1 What IMS is, in the words of its own documents

1.1 Why one subject takes four documents

IMS — the IP Multimedia Subsystem — is the part of a mobile network that sets a call up and carries it as an internet session instead of as a circuit.

No single document says what it is. That is not an accident: 3GPP writes a subject in three stages, and a different group owns each stage.

• Stage 1 is what the system must do, written from the point of view of the people using it and the operators selling it.

• Stage 2 is how the system is put together — which parts exist, and what passes between them.

• Stage 3 is the bytes on the wire: every message and every field, in enough detail that two makers can build apart and still talk.

Those three words come back in every later chapter of this course. Who does the work: fourteen groups, and a core-network shape is where they turn into the names of real groups.

1.2 What each of the four says it covers

The shortest honest answer to "what is IMS" is the opening clause of four documents, read one after another.

The requirements. TS 22.228 is stage 1. TS 22.228

The architecture. TS 23.228 is stage 2, and the document everything else refers back to. TS 23.228

The call model. TS 23.218 narrows the view to one session, from the moment somebody starts it to the moment it ends. TS 23.218

The protocol. TS 24.229 is stage 3: what the two ends actually send. TS 24.229

SIP, the Session Initiation Protocol, is the set of messages that start, change and end a session; SDP is the Session Description Protocol, named beside it. TS 24.229 is 3GPP's rules for using the two, and this register says no more about either than that. ^[4]

1.3 The name the documents use for themselves

The three letters IMS are the everyday name. The definitions clause of the requirements document uses a different one, and it is the name you will meet inside the text. ^[5]

The stage 2 document writes the same name out in full — the IP Multimedia Core Network Subsystem. ^[2]

So the definition draws a boundary rather than describing a machine: every core-network element there to provide these applications over these sessions is inside it, and everything else is outside.

1.4 Four words that carry the rest

The same clause defines what those applications and sessions are, and the four definitions build on one another in order.

Read them upwards and the ladder is clear. Streams make a session, an application handles the media inside it in step, and the service is what the person at the end of it experiences. ^[8]

The fourth word is the one this course keeps coming back to.

Access independence is a promise that has to be made good again every time a new way of carrying IP arrives, and that is what Rebuilt for every new network: circuits, LTE, the browser, 5G is about.

1.5 Two words about the data channel

The architecture document's definitions clause also carries two entries that describe something which is not a call at all. ^[10]

So a session may now carry a web page the network hands to the device, next to the speech and the video. ^[11]

The letters UE are the documents' own. This register holds no expansion of any abbreviation, so where a name is not spelled out in a definitions clause, this course leaves it as the documents wrote it.

1.6 Where this goes next

Nothing above says when any of it was written, or by whom. Those are the next two chapters: The founding: Release 5 and the ring of work around it for the release the subject started in, and Who does the work: fourteen groups, and a core-network shape for the fourteen groups that carry it now.

Everything on this page came out of the scope and definitions clauses of four published documents, and every quotation links to the document it was copied from. This course does not carry the text of any of them — it tells you which number to open. TS 23.228 TS 24.229