Can UE reinitiate attach on same cell from which it received attach reject?

4G LTE
1

Hi LTE Experts.
Once we have attach reject from network, and UE move to idle mode, then I’m finding UE again sending PDN connectivity request and RRC connection setup on same cell, from which it received attach reject with no time difference.
Query: Is it possible or is there any time limit upto which UE cant reinitiate attach on same cell from which it received attach reject?

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I have second query.
From above snapshot why UL logical channel prioritization required at UE end MAC layer, but not at eNB end MAC layer?

2

It will depend on cause reason of reject.
Check these timers:

Timer (EPS Mobile Management - UE Side)

https://www.sharetechnote.com/html/Handbook_LTE_Timer_EPS_UE.html

3

Attach reject cause : Conditional IE error.
It’s observed after attach reject UE reading mib, sib of same serving cell where it’s received attach reject.

4

If that cell is always satisfying “cell selection” criteria, then even with attach reject UE will tend to select same cell. (This i what I think)

5

Reject is from core network because it doesnt like some TFT (from what i understand).
There is no logic in network or in UE to try attach on different cell.
Even if it does, it will likely receive rejection.

6

Attach/detach is transparent to RRC, so RRC setup may keep on happening on same cell, and hence attach request also on same cells.

Others can comment here.

7

In my view if UE is sending RRC connection request on same cell then it’s issue.
Need to check more in NAS specs any blockage after attach reject.

8

Why?
What would you achieve by sending request on different cell if issue is in UE’s HSS profile?

9

Yes good point, need to check.

10

There must be some logic/rule in order to select different cell.
And by all rules, UE cannot select other cell as long as current cell is strong enough.

11

I think I put wrong query…
It’s more like: does particular UE can try again for cell selection immediately (or not) after attach reject?
Need to check: same UE can retry attach immediately or need to wait?

12

UE can only select another cell, in case if serving cell has rejected RRC request.

13

TFT issue seem to suggest that UE is try to request access to PDN which it is not allowed to or there is some other structural issue in its TFTs.

Attach reject is Core NW issue.
You need to check reject reason.
Some rejections have associated timers (see previous link) and UE has to back off for that duration before next attempt.
At next attempt it will check most suitable cell and try again (which can be same cell, it previously tried)

14

True. But isn’t rejection reason important here as well?
If rejection is due to cell barred - UE will not try again.
If rejection is congestion - UE will backoff and may try again?

15

That’s true.

16

Yes it can, no limiting UE to attempt NAS procedure on same cell as long as UE doesn’t receive any NAS reject or congestion/barring/CAG or any other cell becomes better.

17

This can help:

3GPP TS 24.301 version 15.8.0 Release 15

18

Here UE received attach reject.

19

Still, as long as it doesn’t cause RAT mode to be disabled, UE can retry on same cell.
Though it shall attempt Attach Request and not singular SM/PDN Request procedure.

20

Hi All,

My answer is very straightforward.

Yes UE can reinitiate the attach on the same cell on which it received the attach reject .

For an example : Let’s say

UE tried on Cell x and received attach rejected with cause XYZ
Now UE will again retry on same cell for another approach…
but how long it will re-try on same cell it depends on what we have configured at device side…UE will retry until max. attempts are not exceed. If it exceed then UE moved that PLMN in forbidden list for the time being…
…

Correct me please if I am wrong…