CWNP CWNA – Site Survey Tools

1. Site Survey Tools

In this module, we’re going to take a look at some of the tools that you can use to help you with a site survey. So we’ll talk about really the definition of what you should be doing in a site survey. We’ll look at those tools and how they can help us with what we call a coverage analysis.

2. Protocol and Spectrum Analysis

When we take a look at history, like ten years ago, if you were going to plan or design for a wireless network, the chances were there really wasn’t a lot of things that would interfere. Yeah, maybe still a microwave oven, but you didn’t have the bluetooth devices, you probably weren’t using infrared. You probably didn’t have to worry about people bringing in those wireless toys that they play with at home. The cell phones, attached tablets, all of it was really not there to cause that kind of interference.

Today, it’s a different story. The likelihood of interference is very high, and so we have to find a way to see how we can plan our network around that. Now, there are a couple of different tools that do different things for us. There is a wifi based protocol analyzer that is designed to look at the 811 frames to figure out what the ssids are, the bssid information, along with packet and security information.

And so it’s really kind of like a packet sniffer on a wired network. It’s just doing it over the wireless system, but it’s not there to tell you about interference. So there would be a different tool that we’ll talk about that you would use just to hear that noise floor to make sure that you can determine what the signal strength is really going to be.

Now, we can monitor that information, but like I said, it’s for a different purpose. We’re just listening to the signals, not what the actual data is in the signal. And what we would do with those tools is try to document and provide kind of what the rf map looks like in that environment. So again, protocol analyzers. They take the data that’s received by the wifi cards and they help you with understanding the packet analysis. It’s the spectrum analyzers that you want to look at to monitor strength of the rf signals themselves, whether it’s for the data or if it’s for the interference.

3. Spectrum Analysis Part1

So spectrum analysis is probably a really a good place to start. It’s something we do before we even worry about the coverage analysis to let us know potentially where there are going to be some problem points. And you can do surveys locating where all that interference might be. And in the old days, we didn’t do a lot of the spectrum analysis because it cost some money. Whereas today you can buy cards that you can just put into your laptops with the right software software and be able to do the spectrum analysis fairly cheaply. So unlike the early times when we just didn’t do it because of the cost of the hardware, just go.

Now with the PC based analyzer, the only thing I would be careful of is the quality of the radio that you’re going to be using to do the analysis. It’s the same when we get into coverage. I’ve noticed that between different vendors, some antennas have better coverage than others. They’re more sensitive. And so you have to really kind of figure out what would be the best tool, maybe test a few of them if you’re allowed to before you actually buy them. But the Pcbased ones are just decreasing the overall price. I can use my regular old laptop and start running the software to be able to do that spectrum analysis.

4. Spectrum Analysis Part2

So as we look at continuing examples, all we’re trying to do is determine right the amount of interference in decibels that we might see and where they might be located. Notice not only we’re looking at the strength, but you’re at the bottom, you’re looking at the actual band. Remember, in the two, four and the 5 ghz range we have many different channels. And so we’re looking specifically at each of the channels to get a good idea idea about what that interference could be. And at the same time it could also be measuring the strength of the rf signal as well. So you might have the noise floor versus the actual received signal. And if you don’t like the pretty charts, you can always view it over here off to the side to get an idea again for each of the channels within that band that are causing you some problems.

5. Site Survey Part1

So after you do the spectrum analysis on your site survey, the next part is for you to determine where you want the rf coverage to be inside of your facility. Do you actually need it so that if I’m standing at any corner of the office that I have the wifi signal? I mean, is there actually somebody with a desk that’s sitting there? If it does cover that corner, what am I going to think about as far as it going out outside the walls and being able to be seen by other people?

So we talk about where the coverage is going to be, but also the capacity. The capacity is going to be how many people are in that office. Because remember, everybody that’s connected to a single access point is sharing that bandwidth. So even if you have a good, strong signal, you might not have as much throughput as you want because of the number of machines and devices that are connected to it. We use these rf measurements to be able to guarantee that the needs of the coverage are going to be met and also to make sure that we have enough capacity. You might remember I talked about the fact that I might have more access points than you would think I need for coverage, but I’m doing that, putting them in with lower power so I can also meet the capacity.

6. Site Survey Part2

So one way you might start your site survey off is to put an access point in the corner with a low power setting. Remember, some of these could get up to 100 milliwatts if you wanted to. And then look at what the coverage area is going to be measuring as you’re moving away from the access point, how weak the signal becomes through the attenuation. And so that basically means you’re going to walk diagonally since you started in a corner out to the center of the building until your signal gets down to probably about -70 dbms and assuming that’s the signal strength that minimum that you wanted to be able to provide. And and once I get to that point, then at that I could say, okay, now I can temporarily mount my access point there.

Now why did I start in the corner? I start in the corner because remember, you got to worry about even though the wall will absorb a lot of this the outside coverage. So by moving to a place diagonally and I temporarily mount it, I’m basically going to be creating a coverage pattern that should get very close to that corner of that wall. And remember, it’s a temporary mounting because it might not be perfect by the time you’re done. But then depending on the shape and the size of that coverage cell, you might want to maybe say, okay, I’m still missing a little piece in the corner. So maybe if instead of 30 milliwatts, I’ll bump this thing up to 40 milliwatts to get better coverage.

