School of Specs IMS — the call as an internet sessionIn depth

858 checked sources · In depth · 165 minutes

IMS — the call as an internet session

IMS is the part of a mobile network that sets a call up and carries it as an internet session instead of as a circuit, and it is the longest-running area of work this school covers: 345 rows of the 3GPP work plan under 177 acronyms, spread over 16 releases from Rel-5 to Rel-20, touching 241 specifications, with 4939 meeting documents in the local record over 309 meetings. This course is not a guided read of one document, because no single document holds it — TS 23.228 says how the system is put together, TS 24.229 says what goes on the wire, and 1680 documents in this record exist only to change that one protocol specification. It follows the subject the way it actually grew: what the documents say IMS is, how the work is cut out of the work plan at all, one stretch of releases at a time from the founding to what is running now, then who writes it group by group, what is written down, and how to read the record yourself. Every statement hangs on an entry in a register built from files on this machine, and one whole chapter is about what that register cannot tell you — including the first ten years, for which it holds no documents at all.

Start at chapter 1 The whole thing on one page Sit the final test

This course was built from a source register generated 2026-08-05.

Same subject, different depth: quick start · overview ·

The chapters

Part 1 What IMS is, and how this subject is bounded

  1. 1 What IMS is, in the words of its own documents The four documents that say what IMS is, the nine words they define, and the vocabulary every later chapter of this course runs on. 11 min
  2. 2 How this subject is cut out of the work plan One rule decides which rows of the 3GPP work plan are IMS work — the acronym, never the title — and every number in this course inherits its edges. 10 min

Part 2 How it grew, release by release

  1. 3 Release 5: the founding, and the ring of work around it The first row of the work plan, the paper that opened it, and the thirteen sibling rows that show how a 3GPP subject is really shaped. 11 min
  2. 4 Releases 6 and 7: interworking, messaging, and a stream that never stopped Rel-6 connected IMS to the networks people were already on and added messaging and groups; Rel-7 opened the protocol alignment work that has been reopened ever since. 10 min
  3. 5 Releases 8 and 9: emergency calls, continuity and the fixed-network merge The largest block of IMS work in the plan — fitting IMS to the packet core, to regulated emergency calling, to networks that fall over, and to the fixed-network documents brought in from TISPAN. 12 min
  4. 6 Releases 10 to 14: the browser, telepresence and the middle years Five releases in which IMS stopped being built and started being reached from new places — a browser, a room full of screens, a location field, a business trunk. 11 min
  5. 7 Releases 15 to 17: IMS moves onto the 5G core The largest run of work in the record — carrying voice and emergency calls over 5G, asking what a full integration would need, and then writing it down. 14 min

Part 3 Who does the work

  1. 8 The map: fourteen groups, 309 meetings and 374 names Every group with IMS documents on the record, what the shape of those counts says about the subject, and why the list of who filed cannot be added up. 11 min
  2. 9 CT1, CT3, CT4 and CT6: where most of it happens The four core-network groups one at a time — what each owns, the busiest meeting in the record, and exactly how much a meeting entry can tell you. 12 min
  3. 10 SA1 to SA5, the two RAN groups, and the plenaries above them The other half of the map — architecture, media, security, requirements, charging, conformance testing, and the letters groups actually sent each other. 11 min

Part 4 What is written down

  1. 11 The specification family: 241 documents, and the two that are IMS The whole document map of this subject — the two documents at its centre, the family around them, the studies, and the three things this list cannot tell you. 12 min
  2. 12 Maintenance is the main event: change requests and protocol alignment What a mature 3GPP subject actually consists of — 3509 change requests, one work item reopened in every release since Rel-7, and the outcome words that mean less than they look like. 11 min

Part 5 Where it stands, and reading the record yourself

  1. 13 What is running now: Releases 18 to 20 The present tense of this subject — resiliency after real outages, split rendering, an architecture study aimed at 6G-era services, and why none of it may be called finished. 12 min
  2. 14 Reading the record yourself, and what it cannot tell you The four habits that make this subject searchable, then the whole of what the data is silent about — and what to open instead in each case. 14 min

Testing yourself

Every question at once

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

Open all the questions

The final test

15 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
30,959
Checked sources
858
Sources this course cites
399

Every source