Paging — Waking a device that is not listening, and the ten bits that decide when
The device — identity, state, mobility
Where it sits
What it is
What it is. How the network reaches a device that is not listening. TS 38.304 §7.1 for the radio timing, TS 23.501 for what triggers it. A CM-IDLE device has no connection and no N3 tunnel, so the network cannot deliver anything — it can only ask the device to come and get it.
What triggers it. A packet arrives at the UPF, which buffers it and raises a Downlink Data Notification to the SMF; the SMF tells the AMF; the AMF pages every cell in the device's TAI list. The device answers with a Service Request.
The device does not listen continuously. It monitors one paging occasion per DRX cycle — TS 38.304 §7.1: "The UE monitors one paging occasion (PO) per DRX cycle" — and sleeps the rest. Which occasion is its occasion is computed, and this is the seam worth following.
The formulae, quoted from §7.1:
and the device identity that goes into them is, per the same clause:
UE_ID = 5G-S-TMSI mod 1024
mod 1024 is the ten least significant bits — and that closes the seam exactly.
TS 23.501 §5.9.4 requires the AMF to ensure "the 10 Least Significant Bits of
the 5G-TMSI are evenly distributed". TS 38.304 §7.1 is why: those ten bits are UE_ID, and
UE_ID mod N picks the frame while floor(UE_ID/N) mod Ns picks the occasion within it.
So a core-network function's choice of identifier decides when a device wakes up. Allocate the low ten bits sequentially and a batch of devices activated together would share paging occasions, concentrating load into a few moments of each cycle and leaving the rest idle. Allocate them evenly and the paging load spreads uniformly across the cycle.
This is the clearest core-to-RAN seam in the folder, and unlike most it has an arithmetic answer: 1024 occasions' worth of spread, from an identifier allocated by a function that never touches the radio.
Two kinds of paging, and they differ in who is looking. CN-initiated paging comes from the AMF for a device in RRC_IDLE. RAN-initiated paging comes from the base station for a device in RRC_INACTIVE — whose context it still holds — and the device answers by resuming rather than re-establishing. §7.1 notes that a device receiving CN-initiated paging while in RRC_INACTIVE "moves to RRC_IDLE and informs NAS": the core paging it means the core does not believe it is connected, and the device's view must yield.
Beams cost nothing here. In multi-beam operation the device "assumes that the same paging message and the same Short Message are repeated in all transmitted beams", so it may listen on whichever beam it likes — the network sweeps, and the device does not have to have been tracked.
Read on
This concept is read as part of one argument in ref-ue-mobility, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: