 We have talked about how to we send data using signals, and we've looked at some of the characteristics of those signals, including bandwidth, frequency, data rate. Now we want to look at the thing between the transmitter and receiver. So we transmit a signal from our transmitting device, and it's received by our receiving device. But what's the thing between those? Well, what's the media that our signals pass via? That's what we want to talk about. One transmission medium, a single one medium, or the different media. And in one simple example, one transmission medium is a LAN cable, or the copper wires inside this LAN cable. So we'll look about the cabling that we can use to connect a transmitter and receiver, as well as some other issues. We'll first introduce, more generally for all the different media, the issues of the range of frequencies we have available. If you recall from our signals, we said, every signal we transmit is not just one frequency, but it's across multiple frequencies. It has frequency components. And the set of frequencies in that signal, we call the spectrum of the signal, and the width of that spectrum, we call the bandwidth. And that's an important characteristic. It turns out that we can't use just any frequency. There are a limit of frequencies that we can use to communicate. So we'll look at what range of frequencies we have available and talk about some applications that use those frequencies, which we'll also call the spectrum, or more precisely, the electromagnetic spectrum. All possible frequencies we have available to us. So we'll talk about that first. Then we'll talk about different media, giving examples from first what we say guided media. Guided means wired. The signal is guided through some conductor or some material. And we'll give three examples of that. We'll finish that today. And then in the next lecture, we'll look at wireless or unguided media. And we'll look at different concepts of wireless transmission, things like antennas, and how much signal we lose when we transmit across some distance. So today, we'll just do spectrum and guided media. Transmission media. The thing between the transmitter and receiver, that's what we want to look at. And the signal propagates through this thing, through this transmission medium. When you design a communications network, you need to choose which medium is best for you. So some of the factors to consider, well, a medium that will give you the highest data rate, generally we would like to transmit as much data as possible. Sometimes we care about distance. You want to set up a network that covers the entire country. Then you would like to use some medium or some links that go as far as possible so that you don't have to have multiple transmitters and receivers. You can have as few as possible to cover a large area. So distance is sometimes of interest. With respect to signals, we've said one of the trade-offs with bandwidth is that increasing bandwidth increases data rate. That's good, but it also increases cost. So generally, we would like to minimize or we have a constrained bandwidth. We can't just use any bandwidth we choose. We want to minimize transmission impairments. We don't want noise. We don't want too much attenuation. The different types of media will have different amounts of noise and difference amount of attenuation. So choose one that's best. And different media will have different costs involved. For example, optical fiber costs more than our LAN cables, our copper wiring. So we're distinguished between wired or guided and wireless unguided media. When we're tapping on what we know about our signals that we want to send, this picture we've seen before, it shows a signal, a practical signal in the frequency domain. What it shows us is this signal. The blue line shows that the signal has frequency components ranging from down here up to here. But most of them, most of those components have the highest magnitude. And the most energy from about here to here. If we plotted impulses, then the highest ones are in this range of frequencies. So if we define some cutoff, some low and high cutoff, we think that the signal that we transmit has a particular bandwidth. Although that does have components outside here, they're very small that we can almost forget about them. They contribute very little to the signal overall. So a signal that we transmit, two characteristics we care about are the frequency, or the center frequency usually, and the bandwidth of that signal. And we've said larger bandwidth, larger data rate. But this frequency that we can use, and the bandwidth that's available to us, is limited because there are a limited range of frequencies that are usable for sending signals, our communication signals. So we must be aware of what range of frequencies are available, and that's defined by the electromagnetic spectrum. We'll see a picture in a moment. All possible frequencies that we can use, we'll define. And all of those frequencies need to be shared amongst multiple users. Users in different and in the same location, amongst different applications. So in practice, what happens is that usually government authorities, national, and then agree on an international basis, make laws or regulations that says, who can use which frequencies? One example is that the government licenses or auctions off a license to use the mobile phone frequencies. So if a company wants to set up a mobile phone network, they need a license from the government to use that particular range of frequencies, and that has some cost involved. So all the frequencies are usually regulated