Capacity of a UBNT AP vs the number of clients

Note: I am in the process of updating this for AC based radios. This was published in 2014, but much is still relevant.

Almost all the time I get asked: “How many clients can an AP handle?” . My answer is always a very long and drawn out one. There is no set in stone answer. There are many factors which can affect this. I will go into some of these and then explain how to calculate this.

Some things that we will assume.
1.You are calculating on an 802.11N Ap with some kind of polling (TDMA, NSTREME, AIRMAX, etc)
2.You know the MCS values and/or data rates at channel widths.
3.When I say in an ideal situation I mean basically in the lab. This is our baseline. This means no outside noise, everything is working properly, and all the connected clients are excellent.

Before I get into what affects how many clients can an AP handle we need to shift our thinking a little. We don’t think in terms of how many clients can an AP handle. We need to think in terms of how much capacity an AP has. This is very important to think in these terms. If you do so things will become more clear and more quantifiable.

So now, on to what affects the total capacity of an AP.

1.The channel width. In and ideal situation you will get more Capacity out of a 20 mhz channel than you will a 10mhz channel.
2.Noise. In the real world you will have interference. If you have interference the noise floor drops, customer signals can’t reach maximum modulation, and there are retransmits.
3.Plain old signal. Things such as trees, distance, fresnel zone, and antenna gain all affect signal
4.The speed you are giving to each customer.
5.Overselling. The concept of overselling has been around since the dial-up days. You are betting your customers are not all online at the same exact time doing the exact same stuff. So you can oversell your capacity. I will explain this a little more in a bit how this factors in.

Okay, so let’s dive into this. I am going to use a Ubiquity Rocket M5 as an example. Again, this can be applied to any polling type N radio.

Say we have a Rocket M5. At a 20MHZ channel the best modulation this M5 will do is MCS 15 at 130 Megs of over the air. What do you mean Over the Air? Well there is a difference between actual throughput and the Wireless Data Rate (aka over the air). Your actual throughput/capacity will be 1/2 of the over the air rate minus a little for overhead. I factor in 10% overhead for easy figuring.

Back to our figuring. You have 130 megs of capacity on your AP in an ideal situation on a 20 mhz channel. If we do our math:
130 / 2 = 65 Megs of Capacity to sell on the AP.
Now here comes the overselling part.
If we oversell at a 2:1 ratio we have 130 Megs of capacity on the AP.
If we oversell at a 3:1 ratio we have 195 megs of capacity on the AP.

We can do higher ratios, but it starts to become a moving target. With the spread of Netflix, Youtube, Hulu, and other streaming services the average customer is sucking down more and more bandwidth for longer periods of time. Think of a restaurant with so many tables. If your customers are staying longer and longer, you don’t have as much seating capacity to turn over for new people to sit down and consume your food. This is for another blog post.

So, let’s say we are overselling at 3:1. We have 195 megs of capacity. We now need to think about what packages we are selling to our customers. If they are all say 5 meg packages, this means we can safely sell 39 connections to the AP. 195 / 5 = 39. You can figure up the math if you have 3 Meg, 10 meg, or a mixture.

Now to the real world (aka why do my customers hate me and my AP sucks?).

The following is a real AP in the wild.  Blacked out to protect the innocent from script kiddies.

Couple of things to Note (circled in Red).

20 MHZ Channel
Capacity at 45% . This is more important than anything, even CCQ.
43 clients associated.

Let’s apply our math we learned earlier. We know a 20 mhz channel nets us MCS15 – 130 Megs

Here is the kicker.  Our capacity is at 45%.  This means we only have 45% of 130 megs of Over the air capacity.  Take this in half (130 / 2= 65   45% of 65 = 29.25.
This means all 43 of these customers are sharing 29 megs of capacity on the AP.  And the quality isn’t the greatest (37%).  So this means there are retransmissions going on between the client and the AP. The client can’t talk as fast as it is capable of in most cases. This means you can’t oversell the AP as much due to the quality of the signals being poor.  It is important to note I am talking about the quality and capacity of the signals, not signal strengths.

If those 43 people are all paying for, let’s say, 2 Megs download.  That means your AP needs to support a minimum of 86 megs. Thats without overselling.  We only have 29 megs in the current state!

We need to get those capacity numbers up.  How do we do that?

1. Channel selection. A noisy channel will drag everyone down.

2. Antenna gain.  This can be done at both the client and the AP.  A higher gain or better quality antenna can cause the clients to “hear” better.  You might not get an increase in signal strengths, but you are looking for an increase in quality. I use a loudspeaker metaphor.  You can hear a loudspeaker from a far distance, but you might not always be able to make out what is being said.  If you can somehow make out what is being said more clearly, then you don’t have to have the speaker turn up the volume.

3. Shielding. This helps eliminate the amount of stuff a client or AP hears.

4. Channel Width.  Sometimes dropping the channel width down can increase signals, thus raising the overall capacity.  Keep in mind it will lessen the overall capacity of the AP.

5.Simply getting rid of customers that shouldn’t be installed.  We have all done installs that were iffy.  These can drag down the overall capacity.

I hope this has helped understand.  The biggest thing I want you all to take away from this is think in terms of the amount of capacity you have to sell, not the number of connections.

Interesting topic on discontinued gear

So an interesting topic came up on Facebook tonight that got me to thinking. As WISPs grow and evolve, what are your thoughts on hoarding gear you have been using for years when it becomes discontinued? We will examine some ideas as to why this isn’t necessarily all a technical problem. It’s also a philosophical thing with the WISP owner/management.

