For a wireless network to operate, there are many modulation types and frequencies that are used. In this video, you’ll learn about the operation of wireless standards over 2.4 GHz and 5 GHz frequencies.
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If you’re connecting a wireless device for the first time, simply choosing a random frequency and plugging into the network is probably not going to be your best course of action. You may find that you’re conflicting with other devices and other frequencies that are in use, and therefore the performance of your wireless network is going to suffer. The first thing you should do is to perform a site survey. You need to understand exactly what frequencies are in use and how you could best configure your access point to work optimally in that environment.
There are many different tools you can use to help find all of these different access points. It might be a free piece of software, or something commercial that you can purchase. Or you’re getting a full blown spectrum analyzer and really trying to understand exactly what frequencies are in use at that location. Once you have that information, you can then start determining what particular frequencies are open or available or which ones are not being used as much as other frequencies. This way you can plan where you want to put an access point and then decide what frequencies should be used by that particular access point.
Of course, not everybody’s going to follow this formal process when they start setting up their wireless network. And if there are other organizations in the same general area as you, you may find that they are now conflicting with your network configuration. So you may want to plan to do ongoing site surveys. Come back every month or every three months or every six months, and make sure that the wireless environment is running as optimally as possible. One thing you can do is to perform something like this, a heat map.
This is a piece of software I downloaded that allowed me to walk around a particular area. You could even put the diagram of the area as the background. And as you move, it knows exactly where you are in relation to the access point. And it can start to show you in color view exactly where the signal is very good and where the signal is very bad. And you could walk inside of your building, outside of your building, and get a better understanding of exactly where the frequencies are in use for your wireless network.
There are so many different wireless frequencies that we use for our wireless network. It is a subset of the 2.4 gigahertz and 5 gigahertz frequency ranges. And they are very standardized for all of the different wireless types. The IEEE standards will dictate what frequencies are in use, and governments also control exactly how much of those particular frequencies are available in a particular country. We are going to look at 802.11 a, 802.11 b, g, n, and ac. There are a lot of different standards in between that, but those are the main wireless standards that you will run into. And you need to understand what frequencies each one of these particular standards use.
You’ll need to understand, therefore, that each one of these standards uses different speeds. There’s going to be different distances that are supported because of the frequencies and the type of modulation in use. They’re using different channels, which is how we block out the different frequencies in all these different standards. And the frequencies, themselves, are controlled by governmental agencies, which may limit exactly which frequencies you can use in a particular country.
Let’s look at how 802.11 b uses the 2.4 gigahertz range. 802.11 b uses a modulation type over this wireless network of direct sequence spread spectrum or DSSS. This is how it sends the wireless signal from one device to another over this 802.11 b network. It uses this mechanism called chipping. It takes a certain chunk of data, chips it, and sends it out over a number of different frequencies. It’s a very pre-defined order. As this information is being sent, it’s expected to be received in a very pre-defined order on the other side. That is the spread spectrum part of the DSSS.
There are generally 14 channels that you can use. These channels correlate to very specific 2.4 gigahertz frequencies. For example, channel one is 2412, megahertz, channel 11 is 2462 megahertz. So there’s a gap between these different channels and they overlap themselves quite a bit. Each channel is about 22 megahertz wide, and they’re spaced at 5 megahertz intervals. So by simply looking at that, you can see there is extensive overlap for each one of these frequency ranges. That’s why we have certain channels that do not overlap with 2.4 gigahertz.
On 802.11 b, channel one, channel six, and channel 11, for instance, do not overlap with each other, at least not here in the United States. In the US, we can’t use any frequencies above that channel 11, but it can go all the way up to channel 14, which may be in use in your particular country. So you need to look at the different frequencies that are being used, and what you’ll generally find is if it is an 802.11 b network, it is running on channel one, channel six, or channel 11.