in some manner, and the regulations set up such that many different users can make use of those frequencies to send signals, but also that we don't need to fear with other users. If two different users transmit a signal with the same range of frequencies, they will interfere with each other. And the transmission impairments that we saw last topic we said interference is a form of noise. The more interference, the more noise, the less data we can deliver. That's a bad thing. So what is this electromagnetic spectrum? Here's a picture that roughly covers the range of frequencies that we can use for communications. And in fact there are two pictures, the top one and the bottom one. So we'll focus on the top one first and explain what it shows us, and then briefly look at the bottom one. Maybe zoom in a little bit more, same pictures. First, what does it show? Follow this solid black line here. It shows the range of frequencies we have available. So it starts from, and the frequency is, see these dashed lines, starts from 10 to the power of 0 hertz, which is 1 hertz. And the scale goes up to 1,000 hertz, 1 million hertz, or 1 megahertz, 1 gigahertz. 10 to the power of 12 is what? What are the prefixes? After giga, 1 terahertz. We have 10 to the power of 12 will be 1 terahertz, 10 to the power of 15, petahertz, 1 petahertz, and 10 to the power of 18. I can't remember. Exahertz, maybe, EXA. And 10 to the power of 21, I definitely don't know. So we know about kilo, mega, giga, tera, at least. You should know about them. And the others, well, you may know about. So that's what this picture is showing. The range of frequencies from 1 hertz up to 10 to the power of 21 hertz. And these rectangles give us some names of those ranges of frequencies, or the bands of frequencies. So some common names, some will recognize, some may be new to you. For example, the small one in the middle or here, this small rectangle means that the frequencies of visible light are in this range, a little bit less than 10 to the power of 15 hertz. So the light that we see, ranging from what color? What's the lowest color? Red, ranging from red up to, you know, rainbow. What color is at the top? I can't remember either. But I think the top one's purple or violet. Okay, so the range of frequencies of light fall in here, the visible light range. The frequencies below that are given the name infrared, below red. Infrared here, so from about 3 terahertz up to the visible light. And above visible light, ultraviolet. Now that's not used for data communications very often, but that's the name that's given for that range of frequencies. Above that, x-ray and gamma rays. So we're not going to focus on them. For data communications, it's mainly below visible light. So what have we got below? Terahertz is this small range here, but the two common ones that will come across a lot are the microwave range and the radio range of frequencies, radio frequencies, sometimes RF. So they're some common names given to the sets of frequencies that we have available to us. When we send signals at different frequencies, we may impact upon how much data we can send, but it also impacts upon how well our signal propagates. Does it go through the wall or not? How well does it pass through water? And humans are made up mainly of water, so how well will it pass through humans and will humans say interfere or attenuate the signal? So different frequencies will pass through obstacles in different manners. So when we choose a communication system and design a signal, we need to select a frequency that best suits our needs. So let's look at some examples of communication systems and see what frequencies they use. One we've mentioned before is Wi-Fi. And we said that the Wi-Fi access points commonly use a frequency of about 2.4 GHz, 2,400 MHz. So this line here just shows us that Wi-Fi is about... So 10 to the power of 9 is 1 GHz. Note it's a logarithmic scale. So Wi-Fi is about in this range, about 2 GHz. That's all we're showing. And when we transmit a signal from my laptop to the access point on the wall, that signal again contains multiple frequencies. It has a bandwidth of about 20 MHz. And it's centered on one particular frequency. There are different channels like 2.412 GHz, 2.437 GHz. They correspond to different channels. But that's all within this range. 3G or mobile phone systems in general, when we use data access and your voice access, they have different ranges of frequencies. One of them is typically around the 2 GHz. So a common frequency band for 3G is 2.1 GHz, 2,100 MHz. Other ones are 1,800 and 1,900 MHz. And there's some lower at about 850 MHz and 900 MHz. Different telecom companies, AIS, TRU and others, may use different frequencies. Okay, so it's not such a problem now, but in the past maybe three, five years ago, when you bought a phone, you wanted to buy one that supported the frequency that your operator used. But nowadays the phones support all different, the wide set of frequencies. What else do we know? FM radio is about what's your favorite FM radio channel? Anyone? Anyone listen to FM radio? 