First off let us examine the whys you would hoard equipment. One big reason is that you have a significant investment in the gear you are using.  This gear has been proven to work, and you have deployed large amounts of it. As a company grows, the ability to introduce new gear into things facing the customer becomes a slower process. To use the analogy, the larger the company grows, the slower the ship turns.

Another reason is the amount of capital needed to migrate to new gear.  Many times when a product line gets discontinued, there is no clear replacement for it. The Facebook post which brought up this post involved the Mikrotik NetMetal 9s.  These are now discontinued by Mikrotik and have no replacement.  If a WISP were to migrate to something else there would be a significant cost in new access points, but more costly, would-be customer CPE. “But just put up the new gear alongside the old and migrate customers over,” you say. This brings us to the next point.

Frequency plays a big role in any migration path. In a perfect world, everyone has open channels and there is no interference. However, that is hardly the case in many scenarios.  This scenario is especially true of 900mhz.  You only have 902-928 MHz to deal with in the US FCC realm.  At 20 MHz wide this is only one non-overlapping channel.   If you put up another access point on 900mhz on top of your existing you will be interfering with yourself. Besides, the frequency may be the reason you are able to reach customers.

Finally, the pros of hoarding equipment are the soft costs of upgrading. Training, engineering, customer service, and possible re-work of some installs can add to the overall cost.  Anyone who has had to change the pins on a reverse polarity Subscriber Module knows the pain I am talking about.

The Cons

The biggest trap I see operators fall into is they horde equipment and then forget about i.  They have spares on the shelves, and enough to service customers. They fool themselves into a false sense of security and kind of wait for something to fall into their laps.  Then, it seems all of a sudden, something happens, and they are scrambling for a solution.  Sometimes this is a software update current equipment gets, but the older stuff does not. This could be some critical security vulnerability or new code to interface with a new system.  Either way, this equipment is stranded on a software island.

Next up is hardware failure.  As equipment gets old it, is more prone to failure.  A WISP may find their reserves depleted after a weekend of storms or bad luck. What may have been plentiful supplies a month ago is now an issue.

Lastly, the performance of the equipment is a big issue.  In today’s bandwidth-hungry consumer ISP radios are needing to perform better and deliver more bandwidth to the customer. Sometimes a manufacturer discontinues a product because they see the limitations of the band or the equipment. Sometimes the manufacturer sees operators are moving on to other ways of doing things. This could be newer frequencies or data algorithms. Usually, it boils down to the equipment was too expensive to make or wasn’t selling well enough.

So whats a WISP to do?

The number one thing a WISP needs to do is not fall into a rut of doing the same old same old for too long when it comes to equipment.  What worked five years ago, may work okay today, but will it work two years from now? Always have a strategy to dump your equipment if need be for something better.  Whether that strategy makes business sense is a different question. Sometimes the approach is to have money in the bank for when the right equipment comes along. Until then, it’s business as usual. Don’t let yourself keep saying you will figure it out tomorrow.

I believe that WISPs should have three lines of thinking.

  1. What am I doing in the immediate future to run my business?
  2. What am I doing in the next 18 months to keep my business competitive?
  3. What am I doing in the next 24-36 months to grow and keep up with customer demand?

If you have strategies for each of these then hoarding equipment is no big deal.  You have plans in place. Just don’t let yourself fall into a false sense of security. Always be learning about new rules, technologies, equipment, and methods.  As your business grows you can delegate this to others, so you don’t have to be in the thick of it and can concentrate on your business.  If you are that “techie” who is doing all of this, keep an open mind.  Don’t be the typical I.T. guy stuck in your ways. None of this is saying hoarding discontinued gear is wrong, just have a strategy.



RouterOS v7 limited beta

I did an overall video of the New Mikrotik RouterOS v7.

From Mikrotik forum:

We have released a very limited test variant of RouterOS v7. Currently only available for ARM systems with a slightly limited feature set.

What is currently unlocked / available:

– Only available for ARM architecture
– Based on Kernel 4.14.131, which is currently the latest and most supported LTS version
– New CLI style, but compatible with the old one for compatibility
– New routing features, but see below
– OpenVPN UDP protocol support
– NTP client and server now in one, rewritten application
– removed individual packages, only bundle and extra packages will remain

Other features not yet public.

What is not available:

– BGP / MPLS disabled
– Extra packages
– Winbox does not show all features, use CLI for most functionality


Download link:

Water tower install with mounting frame

We recently headed up a job for a client of installing some RF elements horns, Cambium ePMP, and Baicells LTE for a client.  One of the gems of this job was the frame the client designed for the job.  We can’t take credit for this. We just think it’s cool. Some of these pictures were taken during construction, thus post clean-up.

The frame is truly an example of how WISPs are stepping up their installs to become more standardized and carrier-grade. It costs some money but is worth it in the end.


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j2 Briefing: FCC news, Microsoft whitespace, polls

The j2 Podcast for August 29, 2019

Microsoft is pushing it’s Whitespace product as a solution to the Digital divide. This has been branded “Airband”

The commission unanimously voted to distribute more than $20 billion of Universal Service Fund subsidies over the next decade as part of the Rural Digital Opportunity Fund. It also adopted a long-awaited proposal to get more detailed information from broadband providers about where they offer service in order to improve the agency’s coverage maps.  <let’s hope this revamps the form 477 reportin>

iOt is showing it’s age
Amazon is killing off the gimicky Dash buttons.

Verizon turns up 5G

In the ever-changing 5g race Verizon turns up 5G in Atlanta, Detroit, Indianapolis, Washington DC

New poll says the Internet is more important than Air conditioning while on vacation

Mobile Users double since 2013
The percentage of respondents who said their primary online access devices is mobile has effectively doubled since 2013, and many of those are using mobile as a substitute, rather than a complement, to wired broadband service.