TNL address discovery of a candidate en-gNB via inter-system signalling

5G NR
,
1

Hello Core Experts.

Anyone able to explain this?

Reference in the TS 36.300 is given below.

22.3.6.3 TNL address discovery of a candidate en-gNB via inter-system signalling
If the MME is aware that the en-gNB serves cells which provide access to 5GS, the MME may relay the request towards a suitable AMF via inter-system signalling based on a broadcast 5G TAC. Upon receiving a reply from the AMF, the MME may relay this reply to the target eNB using a MME CONFIGURATION TRANSFER message.

What I understood:

Associated TAI and Broadcast 5GS TAI IE will be set in eNB conf transfer if gNB is connected to AMF also.

This is for 4G → 5G ANR in ENDC.

2

This process is to find the IP address of gNB for creating X2 link between eNB and gNB.

3

Yes, it is. Thanks.

But my question is how eNB puts 5G TAI in eNB conf transfer for target gNB?

4

Neighbor gNB details will come from UE and TAC mapping has to be predefined in MME.

5

So actually TAI should be enough here no need having Associated TAI and Broadcast 5GS TAI IE.

Thats the part i did not get.

Associated TAI and Broadcast 5GS TAI IE items are optional, selected TAI is mandatory.

In short: why do we need associated TAI and broadcast 5GS TAI if we have mapping in MME?

6

For case of AMF, associated TAI, broadcast 5GS TAI will never be there.

7

Nope. This is for EN DC, but by using these infos MME asks AMF to retrieve IP of the target gNB.

8

Ok some correction - going towards AMF will only be a case when gNB supports SA and is broadcasting.

Therefore broadcast 5GS TAI is relevant, but associated TAI I don’t know why it is required when going towards AMF.

9

Yes, good point, gNB is connected to both MME and AMF at the same time.

So by using broadcast 5GS MME can ask AMF for gNB is IP.

But do you know we define broadcast 5GS TAI in gNB? It is normal TAI, isn’t it?

10

Yes , gNBs also have tracking areas.

11

I found it here:

https://www.3gpp.org/ftp/tsg_ran/WG3_Iu/TSGR3_107bis_e/Inbox/Drafts/CB%20%23%20102_selectedTAI_TNL_discovery/draft0%20R3-202596%20Selected%20TAI%20CR%2036.413%20Rel-16.docx

Associate TAI is the TAI of 4G eNB having X2 connection with gNB.