5G-GUTI — The temporary name, which is also a routing address
The device — identity, state, mobility
Where it sits
What it is
What it is. The 5G Globally Unique Temporary Identifier — the name a device is normally known by, allocated by the AMF and re-allocated whenever the AMF likes. TS 23.501 §5.9.4. It is the identifier that keeps the SUPI off the air.
Its structure, quoted from the clause:
<5G-GUTI> := <GUAMI> <5G-TMSI>
<GUAMI> := <MCC> <MNC> <AMF Region ID> <AMF Set ID> <AMF Pointer>
<5G-S-TMSI> := <AMF Set ID> <AMF Pointer> <5G-TMSI>
It is an identifier that is also an address. The GUAMI half names the AMF; the 5G-TMSI half names the device within it. So the network can take a temporary identity and route straight to the function holding the device's context — no lookup required. The 5G-S-TMSI is the shortened form "to enable more efficient radio signalling procedures (e.g. during Paging and Service Request)", dropping the country and network codes that are implied on the radio anyway.
The ten bits — the seam between the core and the radio. §5.9.4:
"the NG-RAN uses the 10 Least Significant Bits of the 5G-TMSI in the determination of the time at which different UEs are paged. Hence, the AMF shall ensure that the 10 Least Significant Bits of the 5G-TMSI are evenly distributed."
A core-network function allocating an identifier is being constrained by a radio-layer timing requirement. Paging occasions are derived from those ten bits, so a device's identifier decides when it must wake up. Allocate them badly — sequentially, or clustered — and every device in a batch would listen in the same occasion, concentrating the paging load and wasting the rest of the cycle.
A second constraint, from the other direction. The same clause notes that the RRC connection establishment contention resolution "assumes that there is a low probability of the same 5G-TMSI being allocated by different AMFs to different UEs", and suggests a random seed. So the 5G-TMSI must be evenly spread and effectively unpredictable — both requirements imposed on the core by procedures happening in the radio.
Why it is re-allocated. A temporary identifier that never changed would be as trackable as the permanent one. §5.9.4 lets the AMF reassign "at any time", and TS 33.501 §6.12.3 says when it must. The device may be given a new one "at the next NAS transaction" while it is CM-IDLE.
And there is no paging by permanent identity. TS 23.501 §5.3.3.2.2 notes flatly: "In 5GS there is no need for paging using the SUPI/SUCI of the UE." The temporary identifier is sufficient for reachability, which is what makes it possible for the permanent one to essentially never be sent.
Read on
This concept is read as part of one argument in ref-ue-identity, alongside the rest of its group.
Before this concept, the hierarchy says to learn the following — the full chain, in order: