There are many variables to consider when troubleshooting wireless networks. In this video, you’ll learn about RSSI, EIRP, antenna types, antenna polarization, site surveys, and more.
One of the challenges with wireless networks is that the performance can vary quite a bit depending on where you happen to be, where the access point is, and a number of other variables in between. One way to measure this performance is through the use of throughput. We can send data through the wireless network and see just how much information we’re able to transfer at any particular time. We might also look at the speed of our connection to the access point.
As we get closer to the access point and the antennas, we tend to have a faster connection to that access point, and you may find that some users are so far away from the antennas that it becomes very difficult for the receiver on their device to be able to hear the signal being sent by the access point. There’s a standard for measuring the signal that you’ve received on a device, it’s referred to as the RSSI, or the received signal strength indication.
You should be able to look at the RSSI on every device and determine what devices are able to receive the signal better than others. The RSSI is measured in decibels per kilowatt, or dBm. You can see that the number of decimals is referencing a single kilowatt, or mW. For example, this is the Wi-Fi configuration on my device, and it’s currently showing an RSSI of negative 36 dBm.
The RSSI is shown as a negative number on a logarithmic scale. So the closer you get to 0 is better. Normally, you’ll see a negative value. For example, my negative 36, which certainly fits into the negative 50 and higher would be an excellent value. Negative 70 dBm is good, and anything that’s negative 80 dBm and smaller would be considered a low signal.
You can commonly collect the RSSI values using a wireless survey tool, like the one I have here. So you can see exactly what the signal strengths are and the noise values for each individual access point. This could also point out cases of potential interference, especially if the noise values are very high. This is sometimes built into the operating system or into the wireless access point that you’re using, but you can also get third party products to provide this information as well.
But if those third party tools aren’t providing you with enough information, you may need to use a spectrum analyzer to see exactly what’s happening on those frequencies. Here’s a measurement of RSSI and noise values on this network. You can see the RSSI on my connection is around negative 40, which is a pretty good wireless connection.
Down here on the bottom is the noise on this network, which is down to negative 90, which means, overall, the performance statistics for this wireless connection are quite good. You can compare this to a different wireless connection, where this user has an RSSI of negative 85 or so with some minor peaks up to negative 50 during this very long time frame. You can see this is very close to the negative 90, where the noise floor happens to be, which means that this user is probably not having a very good experience on the wireless network.
Not only do we have to be concerned about the signal that’s being received, but we also have to be concerned about the signal that we’re transmitting. We measure that transmitted signal as an EIRP, or the effective isotropic radiated power. This is the signal strength that we are sending from our transmitting devices, and it’s calculated by taking the transmit strength, the antenna gain, and subtracting the cable loss.
In the United States we have the Federal Communications Commission, or the FCC, that regulates just how much signal you’re able to send on these access points. If you’re on a 2.4 GHz access point, for example, the maximum EIRP is 36 dBm, which is about 4 watts of signal. This can vary based on the bands that you’re using and the frequencies that are in use. As the owner of the access point, you are responsible for making sure that the EIRP does not exceed these VALUES and often on the access point, you have control over how much signal you’re sending from that access point.
Here’s a close up view that shows a transmit power percentage, which can be set to automatic, but you can see on this access point, you can set it to 100%, 75%, 50%. 25%, and 12%. When you purchase an access point, it often comes with these rubber duck antennas, we call them. Those are very common antenna types, and they are considered to be omnidirectional antennas.
That’s because the signal is relatively evenly distributed on all sides of this. The antennas are in the middle, and then anybody who is nearby that device on any side of that device has effectively the same signal reception. This is a pretty good choice for most environments, where you can put the access point in the middle of a floor or the middle of an area, and everyone around that access point would then gain access.
However, if the access point was in the corner of a building, and omnidirectional antenna is probably not the most efficient because you have no way to focus that signal into the parts of the building where it would be used. If you want to change the way that the signal is being sent and received, we can focus that signal by using a different type of antenna. These are directional antennas and allows us, not only to focus where the signal is going, but in some cases can increase the distance that we can send or receive that signal.
