1 The group that writes what your phone says

1.1 Why a protocol needs a group of its own

A phone and a network have never met before the moment they have to work together. Nothing about that first exchange can be worked out on the spot. Both sides need to have been told, years in advance, exactly which message comes first, what has to be inside it, and how long to wait for an answer.

Writing that down is a different job from deciding what the network should be able to do. Deciding is an argument about features. Writing it down is an argument about a single bit in a single byte, and about what a receiver should do when that bit is set to something nobody expected.

3GPP splits the two jobs between different groups, and CT1 has the second one for the core network. Its own name in the tooling is "CT1 — Mobility, call control, session management" CT1 — Mobility, call control, session management (WG1_mm-cc-sm_ex-CN1).

1.2 Who the group is

The group sits under the parent group CT. The register lists four other CT working groups by name — CT3, CT4, CT5 and CT6 — and does not say which of them still meet, so read that as a list of names rather than a roll call. CT1 itself grew out of an older group called CN1, which closed and became CT1 CT1 — Mobility, call control, session management (WG1_mm-cc-sm_ex-CN1).

Three different stores spell it three different ways, and all three turn up the moment you go looking. The specification catalogue calls it C1. Every document number the group handles starts with C1-. The meeting calendar calls it "3GPP CT 1" CT1 — Mobility, call control, session management (WG1_mm-cc-sm_ex-CN1).

Four people hold office with no end date on record ^[1] — the chair, two vice chairs and the secretary.

Post	Who	For	Since
Chair	Lena Chaponnière	Qualcomm Incorporated	2025-04-12 [2]
Vice Chair	Behrouz Aghili	Apple Inc	2025-05-24 [3]
Vice Chair	Sung Hwan Won	Nokia Corporation	2025-05-24 [4]
Secretary	Nevenka Biondic	3GPP Support	2025-10-07 [5]

Forty-four terms of office are on record altogether, so leadership can be followed a long way back through the same file ^[6]. The register used here holds the four current names and no earlier ones, so this course never names a past chair.

1.3 What the group's own document says it covers

The clearest short answer to "what does CT1 do" is the opening of its largest document, TS 24.501. Clause §1 states the ground it covers, and it is worth reading in the document's own words.

Three plain-words glosses, because those names come back on every page. The UE is the phone or the modem in a device. The AMF is the core-network function that keeps track of where a device is and whether it can be reached. The SMF is the function that sets up and tears down a device's data connections.

"Non-access stratum" is the awkward one, and it says where this protocol sits rather than what it does. Clause §4.1 puts it at the top of the control plane between the UE and the AMF — above the radio, and carried by it, but not part of it.

That is the dividing line the whole group is built around. The radio groups write how bits cross the air. CT1 writes what the phone and the core network say to each other, in messages the radio merely carries.

The same scope also covers mobility between the different kinds of access a device can use, and the delivery of policy from the network to the phone §1.

1.4 The postbag nobody mentions

There is a third job in the protocol that surprises people, and it explains why so much unrelated work ends up in this one group. Besides mobility and sessions, clause §4.1 gives the protocol a transport function: it carries payloads that belong to other services entirely.

Short messages travel that way. So do positioning protocols, the container that delivers policy to the device, and several others, all named in the same list §4.1. None of them is mobility management, and all of them ride in mobility management messages.

That is the practical reason the group's name has three things in it — mobility, call control and session management CT1 — Mobility, call control, session management (WG1_mm-cc-sm_ex-CN1) — and the reason a question about, say, emergency messaging can turn out to be a CT1 question after all.

1.5 How much of it there is

The group is old and busy. 105699 CT1 documents are on record ^[7], with stored upload dates running from 2005 to 2026 ^[8], and spread over 133 meetings that have documents against them ^[9].

2005 is where the database starts, not where CT1 started. The group grew out of CN1 CT1 — Mobility, call control, session management (WG1_mm-cc-sm_ex-CN1), whose papers are not in here at all, so no sentence in this course may read a founding date out of that 2005.

The last twelve months alone brought 4235 documents ^[10], from 415 different organisations ^[11]. That is the shape of the place: a large number of companies, each filing a small number of papers, into one room.

The seam that makes all of this necessary — one document saying what the system does, another saying what the two ends actually send — is Architecture and protocol — what stage 2 and stage 3 mean. What the group owns beyond TS 24.501 is What the group owns, and what it actually works on.