2 Stage 3 — the difference between a plan and a protocol

2.1 Why the same feature gets written three times

Suppose the industry agrees that a phone should be able to keep working when the network it is on has a disaster and a rival network opens its doors. Three different questions follow, and one document cannot answer all three.

What is wanted? Somebody has to say plainly that this capability should exist, and under what conditions — before anybody has decided how.

Which boxes do what? Somebody has to name the pieces of the network that take part, say which one decides, which one is told, and in what order.

What exactly is sent? Somebody has to say which message carries the news, which octet of it holds the flag, and what the device does on every possible answer, including the answers that make no sense.

3GPP keeps those three answers in three kinds of document, and calls them stage 1, stage 2 and stage 3. CT1 writes stage 3.

2.2 The ladder, visible inside TS 24.501 itself

You do not have to take the ladder on trust. TS 24.501 lists every document it leans on, with the full title each one carries §2. The titles say the stage out loud:

Document	Title as the reference list writes it
TS 22.261	Service requirements for the 5G system; Stage 1
TS 23.501	System Architecture for the 5G System; Stage 2
TS 23.502	Procedures for the 5G System; Stage 2
TS 23.503	Policy and Charging Control Framework for the 5G System; Stage 2

And the document doing the referring is itself "Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3" TS 24.501.

That reference list is not decoration. Its opening line makes every one of those documents part of this one:

So a stage-3 document does not repeat the architecture. It points at it, and the pointer has force.

The same clause sets out how tightly each pointer binds, which matters more than it looks §2. A reference is either specific — tied to a date, edition or version number — or non-specific.

For a specific reference, later revisions of the target do not apply. For a non-specific one the latest version applies, and where the target is a 3GPP document that means the latest version in the same Release as the document doing the referring.

That last clause is the one people trip over. A non-specific reference does not drift forward for ever; it drifts within one Release and stops.

Two versions of TS 24.501 from different Releases can therefore be pointing at two different versions of the same architecture document while both look identical on the page.

2.3 Where the seam runs

The cleanest statement of the split is one sentence in the overview of TS 24.501 §4.1:

Read that carefully, because it is the whole boundary in two lines.

The elementary procedures belong to TS 24.501. Registration, authentication, security mode control, service request, PDU session establishment — each one is defined here, on its own, with its messages, its timers, its failure cases.

The sequences belong to the stage-2 document. What happens in what order when a device moves from one network to another is a story told over there, using the procedures defined here as its verbs.

Neither document is the summary of the other. A stage-2 sentence can be true while a stage-3 sentence about the same feature says something the stage-2 document never mentions, because the stage-3 document had to decide the case where the answer arrives twice.

2.4 What the third writing actually adds

Three things, and they are the reason a stage-3 document is so much harder to read than a stage-2 one.

A1. Bits. Every message in TS 24.501 is a standard layer-3 message with a fixed shape: an extended protocol discriminator, then a security header type or a PDU session identity, then a procedure transaction identity, then a message type, then whatever fields the message needs §9.1.1. Each message type is a number in a table — registration request is one bit pattern, registration accept the next §9.7. That table is what makes two vendors' code interoperate; it does not exist at stage 2.

A2. Failure. Most of the length of TS 24.501 is not the happy path. Nearly every procedure has a subclause called "Abnormal cases in the UE" and another called "Abnormal cases on the network side", and there is a whole clause on what to do with messages that are malformed, unknown or arrive in the wrong state §7.1.

A3. Who obeys. Stage 3 says which side has to do what. The scope of TS 24.501 names four: the device, the AMF, the SMF and the PCF §1. A stage-2 sentence describing a system behaviour becomes, at stage 3, an obligation on a named box.

2.5 Why the NAS is where the seam is easiest to see

The layer this course is about has an unusually clean definition, which is why it makes such a good example of what stage 3 is for.

TS 24.501 places it between exactly two points §4.1: the device and the AMF, at the reference point called N1, over both 3GPP radio access and non-3GPP access such as Wi-Fi.

Everything below that — how the bits get across the air, how the base station relays them — is somebody else's document. Everything above it, meaning why the network decided what it decided, is the architecture document's. What is left in the middle is a protocol, and that is a stage-3 job.

The protocol also inherits its shape rather than inventing it: the overview says the NAS for 5GS follows the layer-3 protocol architecture model of TS 24.007 §4.1, and TS 24.007 is itself one of CT1's own specifications TS 24.007. The group keeps its own foundations.

2.6 Stage 3 is CT1's whole output shape

Look along CT1's list of active specifications and the pattern in the titles is plain. Every one of these is a stage-3 document, and every one is CT1's:

• TS 24.501, the NAS protocol for 5G TS 24.501

• TS 24.301, the same kind of protocol for the Evolved Packet System TS 24.301

• TS 24.229, the call-control protocol based on SIP and SDP TS 24.229

• TS 24.554 for proximity services TS 24.554

• TS 24.587 for vehicle-to-everything services TS 24.587

• TS 24.193 for steering, switching and splitting traffic across accesses TS 24.193

• TS 24.369 for the ambient internet of things TS 24.369

That is the group in one line: where 3GPP needs somebody to say what actually goes on the wire between a device and the core, the document is CT1's. The full shape of the pile is What CT1 owns — 229 documents and the six that matter, and the document itself starts in TS 24.501 — two protocols living in one document.