Contention Slot Calculator

Has anyone created a calculator of some kind to determine contention slots yet, or a formula that can be used? I am looking for both timing 2.5 and 5 ms as well as 3.65 and 5 ghz in all 450 flavors if possible, even if its some combination of stats I should be monitoring.

For 450 and 450i, if you’re using any firmware made in the last couple of years, there’s an auto contention slots feature that you should enable. For 450m, there’s been various posts in the forum here with guidance on the matter… just do a search with those keywords. There’s no difference in terms of contention slot config based on band. From a sync timing aspect, contention slot configuration has a very small effect on timing differences… you can have a very wide spread between contention slot values between AP’s and still maintain sync. You can see this for yourself by playing around with the PMP450 co-location tool.

PMP PTP co-location tool.xlsx (121.3 KB)

I see responses all over related to frame utilization, and most of our towers are ringed and can potentially see other access points in the same direction so i think auto would not be a good idea due to both of those factors. at this point though we are mostly on 450m 2.5 ms and the best i can see is if you are having issues up the contention slots, I would like to know what I should raise them to. I have read everything going back to 2016 about contention slots and pinning it down is constantly a problem, especially with the 450m line.

I think you have a misconception of how the auto contention slots feature work. Setting 450/450i AP’s to auto still allows them to maintain sync with other AP’s, including PMP450m AP’s (that don’t have an auto setting) this is done by still using a value for the contention slot variable. e.g. We have many 450i and 450m 3GHz AP’s that we sync. For the 450i’s we use auto contention with the contention value set to 6… on the 450m’s we typically use a contention value of 6. As mentioned in my last post tho, the contention slot value has very little effect on timing… so you could use auto on your 450i AP’s and set the value to 6… but for those PMP450m AP’s that need a higher value, like 9 or even the max of 15… you can still maintain sync. You’ll just need to use the provided calculator to make sure you’re still in sync.

How then do we determine what it should be set to? Or is there something/s we can monitor to see if we should raise it? we have one currently on our network that we have raised it from 6 to 10 by 2s, and watched improvements happen every time, it is “over crowded” at 63 sms with only 1 data channel on one 450m sector and we started at the recommendation in the userguide but it wasnt enough. we are taking a WAG by upping it constantly and waiting until we see peak throughput stop climbing in the evenings.

For 450/450i… just use auto… for 450m, you’ll need to use some trial and error. We have 450m AP’s with 100+ SM’s on them and a contention slot value of 12 (the max is 15 BTW) seems to work pretty well on them… but YMMV, if you have a lot of VoIP users, or upload traffic, you may need to increase the value. Like I said though, from a timing perspective, it has very little bearing on things… so you can use wide ranges between different AP’s without losing sync. Again, use the calculator to make sure.

Hi Robert, in the PMP 450 manual there is a reference to the contention slots. For the 450i, set the contention slot to the recommendation based on the initial load, then auto contention handles the rest. Few times a year we go through and audit the access points and adjust the base contention slots if needed.

with 100+ on an ap im curious what kinds of speeds those customers are seeing, especially with a flood test.

We deliver plans up to 60mbps, with most customers average tier around 25mbps. The AP hits around 300mbps aggregate during peak hours. A flood test usually results in around 550-600mbps aggregate.