 The previous topic on data transmission, we spent several lectures covering many different new things for you, mainly about signals. How do we represent signals in a mathematical way? And then trying to relate the physical characteristic of signals, frequency, bandwidth, spectrum with data rate. As well as other things like accuracy and errors, and we come up with some trends. One of the things, one of the trends we said was that increasing the bandwidth will give us more data rate. Impacts on other things as well, but that's one thing that's important. The higher the bandwidth, the higher the data rate. But often we're limited with bandwidth. We cannot just use any bandwidth we choose. And one of the things that commonly limits the bandwidth we have available is the transmission media. The thing that we have between transmitter and receiver, that commonly limits the bandwidth. And we'll see some examples by going through some common forms of transmission media. And we'll see some examples of their bandwidth and see some examples of data rate that we can achieve. So remember, transmission media is the thing between the transmitter and receiver. What types of transmission media do we have? From one of our first lectures. The two types of transmission media. You may see it on the next slide. One of them, guided and unguided. Guided being cabling, wires, unguided being wireless. So we'll look at them separately. First we'll talk about a few examples of guided media and then we'll spend some time on what wireless transmission is and some examples of wireless media or unguided. But first some general issues of any transmission media and some factors that impact upon their design and which ones are appropriate to use. In simple terms we often care about choosing a media that gives us the data rate that we want and covers the distance that we want. You have a computer in one room in your house and you want to communicate to some other computer in another room in your house. So you need to choose the media to connect them. And generally you have wider wireless and in practice you may have a wide choose a LAN, Ethernet LAN, some cabling or wireless, maybe Wi-Fi, wireless LAN. Use an access point light on the wall. And the exact technologies you choose you'll try and deal with, okay you want a high data rate, high enough to support your applications and the distance needs to be large enough to suit your environment, your scenario. So that's two factors that we're interested in. A number of things impact upon what distance and what data rate we can achieve. Bandwidth is one thing. Higher the bandwidth, higher the data rate. We've also seen transmission impairments. Attenuation impacts upon distance. We said attenuation is we transmit a signal at some strength, it attenuates over distance, it gets weaker. So the rate at which it attenuates will determine how far we can transmit from transmitter to receiver. And we'll see some examples in this topic and especially on wireless media we'll see some ways to calculate. If we transmit at some power, how far can we transmit such that the receiver can receive? So impairments impact upon the distance. Interference. If we transmit some data and it turns out others are transmitting at the same time in the nearby vicinity then from the receiver's perspective they will see receive the intended transmission but they'll also receive the other transmissions. And from the receiver's perspective those others will be like noise. And the more interference from others the worse or the harder it is to receive the data correctly. So interference from others will impact upon our data rate and also distance. Number of receivers in some cases impacts upon how far we can send. We will not explain that yet, we may see it when we talk about land technologies. So different transmission media will allow us different bandwidth, will have different levels of impairments, different amount of interference and they will impact upon what data rate and distance we can achieve. Before we look at some examples, one other thing that we may haven't said. Remember our frequency domain plots. Let's see if I can draw one very quickly. You don't have to, you've got many pictures of these. Just remind you of the frequency domain plots where we drew what an impulse and for the different components we drew different modal impulses. Where the height was the peak amplitude and here's the frequency. F1, F2 and so on. That was out, a plot of a signal in the frequency domain. We only did very simple signals with a few components. In practice the signals may contain many components, in theory an infinite number of components. You often in practice do not see plots like this of the frequency of the signal but instead something more like this. Which says the signal is contained in this area. Meaning if this is a frequency domain plot of a signal it means the signal contains frequencies from about this level up until here. It contains many components, say from F1 up until F2. Frequency 1 to F2. So think of this as many impulses close together. But rather than drawing them you'll see