450i 900 MHz - recommended Contention slots?

We are brand new to Cambium products and 900MHz.

The site will have 1 sector AP to cover a specific region and not going to have any other 900MHz around.

Also looking at putting 50 SM onto this one AP.

Bandwidth plan is 15Mbps service.

How many contention slots should I use?

Any recommended tuning or tweaking parameters I should be using?


This should work assuming you plan to use  a 20 MHz channel size, and the noise floor will support this.

At 15 Mbps plans, for 50 customers, this adds up to be about 750 Mbps.  Considering 10-12 oversubscription rate, this would mean you'd need around 60-75 Mbps of capacity to service the sector.

In a 20 MHz channel, this would be achievable if most subscribers are operating in 6X or 8X modulation.  If, however, 900 MHz is being chosen to penetrate obstructions, I would not just assume very high modulations.  Careful planning should be done to set realistic expectations for what will happen.  We have several planning tools available to help you.

Are there obstructions for some or most of the 50 SMs?  Will a single 90 degree sector cover all of them?

If you have LOS, have you considered 5 GHz or other frequencies also?

Contention slots is probably not the biggest concern here, and really depends on how many SMs are connected.  Generally, fewer SMs require fewer contention slots.  There is a good discussion on this here, along with the link to a whitepaper discussing this topic.

The site where this 900MHz is going is just high enough to not have any fresnel zone issues.  We have 5 GHz already at this site and it works great, however 50% of the customers are under a green canopy with no line of sight, due to trees, no elevation issues.  70% of it is evergreen, and the other 30% the leaves will be disappearing soon.

Assumption is correct for 20MHZ channel size, full transmit power and 13.5 external gain, and clients with 13.5 external gain, noise floor who knows yet.  There is a300-foott tower plus another 100 feet of elevation increase behind this site where the big telcos are broadcasting Canopy 100

Use the LINKPlanner software to get an idea of what service you will be able to provide.

"50% of the customers are under a green canopy with no line of sight"

That would definitely be considered an obstruction and depending on the density of the canopy it could have a significant impact.

Noise could be a major factor for you.  Anything from baby monitors to "SMART" Meters could also have a significant impact.

20 MHz in the 900 MHz band is under ideal and magical conditions.

Stay Safe

When we first rolled out the 900 450i product, I asked the same question.  The answer that I finally went with was that it should be somewhere between 1/3 to 1/4 the number of uplink slots.

I found that going with the number of SM recommendation would lead to more control slots than uplink slots when dealing with 5 MHz channels, and that's just ridiculous.

Edit:  I would also caution you against using a 20 MHz channel.  That's really only possible if you aren't doing a full cluster, otherwise the best case scenario would be two 10 MHz channels.  Even then, we ended up going with 5 MHz to start just for the ability to get around noise from other providers.