 If we remember back to our very simple model of a communication system, we have one endpoint a transmitting device, the other endpoint a receiving device, and they're connected via some link, and we said we can classify some link or some medium, and we can classify the different media as either guided or unguided. Guided I think is cables and unguided being wireless communications, no cables, no wires. What this topic is about is giving some examples of different common transmission media that the things that form our links that connect our transmitter to receiver will go through both wired and wireless or guided and unguided media. So we'll go through guided, wired media today, give a few examples. We'll start on wireless transmission principles today and then tomorrow finish with some examples on wireless communication systems, satellite, mobile phone and so on, just very simple concepts. But before we go through them we'll talk about some general factors that is relevant for both wired and wireless, guided or unguided. What are some of the factors that we need to consider when we're choosing a technology to connect our computers together? So we want to create a link from A to B. What technology do we use to connect A to B? Well there are different factors that will impact on our choice. Some key ones are what data rate can be provided and across what distance? So the data rate is how fast we can send our data, say in bits per second. Depending upon the user's requirements we may require a particular data rate. For example, if I want to transfer a large amount of data between two computers over a short period of time, like hundreds of DVDs burnt to a hard drive and want to transfer them across a network, I would like to have a high data rate across that link or across that network so that I don't have to wait for days for that download or data transfer to complete. So a high data rate is commonly desired so that the user doesn't have to wait long that we can transfer our data as fast as possible. So generally we'd like to maximize or choose a technology that maximizes the data rate, high data rates desirable. Another key factor is the distance that our technology can cover. Different technologies, both wired and wireless, are designed to work over different distances. So depending on what we want to connect. So one example is we want to connect this campus with the Runxit campus. It's a distance of about 12 kilometers, 12 to 15 kilometers, let's say 12 kilometers. We've got buildings at both campuses and we want to choose a technology to connect them because we want to transfer our data between the two SIT campuses. So we need a technology that can cover that distance or we need to use a technology that we can may not cover that distance but we can we'll see use multiple links. If we have a technology that has a maximum distance of one kilometer, that is there's some technology where the maximum distance of a single run of the cable is one kilometer. So if you with that technology you cannot transmit signals larger or further than one kilometer in distance. How do we use that to cover or connect our two campuses together? What can we do? So I tell you here's a technology you can buy the cabling system or the cables, the transmitter and receiver but it only covers one kilometer. What do we do to cover our campuses? Okay so some sort of hub or some sort of intermediate device. We have our cable that goes from Bankadee goes for one kilometer and then we have some intermediate device that we connect it to into that receives the data and then a second cable and that intermediate device transmits the signal across the second link and so on until we get into the rung SIT campus because if our technology has a limit on the distance of say one kilometer we to cover this 12 kilometers we need to have multiple runs or multiple links in this case and there are different devices we can connect together but in simple terms it's an amplifier or repeater. We send the signal across one kilometer and then a special device amplifies or repeats that signal across the next one kilometer until we get to the destination. Second technology let's say it covers a distance of so another technology that covers a distance of 40 kilometers then in that case it's obvious that we can run a single link between our two campuses so we can connect our two campuses using both technologies but the problem with the one with a shorter distance is that we need these extra devices in between our two campuses which incur an extra cost and extra complexity of building the network and managing the network so we'd like a technology that covers as long a distance as possible to meet our requirements so normally when we're choosing a technology for connecting transmitter to a receiver we care about data rate distance and maximizing them but of course there are other factors as well cost is another main thing choose one which gives us the highest data rate the appropriate distance but is the cheapest we'll see some different really uh media and compare them from these perspectives how do we get a high data rate and how do we get to transmit across a long distance well there are different factors that impact upon the data rate and the distance that we can send a signal we've seen when we looked at signals the bandwidth of the signal that we send impacts upon the data rate generally when all things are the same the higher the bandwidth the higher the data rate so we would like a technology that supports a high bandwidth so we can achieve a high data rate what impacts upon the distance that which which we can send when i talk to you if i didn't have the microphone on can you you can hear me if you're up close at the front if you're sitting or if you're