this continuous plot in the frequency domain. What it says is that the signal, the majority of the power of the signal, remember the vertical axis represents the signal strength, the peak amplitude, the majority of the power of the signal is contained between frequency F1 and F2. So the bandwidth is the width between F1 and F2. In the same way in our plots we said the bandwidth was the width between the minimum and maximum frequency component. If we had say four components in that case. In general we say the bandwidth B would be there. So in practice with real signals we'll see plots similar to this. Remember a signal that we transmit doesn't contain a single frequency. It contains a range of frequencies. And this plot tries to capture it contains frequencies from F1 up to F2. A bandwidth of B. Sometimes we'll talk about the middle frequency in that range or the centre frequency, FC. We say a particular signal has some centre frequency. It's centred about this point and a bandwidth of some B hertz. What if someone else transmits a signal in the same spectrum as the original signal? If there was a second transmission, I'm not so good for you to draw this, if a second transmission, the green one, transmitted in the same vicinity then those two transmissions would both be received by the receiver. And this would correspond to interference. So what I'm trying to illustrate is that if someone transmits the blue signal to some point and someone else transmits the green signal, the receiver, if they receive both of them because they're overlapping in the frequencies from the receiver's perspective, that green one will be interference and it may be unsuccessful in receiving the data. And you know that in your everyday life. If two people talk at the same time, it's hard for the receiver to receive the data. This is the same concept. If two people transmit using the same range of frequencies at the same time, it's hard for the receiver to receive the data. If two people transmit in a different location or different frequencies, then we may be successful. My scale's running out. If there was a second transmission by someone else, but using a different range of frequencies, not overlapping with the blue one, then that would be successful transmission. Because even though two people transmit at the same time, the receiver, the receiver of the blue signal would be tuned to only receive signals in this frequency range. They would not receive the signals, the purple signal in this case. And it's the same thing you do with your TV. With TV, the TV stations all transmit TV signals to your TV. But when you change the channel, you tune your receiver to receive only a certain range of frequencies. So think of the blue one as one TV channel. The signal's being transmitted. And the purple one is a second TV channel, the signal's being transmitted. From the receiver's perspective, the receiver tunes in to just a range of the frequencies. So if we transmit on a different range, we can have multiple transmissions at the same time. If we transmit in the same frequency range, we'll have interference, which is a bad thing. We will see more about how to do this, maybe after the midterm. We'll look at multiple access and how to split things up, multiple transmissions. My point is that, well, we only have a range of frequencies available for any type of signal transmissions. We need to divide it up so that people don't interfere with each other. So that when I transmit using my wireless radio, that it doesn't interfere with other transmissions when you're using your mobile phone and when I'm using Wi-Fi. So what happens in practice is that governments, organizations come up with regulations that say these particular applications can use this range of frequencies and other applications must use other range of frequencies to avoid interference. And that gets to this slide. This is a picture, a simplified view of the main range of frequencies we have available to transmit in communication systems. Let's explain what it shows first and then look at some of the data inside. It shows a spectrum. The top axis shows frequency. It goes from 1 hertz on the left. 10 to the power of 2 is 100 hertz. All the way up to 10 to the power of 15 hertz. So a kilohertz, megahertz. 10 to the power of 9 is gigahertz. 10 to the power of 12 is terahertz. And I cannot remember the next one up. Better hertz, maybe. So this is the main range of frequencies that we have available for communication systems. Down the bottom is wavelength. What's the equation for wavelength? I don't think we mentioned it in our earlier lectures, but it was in one of the slides. Anyone remember? How to calculate wavelength? Wavelength is the speed of light divided by the frequency. C, the speed of light divided by the frequency, gives us the wavelength. The wavelength is, if you think of our sine wave, the distance that a single cycle travels. The length of that wave, that cycle. That's the bottom axis. So you can see that inversely proportional. Increasing the frequency decreases the wavelength down to 10 to the power of minus 6 meters. We'll look at frequency for now. What do we see in the middle? We see some example applications or systems that transmit communication signals and what range of frequencies that they use. So some that you know about. FM radio. What channel? FM radio. What's your favorite channel? Anyone? 