The 5G system architecture — the cheat sheet

The shape of it

1 What it is and who wrote it

2 What it does for a device

One from each chapter that has one. The course carries 6 in all — the rest are in the chapters.

1.1 What this document is, and why it had to be written
A specification is not the documentation of a product. It is a contract written before the products exist, so that products built separately still fit.
2.1 The pieces a 5G core is made of
The core is a set of software functions that call each other, not a set of boxes joined by cables. Everything else in this chapter follows from that.
3.2 What happens when a phone switches on
A registration lasts days. A connection lasts seconds. That is the whole reason there are two.
4.2 What the network promises, and how one network becomes many
Quality of service in 5G is a description attached to a stream of packets, and the whole of it is about getting that description to the places that can act on it: the phone, the base station and the core.

One from each chapter that has one. The course carries 7 in all.

1.3 What this document is, and why it had to be written
Stage 2 defining no bytes is what newcomers trip over. When TS 23.501 says one function "provides" something to another, that is a responsibility, not a message. The message is in a stage 3 document, and the two can look nothing alike. Never quote this document to settle an encoding argument.
2.2 The pieces a 5G core is made of
"The AMF" is a role, not a machine. Several clauses say plainly that some or all of a function's jobs may be supported in a single instance, which is the document's way of saying a real network may be running twenty instances behind one name — and may split the jobs between them.
3.2 What happens when a phone switches on
"Registered" and "connected" are not two degrees of the same thing. A phone doing nothing all night is registered and idle the whole time: the core holds its context and can page it, but there is no signalling connection to the radio network, no N2 connection and no N3 connection anywhere §5.3.3.2.2.
4.2 What the network promises, and how one network becomes many
A 5QI is not an amount of bandwidth and not a channel. It is a plain number used as a pointer into a table of characteristics agreed in advance — resource type, priority, delay budget, error rate. A standardised value is sent as a bare number and nothing else, because both ends already hold the table §5.7.2.1.

1.2 What this document is, and why it had to be written
The present document defines the Stage 2 system architecture for the 5G System. The 5G System provides data connectivity and services.
2.5 The pieces a 5G core is made of
Network functions within the 5GC Control Plane shall only use service-based interfaces for their interactions.
3.3 What happens when a phone switches on
The 5GC supports a PDU Connectivity Service i.e. a service that provides exchange of PDUs between a UE and a data network identified by a DNN.
4.1 What the network promises, and how one network becomes many
The QoS Flow is the finest granularity of QoS differentiation in the PDU Session. … User Plane traffic with the same QFI within a PDU Session receives the same traffic forwarding treatment (e.g. scheduling, admission threshold).

18 the course explains, out of 221 the document defines — all of them.

The 24 clauses of TS 23.501 this course is built from.

The same subject, overview — Enough to follow the conversation and know what to look up.
The same subject, in depth — The working reference: every mechanism, every parameter.
Every question of this course — 16 of them, on one page.
The final test — 10 questions, 60 per cent to pass.
The whole of TS 23.501 — all 1180 clauses.