standing outside in the corridor if i didn't have the microphone on would you hear me if i was talking normal strength yes would you hear me or not no unlikely you'll hear me if i'm talking and you're out in the corridor why not what what's the the term or what why is that the case why can't you hear me if you're standing out in the corridor interference maybe one thing out in the corridor and in here there may be other people talking and transmitting which makes it hard for you to hear what else why can you hear me sitting here but it's harder to hear me five ten twenty meters away what's the term that we use it depends upon the distance our signal gets weaker across distance attenuation is is the term that we've seen in the previous topic our signal attenuates there's not nothing we can do about that when i transmit a signal it starts at some strength some power level across distance it gets weaker and weaker and weaker okay so attenuation the signal getting weaker is a transmission impairment it impairs our transmission other impairments include interference we tried to do a demo last week where if i talk and then more people start talking it will be harder for one person to hear what i'm saying because there's more interference from other transmitters so if i'm sending a signal from A to B and then others start transmitting signals in the vicinity of B then they can cause interference at B and make it difficult for the receiver at B to receive the information so interference is a problem transmission impairments including attenuation noise and some other factors in some cases the number of receivers in in communication systems has an input impact the more receivers the weaker the signal is on the communications at the next receiver so we need to choose a technology that gives us a high data rate and high distance or long distance and some of the factors the impact include the bandwidth of the signals transmission impairments interference before we look at some more examples of transmission media we've gone through examples before about AM and FM radio someone remind me of their favorite FM radio channel an FM radio channel anyone 93 93 megahertz okay it's usually 93 point something but let's say 93 point something megahertz an example of an FM radio channel now with all our communications systems we don't send a signal at just one frequency we send a signal at a range of frequencies remember our signals made up of we can consider as made up of multiple components each with individual frequencies so when we say that this FM radio channel has a frequency of 93 megahertz what does that really mean if we try and plot in the frequency domain so this is the signal peak amplitude as a function of frequency then the typical shape or way to represent it we've seen seen plots and don't draw this like this they're very simplistic plots of signals with three components most real signals have a range of frequency components and we'll see in practice the plots look more like this where this is showing that this signal contains frequency components from this frequency let's say f l for the lower frequency to f h for the highest frequency that's an f so this signal contains frequency components from here up until here and the bandwidth as we've seen in all our signal analysis is just the highest component f h minus the lowest component so you can think of this as containing impulses all in this area instead of drawing the individual impulses we draw as a curve here so practical signals usually contain a range of frequencies not just one frequency and cover some bandwidth now coming back to our FM radio channel 93 megahertz what does that mean that's usually used to refer to the center point in this range of frequencies i've denoted as fc the center frequency so in this example the center frequency is say 93 megahertz point we'll see why and this radio channel covers a range of frequencies centered at 93 megahertz what's the bandwidth of FM radio well as an example the bandwidth is say 20 kilohertz so the low frequency f l so you can think at centered around 93 megahertz 10 kilohertz to one side and 10 kilohertz to the other side so f l being what is it 93 92 oh nine and f h the high frequency 93.01 megahertz how do we get those numbers i said the bandwidth of our FM radio channel is 20 kilohertz that's a typical value some vary if our center frequency is 93.00 megahertz then with a bandwidth of 20 kilohertz then we 10 kilohertz either side of the center so 93 minus 10 kilohertz is 92.09 megahertz and high frequency of 93.01 megahertz and you take subtract f l from f h and you get your 20 kilohertz most or all communication signals we think of in practice have usually some center frequency and some bandwidth now back to our slides what frequencies do we transmit signals at what are the typical frequencies used for communication signals or this plot shows the range of communication signals the spectrum for telecommunications ranging from one hertz 100 we see a logarithmic scale 100 1000 10000 1 million one gigahertz 10 to the power of nine up to 10 to the power of 15 hertz this is the the range of frequencies used for communication signals the spectrum for telecommunications so where does our FM radio channel fit in there 93 megahertz is about 100 megahertz 100 megahertz is about 10 to the power of eight or is 10 to the power of eight hertz so here 10 to the power of eight hertz is the typical range of FM radio and this plot shows us some examples of some applications FM radio and TV use similar frequencies AM radio and the order of one megahertz say 900 kilohertz satellite TV if you have a dish to receive satellite TV what frequency does the satellite transmit at down to your receiving dish and the order of several