97.5 watt. FM and 97.5 watt. 97.5 megahertz. That refers to generally the center frequency of the signal that's transmitted for that particular channel. So 97.5 megahertz. Think of, okay, this value is 97.5 megahertz. That channel that carries audio has some particular bandwidth. And I think FM radio is around, I always guess, maybe 20 megahertz. 20 megahertz? No, I think 20 kilohertz. I think the bandwidth is about 20 kilohertz with FM radio, meaning 97.5 megahertz is the center frequency, plus or minus about 10 kilohertz either side. That represents the signal that transmits the audio for that radio channel. Of course, there are other radio channels which use different center frequencies spaced apart so that they don't interfere with each other. So that all the radio stations can transmit at once and you tune your receiver to listen into just one of them without the others interfering. So 97.5 megahertz is about 100 megahertz. So FM radio, 10 to the power of 8 is 100 million or 100 megahertz. So this is just saying FM radio across the world usually uses frequencies in this range about 100 megahertz. Note this is a logarithmic scale. AM radio, anyone, favorite channel? But the frequencies in AM radio I think around, what, 900, 1,000 kilohertz. I think 9, ranging from about the high 800s to the high 900s kilohertz. They're about, what, 1 megahertz. So 10 to the power of 6 AM radio is in this range. TV, terrestrial TV is from when there's TV stations with transmitters on the ground. Terrestrial means on the ground, transmitting. Or satellite TV when the satellite in space transmits down to you. We typically use transmissions in this rather wide range of usually several gigahertz to 10, 15 gigahertz at this range here. Some other things you may recognize. Infrared. Infrared remote controls can be used for communications. Not quite, but infrared can be used. Your remote control for your TV uses frequencies in this range. Visible light is up here. The light that we can see is in the frequency in the spectrum in this range. Infrared means what? Below or lower. Infrared is below the red light frequency. So visible light ranges from red light through to, I think, violet. The colors. Infrared below the red light, ultraviolet is above the violet color. Not shown on this plot. Used for different applications, infrared. Power. So getting electricity to your home, for example. And telephone, home telephone systems use frequencies in this range down in the order of several thousand hertz usually. So here's 10 to the power of 3 is a kilohertz. Radio. So this is the broad range here that covers AM radio, FM radio and TV. Mobile phones. What frequency does your mobile phone use? 2100, 2100 watt. 2100 megahertz is a common frequency. 2100, there's other ones that have been around in the past. So I think AIS had, there was 850 or 800 or 900 megahertz. But now common for three years, 2100 megahertz. There are some others in different countries. So mobile phones in the order of, what, around 2 gigahertz. One to 2 gigahertz. 2000 megahertz is 2 gigahertz. So that's some wireless systems. What else have we got here? Optical fiber. You may not have seen it, but I'm sure you've heard of it to connect between cities across countries. So there are optical fibers where a light signal is sent through some thin glass or plastic fibers. And you think the light bounces back and forth along the fiber until it reached the endpoint. So we use visible light to transmit in optical fiber. And the frequency range of frequencies are the same as visible light. What else do we have? Twisted pair, which we'll talk more about today. Land cables. This is referred to, and we'll talk why, twisted pair. So it's just some copper wiring. We transmit a signal along that copper wire, an electrical signal. The frequency range that we transmit from hertz up to usually about 100 megahertz. 10 to the power of 8 is 100 megahertz. Slightly larger in some cases. 500 megahertz, I think. So twisted pair has a bandwidth of around 100 megahertz. The bandwidth is the range of frequencies we can use. What's bigger? The bandwidth of twisted pair, land cables, or optical fiber. Which one's bigger? Hands up. Twisted pair has a larger bandwidth. Look at the diagram. Here's twisted pair. Think of a land cable. Here's optical fiber. Which one's bigger? In terms of the bandwidth. The width. Twisted. So just be careful with this diagram. Even though the line is bigger, it's a logarithmic scale. Twisted pair goes up from zero up to 100 megahertz. A bandwidth of 100 megahertz. Optical fiber, what is this? 10 to the power of 15 hertz minus 10 to the power of 14 hertz. You'll find that's much, much more than 10 to the power of 8 hertz. It's about 10 to the power of 15 hertz. Which is about a million times larger than twisted pair. So just be careful with this diagram. Because