The 5G system architecture — the cheat sheet

The shape of it

1 The shape of the system

2 Serving one device

3 Running it in the real world

Keep these in your head

1.3 What this document actually describes
The core is a set of software functions that call each other, not a set of boxes joined by cables. Every oddity later in the document is a consequence of that.
2.2 Who does what in the core
AMF, SMF, UPF. Where you are, what you are connected to, and the packets themselves. Almost every other function in the core exists to answer a question one of these three asks.
3.2 How one function calls another
An N-name in lower case — Namf, Nsmf — is a producer's front door. An N-name in digits — N2, N11 — is a conversation between two functions. They look alike and are not alike.
4.2 Knowing who you are and where you are
Nothing in normal operation pages a device by its permanent identity. The network reaches a device through the temporary name it handed out, and hands out a new one whenever it likes.

Easy to get wrong

1.4 What this document actually describes
A reference point name like N11 is not a protocol and not a socket. It is a label for the conversation between two functions, carried over their service-based interfaces. Only six names — N1, N2, N3, N4, N6 and N9 — are genuinely separate links.
2.5 Who does what in the core
"The AMF" is a role, not a machine. A real network runs many AMF instances, and a device's context may be picked up by a different one. That is why the document has to say where an instance's state is kept — the state, not the instance, is what has to survive.
3.3 How one function calls another
The gaps in the numbering are bookings, not omissions. N44 to N49 and N100 to N109 are reserved for the charging specification and N90 to N95 for the policy specification, so a missing number is somebody else's to allocate §4.2.7.
4.5 Knowing who you are and where you are
Deregistered is not the same as idle, and idle is not the same as switched off. A deregistered device has no location the AMF can use; an idle device has one, to the accuracy of a tracking area list, and can be paged. The two machines move for different reasons.
5.5 The connection, and what is promised on it
A 5QI is not a bandwidth. It is a scalar that points into a table of characteristics — resource type, priority level, packet delay budget, packet error rate — that both ends already agree on. Standardised values are not signalled at all; only a dynamically assigned one carries its characteristics with it §5.7.2.1.
6.4 One network behaving like several
A slice is not a private network and not a separate core. It shares the AMF, and it is a way of grouping functions, subscriptions and admission limits — not a wall. Isolation between customers is a deployment choice on top of it, not something the S-NSSAI gives you.
7.5 Roaming, other accesses, the edge and the outside
Edge computing normally applies to the non-roaming case and to local breakout. For a home-routed session it applies only under one named arrangement, session breakout in the visited network, defined in another document §5.13.

The document's own words

1.3 What this document actually describes
The 5G System architecture is defined to support data connectivity and services enabling deployments to use techniques such as e.g. Network Function Virtualization and Software Defined Networking. … Separate the User Plane (UP) functions from the Control Plane (CP) functions, allowing independent scalability, evolution and flexible deployments e.g. centralized location or distributed (remote) location.
2.2 Who does what in the core
Regardless of the number of Network functions, there is only one NAS interface instance per access network between the UE and the CN, terminated at one of the Network functions that implements at least NAS security and Mobility Management.
3.3 How one function calls another
The following reference points show the interactions that exist between the NF services in the NFs. These reference points are realized by corresponding NF service-based interfaces and by specifying the identified consumer and producer NF service as well as their interaction in order to realize a particular system procedure.
4.3 Knowing who you are and where you are
A UE in CM-IDLE state has no NAS signalling connection established with the AMF over N1. … There are no AN signalling connection, N2 connection and N3 connections for the UE in the CM-IDLE state.
5.5 The connection, and what is promised on it
The QoS Flow is the finest granularity of QoS differentiation in the PDU Session. A QoS Flow ID (QFI) is used to identify a QoS Flow in the 5G System. User Plane traffic with the same QFI within a PDU Session receives the same traffic forwarding treatment (e.g. scheduling, admission threshold).
6.2 One network behaving like several
A PDU Session belongs to one and only one specific Network Slice instance per PLMN. Different Network Slice instances do not share a PDU Session, though different Network Slice instances may have slice-specific PDU Sessions using the same DNN.
7.5 Roaming, other accesses, the edge and the outside
Edge computing enables operator and 3rd party services to be hosted close to the UE's access point of attachment, so as to achieve an efficient service delivery through the reduced end-to-end latency and load on the transport network.

The words

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

(R)AN: (Radio) Access Network
5G-EIR: 5G-Equipment Identity Register
5G-GUTI: 5G Globally Unique Temporary Identifier
5G-S-TMSI: 5G S-Temporary Mobile Subscription Identifier
5GC: 5G Core Network
5QI: 5G QoS Identifier
AF: Application Function
AMF: Access and Mobility Management Function
AUSF: Authentication Server Function
CHF: Charging Function
CP: Control Plane
DNN: Data Network Name
GUAMI: Globally Unique AMF Identifier
MICO: Mobile Initiated Connection Only
N3IWF: Non-3GPP InterWorking Function
NEF: Network Exposure Function
NF: Network Function
NRF: Network Repository Function
NSAC: Network Slice Admission Control
NSSAA: Network Slice-Specific Authentication and Authorization
NSSAI: Network Slice Selection Assistance Information
NSSF: Network Slice Selection Function
PCF: Policy Control Function
PDU: Protocol Data Unit
PEI: Permanent Equipment Identifier
QFI: QoS Flow Identifier
S-NSSAI: Single Network Slice Selection Assistance Information
SCP: Service Communication Proxy
SD: Slice Differentiator
SEPP: Security Edge Protection Proxy
SMF: Session Management Function
SMSF: Short Message Service Function
SSC: Session and Service Continuity
SST: Slice/Service Type
SUCI: Subscription Concealed Identifier
SUPI: Subscription Permanent Identifier
TNGF: Trusted Non-3GPP Gateway Function
TWIF: Trusted WLAN Interworking Function
UDM: Unified Data Management
UDR: Unified Data Repository
UDSF: Unstructured Data Storage Function
UPF: User Plane Function
W-AGF: Wireline Access Gateway Function

Where to look it up

The 38 clauses of TS 23.501 this course is built from.

§1 §4.1 §4.2.1 §4.2.2 §4.2.3 §4.2.4 §4.2.5 §4.2.6 §4.2.7 §4.2.8 §4.3 §5.2 §5.3.2 §5.3.3 §5.4.1 §5.4.3 §5.6 §5.6.3 §5.7 §5.8 §5.9 §5.10 §5.12 §5.13 §5.14 §5.15 §5.17 §5.20 §6.1 §6.2 §6.3.1 §7.1.1 §7.1.2 §7.1.3 §7.1.4 §7.1.5 §8.2.3.1 §8.3.1

Where to go from here

The same subject, quick start — Under an hour, four chapters, the shape of the thing.
The same subject, in depth — The working reference: every mechanism, every parameter.
Every question of this course — 38 of them, on one page.
The final test — 15 questions, 70 per cent to pass.
The whole of TS 23.501 — all 1180 clauses.