gigahertz so 10 to the power of nine is one gigahertz two three and so on so this is showing some example applications and what frequencies they use not the bandwidths we'll come to that in a moment but what center frequencies typical applications use Wi-Fi anyone know what frequency it uses my wireless LAN Wi-Fi on the laptop 2.4 not megahertz gigahertz 2.4 gigahertz which is where gigahertz is 10 to the power of nine so somewhere around here so when my laptop transmits a signal up to the access point the center frequencies are around 2.4 gigahertz and in fact the bandwidth is from memory around 20 megahertz in that case so different systems use different frequencies to transmit their signals not just wireless systems so far the examples Wi-Fi FM radio satellite are wireless systems also wired or guided media and these three are the examples of guided media that will go through twisted pair co-actual cable and optical fiber twisted pair is these LAN cables we'll go through them in a bit more detail but the LAN cables that you plug into your laptop similar to the telephone lines they use the technology called twisted pair we'll explain that in a moment but and I'll pass this around I passed it around in the in the class yesterday to the CS students I had three of these and I ended up with just one at the end of the lecture so don't take it home with you that's the last one I've got left have a have a look at the wiring at the end in inside so it's it's a LAN cable just cut and we pulled out some of the wires count them as it passes around but when you see the wiring it's just copper wires with some plastic coating on it some insulation we send with twisted pair we send an electrical signal across that copper wiring what frequency do we use for that electrical signal well with twisted pair we send signals in the this range of frequencies from 1 Hertz up to about 10 to the power of 8 Hertz or 100 megahertz about 0 up to 100 megahertz so that's the range of frequencies of the signal that we send across the twisted pair cabling or the wiring the copper wiring what bandwidth if we use all of those frequencies what bandwidth do we have available with twisted pair if we range from the low frequency of 0 or 1 up to a high frequency of 100 megahertz what's the bandwidth 100 megahertz okay it's the high minus the low so which is the typical bandwidth used for twisted pair when we send a signal we can occupy 100 megahertz as the bandwidth coaxial cable is another type of cabling system it doesn't use the copper wires twisted around each other it uses in a different arrangement it's used in some cable tv systems so if you have cable coming into your apartment building into your home then possibly not 100 sure but possibly it's using coaxial cable and it also uses electrical signals across a conductor and the signals range from around 1000 Hertz 10 to the power of 3 up to about 1 gigahertz 10 to the power of 9 Hertz 10 to the power of 3 up to about 10 to the power of 9 Hertz bandwidth calculate the bandwidth approximately anyone you don't need a calculator what do you get 10 to the power of 9 minus 10 to the power of 3 is about no no don't say anything you do with a 6 1 billion oh don't you write it down but 10 to the power of 9 9 zeros minus 1000 this is very small compared to this it's about 1 billion it's 990 million okay approximately because this is very small it gives us a bandwidth of about let's remove this now 10 to the power of 9 mega 10 to the power of 9 Hertz 10 to the power of 9 if you write it in megahertz 1000 megahertz okay a little bit less that's what this diagram shows us when we send our signals across coaxial cable the amount of bandwidth we have available is about 1000 megahertz when we use twisted pair it's about 100 megahertz with all other conditions being equal which one do you think gives us the highest data rate which one's faster twisted pair or coaxial cable have a guess which one do you think it's faster twisted pair or coaxial cable look at the bandwidth the second one coaxial cable why have a all right look look at the numbers the bandwidth is bigger and as a general rule with everything else being the same the larger the bandwidth the larger the data rate okay if we get if we've got more bandwidth to use we can send our bits faster so and it turns out to be true in most cases if we use coaxial cable we could send data at a higher rate than when using twisted pair because the bandwidth is much higher there are other reasons why we can get the impact upon data rate as well like interference and protection from interference both of those twisted pair and coaxial cable we send some electrical signal across some conducting material copper for example a third technology optical fiber with optical fiber we have very thin strands of glass or plastic and with some insulation around them and we pass light through them not electricity but we have a light source at one end point and the light reflects through the fiber and arrives at the other end point so we send a signal which is light and we see that because the signal is used in optical fiber the portion of the spectrum is the portion of or it's a set of frequencies which are about the same as the visible light the same frequencies as the light coming from the the ceilings from the projector and so on so we just send light through optical fiber calculate the bandwidth available in optical fiber ranges so we send when we send light the range of frequencies we transmit about 10 to the power of 14 up to 10 to the power of 15 hertz gives us a bandwidth of what 10 to the power of 15 minus 10 to the power of 14 quite easily 9 by 10 to the power of 14 hertz that's our bandwidth available when we use optical fiber which is how many megahertz