it's a logarithmic scale, be careful that optical fiber still has a very large bandwidth. As a result, we can send data much faster than in twisted pair. So this diagram shows that the range of frequencies commonly used for communication systems. And some example communication systems. There are others, many others. What else do we see? What are these letters at the top in these boxes mean? Anyone recognize them? ELF, VF, VLF, LF and so on. Anyone recognize any of those acronyms? Or have seen them before? Even if you don't know what they mean. ELF, VF, VLF, LF, MF, HF, VHF, UHF. You may have seen those two before, related to old TVs. Usually had a VHF and a UHF setting for the channels. F means frequency. H means, so let's say, HF is high frequency. You don't have to remember these. Let's say them while we're here. HF is high frequency. There were some names given by some standard organization. High frequency, VHF is very high frequency. UHF, anyone want to guess? HF is high frequency, VHF is very high frequency. UHF is ultra high frequency. SHF, super, E, extremely high frequency. Not very smart names, but they work well. And going down, medium frequency, MF, low frequency, very low frequency, extremely low frequency, what's VF? That doesn't follow the pattern. It's voice frequency. The range of frequencies common that the human voice uses. Again, you don't have to remember them, but I think you may have seen some of them in practice before. There are others as well, I think. Think of your infrared remote control at home. You're watching TV. You want to turn the channel. If the TV is in the other room, can you control it? Why not? Because the signal that you transmit, that infrared signal with the frequency range up here, what is it? Several terahertz. The signal will not go through the structure or the materials in the wall. Now, think of Wi-Fi, wireless LAN. You transmit from your laptop to an access point. Will it go through a wall or not? Yes, it will. That is, if there was not an access point in this room, but there's one out in the corridor, most likely I can still transmit to it. So one characteristic of using different frequencies, where is Wi-Fi? Wi-Fi frequency range, anyone guess? Wi-Fi is about 2.4 gigahertz. It's around 10 to the power of 9. It's not shown on this one. Different frequencies give us different physical characteristics, and especially passing through obstacles. So therefore, we need to choose frequencies that best suit our applications. If we want to cover a large distance, using infrared is not very good if we're indoors. So we'll see some of those trade-offs, especially for wireless systems through this topic. Now, this is a very broad pitch of the spectrum. Then within the, if we zoom in, we would see that different governments create laws saying who can use particular frequencies. So you know, I think, with mobile phones and 3G in Thailand, there have been some licensing where the government makes available some frequencies for companies to use, and the companies must pay some money to get the license to use those frequencies. And only the companies that have that license are allowed to transmit on those frequencies so that we don't have interference. I don't have a picture of the Thai allocation. It's available, but it's just not so good to look at because it's not in colour. But I've got one of the US allocation of frequencies done by the government. It's quite a big picture, so again, it's best to look in your own time, but we'll have a quick look. We'll zoom in. This is an old one. There's a new one. This is 2003. It's called the frequency allocation chart. So it's a summary of the frequencies once we zoom in, I think, from 3 Hz up to 300 GHz. So 3 Hz, we'll zoom in, and eventually goes through to 300 GHz. And it's a broad allocation of what types of applications can use particular frequencies. And then within, if we go even further, we see particular companies may have licenses. This doesn't show that. Let's zoom in and see some information. I'll go down the bottom first. This bottom picture, you don't have to read it all, but it's showing the spectrum, like we saw in our previous slide, ranging from Hz up to here's infrared, and it goes beyond that to visible light. The picture above is focusing on just this range. So from the high parts of VLF, very low frequency up to EHF, extremely high frequency. So about 3 KHz up to 300 GHz. That's what the above picture covers. They're the common uses of license spectrum. Let's just go to a couple of points in here. So it starts, it's hard to view. It starts at 3 KHz, and then this says that this range of frequencies, up to whatever it is here, 9 KHz is not allocated to anyone. But then the next range is allocated for a particular type of application, radio navigation, and they all have their different names. And we see other allocations. So from 9 KHz to 14 KHz is allocated for some application, 14 to 19.95 for some other mixed applications. Fixed refers to usually fixed wireless systems. Maritime mobile is mobile access out on boats. So boats need to communicate with each other out