1 What this document is, and why it had to be written

1.1 Why anybody writes this down

A phone bought in Seoul has to work on a network in Lisbon, on radio equipment made in Sweden, talking to core-network software made in China, billed by software made in India.

Nobody owns that chain, and no company can fix it by shipping a better product. The only thing that makes it work is that everybody agreed, in writing and before any of it was built, where each piece stops and the next one starts.

TS 23.501 is the part of that agreement that says what a 5G core network is made of: which functions exist, what each one is responsible for, and how they are wired together.

1.2 What the document settles

Its own first line is admirably short:

The rest of the scope widens that out. The document covers roaming as well as a device on its home network, moving around inside 5G, and living next to an operator's older 4G network.

It also covers quality of service, policy and charging, authentication, and system-wide services such as text messages, location and emergency calls §1.

It also names its two companions in the same breath: TS 23.502 holds the procedures and message flows, TS 23.503 holds policy control and charging §1. The three were written together and are read together.

Almost everything else is somebody else's document. The reference list §2 runs to over two hundred entries — other 3GPP specifications, internet standards from the IETF, and a scattering from the IEEE, the Broadband Forum and the Wi-Fi Alliance.

1.3 The three stages

3GPP splits every subject into three layers of document, and knowing which one you are holding saves a great deal of confusion.

• Stage 1 says what the user gets, in the language of a service: how fast, how many devices, how reliable. It never names a box.

• Stage 2 says what the system is made of and how the pieces interact. That is this document. It draws functions, links, states and parameters.

• Stage 3 turns stage 2 into real messages with fields, encodings and error codes — TS 24.501 for what a device says to the core, the 29-series for what core functions say to each other.

1.4 Who keeps it

This section is background and none of it comes from the document itself.

3GPP is the 3rd Generation Partnership Project, a partnership of seven regional standards bodies from Japan, the United States, China, Europe, India and Korea. The people in the room work for network operators, equipment makers, chip designers and test houses. Decisions are made by agreement in meetings.

Work is split across three groups. TSG SA covers services and system architecture, TSG CT the messages inside the core and towards devices, TSG RAN everything on the radio side. TS 23.501 belongs to the architecture working group SA2, and the SA plenary approves what SA2 drafts §Foreword.

1.5 Releases, and which version to read

Everything cannot change at once, because equipment has to be built. So 3GPP freezes the feature set at intervals and calls each snapshot a release. After the freeze, only corrections go in.

Once a specification is under change control, the first digit of its version is the release it belongs to. Release 15 was the first 5G release, approved at the end of 2017; the version this course reads is 20.2.0, the second update to Release 20 §Annex X.

The plenary meets about four times a year, and nothing changes in between. A version is always the state of the text at the end of one meeting.

Next comes the thing the document actually describes: the pieces a 5G core is built out of, in The pieces a 5G core is made of.