4 What the network promises, and how one network becomes many

4.1 Not every packet deserves the same treatment

A plain internet connection treats every packet alike. That is fine while capacity is free. Radio capacity is not: it is shared, it changes with the weather and the crowd, and it is the scarcest thing in the system.

So the network has to be able to say, for one particular stream of packets inside one session, what treatment it should get — how fast, how soon, how reliably, and who loses out first when there is not enough to go round.

The traffic inside a session is therefore split into QoS Flows, and each one carries an identifier of its own, the QFI.

The identifier rides in the tunnel header on the links between the radio network and the core, so the packet the application sent is never touched §5.7.1.1.

Flows come in two kinds. A guaranteed bit rate flow has capacity held open for it alone. Everything else is a non-guaranteed flow, sharing what is left. Every session is created with one flow that lives as long as it does, and it should be one of the non-guaranteed kind §5.7.1.1.

4.2 The number that points at a table

Each flow is described to the radio network by two values above all. The first is the 5QI.

The second is the ARP, the allocation and retention priority, and it is about a different question entirely. Not how to schedule packets, but which whole flows get admitted and which get dropped when resources run short. Its priority level runs from 1 to 15, and 1 is the highest §5.7.2.2.

4.3 One network, several names

An operator owns one radio network and one core, and has to serve a phone streaming video, a factory arm that must never see a late packet, and a million water meters that wake once a day.

Those three want different things from the same equipment, and often a different customer is paying. A network slice is the answer.

A slice instance is a working set of machinery: core control-plane and user-plane functions, plus at least one way in — the radio network, or one of the gateways that bring Wi-Fi or fixed lines into 5G §5.15.1.

Two slices may differ in the features they support. They may also be identical and exist only to keep two groups of devices apart, one per business customer §5.15.1.

The name of a slice is an S-NSSAI, and it has two parts §5.15.2.1:

• The SST, the Slice/Service type, saying what kind of behaviour is expected of it.

• The SD, the Slice Differentiator, optional, telling apart several slices that share one SST.

Seven SST values are standardised, from enhanced mobile broadband through ultra-reliable low latency to vehicle communication §5.15.2.2.

4.4 Where to go from here

You can now say what the 5G core is, why it is built out of software functions, what the main ones are called, what a device does before it can send a packet, and what the network promises about that packet.

Three places to go next, in order:

• The overview course of this track. The same document at the altitude needed to follow a working conversation — these subjects again, plus how functions find each other, what happens when you roam, and what the identifiers mean.

• The in-depth course of this track. Every mechanism the document describes, parameter by parameter. Read it when you have to work with the specification rather than talk about it.

• The document itself. Every clause number on this page opens the real text, the map shows which parts of TS 23.501 this course covered, and the glossary explains every abbreviation.