in the sea. So we're not looking at all of them. We'll go to a couple of frequencies. There's broadcasting, so AM radio here, from 535 up to, this is in the US, up to 16.05 KHz. Let's see if I can find Wi-Fi. Again, you need to look in your own computer to make sense of some of this. Ah, here we go, found it. Wi-Fi typically uses around 2.4 GHz. 2.4 GHz. The name of the range of frequencies used, or the band of frequencies, is sometimes referred as the industrial, scientific and medical range of frequencies, ISM. It's somewhere in here. It doesn't say Wi-Fi, it says what? Amateur. I think, yeah, the range of amateur radio, meaning anyone is allowed to use this, amateurs like you and me. You need a license to use Wi-Fi. You don't buy a license before you use your mobile phone or no other company needs a license to set up an access point, anyone can do that. So it's amateur or unlicensed range of frequencies. What else uses the same frequency range as Wi-Fi? Anyone know? Which is also amateur. Bluetooth typically uses the same range of frequencies, Wi-Fi and Bluetooth. If you have your Bluetooth and Wi-Fi at the same time, they may interfere with each other. What else uses the same frequency in some places? Some handheld phones, not mobile phones, but handheld home phones, wireless phones do, microwave ovens do. If you put your laptop inside your microwave oven and turn it on, it will not be able to communicate because the microwave oven uses the same range of frequencies and they will interfere. So different applications, not just communication applications, different applications use the same range of frequencies. Therefore they need to somehow coexist so they don't interfere. There are many different applications here, space, government applications and so on. So it must be allocated so that people don't interfere with each other. We may see further examples after we go through wireless. Let's quickly look at three examples of guided media. We may even finish it today. Common form for wired or guided media is using electrical cables. Send an electrical signal through across some conducting material. Copper is a common conductor, but there are others. So we'll see that these LAN cables, you'll see in a moment, inside are just some copper wires. So your LAN card in your computer generates an electrical signal of a particular shape to transmit the data and sends it onto the copper wire, goes to the other end point, the receiver at the other end point receives that signal and interprets the data. There are some problems with using electrical signals. As we transmit electricity across some conductor, that radiates energy out of the conductor. So other sources nearby, other wires nearby, may pick up the energy radiated out from your transmission. So if you transmit across one conducting wire and another nearby wire is there, then that nearby wire will receive the transmission from yours. So when we transmit, it radiates energy out and it also can pick up energy from other transmissions. That's a problem because it can cause interference. A transmission on this or the wires here could interfere with other transmissions. Interference is bad because it means that the quality of the signal and eventually data received is not good. We have errors, for example. So in general, how do we keep the minimum, keep the interference small different ways? Keep the cable length short. The shorter they are, the less interference on others and the less chance to pick up transmissions from others. But keeping the cable lengths small is against one of our goals of trying to communicate across a long distance. Let's say I limit. Every LAN cable to avoid interference must be less than 50 centimetres. You cannot have a LAN cable more than 50 centimetres. If that was the condition so that we don't have interference, then we wouldn't be able to use LAN cables to connect in a useful network. We'd have to have all our computers too close. So we want cables to be long so that we can have one computer here, one downstairs, and connect them. But we want them to be short to avoid interference. Another way to avoid interference is to keep the wires or the cables away from other sources. Sometimes it's not possible. If I plug a LAN cable into this computer, there are other sources of electricity nearby, the audio, the power, other LAN cables. It's not easy to keep them separate. So in fact, we don't want to restrict and force someone to make sure that nothing is within a metre of your LAN cable. That would not be very convenient. The third way is to design the cables and arrange the conducting material in such a way that minimises the amount of energy it radiates out and the amount of energy it picks up from other sources. And the way to do that, how do you design cables so that you don't interfere with others and don't pick up interference from others? Is to add some shielding to the cable, add some outer materials, some coatings such that it blocks any interference. It keeps the signal in. And or to arrange the wires so that they don't interfere with each other. And that leads us to maybe the most