Past posts suggest we should be using 2.5ms with a 40mhz channel in CBRS on a 450m. Is this still the recommendation? We’re running 5.0ms frame on our first AP because that’s what I thought we should use but just reviews the entire 681 page manual and it did not suggest which frame size we should use.
What’s best for us with 40mhz CBRS? 2.5 or 5.0ms frame?
So… it depends… if you’ve been keeping up to date on your firmware (you’re using R22, or R22.0.1), then there are (and will be) more and more throughput optimizations for 30/40MHz and 5ms frames. Furthermore, by using 5ms frames you put yourself in a position to enable LTE sync coexistence if someone comes to town and starts throwing up CBRS LTE AP’s.
If you’re using older firmware (pre-R22) then there’s little gain, possibly the same, and at worse case, less throughput using 30/40MHz 5ms frames vs 2.5ms frames.
Cambiums’ actually going to have a webinar that will discuss many of these firmware optimizations (both recently introduced and in the future).
What contention slot value to use is a bit more of murky subject. Typical rule of thumb is the more VC’s and/or client’s using VoIP (or lots of active upload requirements) the more contention slots you want to use… so yeah, anywhere between 4 and 8 I think most operators settle on. We typically use 6 and we have AP’s with 100+ VC’s or SM’s on them. I’m wondering if Cambium will update their now very old contention slot whitepaper now that there are some significant performance improvements in firmware. @LuciaCambium any comments?
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.