with optical fiber the bandwidth we have available was about 900 million megahertz with twisted pair it's 100 megahertz so about 9 million times the amount of bandwidth available and similar about a million times more bandwidth coaxial cable which one do you think is faster now of the three fiber optic cables the much larger bandwidth available in optical fiber now these are just approximations that don't take these numbers to be exact and under different systems it varies but it's a a good indicator that with optical fiber we have a much larger bandwidth available the bandwidth is the difference between the maximum frequency and the minimum now note that this diagram is on a logarithmic scale so even though this line is the largest of the three this one's very small note the scale here means that this small one at this endpoint is represents a much much larger bandwidth than the other two so the scale can be confusing on this picture what we're going to do is go through and talk just briefly about those three wired technologies cut twisted pair coaxial cable and optical fiber before that what else do we see on this picture so it shows us some example technologies in these six and then some classification of technologies at different range of frequencies so from about 10 to the power of four to 10 to the power of nine generally called the radio frequencies and then we have microwave frequencies so sometimes you'll hear reference to a microwave system it's referring to systems that support signals in these frequencies infrared okay your remote control your infrared pointer and then visible light okay the frequencies of light that we see and there are frequencies of course above that in the spectrum but not used commonly for communication systems so there's and I cannot remember x-ray gamma ray frequencies okay other classifications but not not important for most communication systems this plot also shows on the bottom axis wavelength remember wavelength is inversely proportional to frequency the relationship the wavelength lambda is the speed of light c divided by the frequency and it shows that if we have a a frequency of 100 megahertz the wavelength here is about a third of a meter about 30 centimeters if you do the calculation there sorry about three three meters so 10 to the power of zero is one 10 to the power of one is 10 it's about three meters is the wavelength so sometimes people talk about the wavelength of a signal other times more commonly frequency last thing to see on here when people talk about different ranges of frequencies sometimes they give names to refer to a particular range radio microwave infrared and some other names along the top here lf mf hf anyone want to guess what those three mean lm lf mf hf low frequency medium frequency high frequency just the names that different organizations give to the different set of frequencies low frequency medium frequencies high frequencies vhf very high frequencies there not very ingenious names they have uh uh u hf ultra high frequencies hf super e hf extremely high frequencies okay so it's just getting larger and larger so what's next you can make something up what about the other direction VLF very low frequencies ELF extremely low frequencies here's a confusing one VF the only one that doesn't follow this pattern VF is for voiced frequencies if we look at the frequencies we're in the order of several hundred Hertz or five hundred Hertz up to five kilo Hertz which is the range of the human voice so VF voice frequencies you don't need to remember that those definitions and but just understand when you hear someone talking about UHF VHF understand it's referring to a portion of the spectrum where the indicating the frequencies of the signals used in that system and you can look it up to find the exact frequencies any questions on us our spectrum remember spectrum means the range of frequencies a set of frequencies well this is the spectrum for all communication signals the entire set of frequencies used for any signal let's look at three examples of guided media you would have heard of most of them twisted pair coaxial cable and optical fiber of the three examples the first two are similar in that we take some electrical signal we generate electricity at the source some electrical signal flows through some conducting material say some copper wiring and what are the characteristics of those system both of the first two electrical cables so we can transmit some signal across a conductor copper is a common one now without going back to an understanding all the details of the physics the one of the things that occurs when we transmit electricity across a conductor is that that represents energy flowing across the the wire some of that energy radiates out of the conductor okay so you can think of the signal going through the copper wire some of the energy disperses outside of the copper wire and also if there are other sources of energy nearby that copper wire can pick up the energy from those other sources so we transmit energy out if we're the copper wire and other transmitters can transmit such that our wire receives that energy what that means is that when we transmit across one wire we can cause interference on other cables on other wires and other sources transmitting electricity can cause interference on our cable or in our wire so we need to deal with that because with interference that's effectively noise for our signal I transmit my signal across my copper wire if there's other transmissions received at the receiver they are considered noise by the receiver so the receiver receives my signal plus it receives all these other signals from other sources which makes it much harder for the receiver to work out what the information was that I