I was looking to the X2, and E1 and RRC specs, related PDCP wraparound and came up with the summary as follows:
This was because of following statement in 38.300 section 6.4.1 "Since PDCP does not allow COUNT to wrap around in DL and UL, it is up to the network to prevent it from happening "
“When the gNB-CU-UP detects that the PDCP COUNT wrap around is about to happen, the "Bearer Context Modification Required" message is sent to gNB-CU-CP.
The gNB-CU-CP triggers the message "SGNB MODIFICATION REQUIRED" with cause "Count reaches max value".
The MN sends the"SGNB MODIFICATION REQUIRED" message to remove and add the new DRB. This triggers SN to send the "BEARER CONTEXT MODIFICATION REQUEST" message to gNB-CU-UP. This will contain the DRB whose count is about to wrap around in the "DRB To Remove list". And the new DRB replacing the old one will be in the "DRB To Setup List". On successful response back from the gNB-CU-UP, the gNB-CU-CP will send the "SGNB MODIFICATION REQ ACK" back to MN.
The MN will be sending these changes in the RRC Connection Reconfiguration message to the UE.
The RRC Connection Reconfiguration, will add the DRB whose PDCP count is about to wraparound in the DRBs in the secondaryCellGroup. And in the "rlc-BearerToReleaseList" IE. And also change the other IEs accordingly to remove this DRB. And add the new DRB information, which replaces this, under the "rlc-BearerToAddModList" and other related IEs accordingly to add the new DRB. There would be some interruption time for reconfiguration at UE and RAN side.”
For critical DRBs (which are mapped to URLLC kind of services) where interruption time matters. So I am thinking about solving them. Does the following solutions reasonable for you ? Let me know any comments , corrections, or you have better proposals
1. Is to indicate the PDCP count wrap around much earlier, (but giving some activation time), so network and UE
have time to reconfigure. (OR both can then just configure the new DRB and immediately activate the new DRB when the activation time elapses).
Maybe this can be facilitated with the new cause for the release and add as "PDCP wap around " in the RRCReconfigure message.
1. Or just send the DRB Id that needs to be changed, so UE uses the old DRB configuration, but just resetting the SN.
2. Just reestablish the PDCP entity by resetting the count value (Not very sure of this option, need to explore further).