This video is the first session in LTE KPI Optimization list and explains the LTE ERAB Setup Success Rate KPI. The session covers the signaling flow for LTE accessibility in detail along with messages where the KPIs are pegged. The causes of LTE ERAB Setup failures are also covered in detail along with actions that can resolve those issues.
Firstly, lets understand the definition and points where the ERAB KPI is pegged. After the UE sends the RRC Setup Complete message to the eNB, the eNB sends a S1 Initial UE Message to the MME indicating the purpose of the UE (Attach, TAU, CSFB, Service Request etc) and its credentials. Once the MME receives this message and it decides that a bearer is required, it will send an Initial Context Setup Request to the eNB. This message is considered as the ERAB Attempt as it contains the bearers to be added along with their QCI values. The eNB receives this message and adds the DRB (Data Radio Bearer) based on the bearer profile in Initial Context Setup Request. But before the eNB can add bearers, it needs to activate the security for the connection. This is done by the Security Mode Command which carries the ciphering and integrity protection algorithms. After this the eNB sends a RRC Connection Reconfiguration message to the UE which adds a DRB and it includes the configuration for the DRB like bearer identity, PDCP & RLC configuration (AM/UM etc). SRB2 is also added at this point with this message. The UE receives these messages and reconfigures the connection. Then the UE responds with Security Mode complete and RRC Connection Reconfiguration Complete messages. As the eNB receives these messages, it sends an Initial Context Setup Response to MME and this message is considered as the ERAB Success.
Now let’s understand the common failures that usually cause a ERAB setup failure. Most of the times, the ERAB setup failures can be divided into two broad categories
- Radio Induced ERAB Setup Failures
- MME Induced ERAB Setup Failures
Let’s have an in-depth look at both of them and find ways to tackle them in this video session!