may not have. 15 min
  10. 18 Being served by somebody else's network What changes when a visited operator serves you — the two user-plane shapes, which function sits on which side, and the guarded border between the two networks. 13 min

Part 4 Reaching further

  1. 19 Living next to 4G How 5G was built to stand beside an operator's existing 4G network, which functions had to be merged to make that work, and what a device loses when it walks from one system into the other. 15 min
  2. 20 Computing close by, and opening the door How the core puts an application server near the user, and how software outside the operator's network asks the core for information and for changes. 15 min
  3. 21 Emergency calls, messages, voice and location What the core has to do differently for public warnings, short messages, voice, emergency calls, location and priority users — and which document carries the detail for each one. 14 min
  4. 22 Clocks, TSN and deterministic delivery How 5G hands an accurate clock to a machine, tells the radio in advance when each burst will arrive, and pretends to be an Ethernet bridge inside a factory network. 13 min

Part 5 Special worlds

  1. 23 Very fast, very reliable, very picky How 5G duplicates traffic for machines that cannot lose a packet, and how it learns to see video frames instead of packets for headsets and games. 14 min
  2. 24 Networks that are not for everybody How 5G builds a network for one factory, campus or household — its own name on the air, its own credentials, its own private group of devices — and how a device uses one of those and a public network at the same time. 15 min
  3. 25 Small devices, small messages How a battery-powered sensor sending forty bytes a day gets served — data carried inside signalling, sleep measured in hours, packets buffered while it sleeps, and hard caps on how often it may talk. 15 min
  4. 26 Relays, satellites, femtos and shared networks The odd shapes an access network takes — shared, relayed, moving, repeated, domestic, orbital, deliberately small — and the one thing each asks of the core. 15 min
  5. 27 Overload, energy, deployment and the rest What a 5G core does when it is too busy to cope, how it is deployed and taken out of service without dropping anybody, how it counts its own energy, and thirteen smaller features that live nowhere else. 17 min

Testing yourself

Every question at once

160 questions, chapter by chapter, on one page — for revision after you have read the course.

Open all the questions

The final test

20 questions drawn across the whole course, marked at the end. 70 per cent to pass, and every miss says which chapter to go back to.

Sit the final test

What this course is built on

Chapters
27
Words written for you
70,550
Clauses covered
1145 of 1180
Figures from the document
164
Abbreviations
221
Published versions
50

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