Look through your discussion on low E2ECSSR, It is really an useful and interesting topic. I share my points as following:
1. Fully agreed with ALU’points. There is only SDCCH drop and RTCH assignment failure which will contribute to CSSR. In 2E2CSSR, these two factors are also effected, however we will see that CSSR and E2ECSSR will be degraded in parallel. Our concern is diffent. We have sucsessfull SDCCH assignment (For MTC/MOC only) but there is no RTCH assignment accordingly without any relevant trigged counters in BSS.
2. I found that some BSCs have poor 2E2CSSR, they have poor PSR as well and mostly located in mountainous area with poor coverage.
3. The problem must be investigated in core side as well.
4. After working with core team, we found that some E/// MSC now is using late assignment technique. It mean that serving BSC will only assign RTCH for calling MS if it’s end side is connected (be paged successfully). So if called MS can not be paged successfully (out of coverage or turn-off), RTCH assignment will not happened and result to poor 2E2CSSR