sent to it so the more interference the more noise and the effectively the lower data rate that we can transmit the information correctly the more errors occur so we want to avoid interference it's bad for our communication systems it results in poor quality signals being received so now how do we how do we minimize interference different ways keep the length of the cable short the short of the cable the less that cable will pick up from other sources and of all the less it will radiate to other sources as well so the shorter the cable the better but we said at the start that one of our requirements or one of the things that we want is to have long cables so it's a conflict here we cannot always just use very short cables where's our example if I attach an endpoint to this okay the two adapters on both ends is it very useful for connecting your computer to other devices at this length not very useful you can but you need to put your laptop very close to the other device okay normally you know you want a couple of meters also or if you connect from this PC down to the third floor you see it goes from the PC and it goes up inside here and it goes up to the ceiling and the cable goes in fact through the ceiling down some wall cavities down to the third floor and connects to a device there so we need 30 40 50 meters for those cables the longer the cable the more interference how do we avoid interference other ways make sure that there are no sources nearby that transmit energy that transmit other electrical signals okay keep them away from other sources in practice not always possible look at our LAN cable it actually goes from the computer that you cannot see most you cannot see it goes past a power cable and an audio cable and the VGA monitor cable on the floor and goes into the wall here and one of these in here is a actually there's a video cable in there and I think even a phone a telephone cable all of them transmit electrical signals and they're all very close to each other which means they can all cause interference amongst each other so it's not always easy to keep our cables away from other cables other sources so in fact the best thing to do is to design the cables the arrangement of the cable such that they minimize the amount of energy they radiate and minimize the amount of energy that they pick up from other sources how do you do that use some former shielding add some coating around the wiring to protect protect from interference and or organize the wires such that they don't radiate so much energy out that second one is how we get twisted pair this this LAN cable is an example of using twisted pair cabling you see that there are four pairs of wires in here if you look closely there are eight wires inside the plastic coating so we have some outer white plastic coating and inside the coloured coatings inside them copper wire each pair of copper wires is twisted around its partner and it in this manner the reason for doing that is if we do twist them like that when we send electricity along the pairs that they effectively cancel each other out causing very little interference on other sources and picking up very little from other sources so this is a way of arranging our wiring so that we minimize the interference one way is to twist them around at each other there are other ways as well so this has been around for a hundred years or so this concept of twisted pair inside this LAN cable there are four four twisted pairs there's some colour coating if you look close you would see that each pair has a different twist length so some are wrapped tightly some have a longer twist length and again that's just to minimize the interference between different pairs so two insulated copper wires arranged in a spiral pattern inside a LAN cable we have four sets of those pairs where they used telephone networks your home telephone wiring uses twisted pair LAN networks LANs so you use them every day in computer networks inside buildings homes and so on some old in the past old telephone networks use them between across cities as well but now that's been replaced by other technologies so twisted pair is one of the most common wired media around is cheap it's relatively easy to use easily available and supports moderate data rates there are different variations of twisted pair some have more shielding than others so you'll see references to shielded twisted pair STP and unshielded twisted pair UTP what is this one an example of shielded or unshielded this is an example of unshielded twisted pair in fact when we say shooting this plastic coating is not the shielding it's just some insulation the shielding contains and I don't have an example but contains some metal shielding inside say in the outer white insulation there's some metal inside there to shield the inner wires from interference now what that means in practical what that means that the advantage of the shielding is less interference less interference means less errors better quality signal and ends up with we can send higher data rates so using shielding allows us to send our data faster not using shielding we get lower data rates so unshielded twisted pair is generally slower or supports lower data rates than shielded twisted pair the problem with shielding is with unshielded you can bend them very easily okay you can install them quite easily you can run them anywhere you can feed them through the cavities in the in the ceiling in the walls because they bend quite easily when they have shooting they have metal inside and they just don't bend easy I don't have an example but it's very hard to bend so the shielding makes it very hard to use in practice so if you want to feed it through a wall or around some corners you have to really bend it to do