1 What IMS is: a call carried as an internet session

1.1 Why the work needed a name of its own

3GPP writes down what a mobile network has to do, and it splits that writing into rungs. Stage 1 says what the system must do, stage 2 says how it is put together, stage 3 says what goes on the wire. Different groups own each rung.

IMS is the part of the network that sets a call up and carries it as an internet session. In the definitions clause of the requirements document the entry is filed under IM CN subsystem rather than under IMS. TS 22.228

That is a definition by membership. The IM CN subsystem is not one box and not one protocol: it is every core-network element that is there so an IP multimedia application can run over an IP multimedia session.

1.2 What each of the three documents says it is for

One document sits on each rung, and each opens by saying what it is for.

TS 23.228 is the stage 2, how the thing is put together. TS 23.228

TS 24.229 is the stage 3, what the machines say to each other. TS 24.229

SIP is the set of messages a phone and the network send each other to start, change and end a session. TS 24.229 is 3GPP's rules for using it, and its scope names the Session Description Protocol beside it.

TS 22.228 is the stage 1, what the system owes the people using it. TS 22.228

1.3 The four words those requirements rest on

An IP multimedia session is a set of multimedia senders and receivers and the data streams flowing from senders to receivers. ^[6]

An IP multimedia application is one that handles several media types at the same time — speech, audio, video, data such as chat text or a shared whiteboard — synchronised from the user's point of view. ^[7]

An IP multimedia service is the user experience one or more of those applications provide. ^[8]

The fourth says how far those services have to reach.

Nothing in those four words mentions a mast, a radio or a generation of mobile network. A session is senders, receivers and streams; the access network only has to carry IP.

1.4 Where this sits in the rest of the course

Where it came from, and why it keeps being rebuilt follows the work from Release 5 to Release 20 and quotes the paper that opened each turn of it. Who writes it, and what the paperwork looks like counts who files the paperwork and what shape it arrives in, and Where to read it yourself, and what the record cannot say says which document to open first and what this record honestly cannot answer.