A discussion about Access Point Capacity

Access Point Sectors and Capacity – a look into the real world.

A couple of times recently, most recently just today, I met with customers who were struggling with capacity issues on their access points.  In the case of the folks I spent time with today, they attempted to use our support staff to get to the bottom of the issue.  They abandoned that effort when they did not understand why support staff were asking for the data they wanted – they were hoping for a simple “change this setting” bit of advice that would unleash some untapped power.  Sometimes you get lucky and that happens, but most often the real reason lies a bit deeper, but not inaccessibly so.

In the particular case, they were asked about the subscriber radios attached to the access point and the placement of them and were provided with a copy of the Capacity Planner tool and LINK PLANNER.  Not understanding where this was going, they gave up on the whole effort in frustration.  That’s too bad as they were on the trail of unlocking the answer.

--The Brochure Claims --

I believe the true beginning of the problem is the Data Sheet, and more to the point, the top line data rates often used in marketing.  Often the “eye catching” numbers are based around some conditions you might not be able to meet.  Let’s use a PMP450i     5 GHz access point for example; at its top end on a 40 MHz wide channel it can do a 275 Mbps connection.  So, right off, there are a few things to know.  First, 40 MHz wide channels are nice if you can get them, but many times with spectrum congestion you are forced to use a smaller channel like 10 MHz or 5 MHz.  With everything else being equal, dropping to a 20 MHz channel drops that 275 Mbps in half, or a quarter at 10 MHz wide.  For my example we will stay at 40 MHz wide channel and the 275 Mbps number.  That number, 275, has a few more important things wrapped up in it.  275 is the sum of both the uplink and the downlink.  How you get that is going to vary based on the downlink setting in the access point.  At the default setting of 75% downlink and a perfect RF path for max modulation, that works out to 68 Mbps up and 206 Mbps down.

--A Matter of Time—

For simplicity sake let’s say have our 275Mbps link running, but then another force acts upon it to affect the performance.  Time.  Everything we do takes time.  It is the same thing here.   The longer distance we allow our link to be, the longer it takes to get responses back, therefore we need to pad in a bit of time to wait for signals to return.  We call this the T/R gap (Transmit / Receive).  The farther you want to connect the longer this gap needs to be.  You can see this happen using our LINK PLANNER tool, available for free download on our support website.  In LINK PLANNER the field on a multipoint plan is called “SM Distance.” Just taking a sector with five subscribers all inside a max distance of 7km (5 miles) up to 14km (10 miles) without actually connecting anything that far, we see my modeled sector capacity go from 150Mbps down to 146Mbps, or down to 139Mbps if I double that to 28km or 18 miles.  As you can see, it can pay to not go overboard with the max distance setting.

Another time related variable to deal with is the Uplink/Downlink ratio setting.  This setting adjusts the amount of time the access point dedicates to each task.  In a perfect world with all of the subscribers connecting at a max modulation rate it does a pretty good, but not perfect job of divvying up the capacity.  When our subscribers connect at a wide variety of modulation rates, the UL/DL ratio becomes much less precise when viewed as throughput.  The amount of time will be perfect, but the result viewed as capacity may be very far from it. 

--Modulation Rates –

In the real world, all of our Subscribers aren’t all likely to be received at the highest modulation rate of 256 Quadrature Amplitude Modulation (QAM) with great Signal to Noise Ratio (SNR).  This means that some communications are very fast, and some…not so much.  All of these modulation rates need to happen within a complete communications cycle with our access point talking to each active subscriber in every frame they are active.  The good news is that the Canopy Protocol that our PMP450 radios use does not reserve time for subscribers with nothing to say, so there isn’t time wasted on idle subscribers.   This does mean that we are likely to still not achieve peak performance if not everyone is running clean 256 QAM connections.  And, all of this is bound by the previous topic – Time.  So, what might that look like?  Let’s say we have three subscribers connected that need to communicate on this T/R cycle.  One is at 256 QAM, one is at 64 QAM, and one is at BPSK (the slowest speed).  They are all running best effort, no priority is set – and they all at the same time want to send the text “the quick brown fox jumped over the lazy dog.”  Please excuse the oversimplification, but in my example a single T/R cycle might look like:

256QAM        the quick brown fox jumped over the lazy dog

64 QAM:        t   h   e    q   u  i   c   k    b   r  o   w    n     f  o  x

BPSK:             t                     h                    e                           q

They all got the same amount of time on the air, but because of the differences in modulation the amount of data they could pass varied widely.  It also works out that a 275Mbps access point might now at this point only be able to move 140 Mbps of data.  As you can see, the BPSK signal, given an equal slot to the 256QAM signal, will have a significant impact on access point capacity whenever it needs to send or receive data.    Because of this property some operators have chosen to deploy what we call “Micro-Pops,” access points close to the intended subscriber connections designed to provide very high data rates by being close and strong.  This also means that just because you can connect a subscriber at a great distance – should you?  It needs to be an informed decision.

--LINK PLANNER--

The LINK PLANNER program can be used to model what performance can look like.  By entering in the access point and all the subscribers, plus any additional line of sight barriers, you can model with great accuracy your actual access point capacity.  In the case of one service provider’s network, they wanted to improve performance on an access point that had a dozen subscribers inside of 5 miles, and one out at 15.  That one at 15 miles was causing a very significant performance hit of about 35%.  In the case of this operator, they decided to connect the 15-mile customer with 2.4 GHz ePMP radios and take them off of the 5 GHz PMP450 sector.  The change benefited everyone in this case.

There is significant value in plotting out your network in LINK PLANNER.  It can tell you what your system should be capable of – and what effect changes could have.   This is why our tech support staff was headed in this direction.  Let’s just say we were looking at a system that at best could do 275Mbps, but as configured the top speed dropped to 60Mbps.  With LINK PLANNER, at least we would know if the equipment layout was the problem, or the equipment itself.

There is one more adjustment available in many radios:  Frame Size.  You’ll find this in access point radios only.  The subscribers learn the frame size from the access point they connect to automatically.  The Canopy Protocol that the PMP450 is built upon started out as a 2.5ms frame, and later when we began having to coexist with WiMax systems we added the 5ms frame size to allow synchronization with them.  The longer 5ms frame has additional benefits.  For example, our perfect signal data rate of 275 Mbps bumps up to 296Mbps just by switching to a 5ms frame.  We get that in trade for a about 2ms additional latency.  Not a bad trade all things considered.  Just make sure that if you chose to move to a 5ms frame there is nothing still at 2.5ms that cannot also be changed or you risk self-interference.  We have detailed guides on our support website on synchronizing with WiMax, LTE or the older Frequency Shift Keyed (FSK) Canopy gear that could be consulted if you have some of that gear in the mix you need to live alongside with.

Summary

While access capacity can be a complex subject, it can be modeled and understood.  Freely available tools like LINK PLANNER can give you good insight into how to improve performance of an access point.  If you are new to LINK PLANNER, follow the help links in the program to great content including “how to” videos to get you started.  Just be sure to manage your modulation levels and connection distances if you want to get the best possible performance out of your system.

10 Likes

Very insightful and factual. As a marketing person who develops the content, I appreciate your details.

1 Like