The user guide recommends a starting point for selecting the number of contention symbols as a function of the number of SMs in the sector. As Eric mentioned, the number of SMs is only one factor, the other is the traffic type and average packet size, which may be harder to evaluate and may change over time. We have noticed that the 5 ms frame benefits from more contention symbols, therefore we updated the recommendation on the user guide. If you use the same frame configuration parameters across your network, you may not have much flexibility in selecting this parameter, but if you are monitoring an isolated sector, you can experiment with the number of contention symbols and monitor the Bandwidth Request Success parameter. Note that this parameter does not need to be close to 100%. If the value is low (even below 50%), but the UL is fully utilized, then no changes are needed because the additional requests arriving at the AP could not have been scheduled anyway. If this value is low AND the UL utilization is low, then it is likely that there are too many collisions and the requests are not properly received at the AP. In this scenario it is recommended to increase the number of contention symbols.
This is a 450m issue only, the other APs use the auto-contention feature, which adjusts the number of contention symbols based on the success rate.
I also want to add a note about using 5 ms frame to be ready to co-exist with LTE deployments. This is true, but don’t forget that if you want to co-locate with LTE devices you need to enable the co-location feature in the AP. You will select the LTE frame you want to co-locate with, and the AP shifts the frame start automatically. This needs to be done on your entire network at the same time; if you only enable it on some APs and there are other AP in the area on the same frequencies, you may create self-interference.