In this chapter we shall learn about:
1. 5G GUTI – 5G Globally Temporary Identifier
2. 5G S TMSI – It is a shorter version of 5G GUTI. 5G S Temporary Mobile Subscriber Identity
5G GUTI is a Core Network Temporary identifier assigned by AMF to UE during initial registration.
Below are few points on 5G GUTI:
1. It is 80 bits long consists of PLMN + AMF ID + TMSI
2. AMF can re-assign new 5G-GUTI to the UE at any time under specific conditions.
3. Single 5G-GUTIcan be used for accessing 3GPP and non-3GPP technologies within the AMF.
5G GUTI Structure
Let us understand above image.
5G GUTI = GUAMI + 5G-TMSI
GUAMI = MCC + MNC + AMFI
AMFI = AMF Region ID + AMF Set ID + AMF Pointer
5G-S-TMSI = AMF Set ID + AMF Pointer + 5G-TMSI
If there are multiple AMF in the region [AMF Region ID], then they are supported by AMF Set ID and AMF pointer. So by this operators can re-use the same AMF set ID and AMF pointers in different regions.
When AMF will provide a new 5G-GUTI
1. When it receives Registration Request message of type “initial registration” or “mobility registration update” from a UE, then AMF will assign a new 5G-GUTI.
2. Registration Request for “periodic registration update” from UE. Then AMF will send a new 5G-GUTI.
3. When UE sends “Service Request message” as a response to “Paging Message”, then AMF shall assign a new 5G-GUTI.
Mapping of 5G GUTI and 4G GUTI
As 5G needs to work with 4G networks also, above is the mapping of 5G GUTI with 4G GUTI.
When a user moves from 5G area to 4G area, then the UE will map 5G GUTI to 4G GUTI and forwards to MME.
MME will perform a mapping of 4G GUTI to 5G GUTI to check from where the MME needs to contact to fetch the UE context.
When a user moves from 4G area to 5G area, then UE maps 4G GUTI to 5G GUTI and sends to AMF.
Then AMF will decode the MME ID to get the user context.
For more detailed information, visit TS 23 003, 23 501