100 something, something megahertz. So it's about 100 megahertz FM radio. So that's around here. So this is 1 GHz, 100 MHz, or 10 to the power of 8. AM is about 1 MHz. So it's something about 900 kilohertz. Radio stations are from 900 to about 1,000. Satellite TV, maybe you receive satellite TV, then the signals beam down from the satellite to your receiver and the order of several gigahertz, 2, 3 gigahertz. It may differ in some satellites. So there are examples, in fact, of wireless systems. Most of them don't overlap. That is, my mobile phone doesn't transmit at the same range of frequencies as my Wi-Fi device. If they did, they would interfere with each other. So the government regulations set it up such that if you're using Wi-Fi, you should use these frequencies. Mobile phones use a different set of frequencies, designed so that they don't interfere when people use them in the same vicinity. What else can we look at? Your microwave oven? What frequency does that use? When you zap some food, microwave uses about usually the same frequency as Wi-Fi, in fact, about 2.4 GHz. So if you're cooking some food in the microwave and you're trying to use Wi-Fi, they may interfere with each other. But it's usually not a problem unless maybe your laptop's inside your microwave such that your Wi-Fi won't work. There are other applications. These are just some common ones that we may have heard of. Let's also mention some wired or guided media. And we'll spend a little bit more time looking at each of these guided-meter examples. But just to show you, your telephone line at home, not your mobile phone, but the landline telephone, the one with the cable that goes into the wall and goes out to the telephone network, and also your LAN cables that you plug into your computer, these use copper wiring. So if we look later, there's copper wires in here, and they're called twisted-pair wiring or twisted-pair cables. We'll explain them in a moment. But the range of frequencies which are transmitted across twisted-pair range from about 1 Hz up to about, this is about 100 MHz. If you follow the scale, 10 to the power of 6, 10 to the power of 7, 10 to the power of 8, about. It's logarithmic. So twisted-pair, when we transmit a signal, the bandwidth of that signal sent across those cables is about from 1 Hz up to 100 MHz. So what's the bandwidth? Minimum component, 1 Hz, maximum 100 MHz. The bandwidth is a difference, which is about 100 MHz. So the bandwidth, when we send a signal across twisted-pair, is about 100 MHz. Another wired system we look at, and you may have it if you have cable TV at home, or maybe cable internet access, or even the cabling between your audio equipment, it uses co-actual cable. We'll explain the name shortly, but that's another type of medium that's used. And that carries signals with frequencies of about 1 KHz up to 1 GHz. What's the bandwidth? Or maybe a different question. Compare twisted-pair versus co-actual cable. Two different wired media. Which one has larger bandwidth? Twisted-pair or co-actual cable. When we transmit a signal across these wired media, which one can transmit a larger bandwidth or potentially a larger data rate. Twisted-pair ranges up to about 100 MHz, so bandwidth of 100 MHz. Co-actual cable is 1 KHz up to about 1 GHz. The difference is about 1 GHz. So the bandwidth of twisted-pair is about 100 MHz. Co-actual cable is about 10 times greater, about 1 GHz. And generally, with co-actual cable, you can get higher data rates than twisted-pair. And a third system that we'll talk about in a moment is optical fiber. What's the bandwidth shown in this diagram of optical fiber? Approximate it from the picture. Try to work out the approximate bandwidth of optical fiber. Just roughly. So here we get up to about, what, from 1 up to 100 MHz. That's an M. This is about 1 KHz up to 1 GHz. So the difference for twisted-pair is about approximately 100 MHz. Co-actual cable, it's approximately 1 GHz bandwidth. What about optical fiber? Roughly, it goes... The highest frequency component is about 10 to the power of 15. The lowest is about 10 to the power of 14. Note, we have 10 to the power of 12, approximately 13, 14, 10 to the power of 15. So 10 to the power of 15 minus 10 to the power of 14 is... It's about 10 to the power of 15. Less than, of course. But when we compare it to the others, it's 0.9 times 10 to the power of 15. Compared to... Well, how many GHz is that? 1 GHz is 10 to the power of 9, so this is about 1 million GHz. My LAN cable has a bandwidth of about 100 MHz. My co-actual cable for cable internet or cable TV is about 10 times the bandwidth. Optical fiber is about a million times the bandwidth of your co-actual cable. So with optical fiber, we can send at much higher data rates. And we'll give some examples later. What do we send across optical fiber? Look at the frequencies. It includes the visible light. And light through very thin fibers. Fibers of glass or plastic. And the light passes through them, and that light represents the data. So just some examples of different transmission media and where they sit in the electromagnetic spectrum. There are not so many examples above that. They'd be very specialized. Most of them in the radio or microwave beds. The other part on this diagram, it also shows the wavelengths. What's the equation for wavelengths? It'll come up in the exam. When we do calculations, you'll need to know what is wavelengths. What's the equation? Does your phone show you the equation for wavelengths? Lambda equals the speed of light divided by the frequency. That is, the wavelength lambda of a signal is the speed of light, C, or about 300 million meters per second divided by the frequency. So this plot also shows some example wavelengths. So 3 kilohertz has a wavelength of 100 kilometers, 10 to the power of 5 meters. 