that just makes it not so convenient to install and to manage the shielded twisted pair so in practice you'll see unshielded twisted pair much more is much more common than shielded twisted pair even though shielding gives us higher data rates within these there's also different categories which refer to the quality of the copper wiring and the manufacture of the materials you may have heard of category 5 category 6 cabling we're not going to go into any more details about these three examples just give a quick examples of where they used what some of the advantages and disadvantages are coaxial cable also we send electrical signals across some conducting material it's just that the arrangement is different from the twisting it's arranged in that we have two conductors we have some inner conducting material some insulation around that and then some outer conducting material and we send our signals across both of the conductors and this pattern of one inside the other again minimizes interference so it's a way to arrange our electrical system to minimize interference from other sources and on to other sources they're both on the same axi the two conductors provides much more shielding from interference than twisted pair more shielding less interference higher data rates and longer distances as well as some practical results compared to twisted pair we can go faster and longer with coaxial cable generally a bit more expensive a bit harder to deal with and where is it used cable TV system so inside a neighborhood and coming into your home if you have cable TV coaxial cable is used in some audio video cabling connecting hi-fi components coaxial cable can be used in the past used in long-distance communication say between cities but mainly that's been replaced by optical fiber now optical fiber we have this this fiber in the middle and we send light at one end point and you can think it just bounces off and the light is received at the other end point so our signals are not electrical now but a light signal we have much higher bandwidths available the fibers are usually glass or plastic whereas it used long-distance communications across a city between cities between countries in some special cases inside a LAN or in a telephone network but not so common maybe between say in a data center where there's a lot of servers connected together use fiber to connect them together because we need very high data rates but in your home no what's the advantage compared to the other two we lose much less of the signal as we transmit it the signal attenuates much less than in the two other systems which means the signal can go much further before it's too weak to be received i.e. we can transfer across much larger distances much higher bandwidth available much higher data rates available so a single fiber in practice is equivalent to hundreds of different hundreds of cables so much more convenient just to use one fiber as opposed to hundreds of individual electrical cables and as a result if you can replace a hundred cables a hundred cables with one very thin fiber in fact have multiple fibers in a very small space it's easier to install and a lower cost for installation and we don't have interference from other electrical sources so much better in terms of distance and data rate compared to the other two so as to finish on those three a quick comparison of the three common guided media we have twisted pair coaxial cable both use electrical signals the data rates vary so different systems have different data rates but on the order of one gigabit per second is typical with our LAN cables 100 megabits per second one gigabit per second is common the distances we're talking about for twisted pair up to about two kilometers coaxial cable up to about 10 kilometers in fact usually they're much shorter than that so with twisted pair usually only used over a distance of distances of about 100 meters but we can go up to two kilometers so if we wanted to connect our two campuses together across 12 kilometers we need some form of repeaters or amplifiers if we use either of these technology they're generally cheap especially when you only need data rates in the order of 100 megabits per second one gigabit per second if we compare if we look at twisted pair there's unshielded and shielded twisted pair and also coaxial cable it has similar properties to shielded twisted pair it has more shielding with unshielded twisted pair because it bends it's much easier to install but without the shooting there's more chance of interference with coaxial cable and a shielded twisted pair we protect against interference but with that shielding it's harder to bend and therefore harder to deal with the cabling so some trade-offs if we move up to optical fiber we're talking about data rates of 10 100 gigabits per second and beyond usually individual fibers are combined together so that a single cable has many fibers in one talking about distances about 40 kilometers so if we want to connect optical fiber from Tokyo to LA the west coast of us how do we do it we want a connection between Asia and the US we're not going to use satellite with too much delay how do we connect optical fiber down in the ocean the bottom of the ocean so there's ships that essentially lay optical fiber off the back of the ship and it goes down under the ocean just what's called a submarine cable and there's many of them laid across the Pacific Ocean and most of the oceans around the world and use optical fiber but every 30 or 40 kilometers they need an extra device that receives the signal and then transmits it again because the maximum distance we can cover is about 40 kilometers so in that case it's better to go much further because you need fewer of these intermediate amplifiers