7. Site Survey Part3

Now that we have our first access point in a temporary mounting, what we’re going to do, instead of walking diagonally away, we’re going to walk horizontally along with the wall to again determine where the second access point is going to go. And what we’re looking for, again, is to find when we get to that weaker part of the signal that we may say, okay, here’s a good spot for the second access point. And from there, hoping that as I’m looking at that coverage, coverage that I have now some overlap between the two access points to help me with the roaming part of it, again, we’d mount that one temporarily. And guess what you’re going to do? You’re going to keep walking along until you run into the other wall, I guess, of your building. And again, you might just choose to alter or increase or maybe even decrease the amount of power so you get the kind of coverage that you want.

8. Site Survey Part4

So what we’re generally looking at is when we put an access point in there, we’re generally looking for that cell to have a strength. That is greater than -67 decibels. So that we have adequate signal. Strength for anybody at the edge. And because of the placement, we’re going to have these overlapping areas of coverage. Now you’re going to potentially have different channel numbers. At least I would hope that you would.

And in 2. 4 we have the three channels. And so by putting channel one at the top six and eleven where the overlap occurs they’re not stepping on each other. And then I’d have to put one here again. You do want to have some space here in between the two overlapping channels. That should be about 20 decibels in power. Or as far as the gain, so that I don’t have to worry about again, that overlapping part of it.

9. AP Placement and Configuration

Now, when your site survey is being done, all the cell edge measurements should be recorded, written on a copy of the floor plan of the building. OK? I didn’t say write it on the floor, on a floor plan, a piece of paper, and then have an entry with the exact location of where each access point should be put at. And then something else that’s often overlooked as a part of the design is that whether or not it might be more more beneficial. Instead of having this unidirectional antenna, which is what most vendors sell, when you buy an access point, you might say some coverage might be better if I had a semidirectional antenna. And unfortunately, that’s a thought that a lot of us don’t go through when we’re trying to make the decision of where to place all of the access points. In other words, if I’d gone back to that initial idea of deployment, I could have put an access point in that corner rather than walking away diagonally looking for the weakness or where it gets weak. And with a semidirectional had actually a little bit more coverage area without worry of the people on the other side of the wall being able to have access to my wireless network. So your typical default antenna from almost every vendor is a low gain unidirectional antenna, which at most has about 2. 14 decibels of gain that are or added in. You go to those semi directional, you can see that go up to five decibels or higher.

10. Application Analysis

The idea of application analysis is where spectrum analysis is different than the coverage. But we still consider that they’re both mandatory because as we’re doing the site surveys, we have to look at the applications that we’re going to be doing or using and how that is going to be affected by the coverage and the strength of the coverage that we’re given. So when you consider how many wifi networks are coming along and how important they are in the enterprise, this becomes down to the capacity. I’ve only talked about capacity being so far the number of people or devices that are connecting to the access point.

But capacity also has to be a part of the plan for the support of your applications. Are you going to be doing large file transfers? Are you going to be trying to run real time protocols like voice over IP or video conferencing? So all of those should be a part of your site survey process. I mean, it’s great what I described so far. You know where to put an access point so everybody gets a signal. It doesn’t mean that it’s effective for the support of what you need in your enterprise. 

11. Indoor Site Survey Tools

All right, so a spectrum analyzer is there to help you locate sources of interference. It’s your main weapon that you use in the coverage analysis, and it will be your way of being able to determine what the received signal strength measurement is going to be. So as we are going out here and following this process and trying to get these overlapping cells, in theory, it sounds pretty good, right, that we have this kind of coverage. The problem is, when we look at beyond the theory into the realistic, some of these cells might have a little bit different shape to them because of things that interfere. Like, again, there could be a kitchen here with that microwave oven causing some interference with your cell coverage.

And so spectrum analysis is a part of what we do with the survey tool. So we can go from the theoretical to what’s actually going to be there. And I didn’t even put in there the fact that there’s going to be people there with their desks and their computers sitting on top of the desk and cubicle walls surrounding them, all of these things that make it absorption or even reflection an issue when we’re doing the site survey. So, yeah, we want to know where all that interference is. So get a more realistic view of what our coverage is going to be. And remember, the interference is also going to affect your capacity.

12. Outdoor Site Survey Tools Part1

Outdoor site surveys are going to be using different types of access points or mesh routers. Obviously, most of them are going to be semidirectional or highly directional, depending on what we’re wanting to do. If I’m, again, going to talk about connecting two buildings together, I’m going to probably be using a high directional antenna so that they are able to basically focus that beam and connect the two buildings together with our network. But a part of the site survey is really for you to look at what is the environment? If it’s just these two buildings, we could still have problems with trees, and I hate to sound like I’m against mother nature, but I’m not.