300 megahertz has a wavelength of 1 meter. And we get smaller as we go along here. What else do we see? The second diagram. So we have some names of some of the range of frequencies. They're not very specific. Radio covers from 3 kilohertz up to about 300 megahertz. And there are different applications in there. Some other organizations have given other names to some portions of that spectrum. Some of them you may have heard of. On this picture we see some names and we see that split from 3 kilohertz, 30 kilohertz times 10 up to 300 kilohertz and so on. These frequencies are given names as listed here, where the F means frequency. M is medium frequency. L is low frequency. H is high frequency. So just some names given to ranges of frequencies. VLF, what's under low? What's lower than low? Very low frequency. VLF. Ultra high frequency. Very, very high frequency. Super high frequency. Extremely high frequency. And tremendously high frequency. Not very smart names, but effective for here. And I think you may have seen some of these abbreviations or heard of them before, especially UHF and VHF. They are used in some handheld communications but especially used in TV broadcasts. If you pick up free-to-air TV, not via satellite, not via cable but just from the antennas on your TV, then it's usually using either UHF or VHF. If you have satellite TV or satellite internet access and you look at the specs of your satellite receiver the frequency it receives from the satellite up in space, then usually it's labeled with one of these other bands. So there's some different names. So from 1 GHz up to about 40 GHz, there are some bands of frequencies labeled by letters usually like C band from 4 to 8 GHz. KU band, KA band are some common ones used for satellite communications. So when you subscribe to a satellite operator and you get the satellite dish at home usually that antenna will be designed to receive a signal in one of these bands like C band or KU band depending upon the application. The purpose of this is not for you to have to remember all of these abbreviations and names but to be aware that we have a range of frequencies available for us. And they are separated into different groups and importantly that regulations set up such that only certain applications can use those particular ranges of frequencies. So the government sets up regulations. I'll show you an example of an allocation of the frequencies to different applications. You don't have this because it's so detailed that you need to actually look at it online printing out doesn't help. This is, we'll zoom in in a moment, this is the US frequency allocation chart. It summarises how the US government allocates different frequencies for different purposes. Nothing to see yet but what we'll see in a moment when we zoom in, it ranges from I think 3 kHz or 3 Hz, 3 kHz up here and it goes across and wraps around and we'll zoom in I think this is 3 MHz, 30 MHz, 300 MHz it goes up to I think 300 GHz here. So from 3 kHz up to 300 GHz they define what types of applications can use those frequencies. So this is allocated by the government, the US government. There's an equivalent one for Thailand it's just not in so much colour but it shows the details or not on one picture. And when we zoom in we'll see some of the names here that are quite general and the colour code in corresponding correspond to the different types of applications. Before we look at them we'll look at the the naming scheme, the legend here and we see that some of the colours corresponding to different types of applications. It doesn't specify which company or which specific application has that frequency but the types of applications. You can see there's aeronautical mobile so for planes or planes for satellites for radio astronomy so getting signals from space, amateur radio for maybe handheld radio or home operators for location services so things somewhere in here GPS would be covered the signals that the GPS satellites send down to your phones or your phone knows where you are that's a location service for navigation many satellite type up operations mobile, so mobile phones would fit under here LAN mobile this is aeronautical and I think maritime so out on the sea and so on space and satellite. Broadcasting is like TV and radio broadcasting so that are the types of applications that this picture shows us and we will not look at too many of them but let's try and find somewhere aware of for example TV broadcasting this scale it's 54 megahertz up to 72 megahertz is used or allocated for broadcasting channels 2 to 4 this is in the US and it's similar in Thailand I'll show you another picture that shows that and some different applications for mobile planes space applications and so on TV broadcasting 5 to 6 FM radio in the US so this is our 88 megahertz up to 108 megahertz and some other applications here and a lot of them you see there's a lot of satellite applications satellite space operations for communications from earth to satellites and even between satellites or between objects in space so these regulations are set up so that different users will not interfere with others and if we scroll down we'll see I think this is about 1.7 gigahertz 1.8 gigahertz the mobile ones listed here usually for mobile mobile phone systems so 3G, 4G systems would fit within some of these frequency ranges but there are many others as well and the last one this 2, 4, 1, 7 up to 2, 4, 5, 0 is generally reserved for well for multiple purposes but includes wifi in this range 2.4 gigahertz this one's hard to look at in detail on the screen so maybe you'll look at that in your own time there's a link on the website let's find another one that maybe makes more sense to you for TV in particular for VHF this shows for different regions in the world for very high frequency range of bands in different regions TV channels and what frequencies they use and Thailand