Cheat sheet

IMS — the call as an internet session

3GPP's IMS work in one morning

Depth: Overview
Chapters: 7
Reading: 45 min
Questions: 33
Sources cited: 193

Back to the course

The shape of it

1 What IMS is, and where it came from

2 Who does the work, and what they write

3 Where it stands, and how to follow it

Keep these in your head

1.2 What IMS is, in the words of its own documents
Four documents, one subject. TS 22.228 says what IMS must do, TS 23.228 how it is put together, TS 23.218 how one session runs from start to finish, and TS 24.229 what the messages look like. Nobody works from one of them alone.
2.2 The founding: Release 5 and the ring of work around it
The IMS work item opened as a stage 1 job — what the service must do, not how it is built. The architecture, the protocol, the security and the charging each opened as their own work plan rows beside it.
3.3 Rebuilt for every new network: circuits, LTE, the browser, 5G
Most IMS work is not a new feature. Reaching the old circuit network, meeting an emergency-call rule, keeping a session alive across networks, recovering after a failure — release after release, the same system is made to hold up in one more situation.
4.3 Who does the work: fourteen groups, and a core-network shape
IMS is core-network work. The three largest document counts in this record are CT working groups — CT1 with 2169, CT4 with 746 and CT3 with 719 — and the largest radio-side count is RAN5's 147, which is conformance testing rather than radio design.
5.7 What they write: the IMS family of specifications
TS 29.562, the 5G system's subscriber-server services, is the second-largest target in this register: 230 IMS documents change it, against 92 for TS 23.228, the stage 2 description of IMS itself. Not one of the 5G core documents this work edits has the letters IMS in its title.
6.1 Maintenance, and what is running now
Most IMS paperwork is upkeep, not invention: 3509 of the 4939 documents in the local record are change requests, and 1680 of those edit one document, the stage 3 protocol TS 24.229.
7.1 Following it yourself, and what the record cannot tell you
Search 3GPP by the work item acronym, never by the subject. Four IMS work items — IMS, IMS-CODEC, IMS_SSFDD and IMS_Corp2 — do not have the letters IMS anywhere in their titles, so a title search loses them silently.

Easy to get wrong

One from each chapter that has one. The course carries 9 in all.

1.5 What IMS is, in the words of its own documents
IMS is not SIP. TS 24.229 defines a call control protocol based on SIP and SDP for use in the IM CN subsystem — the protocol is one document out of four, and the other three define the subsystem without mentioning a message at all.
2.1 The founding: Release 5 and the ring of work around it
SP-000216, the paper the work plan names as the one that opened IMS, has no title in this store, and meeting SP-8 has no days and no place. Both may be named and neither may be described or dated. Thirty-two of the papers that opened a work item are like that — the store has the number, the meeting and the file address, and nothing else.
3.6 Rebuilt for every new network: circuits, LTE, the browser, 5G
Rows per release and documents per work item are two different measurements, and neither one is "how much work happened". Rel-16 carries only 13 IMS work plan rows ^[1], and it also holds eIMS5G_SBA, the acronym with 543 documents behind it — more than any other in this register ^[2]. Both numbers are true and they answer different questions.
4.3 Who does the work: fourteen groups, and a core-network shape
RAN2 has exactly one IMS document in this record, at meeting R2-92, tagged with the acronym IMS-CODEC. The register does not record what kind of document it is, so never call it a change request, a liaison or work of RAN2's own — that would be inventing a field nobody stored.
5.2 What they write: the IMS family of specifications
A change-request count is a count of edits, not a ranking of importance. TS 23.228 takes 92 and TS 24.229 takes 1680, and all that says is that the description of a system is edited less often than the protocol that carries it.
6.5 Maintenance, and what is running now
"Deleted" at the front of a 3GPP work plan title is text somebody typed into the name, not a status field. FS_IMSRE_SEC carries that word and an approved study paper and 16 documents at the same time, so quoting one half of that is misleading either way.
7.4 Following it yourself, and what the record cannot tell you
Every document count in this course is a floor, not a total. The record holds nothing earlier than 2015-01-23 and IMS work opened in Rel-5, so the years before that date are missing from every number on this site.

The numbers, and where each came from

Every one was counted out of a file on this machine. Hover a row to read the question that produced it.

Where to look it up

The 151 named sources it cites, by what they are.

Quoted wording 48

Work item 48

IMS IMS-ASEC IMS-LI IMS-CCR IMS-CCR-IWMM IMS-CAMEL IMS-ONOSA IMS-OAM IMS-CODEC IMS-TEST IMS-CCR_Test IMS_CONV_IP_COM-UEConTest IMS-FrWk IMS-Sex IMS-CCR-IWCS IMS_EMER_GPRS_EPS IMS_SCC IMS_WebRTC IMS_WebRTC-CT IMS_TELEP IMS_TELEP_S4 5GS_Ph1-IMSo5G FS_eIMS5G FS_eIMS5G2 IMS_IBCF IMSProtoc IMSProtoc2 IMSProtoc3 IMSProtoc4 IMSProtoc5 IMSProtoc6 IMSProtoc7 IMSProtoc8 IMSProtoc9 IMSProtoc16 IMSProtoc17 IMSProtoc18 IMSProtoc19 IMSProtoc20-CT FS_IMS_RES IMS_RES-CT SR_IMS FS_IMSResil FS_enIMS_ARC AIML_IMS-MED FS_IMSRE_SEC IMS_SSFDD IMS_Corp2

Specification 32

TS 22.228 TS 23.228 TS 23.218 TS 24.229 TS 23.292 TS 23.237 TS 23.380 TS 24.371 TS 26.223 TR 23.794 TR 23.700-10 TS 29.163 TS 34.229-1 TS 23.167 TS 23.334 TS 22.173 TS 29.165 TS 33.226 TR 33.926 TR 23.700-11 TR 23.700-12 TR 29.866 TR 29.867 TS 29.562 TS 29.514 TS 29.513 TS 23.501 TR 22.823 TS 26.567 TS 26.114 TR 23.801-03 TR 23.801-04

Working group 14

SA4 CT1 CT4 CT3 SA2 RAN5 SA3 SA1 SA5 CT6 RAN2 CT SA RAN

Meeting document 8

SP-000216 RP-171635 CP-060101 CP-261069 SP-260686 SP-250878 CP-260044 SP-251237

Meeting 1

C1-112

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 — 33 of them, on one page.
The final test — 12 questions, 70 per cent to pass.
Every source behind the course — 858 of them, each with its method.

The raised numbers on this page

13 IMS work plan rows are marked Rel-16 — rows of table `item` in /var/www/whatthespec.net/data/database/api/wp.sqlite taken in by the acronym rule whose release field is exactly 'Rel-16', read 2026-08-05
543 documents carry the work item eIMS5G_SBA — rows of `tdoc` whose work item column names eIMS5G_SBA as a whole word, asked of /var/www/whatthespec.net/data/database/api/api.sqlite on 2026-08-05; the first and last meeting are those of its earliest and latest upload stamp