or repeaters with optical fiber the equipment for the cabling and the equipment for installing is much more expensive with unshielded twisted pair this has been cut what we can do is just grab another piece and wrap the join the wires together and it will work again okay you can do this in your home just rip off some of the insulation and connect another wire together and so you can make these at home yourself easy cheap with optical fiber if you cut the fibers you need very specialized equipment to join those fibers again so very expensive tens of thousands of dollars to do that for optical fiber because the fibers are very thin glass or plastic fibers and you can't just cut them and then join them back together again okay so very expensive but if we want to transmit data at very high data rates 10 100 gigabits per second rather than using many electrical cables using just a single optical fiber becomes cheaper okay even though there's this high initial cost because we can support large distances and large data rates it eventually becomes cheaper of course it's hard to install so quick very quick coverage of three common wide transmission media any questions again very quick what i want you to know is the general trade-offs of those three media which ones are typically faster which ones cover the most distance what's the advantage of shielding versus no shielding okay i don't ask you to remember all the technical details just the general trade-offs between those three the remaining slides here the next few slides give some more technical details we're not going to cover them that's if you want to look up and see the exact range of frequencies the attenuation across different systems delay and so on and some different physical characteristics of the the signals what we have for the rest of this topic is about wireless so that was some examples about wired now unguided media when we send a signal from a transmitter and the signal disperses so we don't it's not guided along some conductor or inside some fiber it's unguided so wireless transmission what we want to do is introduce the general concepts of wireless transmission applicable to all wireless systems and then we'll go through a few examples of say satellite mobile phone wi-fi all right the first four are just some examples i think you know plenty of examples tv transmission satellite satellite internet satellite tv wi-fi infrared inside your home and so on so there are many examples you know of of wireless communications a simple model of a wireless communication system we take some at the transmitter we have saved some device that wants to send data how do we send it wirelessly or how do we send a signal across the air and what we normally do is we take some electrical signal and some other device called an antenna that converts that electrical signal into some radio signal which propagates through the air and is received by another antenna which has the job of converting it back to an electrical signal so some of the things we care about again how fast can we send our data data rate how far can we send our data so what distance can we separate the transmitter and receiver by so that they can still communicate and they will depend upon a number of factors including the antennas so the antennas have a large role in how good our communication systems is so let's just introduce the concept of what the antenna does and a few simple concepts which we'll finish for today so antenna takes some current and creates some electromagnetic wave that propagates through the air and the receiving antenna takes that wave as an input that energy and converts it back to some current back to electricity at the receiver what type of frequencies are we transmitting wirelessly typically ranging from three kilohertz up to 300 gigahertz we will look at different characteristics of antennas and it turns out whether it's a transmit or a receive antenna they have the same characteristics I don't mean that the transmit and receiver must be the same type of antenna but when we look at the properties we'll introduce things like the size of the antenna the gain of the antenna those same properties apply for both transmit and receive so sometimes we'll just discuss one of now what does our antenna do some electricity comes in and it produces a wave that comes out so it it disperses it disperses energy okay the signal importantly the direction in which that energy or that signal is dispersed has an impact on how well we communicate so the direction and how it propagates how far it propagates depends upon the shape of the antenna so the design of an antenna has a large impact upon how far we can transmit and how we can arrange our antennas so that we can communicate before we go through these give me an example of an antenna that you've seen or you've used or you know about the shape or the size of an antenna the Eiffel Tower okay what frequency does it transmit I have no idea other systems you may have used here in Thailand today in the last hour in the last two minutes does your mobile phone have an antenna yes it does you cannot see it it's built into the into the the handset itself I don't even know the shape of it but that they may be different on different phones you may remember some old mobile phones you had a pull-up antenna okay the old-style ones now they're built in same as my laptop has an antenna built into the back of the screen here okay so it's usually some pattern on there it's hard to visualize antennas on the wireless LAN access point these two poles two sticks here are the antennas called dipole antennas what other ones have you seen recently maybe the red true ubc satellite tv antennas about well this big for satellite tv you have in your home and these