For example, if you want to extend a wireless signal between buildings, you may want to use directional antennas, and if you look at the specifications of the antenna, they may even be able to tell you that there’s an increase in gain by using this type of antenna. For example, if you increase the gain by 3 dB, you’re effectively doubling the power of that transmission. If you want to increase the signal strength between sites, you might want to use a yagi antenna, which is very directional and adds quite a bit of gain.
You might also want to consider a parabolic antenna, where all of the signal is focused to a single feed horn that is in the front of the antenna. These are often used to bridge the gap between a longer distance, where normal omnidirectional antenna wouldn’t apply. For example, extending a signal over a large area or between buildings. You simply place one of these antennas at both ends, and now you have a high gain and very directional connection for your 802.11 network.
Another important configuration for your directional antennas is the polarization of the antenna. The polarization is the orientation of the antenna as it’s relative to the earth. So you can have horizontally polarized antennas that are flat to the Earth or vertically polarized. If you’re setting up two antennas, you want to be sure that both the transmitter and the receiver have exactly the same polarization.
If you were sending traffic between these two antennas, only a portion of that signal would be received because one is set up as horizontal and the other one is set up as vertical. One measurement of the efficiency of your access points is the access point association time with the devices that are trying to connect to that wireless network. Usually, a device associates with an access point and then as you move from one place to another, it may change its association to be with a different access point.
This association process is normally very quick, but there could be times when a delay may occur in the association. For example, if you’re trying to associate with an access point that’s very far away, you may have a problem because of a low signal or a block signal. You might also want to look at the connection between the access point and the wireless controller.
There’s often a number of commands sent to the access point when devices begin associating from one access point to the other, and if there is a latency problem or some type of bug in the firmware of the controller, then you may have a problem with association times. You can always gather more metrics on association time directly from your wireless controller, or you may be able to query the controller or the access points using SNMP, or the Simple Network Management Protocol.
In very dense environments, you may find that there is a problem with the number of people using the access point and delays associated with this large number of users. There’s only so much frequency available on a wireless access point and when you reach a large number of users, you may be using all of that frequency. We often see problems like this occur in very large gatherings like a sports event or trade show.
Everyone who is at that location would like to be able to use this wireless network, and often, they’re trying to use the network all at the same time. You can often get metrics directly from the access point that would show you the available air time, and this usually shows as a percentage. Once you hit 100%, you’re using all of the frequency available to be able to send and receive information from that access point.
Fortunately, there are a number of settings you can configure to help make this conversation more efficient. If you have the access point set for a legacy mode or for low speed support, you may want to disable that function so that the access point can run at its fastest possible speeds. Another good physical layer check is to make sure that there’s no other devices around that could be interfering with the frequencies that you’re using, and if there is interference, you may want to consider changing your access points to work on different frequencies.
If you have multiple access points, you may want to try changing some of the output power settings, especially if you’re finding interference between access points and if you find that a single access point is being completely overloaded and is at 100% of frequency use, then you may want to use a separate access point with different frequencies to be able to split that load. This is where it might be very useful to have a site survey. This is where you can walk around a particular area and see exactly what type of wireless signals you’re seeing in different parts of the building.
This allows you to identify all of the access points that might be in your particular area, even if those access points are used in a different building from a different company. This will allow you to see what frequencies are in use and allow you to make changes to your environment so that there’s no interference from third parties. Of course, you can’t predict what might change with the wireless network or wireless networks that may be outside of your control, so you may want to consider having ongoing site surveys to make sure that nothing has changed with the existing environment or perhaps you need to make changes to what’s currently running.
You might also consider getting third party software that can create heat maps. You would simply walk around your building with a laptop, and it will create a map of where you’ve been and what type of signal strength you’re seeing in different parts of the building.