But trees do have a tendency over time, and I know this is hard to believe. They have a tendency to grow, and as they grow, that becomes a new problem that you might not have originally planned for. Again, if the buildings are maybe a little further apart and a little more metropolitan area, we still have wireless that’s more than capable of spanning that distance. But if you’re wanting to connect these

two buildings, we have another issue that there may be hills in between. Right. And again, trees. Or, believe it or not, your city might make a new skyscraper. So you have to make sure you have a good line of sight to take care of that. Using a lot of the calculator tools, you’ll be able to determine whether or not you have an issue with the Freynel zones, which is a point of interference, not maybe even in the actual way of the signal, but close to the signal. That can also cause problems in the total coverage. So you want to use a lot of the same tools as you did indoors, but also perhaps use the GPS. So you know what the latitude longitude of the coordinates are. topology, maps. You know about elevation. elevation is a big deal because if one of these buildings was on a little bit higher elevation, the angle at which you’re pointing the antenna may, of course, be different at the two locations, so they still have that line of sight.

13. Outdoor Site Survey Tools Part2

So topographic map is something you might use. And instead of being a building floor plan, it’s really a floor plan, if you would, of the outdoors. And it’s going to tell you about the elevations, the positions of where you’re at, again, about whether or not there’s other higher hills that are in your way. Believe it or not, a pair of binoculars might be a great tool for you for an outside site survey to get to where the antenna is going to be and look to see if you have a direct line of sight to where the other antenna is going to be. So don’t think about, or I should say don’t look at just little computer based tools, but look at the entire thing. So one of the things you want to look at, of course though is link analysis. That means you might use software to create this point to point link. And that software can be used with your topographic map to help generate what would be your bridge link profile, meaning where the bridge should be, how tall it should be in elevation, which way it should be pointed, and it can do other calculations like I said, such as the furnacel zone.

And from that, rather than you having to manually do the math, it should be able to start helping you build what your total power is going to be, your eirp. So anyway, same type of software, just a little bit different type of antenna, obviously. Now, don’t kill yourself with the math. Plenty of calculators out there. I call them software calculators because they run on your computer and they can help you building spreadsheets, give you all the calculations you need for your link budget. And again, other types of interference help you calculate what the free space path loss would be, maybe even helping you with that information if we think about inclement weather like rain or snow or whatever is common to your area. And so all of this is going to help you, even can help you with cable attenuation because if you think about it, the transmitter is probably somewhere else in the building and the amount of copper cable it takes to get to the antenna could lead to more loss. So all of these are great things to use in your calculations to make the best outdoor, in this case, as an example, bridge site to site type of setup.

14. Coverage Part1

Now you could do a manual coverage analysis which again would be for you to try to find cell boundaries. There’s two types of major manual coverage analysis you could do. One is the passive where you’re just basically using your radio to collect the rf measurements, which would be the received signal strength, the noise level, the signal to noise ratio, and keep manually moving from one building to the other until you get to the other point to map out all of the information that you’re finding. And you don’t really need an extra client adapter, you just need to have a radio that can pick that information up and be able to tell you what’s existing at Layer One and Layer Two.

An active type of coverage analysis. Again, actively doing this in a manual way is that we try to set up the layer two connectivity so that we have low level frame transmissions or if we have layer three connectivity, then we can start using layer three things like ping to be able to see if the connection actually works. In other words, from one building to the other. If you’ve already done the Layer One, you’ve looked at the signal, the passive part of it, and now these two are talking to each other, hey, let’s test the connectivity and make sure it happens.

And during this time you can still record the Layer One measurements like you were doing with the passive type where you were just listening. Being active means you’re doing a little bit more to send data back and forth and that can help you, right, with things like packet loss or how many retransmissions are being done. And that’s because you’ll have a card that’s able to look at the associated signal from the access point. And with that you’ll be able to make kind of a good documentation as to what are the causes of some of your other problems in the communications.

15. Coverage Part2

The last method of coverage analysis uses applications that can give you rf simulations and modeling design capabilities. This is going to be something we call a predictive coverage analysis, and it’s used based on the information you provide, kind of like the site surveys that you could do with wireless land controllers. You could say, this is the type of access point I have.

This is the type of antenna I have. I can put it in the building and it will theoretically tell you what your coverage is going to be. And so you could still use software to help you in getting that, bypassing the need for you to actually capture the rf measurements. But even if you did that and you followed that plan to the exact letter, I would still go out there and do those same site surveys or those measurements to make sure that you’re getting what the theoretical program told you you were going to have.

16. Confirmation

So what I just described was a confirmation. That means after you’ve done the installation audit or validate the installation, it lets you verify the rf coverage and the data rates that you think that you had when you installed the network. And you can compare those actual values with what you thought you would get from your network design plans.

And hopefully the numbers will either be or be better than what your expectations were. If they’re not, maybe it might be just the simple adjustment of the amount of power that you use with each of the transmitters or maybe a different thought about the type of antenna. But usually in today’s world, we see that the coverage is pretty close to what we expected.

17. Module 16 Review

So in this module, we talked about what a site survey is, looked at some of the tools and talked about how we can use those for coverage and capacity analysis.