For this post, we will be updating it continuously with different techniques and measure that any RF engineer can apply to improve the LTE RF access performance.
Case 1: Access failures due to IPPATH misconfiguration
During a routine check, RF engineer found that one site was showing a degradation of eRAB access success rate and the increase of access failures due to Transport Network Layer. This failure counters as its name says, it is usually related to issues in the transport / transmission layer. For this case, the first step we should follow is to review the performance on the S1 interface.
During access procedure, the S1 setup occurs during the Initial Context Setup Request initiated by the MME.
As a starting point, we should initiate a S1 Interface trace in the eNodeB. This allows us to collect all the signalling between the eNodeB and the EPC (Evolved Packet Core). This way we can understand the kind of failure impacting the LTE RF access performance due to transport reasons. In this particular case we looked into the S1 Setup. To filter the messages, we need to be sure to review only those related with a service request which is related to the S1 Setup procedure.
Checking multiple messages for the S1 setup, we noticed that some procedures failed to setup the S1. However, most of the procedures were successfully established.
During the S1 setup, the MME messages sent to eNB show the transport layer address (IP) of user plane entity of S-GW and the reason of initial context setup failures as shown in below graph.
In this case, we can see how the S1 setup failed when SGW IP is 10.106.0.165
Comparing this message with messages where S1 setup success, we can see a difference between the SGW IPs.
This information can help us to suspect that the issue is related to the IP configuration from the eNodeB to the SGW endpoints. For this site, the configuration is based on IPPATH. Checking the configuration we noticed that the eNodeB has no IPPATH configured for the IP 10.106.0.165.
Once the eNB received an E-RAB setup attempt with the specific IP of S-GW(10.106.0.165), the eNB sends a failure response to MME because the mentioned IP isn’t configured at the affected eNB.
So the solution for this case was to add the missing IP as shown below:
As soon the configuration is commited to the eNodeB, the LTE RF access performance recovered to the normal values.
Interesting Links
- https://rfoptima.com/2024/03/07/guidelines-for-lte-rf-troubleshooting/
- https://rfoptima.com/2024/03/01/4g-rf-optimization-processes/
- https://rfoptima.com/2024/05/01/learn-pci-optimization-to-be-a-4g-rf-expert/
- https://telecommunications4dummies.com/
- https://es.slideshare.net/slideshow/lte-optimization/36620521