fits within the scheme of Western Europe uses the same scheme what TV channels do we have here free-to-air TV channels I don't watch TV can someone tell me TV channel in Thailand 3, 5, 7, 9 7, 9 I'm not sure about this one of them isn't named by the number but there's 11 I think NBT maybe 11 it is actually channel 11 then there are others which maybe UHF which we'll not see in this so you know I think of 3, 5, 7, 9 there's also 11 what frequencies are used follow the yellow ones that applies here so channel 3 for example when the TV station transmits antenna at the TV station it transmits and it transmits a signal and your TV antenna receives that we're not talking about satellite or necessarily cable here think of just picking up on your TV antennas channel 3 uses frequencies from about 59 yeah 54 megahertz up to about 61 megahertz 54 to 61 a bandwidth of 6 to 7 megahertz there's a range of frequencies used say for channel 3 and the other channels use the same bandwidth so each channel is allocated the same bandwidth there's a gap for the TV channels because this is used for our FM radio up to 100 megahertz some other applications some marine so on boats radio and then we get channel 5, channel 7 channel 9 for example is about 200 megahertz 202 up to 208 megahertz for channel 9 so each TV station transmits a signal at the same time but they use different frequencies different center frequencies such that they don't overlap and your TV when you change the channel at least on the old style TVs analog TVs when you change the channel your TV it really tunes into that particular frequency and receives that signal on that range of frequencies so just another example of the use of the spectrum for different communication applications any questions before we look at guided media and we'll look very quickly at guided media going through three examples twisted pair, coaxial cable and optical fiber first two examples of guided media we use electrical cables the way that it works the way that it works is that we plug our cable into a transmitter and the other end into the receiver and the transmitter generates an electrical signal that goes onto the conductor and we'll zoom in in a moment and see some copper wires inside here that conduct the electricity and the electrical signal goes to the other end and the receiver receives a signal of the data so just using electricity to send our data copper is a good conductor of electricity is commonly used in data communications now the problem so that's use electrical cables the problem is when we send electricity through a conductor that the energy goes through the conductor but it also radiates outwards so it's not going through the wire but it's also spreading out and that can cause interference on nearby wires so when we have some wires nearby each other both having a signal across it because the energy radiates out it may interfere on the other wire and interference is bad it creates noise in the same way that our wire when the electrical signal is sent radiates out our wire can pick up energy from neighboring wires so that causes interference so we don't want interference because it means we won't be able to send much data we get a low data rate or lots of errors we receive poor quality signals and that makes it difficult to communicate so the challenge then is how can we still use electrical signals but minimize the interference well, different approaches it turns out the shorter the cables the less interference we'll get the less we'll pick up from others but that's a problem if we want to cover a long distance we can't just have every cable being 10 cm because if I want to connect my computer to the switch down on the third floor we need a longer cable so we can't just restrict every cable to be very short it's ineffective if we keep our cables away from other cables or other sources of electricity we can avoid interference but that's also hard in practice think of the LAN cable going from this PC you can see it at the back here maybe it goes into the wall and then through these conduits and up through the cavities in the wall and down to I think the second floor computer center but there are many other nearby cables audio cables other data cables in here power cables all containing electrical signals it's very hard to keep your data cable away from other sources of electricity and therefore to avoid interference so the third approach is to try to design the cables so that it protects them from any interference so that they don't radiate energy out and they don't pick up energy from other sources two approaches use some materials outside the cables to shield them so provide some shielding so that it doesn't pick up interference and if you return to very basics of physics you see think about the way that electricity flows you can organize the wires so that they sort of cancel each other out and don't cause interference on others and that's what we get with twisted pair and coaxial cable twisted pair and well I talk about it I'll pass around some examples twisted pair is taking copper wires and twisting them around each other many examples so have a quick look and part along got one left examples of twisted pair but I'll show one on the screen as well what you're seeing what I've passed around is just cuts of a LAN cable and if you can't see it in front of you you'll see some things like this it may not be the same so all we've done is cut the LAN cable up and if you look closely the LAN cable inside it so inside this outer coating there's white coating there's in fact multiple wires inside there are in fact eight wires