common wide media you'll see around called Twisted Pair. And we have a few... I got angry before the lecture and cut up my LAN cable, but it gives you a chance to have a look and see what we mean by Twisted Pair. There's a couple more coming around. Just pass around, have a look, have a look at your Twisted Pair, which is just a LAN cable. You see when we cut it up, if you look close at the end, at the cut pieces, inside the outer white coating is in fact some wiring. How many wires can you count them? How many wires inside the outer white coating? There are eight wires. And in fact, those eight wires have some colored coating on them as well. Green, blue, brown and orange or similar colors and some white ones or mixed. Inside those colored coatings is a copper wire. So Twisted Pair refers to a pair of copper wires with some insulation. The colored coating in the ones you see. And if you look closely, you'll see that each pair of copper wires, there are four pairs, each pair, one is wrapped around the other in some spiral pattern. They are twisted around each other and hence we get the name Twisted Pair. Why? Why twist them? Well, the physics of it, if the pairs around, when they both have transmissions on them, then they sort of cancel each other out and they do not interfere with others. And they do not pick up interference from the neighboring wires. So think that there are four pairs of wires in the cable that you see and they're twisted at length such that they will not interfere with the other pairs. So this is a way to design the cabling so that we avoid interference and minimize interference at least. So that's why we twist the pairs around each other to avoid interfering with other pairs. You may call it a LAN cable but generally called Twisted Pair, a pair of copper wires used for telephone systems as well. Very cheap. Today it's very cheap, very common in companies, in homes, in telephone networks. So in-building communications, telephone networks, LANs, all use Twisted Pair. Any questions about the cable you have passed around? No questions? How many pairs? How many pairs? There are four pairs. There are eight wires, four pairs of wires. If you look close and you can't see, you see that they're twisted around each other, each pair at different twist lengths. And again, this is done so that they reduce the interference on each other. There's some outer coating around all of the pairs of wires. The ones that we see here do not have any special material to provide extra shielding from interference. These are called unshielded Twisted Pair. We often refer to UTP. Even though there's some coating, it's not there to provide shielding from interference, or very good shielding. You can find other Twisted Pair which has a harder coating, a different material, such that interference is much less likely. Shielded Twisted Pair, SDP. You don't see it very often though because it's very hard to use. These wires bend and can be installed in cavities between walls quite easily, anywhere they can be installed with shielding the coating is much harder and much harder to deal with. So it's not so common shielded Twisted Pair. And within Twisted Pair, there are different types of materials, a different quality of materials used for the wiring and the coating. And you get different categories. Some you may come across a category 5, category 6. Coaxial Cable. Where do you see coaxial cable? Who's seen coax? Coaxial Cable. Maybe you have coaxial cable. Anyone? TV? TV antennas? Not the antennas, but the TV antenna cable. You plug if you have an antenna port in the back of your TV that cable that plugs in is usually coaxial cable. Similar concept. We have a conductor an electrical conductor instead of wrapping them around each other to avoid interference there's one conductor inside and an outer conductor they're on the same axis coaxial conductors here and again done arranged in such a way to avoid interference coaxial cable is commonly used where audio TV systems, cable TV so connections between components coaxial cable not so common in in lands normally we use twisted pair in lands but more common in audio hi-fi systems satellite connections not the satellite link but connection to the devices in TV. Again, electrical conductor sends an electrical signal just designed in a way to avoid interference optical fibre here we use light a light signal think there's a very thin fibre of glass or plastic here and there's a light source at one end point an LED some laser it creates light and the light bounces off inside that thin fibre until it receives at the other end point and that's our signal of our data everyone wants to go home so let's finish this one tomorrow we'll continue and talk more about the characteristics of those but look at the data rates that we talk about electrical cables we range in the order of gigabits per second data rates we'll see optical fibre has a much higher bandwidth and will allow us in the order of hundreds of gigabits per second as a data rate let's stop there tomorrow we'll go back and just recap on those three examples and then move on to wireless media