CWNP CWNA – Spread Spectrum Technologies Part 2

7. U-NII Review

So when we talked about uni, as I said, was kind of a grouping of channels in a different frequency range that we could use for different purposes, whether it was data or TV. And so you see that we have uni one which is really at the start of the 5. 0 GHz that took up the first four channels. Most often we use that for data communications, but like I said, we could send other transmissions over these unlicensed bands. The uni two of course, was the next set of channels, another four channels, we might have used that for data, maybe we could have used it for video or something else.

And they were just designed to be grouped in a manner that we could try to associate them with different types of transmissions that we wanted to send. So the uni two extended basically used all twelve channels in that range and the uni three was designed with five more channels in mind. And when it comes right down to it, the uni two is the one that we really are going to use for WiFi. And then we have the different types of transmissions that might go through the other uni groups of channels. Now again, I know that many of you in the United States who are using 5. 0 GHz are used to only having eleven channels, but there are actually twelve channels. Some countries, as I mentioned, like Japan, actually could have the 12th channel being used.

8. 60 GHz

So in the realm of 60 GHz, something we haven’t even talked about, it’s a part of what we would call the VHT or very high throughput technology that is going to operate again in another unlicensed band. Now it’s going to have a little bit different physical and mac layer types of enhancements, which we haven’t talked about. Those enhancements and how they work in, you know, connecting to a wired network, but that is kind coming up. But anyway, they have the potential of getting your future speeds for WiFi up to seven gigabits per second. I mean, for WiFi, to me that sounds tremendous.

I realize in Ethernet we’re at one gigabit, ten gigabit, 41 hundred gigabit. So I mean, we have really great speeds. But for wireless, I think that’s really good. Now we’re going to call those ultra high frequency. The problem though, remember, is as frequencies get higher, they tend to have problems getting through objects like walls. So more than likely this will be technology that’s going to be used for short distance communications, usually indoor communications, but be able to provide that high bandwidth that we need.

9. Narrowband and Spread Spectrum Part1

Now, the idea behind narrowband and spread spectrum, so remember that spread spectrum when I talked about it before, was having a channel that was wider than what was needed for the bandwidth, whereas narrowband is pretty much the channel that is just enough for the bandwidth that you want. But these are two of the primary types of radio frequency transmission that we see. Now. Narrowband, like I said, uses very little bandwidth to transmit the data that is carrying.

So if you think about narrow band, and if you go back to the example I made a while ago about Am or FM channels, when you are turning the dial, for those of you just don’t push a button. But if you go back to the old days when you were trying to tune in to a narrow band, it was narrow in that it was specific to one frequency. But the FCC made sure there was a little bit of room on each side of that frequency so as to not interfere with another channel.

So it was very narrow. But then again, sending music and that type of information didn’t take as much bandwidth as we see in today’s data networks. So again, there we might see like a 2 MHz channel frequency instead of a 20 or a 40 MHz wide channel. So it really is very narrow. Whereas in the spread spectrum we have more bandwidth than we need to be able to carry the data and that’s why those channels are wider and many of the channels actually take two or three of those 20 MHz channels to make just one channel.

So again, we can try to prevent the bleed over, if you would, from one frequency to the other. It basically is able to take the data that has to be transmitted and it will spread it across those frequencies that it’s using. So again, more than we need for the data. So an example there of course would be those eight or 211 A’s or BS or GS that we call the spread spectrum. And like I said, most of them are between 20, maybe you can round it around to 22 MHz frequencies. But the idea was that there was more space than we needed for the actual communication.

10. Narrowband and Spread Spectrum Part2

Now, there is a difference between the definition of narrow band and spread spectrum. Remember that in Spread Spectrum, we had more bandwidth than was needed for the actual transmission. And narrowband is usually a part of a licensed type of frequency. So typically the FCC or some other regulatory body would ask or require that a narrow band transmitter be licensed to minimize the risk of the two or more narrow band transmitters interfering. And that would be more of an example of your Am or FM radios. In other words, it doesn’t mean that I can’t send the data, but I don’t need a wide spectrum to send the ones and zeros that represent the music.

So this could be like channel 101 three on your FM dial and then we would have just enough room to be able to make the next one for 101 five. And you probably have noticed this when you’re tuning radio stations, that they tend to skip numbers, but not a lot of numbers, unless there’s just nobody using a license, a bowl frequency within that range. But they put enough distance between them that the two channels don’t bleed over on each other, but is using just enough bandwidth to be able to relay the actual music or the sounds that you hear when you’re playing the radio. Whereas on spread Spectrum, we’re using a lot of smaller channels to represent one channel to be able to have more bandwidth than we need for the data that’s going to be broadcast.

11. Multipath Interference

Multi Path, we’re going to find out later, is going to be actually good for the high throughput and the very high throughput. But remember, we usually have things like walls or desks where the actual signal would bounce off of them. So I might have a direct path to send my signal, but because I’m sending it, usually omnidirectional that signal is also going to bounce off a wall. Bounce off a wall and they’re going to get there at a little bit later time.

Yes, we are operating at the speed of light and if all things are consistent with the speed, this bouncing off a wall is a longer path, so it’s going to take more time to get there. Sure, you can measure that time in nanoseconds, that’s true. But one of the things that we used with Multi Path is by having multiple antennas that can hear those signals, the access point will just pick the one with the strongest signal and ignore all of the rest of them.

12. Frequency Hopping Spread Spectrum

So with frequency hopping spread spectrum, the way it would generally work is it would pick a channel to be able to send data, transmit the data using a very small size channel, a small frequency carrier space, and then on schedule, hop to the next channel and then to the next channel. And then using that same hopping schedule, go back to the next channel to continue to transmit data. And the receiver would know what that sequence of hops were going to be so that it too, could go and continue to change to those different channels. And each one, though, like I said, is a small frequency carrier space. And then there’s also the amount of time it takes to hop from one channel to the next.

Now, one of the ways I look at that, I fly a lot. I hope that looks like an airplane. And some of the airplanes let us actually listen to the control towers. So as you’re flying across the country, I often hear them saying it’s time to go to a different airport’s operation to get more directions about their flight. So maybe they started in San Francisco and next thing I know, they’re telling them to go to Denver, and the next thing I hear, they’re saying, go to Chicago. And so that just tells me that they’re doing something very similar, not at the same speed, but something very similar with the idea of the frequency hop spread spectrum to basically get my flight from one city to the other.