The 2.4 gigahertz range is also used by a 802.11 g and 802.11 n. It uses a different modulation type to send the information though. It uses orthogonal frequency division multiplexing, or OFDM. This is the same modulation type we use for 802.11 a. It’s using the same frequencies as we were just looking at in the 802.11 b. It’s just using that different modulation type of OFDM, instead of using DSSS. So if we start looking at the overlap of channels, it’s exactly the same for 802.11 g and 802.11 in. We have channels one channel six, and channel 11. If it needs to use slower speeds, it effectively reverts back to the 802.11 b mechanism. So you may see also modulations DSSS if you’re using these slower speeds on 802.11 g or 802.11 n.
In a later video, we’ll look at the details of speeds between all of these different wireless standards, but one important thing to know is that 802.11 n provides for much higher speeds than 802.11 g, and it does that in part by using much more of the frequency. You can see that 802.11 n uses a 40 megahertz wide channel. You can see it’s about twice as wide as what we saw with 802.11 b or 802.11 g. And you can really only have one particular frequency, so that you’re not overlapping with any others in this particular frequency range. So channel three is generally what you’ll see for 802.11 n, but of course you can adjust things depending on what other frequencies may be in use in your particular area.
Here’s a better view of all these non-overlapping channels. You can see with 802.11 b is our channel one, our channel six, channel 11, and then the channel 14 at the very top. These channels are 22 megahertz wide. For 802.11 g and n, the channels are 20 megahertz wide, but still we have that non-overlapping channel at channel one, channel six, and channel 11. And as I mentioned 802.11 n has twice the size of the channel width at 40 megahertz, so you can have the higher speeds for those 802.11 n networks.
Another section or frequencies available for our wireless networks are the five gigahertz frequencies. We see these being used for 802.11 a. 802.11 n, and 802.11 ac. Five gigahertz frequencies are not used for 802.11 b or g. For 802.11 a, were using dynamic frequency selection to determine what frequencies to use. There are a number of military uses for these five megahertz frequencies. So by using DFS, were able to avoid conflicting with radar that’s being used for weather or military satellite use. This is using that same frequency modulation we were looking at before of OFDM. It allows us to send multiple data streams even though we’re using a fixed type of bandwidth. There are 23 non-overlapping channels available in this five gigahertz range. And of course, the availability of the frequencies will differ depending on what country you’re in.
To get some of these higher speeds in 802.11 n, we take advantage of something called MIMO. This stands for multiple input and multiple output. There’s a single receiver in 802.11 n, but you can have more than one antenna, So you can send and receive on different channels at the same time. You can support up to four transmit and four received. So there’s four separate data streams that can all be communicating at the same time with 802.11 n. 802.11 ac takes this another step farther with multi-user MIMO, or MUMIMO.
With multi-user MIMO in 802.11 ac, you can have up to four separate receivers inside of an access point with multiple antennas on each of those. And with that particular architecture, you’re able to get multi gigabit speeds out of these wireless networks on 802.11 ac. There’s a large emphasis for backwards compatibility when you start using these wireless networks. That way you could have an older 802.11 b device, but still have it communicate even though you have an 802.11 n access point. It uses a mechanism called high throughput mode to determine how it’s going to work and how backwards compatible it’s going to be. For 802.11 n, this high throughput mode is also called a greenfield mode, because it doesn’t support any of the legacy devices. You have to have everything running at 802.11 n. This is also going to give you the highest throughput for the 802.11 n networks, because it doesn’t have to be the older backwards compatible and slower network communication used in the older standards.
If you do have legacy devices, you might want to consider running your 802.11 n access point in legacy mode. This is a non-high throughput mode. It uses those 20 megahertz channels, so you’re not getting the larger 40 megahertz channels. So you’re not going to get the 802.11 n speeds, but if you have older equipment that can’t run at those higher speeds, this may be a good way to configure your access point, at least until you get everyone upgraded. You might also have an option inside of your access point for a high throughput mixed mode. And in mixed mode you’re able to run 802.11 n and 802.11 b and g at the same time. You’re n devices are not going to run at the full capabilities of n. And your b and g devices obviously won’t run at n speeds either, but this does allow you to have all of those devices running on your network at the same time.
With goodput, you are defining how fast the application can transfer the information. You’re not worried about the network mechanisms or how fast the network is running. You’re only interested in knowing the maximum throughput for your application. And when you look at throughput from an application level, we always refer to that as goodput.
Category: CompTIA Network+ N10-006