Cheat sheet

IMS — the call as an internet session

3GPP's IMS work in under an hour

Depth: Quick start
Chapters: 4
Reading: 15 min
Questions: 16
Sources cited: 67

Back to the course

The shape of it

1 What IMS is, and where it came from

2 Who builds it, and where to read it

Keep these in your head

1.1 What IMS is: a call carried as an internet session
IMS is the 3GPP name for a call carried as an IP session. Its own documents call it the IM CN subsystem and define it as all the core-network elements that provide IP multimedia applications over IP multimedia sessions.
2.7 Where it came from, and why it keeps being rebuilt
IMS was not designed once. Each release names something it could not yet do — reach the old telephone network, take a call from a browser, run over the 5G core, carry AI media — and opens a piece of work to fix that.
3.1 Who writes it, and what the paperwork looks like
The three largest counts in this register all belong to CT working groups — CT1, CT4 and CT3. IMS paperwork is core-network paperwork, whatever the service sitting on top of it is called.
4.5 Where to read it yourself, and what the record cannot say
This record says what was written, by whom, when it was filed and what a piece of work set out to do. It cannot say why anybody decided anything, and it holds no date for any release at all.

Easy to get wrong

1.4 What IMS is: a call carried as an internet session
Access independence is a requirement about reach, not a promise about how. The definition says a subscriber may reach their IP Multimedia services over any access network capable of providing IP connectivity, and stops there. The source register behind this course holds titles, scopes, definitions, objectives and counts — no procedure, no message and no parameter — so nothing here says how any of it is done. ^[1]
2.7 Where it came from, and why it keeps being rebuilt
The record behind this course holds the names of all 16 releases, version numbers and meeting days, and nowhere the day a release froze. ^[2] So nothing here lets anybody say IMS "arrived" in a particular year, or that any release "finished" in one. A release is what the plan says a row is for, and never a date.
3.4 Who writes it, and what the paperwork looks like
That column is free text, so a name there is a spelling and not a company. Ericsson also appears as "Ericsson / Nevenka" on 130 more documents, counted separately, and no name in this register was resolved to a parent company. ^[3] Read the ranking as a ranking of spellings.
4.5 Where to read it yourself, and what the record cannot say
"Approved" beside a paper is not a finished feature, and a full completion percentage in the work plan is not one either. The first is about one document at one meeting; the second is a figure somebody types in by hand. Neither is a status field, because this record has none.

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 50 named sources it cites, by what they are.

Quoted wording 22

"IM CN subsystem" as 22.228 defines it the scope of 23.228 the scope of 24.229 the scope of 22.228 "Access independence" as 22.228 defines it "IP multimedia session" as 22.228 defines it "IP multimedia application" as 22.228 defines it "IP multimedia service" as 22.228 defines it why IMS was proposed why IMS-CCR-IWCS was proposed why IMS_WebRTC was proposed why 5GS_Ph1-IMSo5G was proposed what FS_enIMS_ARC set out to do what IMS set out to do what IMS-CCR-IWCS set out to do what IMS_EMER_GPRS_EPS set out to do what IMS_WebRTC set out to do why eIMS5G_SBA was proposed why FS_enIMS_ARC was proposed why IMSProtoc20-CT was proposed why IMSProtoc2 was proposed the scope of 22.173

Working group 10

RAN5 CT1 CT4 CT3 SA2 SA4 SA1 CT SA RAN

Work item 8

IMS IMS-CCR-IWCS IMS_EMER_GPRS_EPS IMS_WebRTC 5GS_Ph1-IMSo5G eIMS5G_SBA FS_enIMS_ARC IMSProtoc20-CT

Meeting document 5

SP-000216 CP-261069 SP-260686 SP-250878 CP-260044

Specification 4

TS 22.228 TS 23.228 TS 24.229 TS 22.173

Meeting 1

C1-112

Where to go from here

The same subject, overview — Enough to follow the conversation and know what to look up.
The same subject, in depth — The working reference: every mechanism, every parameter.
Every question of this course — 16 of them, on one page.
The final test — 10 questions, 70 per cent to pass.
Every source behind the course — 858 of them, each with its method.

The raised numbers on this page

"Access independence" as 22.228 defines it — the paragraph of clause 3.1 of the parsed text at /var/www/whatthespec.net/data/friendlyspec/json/22228/19.0.0/ that begins 'Access independence', copied word for word, read 2026-08-05
IMS work items are spread over 16 releases, Rel-5 to Rel-20 — distinct values of the release field over the 345 IMS work plan rows, read 2026-08-05. They are named in the order the field spells them and no date of any release is stored
Ericsson / Nevenka filed 130 IMS documents — IMS documents grouped by the source column, largest first, asked of /var/www/whatthespec.net/data/database/api/api.sqlite on 2026-08-05. The column is the name the document spelled, so one company under two spellings counts twice and no name was resolved to a parent company