12 The neighbours — and how little the record says about them

12.1 What the record actually holds

CT1 sits under a parent group called CT, and the register lists four other CT working groups by name: CT3, CT4, CT5 and CT6 CT1 — Mobility, call control, session management (WG1_mm-cc-sm_ex-CN1).

A list of names is all that is. The entry does not say which of the four still meet, so nothing here lets anybody call them the current CT groups. The register was built for CT1 alone: it holds no description, no specification count, no document count and no meeting count for CT3, CT4, CT5 or CT6, nor for any group outside the CT family.

That matters more than it sounds. 3GPP working groups do close — CT1 itself is what CN1 became — and a name on a list outlives the group that carried it.

This chapter is therefore shorter on the neighbours than a reader would like, and says so rather than filling the gap with something unsourced. What it can do instead is show where CT1's edges run, which is visible from CT1's own side.

12.2 The one number that crosses

CT4 is the only sibling that appears anywhere in the counts. In the twelve months to 2026-08-04, CT1 sent it 8 liaison statements ^[3] and received 7 from it ^[4].

That makes CT4 the busiest of CT1's own siblings in both directions.

CT3, CT5 and CT6 appear in neither table. The tables carry the four busiest partners in each direction and stop, so their absence is not a zero and no number may be quoted for them Who CT1 writes to, and what the letters count.

Put next to the other partners, the sibling traffic is small: 8 letters to CT4 against 44 to RAN2 ^[5] and 19 to SA2 ^[6]. CT1's near neighbours by organisation chart are not its near neighbours by correspondence.

12.3 Where the boundary shows from the inside

The specification itself is much more forthcoming about CT1's edges than the register is about its siblings. Three places make the seams plain.

The scope names four boxes and no more. TS 24.501 applies to the device, the AMF, the SMF and the PCF §1. Every other part of a 5G network is outside the document, which is why so much of it consists of pointers.

Those pointers have force. The reference list is not a bibliography — its opening line says the listed documents constitute provisions of the present document §2. A dependency in a 3GPP specification is a legal one, not a courtesy.

The pointers name the seams. The overview hands the architecture to TS 23.501 and the sequences of procedures to TS 23.502 §4.1. Primary authentication produces keying material as specified in TS 33.501 §5.4.1.1, and security mode control points at the same document again §5.4.2.1. None of those three is on CT1's own list of 229 active specifications.

That last check is one a reader can repeat: the register holds every active CT1 specification as its own entry, so a document that is not in it is not one of CT1's.

12.4 What CT1 keeps next door in its own family

The more surprising direction is how much of the surrounding material CT1 keeps rather than hands off. The document TS 24.501 says it follows for its layer-3 protocol model, TS 24.007, is CT1's own TS 24.007 §4.1.

So are the neighbouring protocols a reader might assume belong elsewhere:

• TS 24.301, the same kind of NAS protocol for the Evolved Packet System TS 24.301 — the document CT1 changed most often of all in the last twelve months, with 499 change documents ^[7].

• TS 23.122, the NAS functions a device performs while idle TS 23.122, which TS 24.501 refers to constantly.

• TS 24.008, core network protocols on the mobile radio interface TS 24.008.

• TS 24.502, reaching the 5G core over non-3GPP access TS 24.502, and TS 24.193 for steering, switching and splitting traffic across accesses TS 24.193.

• TS 24.526, the device policies that arrive over the delivery service TS 24.501 defines in an annex TS 24.526.

That is the real shape of the boundary: not a thin group with a narrow subject and many neighbours, but a group that owns the whole device-facing protocol family across two generations of core network, and buys architecture and security from outside.

12.5 The group marks its own slice in the titles

There is one more place the boundary is written down, and it is the most literal of all: some of CT1's own documents say "CT WG1 aspects" in their titles. Those are the documents where a subject belongs to the whole of 3GPP and CT1 is doing its part of it.

• TR 24.801, "3GPP System Architecture Evolution (SAE); CT WG1 aspects" TR 24.801.

• TR 24.890, "CT WG1 aspects of 5G; System Phase 1" TR 24.890.

• TR 23.700-10, "Study on enhanced IP Multimedia Subsystem (IMS) to 5GC integration Phase 2; CT WG1 aspects" TR 23.700-10.

The same phrasing turns up in the new work agreed at the last meeting. Five of the nine work items agreed at C1-161 are titled "CT aspects of…" or "CT aspects for…" One CT1 meeting, by the numbers:

• ambient IoT C1-262440

• proximity services C1-262442

• short messages to an emergency response centre C1-262554

• satellite components C1-262612

• application enablement for MMTel C1-262617

Read that way, the pattern is a boundary marker rather than a piece of bureaucratic phrasing. A feature is decided somewhere in 3GPP as a whole; the protocol part of it lands here, and the title says so.

12.6 What may not be said

Three statements are impossible from this data, and all three are tempting.

T1. Any size comparison. CT1 is not the largest, busiest or fastest CT group, because the register holds no counts for any other group. 229 specifications and 105699 documents are big numbers with nothing to be big against.

T2. Any description of a sibling's subject, or whether it still meets. CT3, CT4, CT5 and CT6 have names here and nothing else — no subject, no counts, and no word on which of them is still an active group.

T3. Any statement about who owns a document CT1 does not. The catalogue in this register carries the owning group for CT1's own specifications only.

All three have the same fix, and it is a single page. The specification status report lists every 3GPP specification with the group that owns it ^[1], and it is the page CT1's own catalogue here was built from.

The two are therefore the same data, one of them filtered. Answering "who owns TS 33.501" or "how many specifications does CT4 have" means asking the unfiltered version of the question this register already answers for CT1.

12.7 Where this meets the rest of the course

The correspondence numbers behind this chapter are Who CT1 writes to, and what the letters count. The list of CT1's own specifications is What CT1 owns — 229 documents and the six that matter.

The pages that answer the questions this chapter cannot are gathered in Following CT1 yourself, and the full list of what the record leaves out is What this record cannot tell you.