School of Specs CT1 — the group that writes what your…In depth

TS 24.501 · version 20.0.0 · 1340 clauses · In depth · 100 minutes

CT1 — the group that writes what your phone says

CT1 is the 3GPP working group that writes the protocol a phone speaks to the core network. Its best-known document is TS 24.501, which says what a device sends when it asks to be let onto a network, what the network sends back, and what either side does when the answer is no. This course is the working reference on both sides of that: what the group owns, how one of its meetings runs, how a proposed edit turns into published text, and how TS 24.501 itself is built. Everything said about the group comes from a source register generated from data on this machine, and everything said about the protocol comes from clauses of TS 24.501 version 20.0.0 — both open beside the text, so no statement here has to be taken on trust. The last chapter says what that record cannot tell you, and that list matters as much as the rest of the course.

Start at chapter 1 Sit the final test Look at the document first

This course was written against TS 24.501 version 20.0.0, and built from a source register generated 2026-08-04.

Same subject, different depth: quick start · overview ·

The document, all of it

One block per clause of TS 24.501, in the order they appear, sized by how much text the clause holds. Lit blocks are taught in this course.

Open the whole of TS 24.501 as a page

The chapters

Part 1 The group and what stage 3 means

  1. 1 What CT1 is, and the three names it answers to The working group behind the phone-to-core protocol — where it sits in 3GPP, who runs it, and why the same group is called C1, CT1 and CN1 in different files. 7 min
  2. 2 Stage 3 — the difference between a plan and a protocol Why 3GPP writes the same feature down three times, what the third writing adds, and how TS 23.501 and TS 24.501 divide the work between them. 7 min
  3. 3 What CT1 owns — 229 documents and the six that matter The shape of CT1's specification list, the families inside it, and why six documents out of 229 take the most change documents. 7 min

Part 2 TS 24.501, the document it is known by

  1. 4 TS 24.501 — two protocols living in one document What the NAS specification covers, the two sublayers inside it, and the rule that makes one of them depend on the other. 7 min
  2. 5 Finding your way around TS 24.501 What each clause of the NAS specification holds, the reading path from a procedure to its bits, and the state machines the whole document is written against. 7 min
  3. 6 Getting on the network, and getting a connection through it The registration and session management procedures of TS 24.501 — who starts them, what they carry, what they leave behind, and what happens when they fail. 9 min
  4. 7 What a NAS message looks like on the wire The fixed header every NAS message carries, how a message is wrapped for protection, how fields are coded, and what a receiver does with a message it cannot understand. 8 min

Part 3 How the work actually happens

  1. 8 From a document number to a published version What a change request is, the trail one real change leaves from a CT1 meeting to the change history of TS 24.501, and how to read a version number. 7 min
  2. 9 One CT1 meeting, by the numbers The rhythm of CT1's meeting calendar, how much paper one meeting absorbs, what came out of the last one, and why its report cannot yet be quoted as final. 7 min
  3. 10 Who CT1 writes to, and what the letters count The liaison traffic in and out of CT1 over a year, the four busiest partners in each direction, and what the specification itself shows about where those boundaries run. 7 min
  4. 11 The work items that fill a CT1 year What a work item is, the six that took the most CT1 documents in the last twelve months, and how far a name in that list can be traced to real text. 6 min

Part 4 Around the group, and the limits of the record

  1. 12 The neighbours — and how little the record says about them CT1's sibling groups, the one number that crosses the boundary, and where the edges of CT1's work can be seen from inside its own specification instead. 7 min
  2. 13 Following CT1 yourself Where CT1's documents live, how to read a document number, what the status words and document kinds mean, and a routine that gets from a question to an answer. 7 min
  3. 14 What this record cannot tell you Every question the data behind this course cannot answer, why it cannot, and where to go for each one instead. 7 min

Testing yourself

Every question at once

73 questions, chapter by chapter, on one page — for revision after you have read the course.

Open all the questions

The final test

20 questions drawn across the whole course, marked at the end. 70 per cent to pass, and every miss says which chapter to go back to.

Sit the final test

What this course is built on

Chapters
14
Words written for you
18,818
Clauses covered
133 of 1340
Figures from the document
45
Abbreviations
201
Published versions
69
Checked sources
354
Sources this course cites
149

The document · Words · History · Documents around it · Every source