1.) RRC Connection Setup:
The base station sends an RRC Connection Setup message but does not receive an RRC Connection Complete message. The possible causes include:
i. The uplink interference of the cell is high.
ii. The downlink coverage is poor.
iii. The cell capacity level indication is set to an over-small value, or the SR channel capacity is too small.
iv. The reselection parameters for the camp state are not reasonable, and the UE fails to reselect a proper serving cell in time.
2.) Initial Direct Transfer and NAS Security:
If there is any problem with initial direct transfer or NAS security, contact core network engineers for assistance.
3.) PDU Session Setup and SRB/DRB Setup:
The following procedure can be followed to troubleshoot PDU session setup and SRB/DRB setup step by step:
A.) Upon receiving the Initial Context Setup Request message, if the eNB directly replies with a setup failure message, the possible causes include that: i. The ERAB admission fails, probably due to the ERAB admission threshold or insufficient licenses.
ii. The UE capability carried by the AMF conflicts with that at the RAN side, for example, the UE initiates access in a cell, but the UE capability sent from the MME indicates that the UE does not support the frequency band.
iii. The UE capability information is too long and exceeds the upper limit of the base station version.
B.) If the SRB/DRB setup fails during the encryption and integrity protection on the air interface, it is necessary to check the encryption and integrity protection algorithms set at the base station side and set them according to the technical notice.
C.) Another common case is that during the SRB/DRB setup, the base station performs RRC reconfiguration for the UE, but the reconfiguration fails, resulting in an SRB/DRB setup failure.
The possible causes include:
i. The UE capability is insufficient.
ii. There is some problem with the system version of the base station.
iii. There is some problem with the physical configuration of the base station.
D.) In addition, in the phase of PDU session setup and SRB/DRB setup, the air-interface quality should be guaranteed, because if the air-interface link quality is very poor, the access should have failed in the phase of RRC connection setup. However, occasionally the messages on the air interface may have no response, so the following actions should be taken:
i. Check the uplink interference of the cell.
ii. Check the downlink coverage.
iii. Check whether the cell capacity level indication is set to an over-small value, or whether the SR channel capacity is too small.