eight copper wires inside in fact the copper wires have some plastic coating on them as well and you'll see there's some different colors like the blue, orange, brown and green here and they go in pairs that is each pair of copper wires usually there's a solid color plus a white with that same color so blue and blue and white orange and orange and white that's what they're on if you look closely at those pairs you'll see that they're twisted around each other that is the blue and it's part of blue and white they're twisted around each other and that twisting is such that when we send a signal down those pair of wires they the signal that radiates out effectively cancels each other out and they don't interfere much with the neighboring pairs so we can send a signal down the blue one and it will not cause significant interference on the neighboring wires either inside this cable or even other cables so that's the idea of twisting the pairs to reduce the interference and you need to go back to some basic physics to work out why and in particular the twist length has an impact if you look very close you'll see that they do have different twist lengths some are tightly twisted some are looser and again that's to avoid the interference so twisted pair we have a pair of copper wires usually twisted around each other a LAN cable has four pairs but other systems may not have four telephone lines and another common example of twisted pair very common in telephone networks specific for LAN cables that's not the topic of today but there are four pairs we send a signal in one direction on one pair so we send the signal on both wires in the pair say the green one I can't remember which colour but say the green one we send in this direction and that device at the other end can send a signal back on a different pair in the opposite direction we get full duplex communications remember full duplex we can send in one direction and at the same time be receiving from the other direction and we achieve that by having different pairs of wires in your LAN cable with some of your computers the older computers it only uses two of the four pairs one for transmit one for receive in devices that support one gigabit per second ethernet it uses all four pairs two for transmit two for receive to get a higher data rate maybe I gave away the answer to my question what data rate can you get for a LAN cable you plug this into your laptop the other end into your switch or router at home how fast can you send how many bits per second no one's done that before tried measured if you look at the spec maybe of your computer the transmitting device supports most devices today support 10 megabits per second 100 megabits per second and 1000 megabits per second or one gigabit per second so normally the devices at each end point will negotiate to choose the highest that both support my laptop supports 10 100 or 1000 if I plug it into this PC it supports 10 and 100 they'll only use 100 megabits per second but if this PC supported 10 100 or 1000 then they would negotiate up to the higher speed of 1000 megabits per second depends upon the devices so 100 to 1000 megabits per second is typical for LAN cables you can get higher there's 10 gigabit gigabit per second ethernet but generally nowadays it uses optical fiber you can use copper wires but most of them use fiber the purpose of the lecture today is just to give you three examples of guided media not to look at the details of them so TwistedPair is one example used in telephone networks, in building networks LANs for example and has different types of cabling these wires have an outer coating, this white coating on the outside it's quite bendable it's referred to as unshielded TwistedPair UTP the white coating doesn't provide much protection from interference it doesn't really provide shielding from interference you can buy a different type more expensive called STP and what that does is it provides more protection from interference and therefore if there's less interference we can send faster so shielded TwistedPair is better for performance the problem with the shielding is that the cables no longer bend, they're much more rigid and if you can't bend the cable very well it's very hard to place it through the conduits and through the walls so it's not very popular to use shielded TwistedPair it's very hard to use unshielded TwistedPair is very cheap and very common and the quality of the copper wires and the coating and shielding leads to different categories so you may hear of category 5 category 6 6 LAN cables so there's one example of a guided media another one which I cannot pass around and have a copy an example coaxial cable maybe you've used it for cable access cable TV so maybe in your apartment building or dorm building there's cable TV coming into the back of the TV there's a coaxial cable between components in an audio or hi-fi system sometimes from a satellite receiver to your TV is commonly used again coaxial cable uses an electrical conductor usually copper and we send a signal across that copper conductor to minimise interference rather than wrapping two conductors around each other they use a different physical property and they have one conductor inside then some insulation around it so this is a cut through the cross section here the inner conductor takes the signal then there's some insulation and then there's an outer conductor and the physics of it is when you send electrical signal the outer conductor protects the energy from radiating out any further and from picking up energy from other sources so this is really two conducting