parabolic dish antennas so that common-shaped antennas of course this size antenna versus a small one on an access point we will see that generally the larger the antenna the further we can transmit a signal and we look at those properties the bigger the antenna the better it is for sending a signal maybe you've seen on mobile phone towers on the top of buildings or on some tower if you look at the top of those towers you'll see some rectangle type antennas or sort of pads that can help draw it like this they're what's called sector antennas they look like a rectangle they effectively transmit in one sector of a circle so many different types of antennas the antennas for your radio on your car old style tv antennas so the shape the size of the antenna has a large impact upon how well it sends a signal a an ideal antenna or a very simple antenna is called an isotropic antenna this is an antenna which if if this was an isotropic antenna what it does is we receive some electrical signal and it produces a wave that comes out the the signal that we're transmitting and the energy in that signal with an isotropic antenna disperses in all directions equally what that means is that if we could see the energy coming out of this isotropic antenna in this direction forward back left right up down in all directions around the energy will disperse equally so if we transmit with some power level from the source and we measure the power received say one meter in this direction and we measure it to be one watt okay then if we measure it one meter behind it will also be one watt it'll be the same in front and as behind and to the left and to the side and up and down and all around one meter away from the source would all get this one watt as the received signal because the energy disperses equally in all directions it's called an isotropic antenna so you can think around the transmitter we've got some spheric sphere that indicates how the energy disperses it's an ideal antenna and in practice all of our real antennas are referenced against this ideal isotropic antenna we cannot build one we can get close but it's the practical limitations mean we cannot build this perfect dispersion of energy so there's an isotropic antenna in theory what about other types of antennas well depends upon how the energy disperses with isotropic it goes equally in all directions in others it goes strong in one direction but weak in other directions so some examples common ones will see an omnidirectional antenna with an omnidirectional antenna we say the energy across one plane say the horizontal plane is dispersed equally so from here going forward going back left and right the energy disperses in the same manner equally but up and down in the vertical plane it's much weaker which means if I measure one meter in front of me the received signal to be one watt but I measure one meter above me the signal strength will be less than one watt it will be weaker so say half a watt it depends upon the design of the antenna so an omnidirectional antenna goes equally in all directions on one plane but say up and down on another plane much weaker you can sometimes we see a diagram that shows it as like a donut so it's round in this area but it doesn't go up or down these dipole antennas on the access point are omnidirectional they around the the pole they essentially transmit equally in these directions but down and up quite weak directional antenna concentrate the power in one particular direction so think of the parabolic dish antennas you see for satellite tv a dish about this big you need to point them at the satellite in space okay for your tv if the satellite is up there if you point the dish that way you will not receive the tv so that's because that antenna concentrates all of the energy in one particular direction so the energy and the resulting signal is very strong in that direction but in other directions behind and to the sides it's very weak so again if we measure the signal strength one meter away in this direction if it's one watt and in the opposite direction maybe point zero zero one watts very strong in one direction but weak in other directions it's directional and they're the different shapes of antennas and the different designs impact upon how that energy is focused or concentrated in a particular direction so sometimes it's highly directional sometimes it's dispersed across an area and in different patterns antenna gain can i give you a quick example instead of going through antenna gain i'm talking about wireless connections antennas and my wi-fi doesn't work on the laptop so that's i wanted to show you some examples of some antennas but you've seen plenty if you go to this website Cisco is a company that manufactures networking equipment including some wireless equipment and from there you'll find some links and one of the pages and i'll try and show it tomorrow give some description about antenna concepts and gives many examples of antennas you'll see pictures of these dipoles sector antennas some dish antennas and so on since my internet is not working and we need a bit more time to explain antenna gain i'm going to stop there and we'll continue that start on talk about how do we compare the design of real antennas against an isotropic antenna to know that and this is what you need to study tonight if you can't remember we're going to talk about decibels so we saw decibels when we looked at uh the shannon capacity equation signal to noise ratio remember decibels 10 times log base 10 of one power level divided by another so we'll use that same concept when we talk about antennas so just remember and if you can't remember remind yourself about the equation for decibels and the concepts behind it let's continue tomorrow morning