Recently, we had a client question why we didn’t mount antennas higher up on a tower with an FM repeater on it. The top of the tower has an FM repeater on it so we mounted the equipment about 25 feet below that.
When you are talking about antennas and transmitters the basic thing to remember is it’s all radiation. Good antennas have predictable drop off patterns and, when paired with a good transmitter, have crisp frequency drop offs. However, there is still radiation emitting from feedline and the antenna on the tower. Many FM repeaters use a dipole design. Some are folded, others are different types. Below is an antenna pattern from a Dipole antenna.
As you can see there are a few patterns radiating from the antenna. These patterns should be taken into consideration when mounting your equipment near FM, UHF, or VHF systems. Radiation may interfere with things such as your cat-5, or your PIM. In an earlier article, I talk about low-pim cables and what affects PIM. This is very important when you are deploying LTE gear. RF radiation from high power transmitters can cause PIM issues if the wavelength happens to coincide with the wavelength of the other transmitter. This does not mean they are on the same frequency. Remember, in RF you have full wave, 3/4,1/2, and 1/4 wavelengths to deal with.
Other things to consider are near and far field patterns. If you want some heavy reading you can read about it on Wikipedia.
Our next issue and the most common issue is the radiation getting into our Ethernet cables as well as our radios on the tower. Below illustrates the propagation of signals coming out of an antenna on the top of the tower. If you notice, some of the radiation is directed underneath of the antenna. Any equipment mounted too close underneath will be bombarded with radiation.
Too much radiation can cause link negotiation issues, signal degradation, and other issues. By moving our antennas out of the patterns of other antennas we can make for a more reliable system. This is one case where higher on the tower is not always better. Just because another antenna is not mounted in front of another it does not mean they are in each other’s radiated patterns.
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.
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.