materials the inner one and the outer one uses so we call it coaxial cable the outer sheath is just the outer protection on that cable it has more shielding from interference than twisted pair and can generally achieve higher data rates it has a larger bandwidth like we saw on the electromagnetic spectrum and can send across longer distance than twisted pair we'll put some numbers to them in a moment use for cable TV audio video some long distance communications like connecting between offices across a city between cities or maybe between countries but it turns out nowadays most of such connections have been replaced with optical fiber coaxial cable is not so common in such long distance communications so optical fiber is our third example it doesn't take an electrical signal it takes light we have very thin hair like the thickness of a human hair a fiber made of plastic or glass and you have a light source at the end point and the light goes through that fiber for example reflecting off and is received at the other end point and that's used to carry our data this optical fiber is generally much more expensive than the other two the materials are more expensive dealing with the fibers very thin fibers is much harder than with the copper conductors so much more expensive but has advantages in that we don't have interference from other electrical sources it's a light signal not electrical signal so as long as we can keep that light signal inside and keep it dark from outside we will not have interference from other sources when light propagates through the material it doesn't lose so much power compared to electrical signals it doesn't attenuate so much lower loss meaning we can send it across larger distances much higher bandwidth than the others so we can send it to higher data rates so a single fiber can support the equivalent of hundreds of electrical cables so instead of having 100 electrical wires we can have just one fiber smaller space occupied which is an advantage in some cases that was the picture of the co-actual cable the inner conductor some insulating material and the outer metal conductor is really providing protection from interference and just the outer plastic coating and not a real picture because it's hard to see the individual fibers but with optical fiber usually they're inside one cable they're the very thin fibers and they need a lot of protection to make sure that the light is maintained inside there's no light from outside sources but they're not broken so the other parts are just the shielding or the protection around those fibers so a single cable may have many tens of maybe even hundreds of fibers inside that cable and that's all this picture is showing that we have fibers and then protection multiple layers of protection for those fibers I've passed around some LAN cable we cut it up if you were energetic you could take those pieces and join them back together again you find the copper endpoints and just wrap them around each other and you could join them back together and it would work we just carry the electrical signal through the copper conductor you can't do that with optical fiber using your hands the fibers are so fine that we need special equipment if we cut them to join them together we have issues with the high cost of optical fiber not just the material but the high cost of installing it changing the length and so on so to summarize just our three examples of guided media two types of electrical cabling one optical cable twisted pair and coaxial cable give us data rates in the order of about gigabits per second twisted pair up to one gigabit per second the cable can go faster the distances are about 2 kilometers up to 10 kilometers that is with a single cable with twisted pair the maximum we can get is about 2 kilometers let's say you want to build a network or a link from Bangkok to Chiang Mai what about 900 kilometers if you wanted to use coaxial cable every 10 kilometers you'd have to have a special device that received the signal and then transmits across the next cable this special device is some form of repeater every 10 kilometers you have a special repeater device so that you can cover the long distance each repeater cost money and is difficult to operate so one of the problems with a short distance is that it's hard to cover of course a very long distance it's more expensive to cover optical fibers we get data rates at the order of hundreds of gigabits per second per cable and the distance is about 40 kilometers some can go further so again going from Bangkok to Chiang Mai every 40 kilometers we'd need some repeater so for one repeater here we'd need four if we used coaxial cable or 20 if we used twisted pair so that's why having a longer distance is an advantage even though optical fiber has high data rates it's very expensive so it's only really effective if we want to carry a very high data rate if I want to send a movie from my computer to my TV I don't need hundreds of gigabits per second I only need tens of megabits per second so a LAN cable is okay but if I want to send all of the internet traffic from everyone in Chiang Mai out through the gateway in Bangkok then hundreds of gigabits per second is maybe important so when the amount of data to be transferred is very high optical fiber becomes cost effective and I think that's the main points that we want to make about those three examples of guided media we're not going to look at the characteristics just be aware of different types of guided media and maybe some of the tradeoffs which ones faster which ones cover the longer distance which ones are cheaper