PMP450i 900MHz Maximum bandwidth from AP 7MHz

We have a PMP 450i 900 MHz AP that is showing 100% Fame Utilization during peak hours. We have all of our 900 MHz APs set to 7MHz channel size. On this AP we only have 11 SMs connected to this AP. Mostly we have SM speeds set to 5/2 Mbps. The AP during peak hours is hitting 19.5 Mbps. What is the maximum bandwidth for a 7MHz channel on 900? 

Peak hours are definitely when you want to utilize your fame...  (sorry, couldn't resist)

That question always seems simple, but it (and the answer) unfortunately is not.  Several factors affect the 'maximum' throughput on a sector, including:

Max modulation (if all clients are 8x then max throughput is higher than if clients are 6x, 4x, etc)

Downlink/uplink ratio

Contention Slots

Frame Period

Signal (DBm or RSSI) and SNR are critical, but if signal is too weak, or SNR too low, then modulation is lower as well, so for a quick-glance assessment the modulation is easier to check.

On an AP here we're currently running 5ms frame, 2 contention slots, 85% downlink, and two clients.  Each client is at 8x, and each can test up to about 40Mbps down and 5Mbps up.

If you run 2.5ms instead of 5ms frames, then your latency will be lower but you lose twice as much to frame overhead.

If you increase contention slots (necessary to support larger numbers of clients) then those slots are no longer available for client data - not a huge difference, for example one contention slot would lead to 51DL slots and 9UL slots, while two contention slots would lead to 50DL and 9UL slots, at 85/15 DL/UL ratio.

The impact of modulation on sector capacity is huge, but only if the client is active - a link that's idle 24hrs a day doesn't impact the sector much at all, but one that's downloading 5Mbps constantly, streaming video perhaps, matters greatly - whether that client has 8x vs 1x connection means a really huge difference in sector utilization, because at the lower modulation the same frame (timeslot-wise) is used but carries much less data, or conversely the same throughput uses more frames at lower modulation than at higher.


Assuming everyone is connecting perfectly at 256QAM... then around 30mbps aggregate.... but this rarely ever the case, especially with 900MHz. If you'd like to get a better idea of what to expect using different channel widths, plug in your numbers into Cambium's free capacity planner located HERE.

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If your 'real' question is how to improve it, I'd suggest looking for the weakest clients on the sector, and the most data-hungry clients on the sector, and any that overlap you roll a truck to try to improve their connection.

One heavy user with a weak connection can kill a sector, and improving that one customer by a few dB can sometimes greatly increase overall sector capacity since their heavy use can be squeezed into less frames at higher modulation.

If you have multiple heavy-hitters on the sector, and one can see an adjacent sector, consider pushing one to a weaker sector if that sector is lightly loaded - using 1/2 the capacity of an unloaded sector is usually better than using 1/4 the capacity of an overloaded sector.


This is great advice newkirk... because the sector is a shared resource, the poorer modulating subscribers take more time to transfer data. If these poor signal users are also the heaviest users, the impact to the sector will be greater.

That link capacity planner tool can tell you what to expect at a sector level (by inputting the ranges of all 11 SMs in the sector). This is really a simplified snapshot.  If you want something a bit more accurate, you can plot everything in LINKPlanner, and this should give you a more accurate expectation of performance.  If the sector throughput still doesn't match the expected values, you may have interference that the tools are not accounting for.

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